Note the use of a cradle on the TV aerial which it is good practice to use for all “high gain” aerials (particularly X Beams) due to their higher wind loading. Mounting cradles strengthen the aerial and reduce the twisting forces exerted on the pole and clamp. End mounting aerials can exert a very high torque and this was exceptionally annoying when we were undertaking aerial tests whilst there was wind about ! On the other hand Log Periodic aerials should always be end mounted. On a related point if any aerial is fitted with a cradle this should always be fitted perpendicularly to the dipole so as not to interfere with the tuning of the directors. Similarly the pole can also interfere with the aerial`s tuning as with this DAB antenna.
As more elements are added to a Yagi aerial (provided they are “tuned” correctly) the acceptance angle narrows and the amount of signal “collected” (the ”gain” ) increases. Gain is measured in decibels or dB [this is a logarithmic scale] and for aerials it should be measured as dBd, which is the increase in signal received from an antenna compared to that which would be measured from just the dipole on its own. Note that 0dBd does not mean that zero signal would be received by the aerial, it just means that the aerial only picks up the same amount of signal as the dipole would on its own, i.e. with no reflector or any directors. In fact the vast majority of FM and DAB aerials are just dipoles which only have a gain of 0dBd, yet they work fine in most cases. On the other hand a TV aerial of only 0dBd would need a very strong signal to work OK.
Always remember that there is more than one way to measure gain, e.g. dBi and dBd, the latter being the more honest parameter. To convert dBi to dBd deduct 2.15 from the dBi figure.
Gain is not everything but it is the simplest most objective measure of the efficiency of the aerial, though you should always be careful when comparing the quoted gains of aerials.
For any aerial the tuning of the elements is critical. This is a highly specialised job and it is fundamental to the performance of the antenna. If a Yagi aerial is designed to work over a large section of the band (a “Wideband”) it cannot be tuned precisely and therefore its gain and directivity will be lower. You have to remember that Yagis were originally designed as single frequency aerials. Thus if you really need a high gain aerial and are able to use a grouped antenna on your transmitter, a grouped aerial is always preferable. This is particularly true in the case of A group transmitters, but also (to a lesser extent) K group and B group transmitters. Having said that, most people don`t need a high gain aerial and a Log periodic would be preferable.
Ever since Dr Yagi patented his eponymous aerial in 1926 people have been trying to improve and adapt it. At the present time there are four main versions, the “conventional” Yagi, the “Double” Yagi, the “Tri Boom” Yagi and the “X Beam” Yagi. All these use the tuned director / dipole / reflector (see above) to produce gain. Basically the Double Yagi (DY), the Tri Boom and the X Beam (XB) are all attempting to increase the number of directors (and therefore the gain) without making the aerial too long. The Double Yagi places two sets of directors side by side whereas the Tri Boom stacks three sets one above the other.
The X Beam is a variation on the Double Yagi using either one V shaped director or two shorter directors to mimic the longer flat directors of the Double Yagi and it is an attempt to limit the width of the aerial, and make it cheaper to manufacture.
Now we come to the contentious area of “element count”. It was always the convention that each director was counted as an element, the dipole was counted as one element and the reflector (in its entirety) was counted as one element. Unfortunately in this marketing* dominated world of ours this simple concept has been eroded. Many manufacturers now count all the individual elements of the reflector as separate elements, and when it come to X Beams some even count each “X” as four elements, when arguably that is double the real figure !
In any event, aerial gain is also dependent on the length of the antenna, and if it`s a grouped aerial, it`s not just about how many elements it has.
So, the moral of the story is : be very wary of manufacturers` “element counts”.......
* read Bulls**t
I am not an expert on aerial design, I just test them and know what works. In my experience, for a given number of elements, the highest peak gain figures are achieved by the conventional Yagi. The next highest efficiency is the Double Yagi, followed by the X Beam and in last place is the Tri Boom. The best theory I have heard to explain these “league positions” is that since RF is longitudinal in waveform, the closer the director chain is to the direct line of sight to the transmitter (along which the wave is propagating) the more efficiently it will work. I must stress here that we are talking for aerials of the same number of elements, obviously a Double, Triple or X Beam would normally have more elements and may (but may not) therefore have more absolute gain, but remember that when talking about aerial gain one must bear in mind that peak gain and average gain are not the same thing. I have found that Double Yagis and X Beams do seem to “broaden” the gain curve* over the conventional Yagis, and the A group and B group gain curves show this quite clearly. This wider gain curve is obviously useful for wideband applications, which is why we recommend the DY14WB / XB22WB but not the Yagi18 in a wideband version, whereas we do recommend the grouped versions of the Yagi18.
* This is also be down to the fact that most X Beam type aerials have a larger reflector which will tend to increase gain at the bottom of the band (because it`s tuned for those frequencies).
So why do we see so many X Beams and Tri Booms around (most of which aren`t even needed, see Log Periodic aerials) whilst conventional Yagis are becoming less common and Double Yagis are strictly for the cognoscenti ? There may be a lot of Tri Booms about, but just because a particular product is common it doesn`t necessarily mean it`s any good, "stick on your windscreen" car mirrors for instance. Who came up with that crap idea ?
