Aerial test site on the moors above Sheffield, at an altitude of around 345m (1140ft).
Aerial testing, why it all started……..
Over the years we have tested dozens different models of TV aerial on Belmont (A / Wideband), Waltham (CD / Wideband), Bilsdale (A / K), Crosspool/Sheffield (A / K), Emley Moor (B), Stocksbridge (CD) Oughtibridge/Wharncliffe (CD).
The results are interesting to put it mildly, we rely on our own data now and more or less ignore manufacturers figures. This is particularly the case where they only give a (peak) gain reading. Even if the latter is accurate, this figure can be misleading, especially with wideband aerials. What you really want is the gain graph, but many manufacturers won`t release this information, arrogant gits. Also see Aerial Polar Diagrams.
Differences in the test results of one dBμV on any individual frequency should be disregarded, manufacturing tolerances or a truck going over a hill somewhere between the test site and the TX could easily account for those ! Furthermore the response of any aerial is not linear, its gain curve will have bumps and dips in it. That said, a 1dB difference across the whole band is significant, and an increase (or decrease) of two or three dB is very significant. It should be remembered that 3dB is a 40% higher signal level, and such an increase at the aerial (as opposed to through an amplifier) is very difficult to achieve. Obviously if you live in a decent reception area these amounts of signal variation are not important, but if that`s the case, you should just use a Log Periodic anyway !
On the left is Ferrybridge power station and on the right we have West Burton. Belmont is situated just to the left of the latter, except it`s a further 26 miles away..... From the test site you used to be able to see an incredible 7 (seven ! ) power stations, these are marked with white pins on our TX map. Unfortunately, as of summer 2012, that`s now down to five due to the cooling towers at Thorpe Marsh and High Marnham being demolished. From the NE to the SE they are/were :
Why peak gain figures are (almost) meaningless,
and peak gain figures.
In order to give some idea of the gain of the tested aerials we also included in the tests a 46 element (it`s actually a 28 element....) Tri Boom Yagi which is very similar in performance to a DAT45. We do not stock Tri Booms (personally I think they`re overrated) but for those who know of them, the figures could be illuminating. I didn`t test the DAT 75 because it was too unwieldy, but Televes report it as being about the same as a DAT 45 at CH 21, rising to +1dB at 45 and peaking at +2dB by CH 62. Incidentally this trend for extra directors to add gain at the top end of a widebands gain curve (and less so at the bottom) was reflected in our own tests of other antennas.
It should be borne in mind that gain is not everything, which is why Logs are still one of the best aerials despite being having relatively low gain figures. Impulse noise rejection, directivity, out of band rejection, cross polar rejection and front to back ratio and can all have significant effects on the quality of the received signal. Having said that, gain is a function of the others (apart from impulse noise rejection) as it demonstrates that the antenna is performing efficiently. Indeed gain must be a function of the others. Why must ? Well an aerial is a passive device, the only way it can increase its gain in a particular direction is at the "expense" of gain in the other directions. As an aerial`s gain increases, its acceptance angle (and its cross polar rejection come to that) decrease. They must do, there is no other way for the aerial to increase its gain.
Finally, all of the above is before you even start talking about wind loading considerations and/or build quality. Fortunately, we definitely know what we`re talking about when it comes to the latter, there are no “Bacofoil” aerials here......
This table compares six Yagi 18s of the same model, but of all five groups plus Wideband. Note, we don`t actually sell the Yagi 18 wideband (because the DY14WB outperformed it by a significant margin) but the gain plot is included because it directly compares aerials of the same model. Interestingly these grouped Yagi 18s performed just as well as grouped XB10s, and since the Yagi 18s are cheaper, have lower wind loading and are (arguably) more robust in construction, we do stock those. The test methodology, and points to bear in mind when interpreting the results, are explained above. The figures are obviously for this particular model of antenna. Other models varied in performance and this was particularly the case for gain outside the designed for band. As an example, the point above which significant gain fall off occurred, for A group aerials this varied from CH40 to CH42, and for B groups from CH56 to CH59. For gain within band, other tested 18 element A groups (which we don`t stock) gave around 3dB (av) more than a DM Log, the other B groups gave 4 to 4.5dB and other C/D groups gave 4 to 4.8dB.
