Hillsborough
Sheffield
S6 2LL
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Incorrectly Fitted Cradle Tests
In this test we deliberately installed the aerials cradle incorrectly, that is to say parallel
with the dipole/directors as opposed to perpendicular to them, see the pictures below.
As mentioned on Aerial Arrays, many years ago we deliberately mis-
our Belmot XB10A (by placing the cradle parallel with the dipole/directors) and we were
amazed at just how much worse the picture was. We really shouldn`t have been surprised,
if you think about it the whole tuning of the antenna is changed by such a bodge.
So the purpose of this experiment was to put some sort of figure on just how much signal
one can lose by placing the cradle in the wrong position. The actual loss (in dBs) varied
according to the frequencies involved, which is what one would expect. It must be stressed
that the level of loss would probably vary depending on the model of aerial but it`s a fairly
safe bet that it would always be significant. That said, if you`re in a strong signal area
anyway then your picture would still be fine. The only problem you`d have is any passing
aerial installers laughing at your antenna.....
An average signal loss of 3.7dB is very significant.
In fact to achieve a increase in gain of that amount
from an aerial takes a lot of effort. As an example
a Yagi 18B only averages + 3.6dB gain over a
Just out of interest we then positioned the aerial
on the other side of the mast (picture C) and there
was an additional signal loss of about 2dB (on
average), just for moving an aerial about 14” to
one side ! ? CH 41 (which is analogue C4 off
Emley) was particularly badly affected, a total of
9dB down on the meter, and the picture was very
much worse. I have no explanation for this other
than the aerial was then 30” from the chimney as
opposed to 36”. But the fall off didn`t occur to
anything like that extent when the aerial was
mounted correctly (as in picture D).....
A classic case of RF being a black art ?
As a postscript to the above, whilst trying to
improve my own aerial`s signal I tried putting it
on the other side of the mast, where it would
be further away from the chimney, but it
actually lost an average of 1.1dB ! (individual
frequencies varied from +3dB to -
A = Cradle installed correctly,
right hand side (RHS) of the mast.
C = Cradle installed incorrectly,
left hand side (LHS) of the mast.
B = Cradle installed incorrectly,
right hand side (RHS) of the mast.
D = Cradle installed correctly,
left hand side (LHS) of the mast.
Just how much difference does a longer aerial pole make ?
In these experiments (and Aerial Installation Ridge Tests) we set out to discover the answer,
although it must again be emphasised that these specific results are only applicable to this
particular installation. Remember RF is a black art, particularly where an aerial doesn`t
have “line of sight” to the transmitter.
We erected two DM log Periodic aerials 3ft 3in apart, with the lower one (when aligned on Emley or Belmont) clearing the roof line / ridge by 2ft 8in.
There was no ridge in the other direction, towards Sheffield /Crosspool TX. These apparently
arbitrary dimensions were governed by lack of access to our mast, it wasn`t easy (or safe) to go
any higher without having to dismantle our aerial array ! It should be remembered that the most commonly adopted method of raising an aerials height is to swap from a 6ft pole to a 10ft.
Bearing in mind that one then needs a bigger
bracket (thus losing some of the additional
height) this would normally give a nett increase
in height of between 3ft 3in and 3ft 9in,
i.e. we`re not actually far out on that one !
For Emley Moor we`re in a medium signal level area (thus we utilise a Log 40 on that
transmitter) but for both Belmont and Sheffield / Crosspool the reception is weak.
None of the three transmitters gives direct “line of sight” but in this respect Emley is the best, Belmont is in the middle, and Crosspool is the worst.
I have not published all of the frequencies for Belmont and Crosspool because some of the readings were too low to be reliable.
As can be seen in the results table,
off Emley the additional 3ft 3in height
gave an average increase of 1.7 dB.
An increase of this magnitude from the
aerial, as opposed to from an amplifier,
is significant, particularly from an
increase in height of only 3ft 3in.
I was surprised.....
To put 1.7dB into context, that is the
A group increase you`d get upgrading
However, as the ridge tests revealed we
had, by sheer chance, put the upper
aerial near the “sweet spot”, much
higher and the signal actually falls
slightly ! Similarly the bottom aerial
was just at the height where the ridge
on the roof begins to affect the
reception. Obviously the differences in
gain vary with the frequency and
wavelength of the signal(s) but all
channels off Emley Moor were up,
between 1.1 and 2.5 dB.
