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More Signal In The Loft & Behind The House ! The Black Art Of RF Reception......
It is undoubtedly true that aerials work much better without a roof in the way of the
transmissions but antennas can still work reasonably well in lofts provided your signal strength is reasonable or, preferably, good. In medium or poor signal areas we do not recommend aerials in lofts, but some people have no choice as there may be
planning restrictions on outside antennas or access problems may be too severe.
Bearing in mind the loss in signal caused by the roof it may be advisable to go for a slightly higher gain antenna than normally recommended for an (outside) aerial in your location.
Remember that if your aerial is mounted outside but pointing back through the roof,
or with insufficient clearance over it, then it may actually be preferable to install it in your
roof space !
There is no such thing as a “Loft Aerial”, there is only an aerial built to go outside which
may work OK in a loft if the signal level in your area is sufficient. That said, some aerials
Just as with the cable the
attenuation level
experienced by the signal (as
it passes through the slates)
is proportional to the
frequency.
The higher the frequency the
higher the loss. VHF signals
(=FM / DAB) will suffer the
least attenuation and UHF
(=TV) more attenuation.
Once satellite frequencies are
reached (which are 10x higher
than the IF frequency received
at the receiver box from the
dish) no usable signal will get
through the roof.
One interesting point revealed by the above graphic is that theoretically the bottom end of
the TV band (= channel 21) should suffer less attenuation than the top end (= channel 68)
and in fact the same applies to transmission through the air, signals at CH21 travel further
than signals (of the same power) at CH68.
Different roofing materials will attenuate the signal by differing amounts, though as far as I
know no one has published any data on this, contact us if you know different ! Generally
speaking the losses (at TV frequencies) would be 3 to 10dB (decibels) which is about 30 to
70% of the signal. During tests inside a (Welsh slate) roof the TV signal was 8dB down on
that from an aerial on the chimney, though the latter was installed 9ft higher.
However some materials can give significantly more loss than 8dB and no signal at all will
be received through metal sheeting. Some types of roofing material are semi porous and
the attenuation level can rise if they get wet, though I have to say there was hardly any
difference through the wet/dry Welsh slate roof when I was doing my testing.
It goes without saying that the aerial alignment should normally be through the slates
and not through the end wall(s), and this is particularly important if the house is a terrace
and there are even more walls beyond the first ! That said, RF is such a Black Art that it`s
always worth trying anything if you`re desperate. Remember we`re dealing with a roof
space where RF waves are bouncing around all over the place, anything`s possible !
It is very important to remember that signal levels in lofts can vary significantly between
locations within the roof space and the best location can vary even for different models of
antenna. Furthermore the signal level can be markedly different for different frequencies
(i.e. a particular analogue channel or one digital MUX) at different locations ! This variation in
signal level can be particularly marked if there are metal objects in the loft, e.g. water tanks.
If the latter is in line with the transmitter then it would normally have a very negative effect
on the received signal. It is also advisable to try and keep the aerial away from mains
cables, at least 3ft, or more if possible. The cable could radiate interference if an
unsuppressed load is in use (or an arcing switch) but remember that mains will only
usually interfere with TV reception if there is noise on it, see interference tests on cable.
It`s a good idea to start by placing the aerial in the middle of the loft, and at about 2/3 of the
height of the roof space, then work from there. Bear in mind that in order to avoid aligning
onto the transmitter through any end wall(s) you may have to place the aerial towards one
end of the roof space. If you are struggling for signal then time must be invested in moving
the aerial all over the loft, up/down, left/right and forward/backward in order to find the area
of highest signal.
Size does matter, but in this case the shorter the better....
It is still possible to use an amplifier in the loft, if one is needed, even if you have no
mains socket by utilising a mast head amp, although in this context the term “mast head”
is obviously a misnomer. Amplifiers are not the answer to everything though.......
Splitting the signal in the loft using the aforementioned mast head amp (or in strong
signal areas just a splitter) can be very convenient because it makes it easier to feed both
sides of the house and/or feed the upstairs rooms straight through the ceiling(s).
i.e.
1ft 2” shorter
i.e.
