Hillsborough
Sheffield
S6 2LL
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Main Page List
Aerials (Also see FM / DAB aerials)
The subjects on this page are listed in the following order :
Basic Aerial Facts (Aerials In Lofts, “Set Top” Aerials & Aerial Positioning / RF Dead Spots)
Basic Aerial Theory, Yagi type (including definition of aerial “gain” ) Log Periodics
Aerial Groups/Widebands (including Wideband V Grouped Gain Graph)
Signal Strength : “Am I in a strong signal area ?” (Also see “Which transmitter am I on ?”)
"High Gain Aerials” (includes Phased Array & Co-
Types of Aerial (Quality, Size and Group, incl aerial groups, minibuses & vasectomies....)
ATV`s choice of aerials (sales and installations), and why we chose them.
(also includes approximate wind loading and gain figures from our own research)
Yagi aerials with main components marked.
(13 element TV aerial, above, and 3 element DAB aerial, on the right)
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(s). 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 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, see picture above, and of DAB antenna plus our tests on an incorrectly fitted cradle.
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 19.8mm at
the front up to 22.2mm at the back with a 24.6mm (OD) dipole. At the other end of the TV frequency band the wavelengths are that much shorter, and therefore the dimensions of the
elements for a Yagi18C/D are 14.0mm, 16.0mm and 16.3mm respectively.
As more elements are added to a Yagi (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 (note that 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. 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.
Incidentally, always be careful when comparing the quoted gains of aerials.
In any aerial the tuning of the elements is critical. This is a highly specialised job and it is critical to the performance of the antenna. If a Yagi aerial is deigned 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, K and B group transmitters.
Having said that, most people don`t need a high gain aerial and a Log would be preferable.
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.
The fact that one of the dipoles can be resonant for any part of the broadcast band means that Log Periodics have a very flat signal gain curve. If you add in their tight polar response (i.e. they are less likely to pick up signals you don`t want), their high “cross polar rejection”, their excellent impulse noise suppression and their low wind loading you would think they are the aerial to go for. Unfortunately Logs have a relatively low gain figure and the they`re only available in wideband. Thus they are only really suitable for areas with a fairly good signal, though the Log 40 is OK for medium signal strength areas as well. Apart from this Achilles heel the Log is one of the best antennas, particularly for digital, which is why we (and the broadcast authorities) use them wherever possible.
All of which begs the question, ”Why don`t you see more Logs around ?”. Well I think
there are three main reasons. First, the log is a fairly recent development, more will appear as time passes. Second, 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 ! See ATV`s choice of aerials and TV aerial tests.
sculpture this is the Llanddulas
repeater in North Wales. It uses the
bottom Log Periodic to receive its signal from Moel Y Parc transmitter and it then re-
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 !
For some reason I really like this picture, the abstract look of it or the blue sky,
but it`s possibly one of my favourites.
For more detailed information on transmitter groups and polarities nationally
see Digital Nationwide or the specific page on Nationwide Transmitters.
Note, in this article a channel number refers to a frequency not a programme.
The TV UHF broadcast band in this country stretches from 471 MHz (= Channel 21 which has a wavelength of 64cm) at the bottom, to 847 MHz (= Channel 68 with a wavelength of 35cm) at the top. 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....
This 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 were (and still are ! ) as follows :
Graph of typical gain curves for “high gain” Yagi 18 element aerials
Notice the
significantly 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.
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 / top end.
For gain curves of
our recommended aerials see :
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-
of the way some Freeview STBs scan for channels. If they find more than one of a particular MUX they will often put the first one they come across (which may not be the strongest) in the main selection position, whilst frequently leaving a more robust channel further up the menu or possibly ignoring it altogether. If you live in a strong reception area and some of your digital is fine, but some is not, it is always worth checking for this problem and most STBs can tell you which transmitter they`re picking up a particular channel from.
A good “worked example” of this is Sheffield where Emley Moor (a B group and the
preferred transmitter) is NNW and Bilsdale (mainly an A group) is NNE. Particularly if the customer has a wideband aerial, when the STB scans it will often find the weaker Bilsdale MUXES first and ignore, or relegate, the stronger Emley Moor output. The simplest way round this is to use an attenuator (possibly only whilst the box is scanning) to tip the unwanted weaker transmissions down the cliff edge whilst leaving the required ones at the top. Alternatively, if you have the option, try and scan your box manually.
You may have wondered why some aerials are mounted (or polarised) Vertically and some Horizontally.