Well some cynics say that X beams and (particularly) Tri Booms look more impressive and so that`s why people use them, especially aerial installers who can then charge more for them !
Aerials as fashion victims ? Me a cynic ? Not in this world.......
Note, in this article a channel number (CH) refers to a frequency not a programme.
Although the TV UHF broadcast band in this country now only goes up as far as CH60 it used to stretch from 471 MHz (= CH21 which has a wavelength of 64cm) at the bottom, to 847 MHz (= CH68 with a wavelength of 35cm) at the top. Now anyone who knows anything about RF (Radio Frequency) will tell you that it is asking a great deal of an aerial to perform well across such a wide frequency band but certain people have a vested interest in trying to ignore scientific fact.... The poorer performance of widebands is clearly shown on the graph showing Grouped v Wideband aerial gain curves. To get around this, when UHF TV transmissions were introduced in the mid 60s, great care was taken to minimise reception problems by utilising grouped transmitters and receiving aerials. That is to say all the the broadcasts from any particular transmitter were fitted into one group of frequencies, stretching across about one third of the full band. For example Crystal Palace was (and still is ! ) an A group. The groups (for now) are shown in the table below. Note that the groups have changed slightly over the years, these apparently minor changes can sometimes be significant .
Looking like an abstract piece of sculpture this is Llanddulas a repeater in North
Wales. It uses the bottom Log Periodic to receive its signal from Moel Y Parc transmitter
and it then re-
For some reason I really like this picture, the abstract look of it or the blue sky ?
When it comes to aerial choice remember the basic rule (which the broadcast authorities also use) if you can use a Log, use a Log !
See this aerial report.
On the other hand a Log is not always the best aerial if you`re in a poor spot.
I`d only add that if you really do need more gain than a Log will give you (therefore losing some of its inherent advantages, most significantly its superior impulse interference suppression) try to make sure the aerial you do use is a really high gain type.
The fact that one of the dipoles can be resonant for any part of the broadcast band means that Log Periodics have a pretty flat signal gain curve. They also have a tight polar response (i.e. they are less likely to pick up signals you don`t want [particularly from the rear]), a decent “cross polar rejection” and low wind loading. Finally Logs have good suppression of impulse noise which can be a cause of intermittent blocking and freezing on digital signals. Manufacturers have been known to claim that Log Periodics short this noise out to earth, but it`s just as likely that the aerials` tight polar response just means they`re less likely to pick it up in the first place !
Unfortunately Logs have a relatively low gain figure, the shorter models being particularly poor in this respect. Even going for a grouped Log won`t give any more gain than the wideband versions because the manufacturers have simply chopped off the part of the aerial which isn`t required to receive the relevant group, which is why we don`t bother stocking them. Adding extra elements does seem to improve the gain a little (though some say that goes against the theory of how they work ! ) but you can`t take that too far because the aerial would then become too long and Logs shouldn`t be used with cradles because that can short out some of the signal to earth. Thus Logs are only really suitable for areas with a reasonable signal, though the Log36 is OK for medium signal strength areas as well. Apart from this Achilles heel the Log Periodic is one of the best antennas, particularly for digital, which is why we use them wherever possible, as do the broadcast authorities, for both receiving and transmitting.
Well I think there are three main reasons :
First, the Log Periodic is a fairly recent development, more will appear as time passes.
Second, most aerial installers are notoriously tight. Logs are more expensive than the equivalent small Yagi and they`re twice the price of a Contract aerial......
Third, many installers are stuck in their ways and use what they`ve always used, especially if it`s cheaper as well !
Log periodic aerials use the same basic mechanism (of director + dipole + reflector) as a Yagi but instead of having one dipole (onto which all the RF is “focused” ) they use all the elements as potential dipoles. I say potential because for any particular frequency only one of them will be “resonant”, i.e. producing significant signal down the cable. At this particular frequency the element in front will act as a director and the one behind as a reflector.
This is the main reason that our recommendations jump from a Log36 to a high gain grouped aerial, or a really high gain wideband aerial if your transmitter is a wideband.
If you use anything smaller the nett advantage (over a Log 36) is debatable.
Graph of typical gain curves for “high gain” Yagi 18 element aerials
Notice the lower gain of the wideband antenna over the grouped aerials, and that this is particularly marked at the bottom of the band, i.e. group A. It should be noted that group A was extended up to CH37 (from CH34) in 1995.
The E group is a “semi wideband” aerial which sacrifices a bit of gain at the bottom so as to increase it at the middle and top end.
Also see :
If one looks closely at the various grouped aerials one can see that the elements become smaller as the frequency rises and the wavelength shortens. The clearest example of this are amateur radio antenna arrays. Thus it can be appreciated how an aerials gain is not just down to the number of elements or its length. The most important factor is the “tuning” of the elements. This is why an A group aerial (no matter how high its gain) will not receive the “out of band” frequencies. Furthermore, the higher up the band they are, the lower its response to them will be.