2ft 8” shorter
Loft Aerial Positioning Tests (Also see this "loft mounted aerial" report)
Just how much difference does aerial positioning in a loft make ?
With the usual caveats about RF tests, I decided to try and put some figures on it.
A similar aerial installed on the roof at this location (which would admittedly be about 9ft higher than the 30” datum) averages about 58 dBμV, so installing it in the loft loses about 8 dB,
which is about 60% of the signal. The roof is made of Welsh Slate.
The test loft is in a poor to medium signal area and I must admit I wouldn`t normally
recommend fitting an aerial in the loft with a signal level this low unless you have no
alternative. Furthermore the location does not have line of site to the transmitter, which
from a test point of view makes readings even more erratic. Mind you most loft locations
probably don`t have line of site to the transmitter (because the aerial is usually mounted
lower than it would be outside) so from that point of view it`s more realistic !
Basically I mounted our usual test aerial (a DM
Log Periodic) on a temporarily modified camera
tripod and moved it all over the loft taking signal
readings as I went. To be frank I ran out of time to
do as much as I had originally wanted but on
reflection I found out enough as it was.
I started off in the middle of the loft space (about
18” above the joist line) and then moved the aerial
up one foot at a time. The tripod wouldn`t go any
lower than 18” but it was pretty obvious (if anything
in RF is obvious, which it isn`t...) that the signal
was dropping as the aerial was lowered anyway.
The loft isn`t broadside on to the transmitter
(Crosspool) which meant aiming the aerial
about 45 degrees across to the roof. It also
meant that when testing it through the end
wall it wasn`t aiming perpendicularly through it.
The figures in blue are for the signal strength (in dBμV) at a point 30” above the roof joists and in the middle of the loft.
The other figures reveal the difference in signal (in dB)
at points 18”, 42” and 54” above the joists, i.e. 12” below, 12” above and 24” above the 30” datum.
The aerial was then moved forwards right up to the roof (at the 54” height) and the relative figures for this location are given in the
end column.
The results table of the main test tells its own story really. The peak reading on average
was at 30” above the joists, which was more or less bang slap in the middle of the loft.
But the signal levels on some of the frequencies was anything but peaked up at that height.
Even more significant was the drop off of on some of the channels (CH31 in particular) as
the aerial was raised. Two readings were taken on every channel but on CH31 I took four,
just to be sure, and it was consistently way down. Let`s just think about this a bit, moving
the aerial up one foot knocked 5 dB off the signal on CH31 (which is analogue BBC1 off
Crosspool / Sheffield). Then moving it up another foot knocked another 5 dB off it ! ? !
That`s 10dB in total and that`s a huge amount. To put that into perspective, that`s
probably more of a difference than you`d get swapping from a crappy little (wideband)
Just to make it more interesting the signal on most of the other channels varied by up 3 or
4dB (up or down according to location) and that`s also a significant amount. It`s the kind of
difference in gain between a Log40 and an XB22WB !
Similar differences in signal levels were found when the aerial was moved to the sides and
front to back, and as an example I moved the aerial as far forwards as possible up to the
roof. I`ve read elsewhere that this position can sometimes give maximum signal, all I can
say is that it didn`t in this particular case !
Next I deliberately aimed the aerial towards the transmitter through the end wall, which is double thickness brick.
It must be admitted that some of these signal levels are getting so low that the readings
are (even more) unreliable but it still be seen that aligning the aerial through the end wall
knocks the signal down significantly, which is to be expected. I was surprised, yet again, that there was a 4 dB (average) gain increase by moving the antenna only one foot to the side, particularly as Position 2 was where one would expect the signal to be dropping.
Note the difference in signal can vary significantly according to the frequency / wavelength.
The signal on CH60 was the same at all three positions, i.e. it was the same through the end wall as in the middle of the loft ! On the other hand the end wall reduced CH39 by a whopping 15 dB at Position 1 but recovered to “only” minus 6 dB at Position 2, that`s a
huge 9 dB difference just by moving the aerial twelve inches !
I know that CH67 (analogue C5) actually showed an 18dB difference ( ! ) , but the signal
level was so low for the readings in Position 1 that it`s not reliable.