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 minimise
co-
discussed below. When we`ve tested aerials we found this rejection to range from
20 to 35 dB, “high gain” aerials were the best, Logs were in the middle and small Yagis
were at the bottom. It should be stressed that CPR can be greatly influenced by nearby objects, e.g. the chimney or the roof, because RF waves can be cross polarised when
they are reflected, even off the ground ! This helps explain why our figures for Cross Polar Rejection are lower than those of the manufacturers, because their`s are obtained in a lab
where such cross polarisation does not occur, unlike in the real world....
We once went to a job where the aerial installation had fallen down and was now vertically polarised on Belmont (which is horizontal) but working perfectly because the roof was
cross polarising all the received signal ! Yet again, RF is a black art, it`s not a science....
An example of the use of differing polarities is Sheffield/Crosspool which is a repeater
(or relay) off Emley Moor and is vertically polarised. It shares some of its frequencies
with Waltham and Belmont, but they both broadcast horizontally, thus the vertically
polarised antennas on Sheffield are less likely to pick them up and cause co-
a wide band aerial will not only have to endure the biggest drop off in signal strength (see graph) but also more potential ghosting problems as well. Having said that if you live in a reasonable signal area you should still be OK, unless you have trees or high buildings in
the vicinity. Yes that`s right, I love trees as much as anyone, but trees CAN lead to reception problems !
Signal Strength : “Am I in a strong signal area ?”
The potential range of signal strengths available throughout the country is huge.
Some people live in such a poor area that even a “high gain” grouped XB16 aerial with a Mast Head amplifier will still not get them a good picture. 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 !
Consultation of the “ATV`s Choice Of Aerials” page reveals that we recommend different antennas according to how good the reception is in your area. Thus, in order to choose the correct aerial, it is very helpful to know the signal strength in your location. For those who
do not know the latter, there are a few pointers one can use to discover this. Don`t forget that aerials in lofts never work as well as those outside and so if you intend to site your antenna inside it may be wise to go for a slightly higher gain type.
Remember you can have too much signal, the latter can actually contribute to interference.
You could try one of the signal strength prediction websites, but I have to tell you they
aren`t very accurate, though to be fair, you wouldn`t really expect them to be.
Checking the signal quality 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 !
But remember, if aligning onto an alternative transmitter, you must retune your TV !
If you get reasonable pictures on all available channels you`re in a pretty good signal area.
Your average set top will give between 2 and 4 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.
Haven`t got a set top ? They`re only about a fiver, so go and buy one then you tight git !
Do your neighbours get good signals on all the available channels and do they need an
amplifier or “high gain” aerial to do so ? This is probably the next best indication of signal strength in your location. Having said that some 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”......
Also remember there are RF dead spots around.....
Are you up high or in a dip ? This is probably more important the distance from the transmitter, within reason obviously ! The higher up you are the better your signal is likely
to be and the kind of massive signal levels you could expect if you were at 1100ft are given on our aerial test page. Unfortunately being in a valley has the opposite effect......
Can you actually see the transmitter. If so, and the transmission pattern is not restricted in your direction (the restrictions which we know of are listed on Digital Nationwide and/or
the relevant transmitter page) then it`s a fair bet you`re in a strong signal area !
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 1MW 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 ? (see importance of “line of sight” ) 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 ghosting. 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 perfectly or it may not, you can only do your best and hope, after all, who knows the secret,
of the Black Magic box....... (also see Major Transmitters: Which Aerial To Use)
Due to the large wind loading
on Tri Boom aerials* we do
not really recommend them.
Personally I think they`re a bit of a gimmick anyway, and our tests backed this up,
remember that things aren`t always what they seem.......
Quite apart from anything
else this particular aerial is
on Hebden Bridge transmitter which is an A group, and at these frequencies a Yagi18A would blow away a DAT 75,
or any other wideband aerial....
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 some of these cheapo X beams
are what we call “Bacofoil” aerials, try bending the elements and you`ll see what we mean.....
We are quite particular about which model we sell.
Two of the aerials have lost a number of directors and as for the one above right, for once,
words fail me.
Note the pole on the install above left, it is of
the 1” variety, either steel or (that equivalent of
the 3” fence post) the 1” alloy pole.
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.
NB The wavelength at the bottom of the
UHF band (i.e. channel 21) is 64cm and
at the top (i.e. channel 68) it is 35cm
For more information see Bill Wrights articles on Stacking and Eliminating Ghosting.
This is the Birchover repeater in the Peak District.
Note the use of two phased arrays. The bottom pair of 18 element Yagis 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. Stanton is itself a repeater and receives its signal
off Waltham. The top array uses a couple of Log Periodics to retransmit the signal into Birchover village.
