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-
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 in a loft 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, diffraction 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 any 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 or 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 multipath effects because there is no objective measure of that, and it would take too long to do it anyway !
To anyone who has spent any time on this site it may not come as any surprise to hear I rarely just accept what I`m told, by anyone. Thus, when I used to read that aerials installed on the same pole should be separated by 3ft I didn`t accept that either. Quite apart from anything I didn`t see how that could apply for aerials of opposite polarity. Now this is particularly significant because many people install a vertically polarised FM half wave dipole (for FM and DAB reception) on the same mast as their TV aerial (the great majority of the which are horizontally polarised), but FM half wave aerials are 5ft tall so it wouldn`t be easy to separate that from the TV aerial by 3ft ! So, almost as an afterthought, back in summer 2008 when I happened to be up at the test site I played about taping a one inch pole to an aerial in the same and opposite polarities to the dipole/directors. Sure enough if the pole was opposite polarity it had almost no effect on the signal received by the TV aerial but if the polarity was the same then around 3dB was knocked off.
The Tests :
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 + 3.1dB gain over a Log36.
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 ! ? CH41 (which was 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 -
Just how much difference does a longer aerial pole make ?
In these experiments 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.
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.
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.
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 !
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.
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, as already stated, it is not normally necessary to mount multiple TV aerials anyway !
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 !
Aerial report : confirms my misinstalled aerial tests !
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 10ft “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 methinks
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 Log36), 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 !
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......
Cradle installed correctly, right hand side (RHS) of the mast.
Cradle installed incorrectly, right hand side (RHS) of the mast.
Cradle installed incorrectly, left hand side (LHS) of the mast.
Cradle installed correctly, left hand side (LHS) of the mast.
The original experiments were all very well but since then I`ve frequently wondered, and in fact frequently been asked by customers, if the same minimal effect (at opposite polarities) would apply to a 2" diameter pole as opposed to a 1"pole, or indeed an FM half wave at only half an inch in diameter. Furthermore, originally, I didn`t spend much time researching the drop off in the attenuating effect as the pole was moved further away from the dipole and lastly, because the tests were just a quick afterthought, I only used an A group aerial on the A group frequencies.
So, the stage was set, and in September 2012 (after much planning ! ) I really went to town on it.
I used a 3ft section of 1" pole and a 3ft section of 2" pole, both of which were tested at same and opposite polarities and at 3 points along the aerial, namely between the 3 & 4th director, the 9 & 10th, and the 13 & 14th. I say along the aerial, I actually used 2 aerials, a Yagi18A for the A group frequencies and a Yagi18E for the B and C/D group frequencies.
And that is a lot of testing, believe me......
Well, I have to tell you, due to the Black Art nature of RF, it`s rare indeed to get such a consistent set of results !
The only slight anomaly is the opposite polarity 1" pole lost more signal between the 9 & 10th director than it did when closer to the dipole between the 3 and 4th, and no, I don`t know why, one could speculate endlessly. It`s only 0.3dB, which really isn`t that much, but it is an average over 27 frequencies, so that`s a bit more significant.
I did a few tests on vertical polarity transmitters and unsurprisingly they broadly followed the same trends.
It was satisfying to report that the results of the original tests were supported here, so I haven`t been giving out misleading information between then and now !
Opposite polarity obstructions hardly affect the signal at all, though the 2" pole affects it more than the 1".
Same polarity obstructions affect the signal significantly, again the 2" pole affects it more than the 1".
As the obstruction is moved further along the aerial (away from the dipole) the effect of any obstruction reduces.
The difference in effect as the frequency increases isn`t consistent with the 1" pole, but it`s not that significant either. With the 2" pole, particularly near the dipole, there tends to be a bigger drop off in signal as the frequency rises, but we`re talking less than a dB at most.
Notwithstanding the above results, I still wouldn`t place an obstruction (e.g. an FM dipole) actually through the elements of an aerial if I had any choice in the matter, for aesthetic reasons apart from anything else !
Panoply of permutations.
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.
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) or Bacofoil aerial where the reflector elements have fallen off.… But how much signal is it actually losing ?
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.......
Thus, the lack of a reflector has a significant effect on the gain, though in a strong signal area a reflectorless aerial may still well work OK. On the other hand, 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 a negative 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.
on Emley Moor
Funnily enough we get asked about this quite often, particularly by caravanners / boaters. I think it`s because they don`t want an end mounted aerial due to the fact it may overhang an adjacent pitch or the tow path, the former being for caravans and the latter for boats (unless there`s been a violent hurricane in which case they`re interchangeable…..). It`s also an indisputable fact that end mounting an aerial does produce more twisting force on the pole when the wind blows, which is why we don`t usually recommend end mounted aerials (for larger antennas), and if we do we advise using saddles with the pole. However, Log Periodic aerials are pretty low wind loading so it`s not usually a problem with installs on buildings, though, on the other hand, the average caravan aerial mounting is not anywhere near so rugged.
The received wisdom is that using a cradle with a Log Periodic is a bad idea because it can theoretically alter the impedance of the aerial. Thus, up to now, we`ve advised people to insulate the cradle from the aerial to be on the safe side. So, I bolted a cradle to a DM Log and checked the signals, the results are to the right. It`s quite obvious there is no significant difference in signal, though that is not quite the end of the story. Theoretically if the screen of the feeder (the cable) is earthed and so is the pole, to which the cradle is bolted, the signal could be affected. I`d have thought it unlikely, but if you have a cradle mounted Log and get unexplained low signal try removing it from the cradle or electrically insulating the cradle from the aerial`s boom.
Anecdotally a few of our customers have used their DM Logs with cradles and not reported any problems with lower signal level.
Note 1 : routing of the cable back up the boom can affect the signal negatively on a Log Periodic, so, if using a cradle with a Log, route the cable as far away from the aerial as possible.
Note 2 : Neither the DM Log, nor any other log periodic aerial, comes with a cradle as standard ! We do, however, sell cradles separately.
How often do you see an aerial with the front half missing ? Have you ever wondered how well it`s actually working ? On another tack, if you want to put an aerial in the loft but lack space how will it perform if you only use the rear half ?
Whilst up at the test site I farted about testing just the back half of two aerials, one of those common “Bacofoil” types (aka Screwfix / SLX) and a DY14WB. Later, on the shop roof, I did the same with an XB10B.
Over the two W/B aerials the approx loss (av) was :
A grp = minus 1dB
B grp = minus 1.5dB
C/D grp = minus 2dB
All CHs = minus 1.4dB
Interestingly the grouped aerial lost a bit more signal, minus 2.9dB over the B group. If I had the time I could extend the tests to a load of grouped aerials to see if a trend emerged, but, to be frank, at the moment anyway, I can`t be arsed.
Basically the aerial loses a significant, but not massive, amount of gain, and more signal is lost higher up the band for which the aerial is designed. Remember in aerial design the law of diminishing returns (as regards extra directors for more gain) applies with a vengeance.