Black Hole
May contain traces of nut
(Sources: http://www.jimsaerials.co.uk/freeview/reception/aerials & coax/aerial.htm, https://www.aerialsandtv.com)
Standard UHF broadcast frequencies for TV are 8MHz apart, spanning Channel 21 (474 MHz) to Channel 68 (850 MHz). The frequencies for intermediate channel numbers can be calculated from 8MHz per channel (see footnote).
Note: these are the traditional broadcasting channel numbers – not the LCNs (Logical Channel Numbers) used to identify a specific digital TV service, eg 231 = BBC NEWS. If there is any chance of confusing the two, I recommend prefixing the number with either "UHF" or "LCN". Thus: UHF21 = 474 MHz, LCN21 = 5 USA. Other sources use "CH" instead of "UHF", but this does not avoid confusion.
The benefit of digital TV is that each UHF channel is used to transmit the data for multiple TV and radio services (and other information such as the EPG), instead of a single service as was the case pre-digital. Because of this, broadcast channels are also called "multiplexes", or "muxes" (for short).
Channel numbers specified with a "+" prefix indicate the channel frequency is shifted from its nominal standard frequency. This is to reduce co-channel interference from another transmitter (this practice would have been impractical before digital broadcasting). The frequency shift should be insufficient for a digital TV tuner to fail to find it using the basic channel number.
UHF TV aerials are optimised to receive specific ranges of frequencies, dictated by the exact dimensions and positions of the antenna elements. This is a consequence of basic physics - each aerial design is tuned to a specific frequency and have a range of frequencies over which reception will be acceptable. Anything else is a compromise, and trying to design an aerial to achieve a wider frequency range results in it having a reduced sensitivity (poorer signal strength to the TV). This is OK in a good signal area, and digital broadcasting makes it possible to get away with a weaker signal than analogue would have done (before "snow" became unacceptable). So: the wider the range of channel numbers the aerial is specified for, the weaker the signal (or the bigger the aerial has to be for the same signal).
.
Here is the same information presented in graphical form (the colours are the nominal colour code for each aerial group, which might be on the aerial itself in the form of a coloured plastic cap on the main boom).
.
The "700MHz Clearance" programme (to make way for more mobile phone traffic) is shifting transmitter frequencies downwards. It is entirely possible that a Group C/D aerial (which has served perfectly well for many years) will no longer provide adequate reception after a retune event, because it does not cover the required channel numbers after the retune.
To find out what channel coverage you need, start by using the coverage checker (click) to determine the set of broadcast channels used in your area, by plugging in your postcode and house number then scrolling down and clicking "detailed view". You will need to know which of the suggested transmitters your aerial points at. For example, for me (note the presentation has changed since I wrote this post):
If you are unsure which of the transmitters you receive from, take a compass bearing for the direction your aerial points, and compare with the figures in the "Bearing" column. If there is still doubt, look at the "Distance" column and assume the nearest (unhelpfully, this table does not include the transmitter power - a distant high-power transmitter might still override a local weak transmitter... and the two listed at the end of the table above are very weak!).
The table also provides guidance on what aerial group you need (although it is not clear what the last column means – after what, exactly? "Final prediction"??). H means the aerial polarisation needs to be horizontal, and V for vertical (the aerial is mounted so the director and reflector elements – ie the spiky bits – are either horizontal or vertical). So, for Wenvoe, the table suggests a Group B or a Group K aerial (even though Group B might not be good enough to pick up COM7 on UHF21 – see below).
Once you have determined your transmitter, further down the table you will find more detailed information for each of the transmitters. I prefer to take the West region rather than Wales, so I have selected out my transmitter (Mendip):
So, what does this tell us? First of all, the top line says that to receive all muxes (multiplexes – BBC A, D3&4, etc) I currently need to pick up channels UHF32 to UHF58 inclusive. Comparing those with the group table at the top of this post, that means a Group T aerial*. The green bars marked "100" indicate I should get very good reception of all muxes.
* I had my aerial changed from Group C/D to Group T to accommodate the downward shift, but the installer just called it "wideband" (and apparently failed to comprehend my references to Group T) even though the spec sheet clearly says "CH21...CH60" - ie Group T not Group W. Strange, because I chose an experienced firm for the job... but not unreasonable in as much as Group W is no longer needed and ultimately neither will Group T (see later). Whatever, I now have the muxes that had gone missing with the old Group C/D aerial. For reference: £80 got my aerial changed and saved me multiple trips up a ladder (with the attendant risk).
This is still not necessarily the final story though. Regardless of the term "final prediction", those in the industry have this to say:
** https://www.aerialsandtv.com is a mine of useful information, but can take some digging through! The website is written by an aerial company operating around Sheffield, and they appear to know their stuff and are happy to share it - obviously real enthusiasts and engineers.
So is that it then? Take the aerial group recommendation from the Coverage Checker, or match your (best guess) selection of mux channel numbers against the aerial group chart? That might well be what a fly-by-night installer would do, but we can do better.