The Belmont results were less consistent than Emley`s, unsurprisingly as “line of sight” is
more problematic, i.e. the signal has more chance of being reflected, refracted or even cross polarised. In fact one of the five measured frequencies off Belmont showed no increase at all.
Having said that, the average increase was 1.2 dB, which is still worthwhile if you`re really
struggling for signal.
As mentioned in the article on the Topography Map there is a hill between us and Crosspool / Sheffield and this was certainly instrumental in the results off this transmitter being the most inconsistent. The thing is that being hidden from the transmitter by high ground is one of the
main reasons that people fit taller poles..... Five of the ten measured frequencies off Crosspool were increased and one showed no difference, but four actually decreased !
The average increase was 0.3dB, which is so small as to be unreliable.
Conclusions
I think that it`s reasonable to conclude that in most cases mounting an aerial higher will usually
give more signal, but it`s not a certainty by any means and moving it laterally may be just as
effective. The latter is particularly true in the case of ghosting/multi path problems.
However, unless the additional height is required to clear a roofline (or other obstruction) by at
least three feet, then I wouldn`t, generally speaking, expect miracles.
Also see this Aerial Report. Back to the top
All these tests are investigating the effect of positioning on an aerial installation`s gain
(that is to say the amount of signal it collects) and they prove conclusively that the position
of an aerial can be vital to its effectiveness. The tests on this page are all outside but aerial
positioning can be even more critical in a loft, and inside the optimum position can even
vary between aerials ! I say can be, rather than always, because the adage that RF (radio
frequency) is a black art it`s not a science, is demonstrated time and time again in this
section. I think Bill Wright`s comment about RF is apposite :
“If you could see RF, you`d see some incredible and totally unexpected things”.
Consequently none of the results published here can be regarded as definitive as every
installation is different. There are so many variables e.g. the frequencies / wavelengths
involved, the transmission path to the aerial, the distance to the wall / roof / trees / next
door`s house etc etc. All of these factors can result in more reflection, refraction or cross
polarisation of the signal. Also see Fresnel zones.
What can be established is the basic principle that an aerial will almost always operate
most effectively high up and in free space, i.e far from other objects which affect RF.
Notes. All test results are averages of at least two readings. The figures are for peak
reading for analogue channels, and the integrated channel power (over the 8MHz bandwidth)
for the digital channels. This is purely of academic interest because we are only interested
in the difference between the readings in any of the tests.
No account is taken of possible ghosting/multipath effects because there is no objective measure of that, and it would take too long to do it anyway !
Polarity Of Obstruction(s) Tests
We`re often asked how far to separate aerials mounted on the same pole. The answer very
much depends on if the antennas are of the same polarity or not and this test proves that
to be the case. Of course the potential obstruction doesn`t have to be an aerial, it could be anything metal (e.g. the aerial pole), though masonry also attenuates RF. Obviously any electrically inert object (e.g. plastic) should not have any effect on the signal.
I have been meaning to research the different losses resulting in same or opposite polarity obstructions, so whilst up at the test site (testing the new version of the XB16A) I had a bit of a mess about with a section of pole.
I taped it onto the aerial, first perpendicular,
then parallel to the dipole/directors. Thus the section of pole acted as an obstruction to the aerial of the opposite, and then the same polarity.
As expected the fall off in signal when the pole was the opposite polarity (to the aerial) was
very little. In fact it is not statistically
significant. That said, I still wouldn`t place an obstruction (e.g. an FM or DAB dipole) through my aerial if I had a choice in the matter, for aesthetic reasons apart from anything else !
On the other hand, if the polarity of the obstruction is the same as the aerial then the drop off in signal is significant, see table.
Don`t forget that 3 dB is about 40% more
signal, and that is more than the difference in gain between a Yagi 18A and the XB16A.
If fitting two aerials (of the same polarity) I`d try
and separate them by at least 2ft, just to be
on the safe side, see Aerial Separation Tests.
The section of pole (used in this experiment) was attached to the middle of the Yagi 18A (the 8th director), but just out of interest I briefly moved it to the end (the 18th director) and
re-
Conclusions
Obstructions of the opposite polarity have minimal effect on the level of received signal, though this cannot be said for same polarity obstructions.