Contract Aerials
The (top) 18 element version is horizontally polarised whereas the (bottom) 10 element type is polarised vertically
In Quality terms "contract" aerials (the type with the plate type rear reflector) are
generally at the bottom of the pile. The only aerials with a weaker standard of construction
are the cheapo X-
In comparison with (say) a Yagi18, contract elements are of thinner/slimmer gauge alloy
and the saddles holding them on the boom are smaller and less supportive.
The cradle (if fitted) is smaller and the clamp is fixed which does not allow the aerials elevation to be altered. The latter can help to maximise signal acceptance and reduce interference, usually by tilting the aerial upwards at the front. The rear reflector is made of thin alloy plate which as well as being physically weak, has a relatively high wind loading
for it`s small size. Generally speaking they do not have a "balun" in the dipole. These are designed to maximise the signal by impedance matching the dipole to the cable, hence the name balancedunbalanced. They also help eliminate impulse interference which can be one of the causes of intermittent blocking or pixellation on digital signals. That said the best aerials for eliminating impulse interference are Log Periodics, which effectively short it out to earth. Also see “double screened” cable and screened amps/splitters.
Historically most aerial installers have fitted contract aerials because they`re
cheap, they don`t take up much room on the van and they don`t take long to assemble,
“time is money”. The advent of Digital and the wider availability of higher quality
antennas has encouraged more riggers to abandon contracts, but they are still the
cheapest and so continue to be widely used. Our own tests prove that contracts have
inferior gain to the the equivalent Yagi18s (for instance) although there is often
(but not always ! ) only a small difference -
see Basic Aerial Facts. In addition “Anti-
Having said all the above virtually all 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 as well ! 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........
* 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 !
Our Yagi18B would work better, look neater, have much less wind loading, and be cheaper as well ! And an XB16B would have even more gain than a Yagi18B....
Next we come to the Size of the antenna.
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.
There is no point in fitting an aerial bigger than is actually required. Too much signal can
be detrimental and there would also be an increase in wind loading.
As mentioned above (see "High Gain" and Aerial Theory) the number of elements is
only a guide to an aerials gain, particularly with an X-
Finally we have the aerials Group.
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.
To continue the car analogy used above, having a wideband aerial on a grouped transmitter is the equivalent of running round in a minibus (rather than a car) 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, in fact Ofcom has confirmed that
most widebands will go back to their original groups at DSO. 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 you have no real choice (at the moment) 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 you could diplex a A group with an E group, or even an A group with a wideband, if you`ve already got one of those which works OK with the higher frequencies.
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, particularly for A, B or K group transmitters.
Also see Major Transmitters: Which Aerial To Use and Which Transmitter Am I On ?
Also See ATVs Choice Of Aerials, And Why We Chose Them
For further reading on aerials/antennas see Wikipedia & Astrosurf articles.
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, just as important as the number of elements, is the total number of "element bays". This is particularly relevant
in the case of X-
of marketing over performance, just like Tri-
As mentioned in the section on aerial groups, one of the biggest factors determining an antennas 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 group (or even the B group ?) frequencies. I invite you to study the test results for
all groups, A group, B group and K group aerials, then make up your own mind.
One other beneficial side effect of the narrow acceptance angle associated with high gain aerials is the reduction in the chances of ghosting caused by multipath reception. Thus grouped aerials are also less likely to give ghosting problems. That said, the most effective way to reduce ghosting is to resite the aerial, unfortunately this is not always possible !
Sometimes a "High Gain "aerial can be used (even in a strong signal area, possibly
with an attenuator) to minimise ghosting or co-
Also see Belmont transmitter as an example of co-
In any event (by utilising high gain aerials or amplifiers) it should be stressed that too much signal is as bad as not enough because it can generate Cross Modulation Interference.
The ultimate form of high gain aerial is the Phased Array though these are generally
more effective at reducing ghosting rather than increasing the gain. A phased array is two
(or more) aerials linked together on to one down lead. Assuming they are perfectly in
phase then in theory this should give 3dB more signal (which is a significant increase)
but losses through the splitter/combiner generally reduce this figure. It cannot be over emphasised how important it is for the two antennas to be perfectly in phase at the point where the combiner/splitter joins them together. Any phase difference may very well give less gain and more ghosting. In fact if the two aerials were 180 degrees out of phase it would be theoretically possible to get no signal at all !
It must be stressed that setting up a phased array can be difficult and time consuming and there are no guarantees how well it will work either ! That said they can be very effective against ghosting because they reduce the beam width and increase the front to back ratio.
Finally, the reflected signals should be received out of phase therefore reduced.
Important points to note are that the two cables from each of the aerials to the combiner/splitter should be exactly the same length.