Conclusions
Moving an aerial around a loft will generally affect the signal level quite markedly, even more
so than outdoors, and the point of maximum signal will vary for different frequencies.
In a strong signal area this may not matter too much, but unless you have the luck to live in
one of those then time spent searching out the area of greatest signal is a good investment.
Aligning the antenna onto the transmitter through the end wall usually attenuates the signal
more than aligning it through the roof.
If getting desperate don`t ignore any possibility as to where the location of maximum signal
may be.
RF Reception Really Is A Black Art.....
The usual caveats about RF tests are proved conclusively here......
It must be stressed that the test site in question did not have line of sight to the transmitter, and this makes RF even less predictable then usual. Though not usually, I hasten to add,
as unpredictable as these test results showed.
The figures in blue are for the
signal strength (in dBμV) at a
point in the middle of the loft.
The other figures (in dB) are
for the difference in signal at
two test points when the
aerial was deliberately aimed
at the transmitter through the
end wall.
The alignment to the
transmitter was at an angle of
about 45 degrees to the wall.
Position 1 was nearer the
edge of the wall (at the
transmitter end), whereas
Position 2 was more
towards the centre.
I then hoped (some hope....) that subtracting the signal levels it would give the attenuation through the roof slates.
Getting an aerial (temporarily) at the same height as one in the loft was not that straight forward, but, at some risk to myself, bolting a 10ft pole (with a DM log aerial mounted on it) to a previously erected ladder seemed to work reasonably well.
These tests were originally designed to find the attenuation through a roof, in this case Welsh Slate. The plan was to check the signal level on the far side of the roof (from the transmitter), then in the loft, and then finally on the near side of the roof.
I started off taking the readings behind the roof, at the East end of the building, as far away from a line of sight to Crosspool transmitter as possible. Then I worked towards the West end of the house taking readings (at the same height) every two feet. I expected the signal to get much larger when the aerial started clearing the roof line and to be fair it did go up,
but no where near as much as it should have done. By the time I got to the West end of the building the aerial was a good 4ft above the roof, yet the signal was only about 2dB up ! ? ! It was dawning on me that the original purpose of these tests was in ashes.
All that bleedin` effort, for nothing.
I went for my lunch.
Now signal levels can sometimes change for no apparent reason, so I did wonder if they`d still be the same after I`d remotivated myself to continue. When I do aerial tests I always take at least two readings, but in this case I just spot checked the West end and the East end of the house (including plus 1, 2 & 3 ft), and I have to tell you they were exactly as they were before.
I removed the pole and aerial from the ladder and took everything round to the front of the house, that`s the transmitter side of the building remember. That said, there are houses in the way on the other side of the road and large trees beyond them. I reattached the pole/aerial (at no small risk to myself) to the ladder and tentatively took the readings.
They were lower than they were behind the house........
HOW CAN THAT BE !
Thoroughly demoralised I just took readings at the East end and at the West end (including plus 1, 2 & 3ft), then I called it a day. What a waste of time.
It was only later that I realised that this was a textbook example (if that`s possible) of RF being a Black Art, so perhaps the time wasn`t wasted after all !
RF is indeed a Black Art...
Note that there is
more signal at the back of the house than at the front ! ?
In fact there`s
more signal in the loft than at the
front of the house !
Obviously the
most signal is
produced by the
aerial on the
chimney, it`s a
whopping 9.3
decibels up. It is
installed 9ft higher, but even that doesn`t have clear line of sight to the transmitter.
Hold on, there`s
nothing “obvious”
in RF reception.
The above figures are the averages for the 11 transmitted channels (5 analogue & 6 digital) from Crosspool. There was even more variation in the signal strengths of the individual frequencies. Just across the back of the house CH27 varied by “only” 5dB (still a significant amount.....), but CHs 45 & 67 varied by a huge 14 dB. Under these sort of circumstances you really should have a lot of sympathy for your aerial installer, or yourself if DIYing !
So how can this be ? Well, whilst I am pretty sure the fact there`s no line of sight to the transmitter is significant, I`ve really no idea. I thought that the most likely cause was
refraction off the roof ridge, but the signals off Crosspool are vertically polarised, so I`d have thought that made that less likely ? The only other thing could be reflection off the houses (or the large tree) behind, but they don`t seem to be in the right plane (see below) to accomplish that.