Any obstruction has more of an effect the closer it is to the dipole. Back to the top
The Tests
How high above a roof line (or other obstruction) does an aerial have to be mounted in order for it to avoid the worst effects of it on reception of
the incoming signal ? ?
With the usual caveats about RF tests, we set up an experiment where a DM Log Periodic aerial was lowered in one foot increments from 10ft above the ridge line, down to two foot beneath it.
The results are shown below in a graph plus
(most of the readings) in tabular form, the graph arguably shows the fall off in signal more clearly.
As expected the decrease in signal varies with frequency / wavelength. The largest increase in signal occurs in the first two or three feet above the roof line, then it dips slightly. This coincides fairly closely with the vicinity of the stay wire holding up our semi stayed mast, so that`s the most likely culprit, though when it comes to RF I`m not prepared to say anything for sure....
The peak reading (up to 10ft anyway) was at 7ft after which a slight downward trend was
observed. I`m unsure why this occurred.
My first guess was the effect of the aerial at the top of the mast, but I feel this is unlikely to be the culprit as even at the very top of the test mast there was still a good two foot gap, and
the top antenna is vertically polarised,
as opposed to the horizontal polarisation of the test DM Log. A more likely explanation is some sort of “RF dead spot”, but, to be frank, your guess is as good as mine, RF is a black art....
I had always advised aiming for a two or three foot clearance above an obstruction and I was pleased to see my guesstimate was vindicated, but what I didn`t expect was there would be such a huge fall off within 6” of the ridge level. Arguably if you can only get your antenna 6” above the roof line then you may be better off putting it in the loft. The fact the attenuation peaked just below the ridge proves that electromagnetic radiation does indeed travel in
straight lines (apart from when its being refracted ! ), but also that, as expected, the roof is thickest there, ridge tiles, roof trusses etc. If we could have gone lower I reckon the signal would have risen a bit more, assuming there wasn`t anything metal in the loft obviously !
Conclusion.
See above graphic !
Try to mount an aerial at least two feet, and preferably three feet, above a roof line.
Aerial Separation Tests (also see Bleedin` Blackburn Bodge)
Other than for diplexed antennas (or phased arrays) there should be no need for more than one TV aerial on any pole, so in some ways this article is extraneous ! However one does frequently see multiple television aerials installed on the same mast so it is of interest to
find out how much signal they`re losing, if any. Obviously different aerials may give different results, but I wasn`t going to go down the route of trying it. If you imagine the number of aerials on the market how long do you think that would take !
I used two “Contract aerials”, not because I recommend them (quite the opposite in fact....) but they can be stacked close to each other because the reflectors are so small. Both of
the aerials are widebands so as to test the array over the full range of frequencies.
Two aerials mounted within 6” of each other. The pair on the left horizontally polarised and those on the right vertically polarised. Note how the yellow arrows show the vertical pair are
in phase with each other whereas the horizontal pair are out of phase.
The results table (above) more or less tells the story, i.e. At 6” separation there isn`t that much of an effect, but that isn`t quite the whole story......
At most of the frequencies the signal levels were the same, and in fact at some they were actually up slightly, such is the black art nature of RF and aerial tuning ! Don`t forget much depends on the wavelength of the signal. However, whilst the 0.6dB average fall off in signal (over the B group frequencies) is not that large this average figure does disguise the fact
that two of the frequencies (CH 37 and CH 40) were actually down by 2dB. The fact that the two frequencies most affected were so close to each other would seem to prove that signal wavelength is critical to any observed effects.
Now while it must be admitted that in the great majority of locations this 2dB loss of signal would be irrelevant, in a really poor signal area it would be worth having.
Moving the second aerial down to 1ft separation the losses decreased significantly, in fact we`re at the limit of experimental error here, but there was still an effect. Some frequencies were up slightly and some down.
I then tried moving one of the aerials so that they were out of phase, that is to say one was moved forward so the elements were not immediately below those of the other, see the yellow arrows on the pictures. This did have an effect but it was only marginal.
Finally I experimented with the both the aerials vertically polarised. Unfortunately at the test site only two of the six transmitters are vertically polarised so we`re working with less data,
(that`s why there`s only C/D group results) but the difference in signal levels seemed less,
if anything, than with the horizontally polarised pair of antennas.
I didn`t have the time (or inclination) to test it, but if mast head amplification is involved the received wisdom is that aerial separation becomes that much more important.