We are more than willing to give advice to those actually purchasing from us.
Could those only seeking information please just ring an aerial installer local to
them or try calling reception advice on 08700 100 123 (BBC) or 0844 881 4150 (ITV).
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.
People will happily spend hundreds of pounds on their television but many are 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, is often (but not always ! ) quite small, and will not usually transform a really poor picture into a perfect one. But it will make some difference,
and that difference (particularly with Digital signals, see “Digital cliff edge") could make all
the difference. In any case the extra cost of a good quality aerial installation is as nothing compared to the cost of the average TV.
1 Other than in strong signal areas we do not recommend fitting TV aerials in lofts.
This is because about 3 to 10 dB (approx 30% to 70%) of the signal will be lost. In some cases the attenuation can be even higher than this, though FM / DAB aerials usually suffer less signal loss due to the lower frequencies involved. On the other hand the higher frequency of satellite signals means nothing will be received through a roof.
If an aerial is fitted in the loft it should be aligned through the slates rather than any wall(s) and no signal at all will be received through any metal sheeting.
It is worth considering the use of a grouped antenna to maximise the signal and it should
be stressed that aerial positioning in a loft is even more important than it is outside.
Spend as much time as necessary in moving the antenna all over the loft to try and find the spot of maximum signal, this may vary for different channels and even different aerials.
TV reception in lofts is even more unpredictable than it is outside.....
Note. There is no such thing as a “Loft Aerial”, there is just an aerial (built to be fitted outside) which may work in a loft if the signal strength in the area is sufficient.
Even more problematic is to get a decent picture off a “Set Top” aerial and if you get
perfect Digital off one you`re lucky indeed and you must live in a very strong signal area.
Using a “booster” antenna is also unlikely to give you a perfect picture for the reasons given in the amplifiers page. I`d recommend a feed from your outside aerial if one of the standard type “set tops” doesn`t work, though one of the latter should outperform a “loop” antenna.
An Omnidirectional type aerial is even less likely to be effective, they 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 ghosting. This is particularly the case if trees or high buildings are near to the aerials reception path from the transmitter (TX). 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. Unfortunately this is a time consuming (i.e. expensive) business and there is no guarantee
it will be effective, also see cranked poles. Try tilting the aerial upwards at the front, it can sometimes make a worthwhile difference, and always peak the alignment up on the
weakest channel. Bearings worked out on a map (also see article on using Google Earth
for this) are a good place to start but remember RF is a black art, it`s not a science....
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.
Basic Aerial Theory (Yagi Type)
The vast majority of aerials are "Yagis" which basically work by "focusing" the RF waves coming towards them on to 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 (e.g. from the rear) reaching the dipole. Yagis, 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.
If one look`s 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 it`s length. The most important factor is the “tuning” of the elements. This is why an A group aerial (no matter how high it`s gain) will not receive
the “out of band” frequencies. Furthermore, the higher up the band they are, the lower it`s response to them will be. A common example of this is those on Crosspool who have an
A group antenna. Often they will still receive the BBC1 Digital programmes because they
are on MUX1 which is broadcast on channel 39 (from this TX) and that is not that much higher than channel 37. The latter figure is significant because it is the top of the A group.
Generally speaking the roll off in 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 B group aerial (and even a C/D, to a certain extent) will work reasonably
well right down the A band. But its response above the B group will only be significant
up to about channel 55 to 57, see ATV aerial test results, Group Responses. These figures will obviously vary according to the particular aerial and the signal strength in the area.
Also see local Transmitter Channel Allocations, Digital Nationwide and Digital MUXs.
Finally there is the WIDEBAND aerial which works (to a greater or lesser extent....) across the whole band. 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 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, or if there is I`ve yet to come across it. In fact I would say even that a
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 with a (“high gain”) wideband if you`ve already got one of those !
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, the curves for the DAT45 and DAT75 reflect this as well.
Anyone aligned on a transmitter which has changed group (see Digital Transmitters) will
not receive all of the digital services (or possibly analogue C5) without having their aerial changed. I have deliberately avoided using the term "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
has never been an “upgrade”.
To put it into laymans terms a wideband aerial is the equivalent of “a jack of all trades but master of none”. (also see aerial groups, widebands, minibuses & vasectomies)
That said, if you`re in a reasonable signal area a wideband will work fine, and in fact we actually recommend Log Periodics (which are widebands) in good or medium areas.
OK, I admit it, apart from Log Periodics (which are “Mega”, as we used to say at school)
I don`t like widebands, they offend my perfectionist sensibilities. There, I`ve said it, and you know they say confession is good for the soul, well, I feel better already.