Note the coloured “bungs” in the ends of the booms, they are to denote the group of the aerial. If an aerial has a coloured bung it is a grouped type. On the other hand the vast majority of aerials with a black bung are widebands, but not all of them ! (particularly foreign made).
Also see :
You may have wondered why some aerials are mounted (or polarised) horizontally and some vertically.
To work efficiently aerials must be polarised in the same plane as the transmitter (TX) they are receiving from. All main TXs such as Emley Moor or Crystal Palace are horizontally polarised whereas repeaters (so called because they receive the signal from a main TX then rebroadcast it) are usually vertically polarised. The reason for this use of vertical and horizontal polarisation is to take advantage of the fall off in response (to a signal of the opposite polarity) to
One last point to bear in mind is that vertically polarised aerials will be more
susceptible to picking up off beam transmissions (the main cause of co-
Virtually every TV aerial ever manufactured can be polarised horizontally or vertically.
Unfortunately there is an inverse square law over the addition of elements to achieve more gain. Adding 2 elements to a dipole (to give a 3 element aerial) will give a relatively large increase in gain, but adding an extra 2 elements to an 18 element aerial will make very little difference. Once over 18 elements ehe law of diminishing returns really starts applying itself in a big way which is why we end up stocking XB16s, anything smaller wouldn`t give significantly more gain over a Yagi 18 or XB10.
As mentioned in the section on aerial groups, one of the biggest factors determining an aerials efficiency/gain (particularly at the bottom of the band) is if it is a grouped antenna or a wideband. In fact there is no such thing as a “high gain” wideband aerial for the A or K group frequencies.
That said, grouped aerials may have more gain but I`ve never seen the point of 10 element
From the point where the aerial would be mounted can you see the transmitter ? If so, and the transmission pattern is not restricted in your direction, then it`s a fairly safe bet that you`re in a strong, or very strong, signal area (though this may not be the case if it`s only a small repeater transmitter). In fact it may even be the case that you don`t need a different aerial, you need an attenuator !
Aerials never work as well in lofts as they do outside so if you intend to site your antenna inside it may be wise to go for a slightly higher gain type.
Thus, in order to choose the best aerial, it is very helpful to know the signal strength in your location. For those who do not know the latter, and most people don`t, there are a few pointers one can use to discover this. You could try one of the signal strength prediction websites, but I have to tell you they aren`t always very accurate.
Remember you can have too much signal, the latter can actually contribute to interference. That said, it`s usually pretty easy to add an attenuator to reduce the signal level and that (if used with a high gain aerial) will also improve your signal to noise ratio.
Note, a small aerial with a built in amplifier is not a “high gain” aerial, but what is a high gain aerial ?
“High gain” is obviously a relative term but generally speaking an aerial with 18 elements (or more) is considered to be so. However element count is a bit of a contentious issue, so I`d take most manufacturers element counts with a large pinch of salt.....
It must be stressed that setting up a phased array can sometimes be difficult / time consuming and there are no guarantees how well it will work either, but, they can be very effective against multi path because they reduce the beam width and increase the front to back ratio, plus the reflected signals should be received slightly out of phase and therefore reduced.
The transmitter on the right is Birchover, a repeater in the Peak District. The bottom pair of aerials are 18 element Yagis in a phased array and are used to receive the signal from Stanton Moor. The latter is only just visible above the trees thus dictating the use of a pair of “high gain” Yagi receiving aerials.
Types of aerial (Also see Yagis, Double Yagis, Tri Booms & X Beams)
In Quality terms "Contract" aerials (the type with the plate type rear reflector) are generally at the bottom of the pile (apart from Bacofoil aerials, obviously.
The elements on a Contract are of thinner/slimmer gauge alloy, in fact they have about 35% less metal than those on our Yagi18s.
The Contract cradle (if fitted) is smaller and doesn`t have a tilting clamp, the latter allows the aerials elevation to be altered. The latter can sometimes help to maximise signal acceptance and reduce interference, usually by tilting the aerial upwards slightly at the front.
The Contract aerial rear reflector is made of thin alloy plate which as well as being physically weak, has a relatively high wind loading for its small size.
Most Contract aerials (particularly older ones) do not have baluns in the dipole.
The performance of Contract aerials (gain wise and ghosting wise) can be very inconsistent. The best of them aren`t too bad, a good Contract 18 may only be about 1dB behind the best of the Yagi 18s (i.e. these ! ), but the worst of them are truly appalling. It`s not just the average gain figures (which can be up to 3dB down on a quality aerial of the same size) but they can have huge random dips in their already low gain curves, glitches of a further 4dB in some cases.
Unfortunately it`s impossible to tell whether any particular Contract is a good un` (relatively speaking of course ! ), or a bad un`, though if it`s got a balun in the dipole it`s a good sign. Historically most aerial installers have fitted contract aerials because they`re cheap, in fact in the 70s and 80s the price difference between Contracts and “quality” aerials was even bigger than it is today. Furthermore Contracts don`t take up much room on the van and they`re quick to assemble, remember “time is money”. The advent of Digital and the (relatively) smaller price differential of higher quality antennas has encouraged more riggers to abandon contracts, but they are still the cheapest and so continue to be widely used.