Conclusions
Mounting aerials of the same polarity close to each other can have an effect on the signal, though it may only be marginal. Try to separate the antennas by at least one foot, and preferably two or three. If the aerials are of opposing polarities antenna separation is of
less importance, see the appropriate article on this subject.
But it is not normally necessary to mount multiple TV aerials anyway !
One often sees aerials about without their reflectors, especially Contract types where the
installer hasn`t bent over the tang on the plate, which is supposed to prevent it falling out....
But how much signal is it actually costing ?
Whilst up at our test site I had a play about with a Yagi18B on Emley with its reflector in various stages of dismemberment. Crude tests I admit, but interesting.......
Conclusion
Always fit the cradle perpendicular to the elements, even if you have to fit the aerial to a
side pole on the mast to enable you to do that. That said, in a decent signal area it`d still
work either way ! Back to the top
1 I started out with a correctly fitted full reflector.
2 First I put on the reflectors "inside out", and there was no significant difference.
3 Next, I deliberately fitted the reflectors facing the wrong way round (Speedbird style.....)
and the aerial lost 1.4dB.
4 Then I removed 4 (of the 6) reflector elements, interestingly the aerial only lost 0.8dB.
5 Finally I found that no reflector at all knocked 3dB (average) off the gain.
Thus, in a strong signal area, a reflectorless aerial may well work OK .
However, if the antenna has a reflector but it`s at the wrong distance from the dipole, such that
reflected waves are 180 degrees out of phase, it would actually have more of an effect than not
being there at all !
Note that these tests are purely for forward gain, they do not test attenuation of signals from the
rear. I did start off trying to check the latter but it quickly became apparent that the readings were
inconsistent (to put it mildly) and therefore that test was of no practical value.
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Many people know that raising an aerial in height can affect the signal level, but an aerials
lateral position can also have just as much bearing on reception. I have suspected the latter for
years but wanted to know just how much the signal can be affected so we set up a five foot long horizontal pole, moved the aerial in one foot increments to the side and measured the results.
A length of five foot was chosen because we sell a 6ft “Supercrank” pole with an offset of about
2ft, i.e use of the latter can move an aerial around two foot sideways, either way.
The usual caveats on aerial positioning experiments must be emphasised.
This experiment more than any other shows that just because your “mate from down
the road gets a good picture”, it isn`t any guarantee that you will !
That said, your next door neighbour`s signal levels are usually a pretty good indication of your own likely signal strength.
Despite the DM Log (pointed at Emley Moor) being nearly three feet above the chimney I had
still expected the signal level to increase as the aerial was moved away from the stack and this
is what happened. What I didn`t really expect was for it to start dropping off again as the aerial
moved more than 2ft out to the side !
Another classic example of RF being a Black Art...
At first I thought it was the presence of one the diagonal stay wires which we use to stabilise
our mast. So at slight personal risk (you have to suffer for your art....) I climbed over the ridge
and disconnected it. Result ? No difference !
Just to confirm the trend I extended the pole even further away from the stack and the signal
promptly dropped even more.....
Examination of the graphic below shows that as the aerial is moving out from the stack it is also
getting closer to the ridge. Despite the fact that the aerial was still 4ft above the ridge, which I`d
have thought was a reasonable clearance, I still think that the presence of the roof/ridge is the
cause of the fall off. I say think because one cannot know anything about RF for sure.
The graphic below doesn`t show that the aerial is pointing over the ridge towards the transmitter
at about 45 degrees (see picture of “Ridge Tests” ), I`m unsure how relevant this is but I thought
I`d mention it anyway !
Gaining 2 to 3 dB by moving an aerial two or three foot is very significant. To put it into
perspective that`s the increase that one would get by swapping from a 46 element wideband
Tri Boom (or a Log40), to a decent grouped Yagi 18B !
Unfortunately it`s a time consuming business finding an RF “sweet spot”, and there`s no
guarantee that one even exists on the installation one is undertaking. Furthermore if the
antenna is pointing through trees who is to say that they`ll just grow a bit, and then you`ll
have to start all over again !
Conclusion
The lateral position of an aerial can be just as important as the vertical (height) of it in the amount of signal that it will receive. This is particularly the case in hilly areas or in the vicinity of a roof, or a chimney, etc. Also see RF is a Black Art......