So your guess is as good as mine......
Conclusions
The conclusion is that there is no conclusion. I accept the above situation is rare but it exists, you`ve just read it with your own eyes.
Actually there is a conclusion, well a sort of conclusion. This test proves the adage that RF
is a Black Art and (almost) anything is possible. It also underlines the importance, yet
again, of “Line Of Sight” to the transmitter. The fact this site doesn`t have that is, I feel,
very significant.
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Loft Aerial Installations
The subjects are listed on this page
in the following order :
Our recommendations for loft aerials.
Mounting an aerial in the loft.
To maximise your chances of being able to locate the aerial at the signal “sweet spot” it
may be advisable to go for a slightly smaller aerial. This is a little problematic because there is no such thing as a small “high gain aerial”. That said, some are smaller than others.
For example for loft installs in weaker signal areas, despite the fact they perform similarly,
we would recommend an XB10A, B or K over the equivalent Yagi18, A or K. This is
because they`re about 1`2” shorter and they can also be end mounted, although if mounting an XB10 outside we`d recommend cradle mounting it. On the other hand, for outside installs we`d normally go for the Yagi over the X Beam, because the Yagis are particularly strongly constructed and they`re a bit cheaper ! Though the latter fact should really be a minor point when you bear in mind an aerial installation should last 20 to 30 years (or more), and even longer than that if it`s in a loft.
For loft installs on wideband transmitters we`d go for a DY14WB over a larger aerial such
as an XB16E or an XB22WB, its gain may be marginally less, but its 2` 9” shorter.
Our aerial recommendations for lofts are dependent on your signal strength.
In very strong signal areas on any transmitter we`d go for a DM Log Periodic (or a Log 40).
For strong to medium areas on any transmitter we`d recommend a Log 40.
For medium and (especially) weak areas our aerial recommendation would depend on
which transmitter you are on, see article “Which Transmitter Am I On”.
For A group transmitters (e.g. Crystal Palace in London) we`d recommend an XB10A.
For B group transmitters (e.g. Sutton Coldfield in Birmingham) we`d go for an XB10B.
For K group transmitters (e.g. Bilsdale SSE of Middlesborough) we`d choose an XB10K.
For all other transmitters I`d go for a DY14WB.
Remember trying to receive TV signals from a loft aerial in a poor reception
area is never going to be easy, so you can only do your best, possibly utilising
a mast head amp, and then hope.......
Many aerials in the loft use cable originally installed in the house walls and the bad news
is that this is almost always crappy “Low Loss Co Ax”. The article on cable explains why
cable is so important, particularly for impulse interference with digital signals.
The good news is that cable run in wall cavities is less likely to have degraded due to UV
exposure and other weather related issues.
There`s no escaping the fact that it`s best practice to change the cable (particularly if it`s
“Low Loss”) but if changing the cable is really difficult I`d try your install with the old stuff
first and see how you go.
Forget about cheap signal level meters when trying to align your aerial, the best way
is a portable TV in the loft with you, but make sure it`s tuned to the transmitter you`re trying
to pick up ! Bear in mind that it is far easier to peak up an aerial (or fault find on a system)
with an analogue channel, and in the vast majority of cases (assuming the aerial covers
both analogue and digital channels) optimising your analogue will also optimise your digital.
Obviously if DSO has already occurred in your area you`ll have to use the signal level
reading on your digital box/TV. You could try a signal strength meter, though I have to say that those cheap £30 meters really are rather crude, a decent meter would cost £300 !
Remember that the maximum signal “sweet spot” for whole TV band may be in one place,
but for an individual frequency (i.e. a particular analogue channel or one digital MUX) it may
be somewhere else !
Although it is fine to mount the aerial by suspending it from the rafters by string
(preferably not by wire though ! ) aligning it whilst using this method is obviously a little
problematic, so most people use a loft mounting kit. I would normally advise using the
“L section” kit because it`s more flexible in its use. Remember you may need to suspend
the aerial from the roof truss, but by reversing the “L section” in its bracket (it can also be cut to a straight) you can also use it to mount on a joist.