I have to say it makes me laugh when I see a big expensive house,
with a big expensive car on the drive,
and on their roof
is a cheap crappy Contract aerial !
Without wishing to contradict myself, Contract aerial may be the Ladas of the aerial world, but at least they`re better made than Bacofoil aerials. DIYers who use Bacofoils have the excuse of ignorance (assuming they haven`t read this website ! ) but more and more "bottom end" aerial installers are starting to use them* rather than the Contract aerials they traditionally fitted, and they have no excuse at all because they know how flimsy they are. It`s an absolute disgrace. At least the errant installers who fit (or fitted) Contracts were using UK manufactured aerials, now they`re using Chinese made Bacofoils !
* Maybe they`re hoping to be called back when it gets blown apart by the wind. Or maybe they think that they look more impressive (from the ground that is....) so they can charge more for the install, or even so they can, falsely, describe it as a "digital aerial"
the wrong side of the Cliff Edge while keeping the required one at the top but if this works (with just an attenuator) it`s more likely that the tuner was suffering cross modulation due to excessive signal.
Perhaps the best approach is to resite the aerial (say down the side of your house) and attempt to shield it from the rogue transmitter.
The ultimate form of high gain aerial is the Phased Array though these are not generally installed just to increase gain. A phased array is two (or more) aerials linked together on to one downlead. The cables from each of the aerials to the combiner should be exactly the same length, though this is slightly less important with Log Periodics than with Yagis. The distance between the two antennas should be about one metre. Once the array is in situ experimentation should be undertaken to optimise the signal by varying the distance between the aerials and moving the antennas (front to back) relative to each other. Assuming the aerials are perfectly in phase then in theory it`s possible to obtain 3.5dB extra gain compared to a single aerial (which is a significant increase) but in practice losses in the combiner [usually a splitter “in reverse”] will generally reduce this figure to around 2dB or less. However, any increase in the signal to noise ratio (that has been achieved by the phased array) would be maintained.
It cannot be over emphasised how important it is for the two antennas to be perfectly in phase at the point where the combiner joins them together therefore a decent meter (not a £20 job.....) is essential when setting up a phased array. Any phase difference may very well give less gain, in fact if the two aerials were 180 degrees out of phase it would be theoretically possible to get no signal at all, not that this would ever actually happen in the real world !
source more than that from the unwanted one. If the resultant level is too high the attenuator then knocks both signals back down but maintains the increased difference between them. It is occasionally possible to just use an attenuator to tip the interfering signal onto
Having said all the above, even well made aerials aerials can be strengthened by junking any supplied wing nuts and fitting conventional nuts instead. These are far stronger and can also be tightened more effectively, do not over tighten them though ! If “Nylocs” are used (the type with the nylon insert to prevent them working loose) the fastening should never fail.
To be honest I cannot understand why manufacturers use wing nuts at all, lets face it 99% of people fitting an aerial would have a spanner anyway and all professional installers have a decent ratchet spanner, so fitting a conventional nut would actually be easier to tighten up !
Yet another case of manufacturers not talking to their customers ?
Note. Nothing can compensate for an aerial which is manufactured in a flimsy manner to start off with. The easiest way to find out how well made your antenna really is ?
Try bending the elements, you may be shocked by how flimsy they are..........
Most models of aerial are available in 2 or 3 sizes (e.g. 10 element or 18 element) to suit the particular installation. An appropriate analogy would be the same model of car being available with different engine sizes, although these days the number of elements is only a guide to an aerials gain, “element count” being as much to do with marketing as anything else. There is no point in fitting an aerial bigger than is actually required, though an installer (or an aerial supplier) trying to sell the job up may disagree with that ! Too much signal can be detrimental and there would also be an increase in wind loading.
Remember, most people do not need a big “high gain” aerial.
Historically most ranges of aerial have been available in all the major groups and wideband but there is now a trend by some manufacturers and suppliers to only deal in widebands. One reason includes the fact that more transmitters now need a wideband but just as importantly (for them) is the fact that it`s much easier to manufacture just one aerial group. Even more significantly it`s far easier to sell just widebands rather than have to go into all the complexities of which transmitter the customer is on and which group would be a suitable choice etc. The downside, as we have seen, is that widebands have inferior performance because they are a design compromise, see TV Aerial Tests.
This particular aerial is on Hebden Bridge transmitter which is an A group, and at these frequencies a Yagi18A would blow away even a huge aerial like a DAT 75, or any other wideband aerial.... Incidentally, note how gravity and/or the wind has taken its inevitable toll......
We do not really recommend Tri Boom aerials* this is partly due to their large wind loading but also because I think they`re a bit of a gimmick, and our tests backed this up. Remember that when it come to aerial performance things aren`t always what they seem.......
* I regularly visit Knaresborough and for some reason I cannot fathom there are a few houses in that locality which use this particular antenna on Emley Moor.