I must confess to some surprise that the Yagi18 K performs as well as it does up to CH62, but this was consistent over three sets of tests on three different days. It will be noted that the B group “beats” the K (on average) over the K band. However this is primarily due to its excellent performance in the B group, note how the B trounces the Tri Boom (e.g. DAT45), which is a larger and more expensive aerial don`t forget. The K is superior to everything bar an A group at the bottom of the band, and this is the area that is the weakest by far for all wideband (or semi wideband) aerials. Note that the A group antenna is a huge 3.8dB up on the wideband (of the same size) for the A group frequencies, it`s also a very significant 3.2dB up on the (larger) Tri Boom antenna. In fact such is the widebands inferiority that there is no such thing as a “High Gain” wideband aerial for the A group frequencies. The C/D`s performance was inconsistent, it was sometimes only on a par with the E and the W/B, but sometimes it was in front ! It would have been easy to “massage” the results to say what they should, but being supremely honest, I decided not to........
We tested the aerials against each other at the same time, in the same location, and in actual reception conditions. We have also retested the same antennas and find the signal reception can vary for no apparent reason (possibly the temperature or humidity ? ) so our figures are based on more than one test and our own everyday experience.
In summer 2007 I decided to institute a major new test programme and identified a site on the hills above Sheffield which would enable me to do this as it has “line of sight” off four main transmitters and two repeaters. Between them they cover virtually all the TV band frequencies.
Some of the most significant data concerned the response of grouped antennas above and below the designed for frequency band, this is particularly relevant for some transmissions which aren`t within the transmitter`s original (analogue) group. As an example it proves that a B group aerial (certainly the ones we sell ! ) will work perfectly well on CH 55, and not too bad on 56 either [see the relevant graph]. Crappy Contract aerials might not though !
At least two readings were taken -
Well, I can only say that when I`ve retested aerials, the results, when comparing similar types of aerial, were pretty much the same, as indeed they are when I`ve tested the same aerials on the roof of our shop, a completely different location (and height off the ground).
Furthermore, the results are actually more or less what you`d expect.
Now, when these tests were first published some people said "they`re not valid because you`re testing aerials only 6ft from the ground". For various reasons (including the fact the results were more or less the same on our shop roof) I didn`t accept that argument, but I couldn`t prove I was right, until September 2011 :
Whilst retesting a Yagi18A and a XB10A I decided to take a 2x2 bracket and a 6ft pole* along so I could mount the test aerial on the latter and then slide it from 6ft to 12ft, and back again. I tested them against each other at 6ft, and again at 12ft, then, repeated the process to increase the accuracy.
It`s very important to understand the difference between the received signal at the two heights, and the difference in the received signal between the two aerials at those heights. The former obviously did vary, and by quite a bit, but the latter hardly varied at all, even less than I thought it might. On reflection this isn`t that surprising because testing similar aerials against each other at the same time, is the most accurate way to test comparative performance.
Note that the signal level on Belmont actually dropped as we raised the aerial, though it increased on Bilsdale, yet the difference in gain between the two aerials was still the same !
* We wouldn`t normally advise using a 6ft x 1" pole, or only one 2x2 bracket come to that, but this was only a temporary set up.
The difference of 0.04dB (between 0.57 and 0.53) is nothing. In fact most aerial
test agencies will only guarantee an aerial gain figure to +/-
QED Back to the top
Aerial Groups (the same model of aerial but all the different groups)
It should be noted that as antennas leave their designed for band their performance can become erratic and the further out of band you go the more pronounced this becomes. This can give spurious readings, particularly in a spot where multiple TXs can be received, because the aerials directivity can become very poor and inconsistent. This is the main reason why we have not published some readings (e.g. C/D off an A group) as they are meaningless.