Why is that so surprising ? All DAT aerials are widebands and Emley Moor is a B group !
To continue the car analogy having a wideband aerial on a grouped transmitter is the equivalent of running round in a minibus when you`ve only got a wife and two kids ! That said we would normally fit them in strong or medium signal areas because the best aerials to use in these circumstances are Log Periodics and they are wideband anyway. Furthermore the customer is covered if their transmitters broadcast frequencies are changed in the future but remember that at the present time there are no plans to change any more transmitters to widebands. To finish the car analogy, the husbands had a vasectomy !
See “The Great Wideband Debate” (or passing the buck).
If your transmitter is a wideband (and there aren`t that many of them since switchover) you have no real choice but to use one, but in fringe areas you may pay for it with reduced signal quality. Bear in mind that sometimes a K group or E group will suffice, or (occasionally) you only have to forego one or two of the available channels to enable you to stick with a grouped antenna. Alternatively, if you ever needed to, you could diplex a second aerial onto your existing antenna. As a general rule, if you are in a poor signal strength in your area then I would take advantage of the superior performance of a grouped antenna if your transmitter gives you this option. This is particularly the case if your transmitter is an A, B or K group, and especially the A group.
See ATV`s Choice Of Aerials, And Why We Chose Them
OK, I accept that aerial is one of the more difficult words to spell, but here`s how NOT to do it :
aeriel, aereal, aireal, aireil, airial, arial, areil, ariel, areal & ariael.
grouped antennas. After all the main reason for going grouped is to maximise signal (and/or minimise acceptance angle), so surely an 18 element should be fitted in these circumstances !
Whilst on the subject of "High Gain" aerials which aren`t, XB5 aerials ( = X Beam 5 bay) are not "High Gain" aerials and don`t let anyone tell you they are, see the proof . Of course that is not to say they won`t work if you`re in a reasonable signal strength area (or just lucky), but, to be frank, when Log36s are available I don`t understand why anyone uses them.
They`re certainly used quite often, but just because something is used often it doesn`t necessarily mean it`s any good. Flat roofs being a good example of the latter methinks.
Flat roofs really are crap aren`t they ? I actually think they should be banned by the Building Regulations. Let`s face it, it really is basic isn`t it ? When I last checked, water had a pronounced tendency to flow downhill........
One other beneficial side effect of the narrow acceptance angle associated with high gain aerials is the reduction in the chances of multipath reception (this used to give ghosting with analogue pictures). That said, the most effective way to reduce multipath reception is to resite the aerial, unfortunately this is not always possible, or guaranteed to be effective !
Sometimes a "High Gain "aerial can be used (even in a strong signal area, possibly
with an attenuator) to minimise ghosting or co-
Knowing whether your aerial is a wideband could be important, either if you need a wideband to receive all the available channels, or, on the other hand, if you have the option to improve your signal by swapping from a wideband to a grouped aerial. A classic case of the latter being those on Crystal Palace transmitter who live in poor signal areas.
If your aerial was obtained from a non-
If your aerial has black bung in the end of the boom (as opposed to a coloured bung) then it`s likely to be a wideband.
Also see element sizes.
The frequency response of the aerial may well be stamped on it, this is often on the dipole.
How old is your aerial ? The older it is the, more likely to be a grouped aerial. Thirty years ago very few widebands were fitted, though I assume any DIY shed aerial must have been one.
If your transmitter is a wideband (e.g. Belmont) and you`re picking up all the channels, to a greater or lesser extent, then your aerial must be a wideband. Either that, or you live next to the transmitter ! A grouped aerial may not give any significant signal at all outside of its band. But also bear in mind that different widebands do give different amounts of signal.
Apart from the above, checking the signal level using a “set top aerial” (preferably in the loft) is the easiest and most accurate test that anyone can undertake without an RF meter or local knowledge, or getting out on the roof ! If you get reasonable pictures on all available channels you`re in a pretty good signal area. But remember, if aligning onto an alternative transmitter, you must retune your TV !
The average set top aerial (like the one pictured) will give about 3 dB of gain, a DM log will double this, and putting it outside gives another 30 to 70%, or more (see aerial ridge tests). The relative signal levels of all our other aerials can then be found on aerial gain tests.
Do your neighbours get good signals on all the available channels and do they need amplifiers or “high gain aerials” to do so ? Having said that many people use amps or high gain aerials when they don`t even need to, particularly if an installer is trying to “sell the job up”.
On the other hand, remember there are RF dead spots around.....
Are you up high or in a dip ? This is probably more important than the distance from the transmitter, within reason obviously ! The higher up you are the better your signal is likely to be and , assuming you`ve got “line of sight”, you`ll get massive signals if you`re at 1000ft.
Unfortunately being in a valley has the opposite effect......
How far away is the transmitter ? This is actually less important than many of the other mentioned points, but obviously the signal weakens the further away you are. Provided you have a clear view towards the transmitter, even a distance of 30 miles from a 200kW main transmitter (such as Sutton Coldfield) should still give you a pretty strong signal.
Lastly, the 64 thousand pound question, do you have obstructions between you and the direction of the transmitter ? Line of sight is (almost) everything
Potential problems could be caused by hills, high buildings or trees ? In the case of the latter I would go for the highest gain aerial (the use of a grouped antenna, if possible, becomes even more important) in an attempt to minimise multi path reception. This use of a high gain or grouped aerial is recommended even if you then need an attenuator to knock the signal level back down.
In any event where trees are involved (or RF dead spots) all bets are off. It may work fine or it may not, you can only do your best and hope, after all, who knows the secret,
of the Black Magic box.......
(See the Bill Wright`s advice leaflet on reception through trees)
It will be noted that the element for the A group is the longest because it is designed to be resonant with the lowest frequencies, which have the longest wavelengths. Incidentally the directors may all look the same size but in a correctly “tuned” antenna they vary, for example the Yagi18A directors come in five different sizes, from 19.8cm at the front up to 22.2cm at the back (with a 24.6cm [OD] dipole). At the higher end of the TV frequency band the wavelengths are that much shorter, and therefore the dimensions of the elements for a Yagi18C/D are 14.0cm, 16.0cm and 16.3cm respectively.
The roll off in an aerials sensitivity is far greater above the aerials designed
for group than below it, and our own aerial tests confirm this, see gain curves.
For example a decent B group aerial will work reasonably well right down the A band.
But its response above the B group will only be significant up to +/-
Finally there is the WIDEBAND aerial which works (to a greater or lesser extent....) across the whole band. Anyone aligned on a transmitter which has changed group (and most have not changed group, see Digital Transmitters) may not receive all of the digital services with their original analogue group aerial. I have deliberately avoided using the term “aerial upgraded" because this implies there was something inferior about their old antenna which is not necessarily the case. I have seen installers replace an A group TC antenna with a wideband Contract aerial, and that`s never been an “upgrade”.
Ironically the future of wideband aerials (which were "the future", apparently) is now in doubt because not only does the TV frequency range now stop at CH60 but it`s actually beneficial to no longer pick up signals above CH60 because of high power 4G signals. Anyway the new wideband is going to be a
T Group (T for Total) covering from CH21 to CH60, which, size for size, should also enable an increase gain over a wideband. Of course some (existing) K group aerials are more or less T Groups anyway ! Some people “in the know” think that in the medium long term everything will actually go K group, and even further into the future, A group. It should be remembered that widebands, T groups and K groups will all still work even if the band does go A group, though obviously the Ks will work the best, the widebands the worst, and T groups in the middle
Signal Strength : “Am I in a strong signal area ?” (also see signal levels)
This is one of the most important articles on this entire website, read it carefully !
Signal strength and signal quality aren`t be the same thing but generally speaking
if you`ve got high signal strength at the (unamplified) aerial it`s likely the signal
quality will also be good, excluding co-
Post switchover there are many more locations in strong signal areas !
The potential range of signal strengths available throughout the country is huge.
Others, living within sight of a main transmitter, will get a perfect picture with a “set top” aerial or (in some rare cases) just the lead into the tuner, i.e. no aerial at all !
For those who live in areas capable of reception from multiple transmitters the fact
that a wideband can pick up right across the TV spectrum can, ironically, be a negative.
This is partly because it gives more chance of co-
A good example of overlap is Sheffield where Emley Moor (a B group and the preferred transmitter) is NNW and Bilsdale (a K group) is NNE. Particularly if the customer has a wideband aerial, when the STB scans it will often find some of the weaker Bilsdale MUXES first and ignore, or relegate, the stronger Emley Moor output. If this happens relegates the box will often put them up in the 800s, e.g. BBC1 on 801, so if you suffer from this can you swap the channels around ?
If your box or TV gives you the option the simplest way round all this is to set your box to receives the correct region, or tune your box manually, but, unfortunately, not all TVs/STBs let you do this. So the next simplest solution (if your transmitter is A, B or C/D group) is to use a filter [or a diplexer as a filter]. Alternatively try pulling the aerial out of the TV, set the latter to tune, watch the tuning display closely, and only replace the aerial as it scans through the required part of the band. You could also try using an attenuator (only whilst the box is scanning) to tip the unwanted weaker transmissions down the cliff edge whilst leaving the required ones at the top.
Unfortunately not all X Beam type aerials are created equal. Maybe it`s something to do with the basic design of X Beams (it may be costly to manufacture one which is sufficiently rugged) but the truth is many of them are rather flimsy to put it mildly. In fact a great many of these cheapo X Beams really are “Bacofoil” aerials, because their elements are so thin. This is particularly the case with most Philex / SLX / Labgear X beam type aerials.