In order to make the results comparable all readings are in dBuV and are all relative
to a DM Log Periodic aerial. The latter has a pretty flat gain curve and thus is
an ideal control antenna. Quite apart from anything else, wherever possible (i.e.
in reasonable signal areas), Logs should be used as a matter of course, so the table(s)
will show how much gain you have to make your decision on antenna choice. Note how
the Log performs pretty well (in terms of gain) against small/medium wideband Yagis
at the bottom of the band(s) but falls behind as the frequency rises. For those who
are interested I reckon that, conservatively speaking, the gain of a DM log would
be about 6 dB at the bottom of the band, then rising by about one dB in the middle,
before dropping a little at the top end. These are the figures I used when calculating
the absolute gain for the graphs. The absolute gain figures are conservative estimates,
though they are accurate relative to each other. Remember that all aerial test readings
are approximate anyway, as far as I`m aware no lab will even guarantee accuracy better
Note this is dBd, gain compared to a half wave dipole, NOT the (dishonest ? ) dBi....
Such are the signals available at 1140ft that even the DM Log gave/gives the signals shown in the table below. Bear in mind that the Yagi18s were giving up to 7dBμV more than that and the XB16s up to 9dBμV more, that`s NINE ! Just to put it in perspective, one should be aiming for 60dBμV (on digital), more than that and cross modulation interference can result.
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.
What`s a vewpoint ? (How many power stations can you see ? ! ? )
Ferrybridge (24m to the NNE, 2GW, opened in 1966) due to close by 2016
Eggborough (26m to the NE, 2GW, opened in 1968) due to close by 2016
Drax (32m to the NE, 4GW, opened in phases 1974 & 1986, largest in Europe)
Thorpe Marsh (22m to the ENE, 1GW, open from 1963 to 1994) (C/towers demolished Aug 12)
West Burton (32m to the ESE, 2GW, opened in 1967)
Cottam (34m to the ESE, 2GW, opened in 1969)
High Marnham (36m to the ESE, open from 1962 to 2003, 1GW) (C/towers demolished July 12)
Incidentally I`ve been on a trip round Drax and West Burton, they were mind blowing !
If you`ve found this site informative and, hopefully, interesting as well,
When I say tests I`m afraid I just mean swapping baluns about between aerials (or omitting them) and seeing how much, if any, difference it makes……
Now, I knew that putting a balun in a DAB dipole cuts down the FM considerably, in fact that was the start of all my aerial testing back in April 2007. To be frank, compared to what I know now, I didn`t really know that much about aerials. I`d sold a DAB dipole to a chap who wanted DAB and FM. I assumed that because the frequencies were fairly similar it`d work for both. But the chap brought the aerial back saying the FM was rubbish (to be fair he wasn`t in the strongest signal area) and I can remember being stood in the shop thinking how can that be ? So, whilst he waited, I grabbed our spectrum analyser, connected it up to the DAB dipole, went up on our flat roof (in the pouring rain……) and “waved it about a bit” whilst checking the spectrum. Sure enough, not much FM, though our flat roof is fairly low down so I wasn`t 100% convinced either way. I swapped the the guy`s DAB dipole for a FM Half Wave and asked him to try that. He never came back so I assume it was all OK. But I, on the other hand, was not OK. My curiosity was positively burning, which led to all our FM/DAB tests (then TV aerial tests, then pole tests etc etc ! ). Anyway, the upshot is I learnt it was the balun in the DAB aerial which was filtering out much of the FM signal. To continue the story, years later, one of the companies which manufactures aerials for us accidentally put a load of DAB baluns in FM aerials, straight away I told them that would be a problem, and did some tests to prove it.
The summary of all my balun swapping / omitting is set out below :
FM folded dipole with / without FM balun = loses about 2dB on FM (without the balun)
FM folded dipole with / without FM balun = gains about 12dB on DAB (without the balun)
FM folded dipole with a DAB balun = loses 7.3dB on FM (and DAB was no better ! )
DAB dipole with B group (UHF) balun = loses about 9dB on DAB and 2dB on FM
Yagi 18A with a wideband balun = no significant difference *
* This last test came about in April 2014 when the manufacturer of the Yagi18A announced they were no longer manufacturing A group baluns, they`d be fitting wideband baluns instead. I was concerned that might result in a drop off in gain so went and tested it. I was relieved to find no there was no significant difference.