If you`ve found this site informative and, hopefully, interesting as well,
The aerial and its downlead (see Cable) are taken for granted by almost everyone, but they are just as important as the TV and in fact (electronically speaking) are actually part of it. Many people will happily spend hundreds of pounds on their television but are, apparently, unwilling to spend any more than is absolutely necessary on their aerial installation. This is an important point because the difference in gain between a cheap "contract" aerial and, say, a Yagi18 of the same group, can sometimes be quite small, but it will make some difference, and that difference (particularly with digital signals, see “Digital cliff edge") could make all the difference. In addition there are big differences in build quality between many of the aerials available on the market. Remember that a decent aerial installation should last 20 to 30 years or more *, and at the end of the day what`s the cost of a good quality aerial install compared to the cost of the average TV, or your sanity when your Digital picture keeps freezing ? ! ?
* With the possible exception of the cable, particularly if running over a roof.
1 We would normally only recommend fitting TV aerials in lofts in decent signal areas. That said, since the Digital Switchover Over when the power was increased many more people find a loft aerial is sufficient. You need to be an even stronger signal area to get a reliable signal from a “Set Top” aerial but post switchover many people find one can give acceptable results. “Booster” antennas generally work no better then unamplifed set tops for the reasons given in the amplifiers page. One of the standard small Yagi type “set top aerials” should have the best chance of working (and will generally outperform the kind of “loop” antenna that many TVs come supplied with), but if that doesn`t work I`d recommend a feed from your outside aerial. Finally, Omnidirectional type aerials are even less likely to be effective than "set tops", in fact Omnis actually have negative gain. If you get ever get good results from one you`re probably in an excellent signal area, a coat hanger would probably work as well ! See Aerials for Boats & Caravans.
2 Aerial positioning can be of critical importance, particularly when anything is close to the aerial, e.g. the chimney. Bear in mind that at UHF frequencies the wavelengths are between 14” and 25” so moving an antenna a few feet (vertically or laterally) can sometimes make a big difference to signal strength or multi path reception.
Aerial positioning can be particularly important if trees or high buildings are near to the aerials reception path from the transmitter (TX). Furthermore, although they are rare RF (radio frequency) “dead spots” do exist, that is to say a fall off in signal level without anything obstructing the `line of sight` to the transmitter. They are caused by the reflected (out of phase) RF waves interfering with each other. “Dead spots” tend to be more common in hilly or built up areas but reflections from the ground can give the same effect. Use of a “High Gain” aerial can help but what is really required is to resite the antenna (also see cranked poles). Unfortunately this is a time consuming business and there is no guarantee it will be effective. As an aside, when it comes to aerial positioning a DIY installation can sometimes be more effective than a professional one. Why ? Well most installers haven`t got time to experiment placing the aerial all over the house, and if they did have time they`d (quite rightly) expect to get paid for it, whether it worked or not. The same principle apples to experimenting with the reception on different transmitters. On the other hand a DIYer is usually prepared to spend his own time on the chance it might work.
You can also try tilting the aerial upwards at the front, it can sometimes make a worthwhile difference.
Cheap signal level meters are not generally worthwhile, except for boaters or caravanners.
£30 meters are of questionable accuracy, often only work in huge 10 or 20 dB increments and, even more significantly, they`re not frequency dependent, i.e. they only give one reading for all the received transmissions. The only meters I`d recommend would cost about £300, or more. Thus you`re better off aligning your aerial using your TV`s signal level indicator, though even these are not always to be relied on. Many TVs` signal level readings really aren`t that accurate and can even report low/no signal when in fact there`s too much ! But, at the end of the day, the only thing that really counts, is how often your picture freezes or your sound breaks up. So use that as the best guide as to optimum aerial alignment (or amplifier gain setting, or even removal of).
Note : 4G transmissions can make cheap signal level meters worthless because the 4G signals are within the frequency band which the cheapo meter is looking for. It may report large signals in a particular direction but that might just be a 4G mobile phone mast ! You can get round this by using a filter between the aerial and the meter (we recommend a CH56 filter), though meters sold after summer 2013 may have built in filters, check with your supplier.
It must be admitted that fault finding on analogue signal was far easier, but that`s gone......
Lastly, always remember that ”line of sight” is (almost) everything. Which transmitter have you got the clearest view of ? This is far more important than which is the nearest......
It cannot be over stressed that gain at the aerial is much more important than any gain added by subsequent amplifiers. It is only at the aerial that the critical signal quality can be achieved, see amplifiers and cross modulation.
The vast majority of aerials are "Yagis" which basically work by "focusing" the RF (Radio Frequency) waves coming towards them on to the dipole. The electro magnetic RF waves induce electrical currents into the elements of the aerial and it is this which becomes the signal at the dipole. The latter is the driven element which actually "collects" the signal. All the other elements either act to "focus" the required RF or prevent unwanted RF reaching the dipole. Yagi aerials, particularly high gain types, are very directional and it was this property which the Germans used when they were attempting to detect Resistance radio operators in WW2. Operatives would be monitoring for any transmissions and by utilising the directivity of the antennas they could plot which direction they were coming from. If two or three teams were doing this simultaneously the plots could be drawn on a map and the transmitter was at intersection of the lines. No wonder the radio operators had such a short life expectancy, very brave, I wouldn`t have done it, no chance.
The dipole on the left has no balun, the one in the middle has got one.
The PCB on the right is an alternative type of balun shown from the underside.
Incidentally, when wiring up an aerial with a balun I`d try to maximise contact (of the cable`s outer braid) with the balun PCB [below the cable], as opposed to the saddle clamp, because the screws of the latter are not always in good electrical contact with the PCB !
“Conventional” element count = 30 ?
“Conventional” element count = 22 ?
We are more than willing to give advice to those actually purchasing from us. Could those only seeking information please just find the answer somewhere on this site, or ring an aerial installer local to them, or call the reception advice phone numbers.
Yagis, Double Yagis, Tri Booms and X Beams including :
All Grids aerials are widebands and most consist of 4 X dipoles mounted in front of a large reflector, though some smaller versions have only 2. The dipoles are linked together in such a way that a signal picked up from within the aerial`s acceptance angle will be in phase at the point where the aerial is connected to the cable (the "feed point") and this gives the aerial its gain and directivity. Element count is largely irrelevant with these aerials but using conventional element count it`d be either 5 or 8, take your pick. The Grid cannot be considered a high gain aerial, particularly at the very bottom of the band (it`s actually behind a DM Log at that point), but by the time the B group is reached the gain curve has reached respectable (medium) levels. Since a Grid is effectively a little phased array that means the front to back ratio is high (that is to say these aerials are good at rejecting signals from behind), but the fact that each of the 4 aerials consists of only a dipole and a reflector means that the acceptance angle is large at around 45 degrees, and that isn`t usually what you want. Maybe that`s the explanation for why I found that the signal levels from the Grid on test did seem to vary more than for other aerial types. On the other hand such a wide acceptance angle does mean that if you want to receive from two transmitters which are within that 45 degree sector then you don`t need 2 aerials ! In analogue days the Grids was reputed to be an "anti ghost" antenna particularly for vertically polarised signals. These days digital signals don`t suffer from ghosting and they`re supposed to be less sensitive to multi path reception (which is effectively what "ghosting" was), but that doesn`t make them immune to it [apart, in theory, from Single Frequency Networks]. Grids have good Cross Polar Rejection, they`re not that far behind the best of the high gain Yagis particularly for vertically polarised signals.
Last, but by no means least, Grid aerials are perfectly capable of picking up digital signals, so don`t let anyone tell you different....
There is one other reason why some people still want to fit Grid aerials though, there`s only about 12" from the pole it`s mounted on to the front of the aerial, and that can make for a neat install if the receiving transmitter is in a perpendicular direction straight off the wall.
So that`s explained all about the different types of Yagi aerial, and there`s also the Log Periodic of course, but there`s a third type of aerial which you sometimes see around, the Grid aerial. Also known as Fireguards, or Stacked Arrays, or Panel Aerials, or even Billboards, yes there`s almost a name for every day of the week, well the working week anyway.
Decent Yagi aerials have a "balun" in the dipole which is usually a small PCB built into the dipole assembly and it`s designed to maximise the signal by impedance matching the dipole to the cable, hence the name from the term balanced unbalanced. Baluns are reputed to help eliminate impulse interference which can be one of the causes of intermittent blocking or pixellation on digital signals. Some say the best aerials for eliminating impulse interference are Log Periodics, because they theoretically short it out to earth (assuming the aerial is bolted to the brickwork of the house), but this is not a universally accepted theory ! Note that neither half wave open dipoles nor Log Periodic aerials need a balun because they have a natural impedance of 75 Ohms across the full band (Log`s individual elements are, in effect, half wave dipoles).
We recommend that all Log Periodic aerials are installed slightly tilted up at the front end.
The subjects on this page are listed in the following order :
As mentioned above, no Yagi aerial will work efficiently across the whole band. In the RF game (as in life generally ! ) you never get anything for nothing. At all points across the band the gain of a wideband is inferior to a grouped antenna (of the same model) but the fall off is greatest at the bottom of the band, i.e. group A. In fact such is the widebands inferiority that there is no such thing as a “High Gain” wideband aerial for the A group frequencies. In fact I would say that even the average Contract 10A would be on a par with any wideband antenna..... The only way to get a genuine high gain wideband aerial is to diplex an A group with an E group, or even an A group with a wideband if you`ve already got one of those, although very few people would actually need to do this. Incidentally we have noticed that when extra directors are added to wideband aerials (to make them “high gain”) the gain increase tends to be more at the top end than the bottom end and the gain curves for the DAT45 and DAT75 reflect this as well. To put it into layman`s terms a wideband aerial is the equivalent of “a jack of all trades but master of none”, see aerial groups, widebands, minibuses & vasectomies.
OK, I admit it, apart from Log Periodics I don`t like widebands because they offend my perfectionist sensibilities, although I must say I`m pretty impressed by the DY14WB. The latter aerial has exceptional performance for its size.
For reasons I don`t fully understand, some grouped aerials seem to work better than a wideband aerial of the same gain. Theoretically there`s only a small difference between a C/D group and wideband aerial of the same size, but they do sometimes seem to work better ! ? !