Aerialsandtv.com provides a lot of advice on aerial siting and such like which shows the lengths they are prepared to go to as competent installers, and which are probably impractical for a DIYer (this may explain why they are willing to share). However, if it comes to simply selecting a suitable aerial, they have provided performance data for a wide range of aerials here:
https://www.aerialsandtv.com/gaincurves.html
An important thing to note is that aerials designed for a specific group (A, B, C/D, etc) do not simply receive that range of frequencies (as illustrated in the groups diagram above) - they have variations in their efficiency within the band, and reception continues (to some extent) outside their nominal band. So, for example, an 18-element Yagi design aerial for Group E still receives passably down to channel 35 or below! The next thing to note is that you do not need a high gain aerial if you are in a high signal area (which could give you signal overload), and if there is too great a difference in the gain performance curve for the frequencies of your particular collection of muxes it could be that making sure the strongly-received muxes do not overload your receivers results in the weaker-received muxes dropping out.
Possibly the most important thing to note is: don't buy cheap (AKA "contract"). Your TV aerial is (typically) inaccessible and out in all weathers, and the last thing you need is to have to replace it when (for just a few pounds more) you could have had a sturdy one that's good for 30 years. It's not the purchase cost of the aerial that's important - what's important is the cost/effort/inconvenience of fitting it.
Footnote: Calculating Frequencies and UHF Channel Numbers
Digital TV combines multiple data streams on a spread-spectrum signal occupying the whole 8 MHz channel bandwidth. The centre frequency for a particular channel number, or the channel number for a particular frequency, can be calculated as follows.
PAL analogue TV (no longer relevant) used separate vision and sound carriers displaced from the channel centre frequency by -2.75 MHz and +3.25 MHz respectively.
Standard UHF broadcast frequencies for TV are 8MHz apart, spanning Channel 21 (474 MHz) to Channel 68 (850 MHz). The frequencies for intermediate channel numbers can be calculated from 8MHz per channel (see footnote).
Note: these are the traditional broadcasting channel numbers – not the LCNs (Logical Channel Numbers) used to identify a specific digital TV service, eg 231 = BBC NEWS. If there is any chance of confusing the two, I recommend prefixing the number with either "UHF" or "LCN". Thus: UHF21 = 474 MHz, LCN21 = 5 USA. Other sources use "CH" instead of "UHF", but this does not avoid confusion.
The benefit of digital TV is that each UHF channel is used to transmit the data for multiple TV and radio services (and other information such as the EPG), instead of a single service as was the case pre-digital. Because of this, broadcast channels are also called "multiplexes", or "muxes" (for short).
Channel numbers specified with a "+" prefix indicate the channel frequency is shifted from its nominal standard frequency. This is to reduce co-channel interference from another transmitter (this practice would have been impractical before digital broadcasting). The frequency shift should be insufficient for a digital TV tuner to fail to find it using the basic channel number.
UHF TV aerials are optimised to receive specific ranges of frequencies, dictated by the exact dimensions and positions of the antenna elements. This is a consequence of basic physics - each aerial design is tuned to a specific frequency and have a range of frequencies over which reception will be acceptable. Anything else is a compromise, and trying to design an aerial to achieve a wider frequency range results in it having a reduced sensitivity (poorer signal strength to the TV). This is OK in a good signal area, and digital broadcasting makes it possible to get away with a weaker signal than analogue would have done (before "snow" became unacceptable). So: the wider the range of channel numbers the aerial is specified for, the weaker the signal (or the bigger the aerial has to be for the same signal).
| "Group A" aerial | UHF Channels 21-37 |
| "Group B" aerial | UHF Channels 35-53 |
| "Group C/D" aerial | UHF Channels 48-68 |
| "Group E" aerial | UHF Channels 35-68 |
| "Group K" aerial | UHF Channels 21-48 |
| "Group T" aerial | UHF Channels 21-60 |
| "Group W" aerial (wide-band) | UHF Channels 21-68 |
Here is the same information presented in graphical form (the colours are the nominal colour code for each aerial group, which might be on the aerial itself in the form of a coloured plastic cap on the main boom).
.
The "700MHz Clearance" programme (to make way for more mobile phone traffic) is shifting transmitter frequencies downwards. It is entirely possible that a Group C/D aerial (which has served perfectly well for many years) will no longer provide adequate reception after a retune event, because it does not cover the required channel numbers after the retune.
To find out what channel coverage you need, start by using the coverage checker (click) to determine the set of broadcast channels used in your area, by plugging in your postcode and house number then scrolling down and clicking "detailed view". You will need to know which of the suggested transmitters your aerial points at. For example, for me (note the presentation has changed since I wrote this post):
If you are unsure which of the transmitters you receive from, take a compass bearing for the direction your aerial points, and compare with the figures in the "Bearing" column. If there is still doubt, look at the "Distance" column and assume the nearest (unhelpfully, this table does not include the transmitter power - a distant high-power transmitter might still override a local weak transmitter... and the two listed at the end of the table above are very weak!).
The table also provides guidance on what aerial group you need (although it is not clear what the last column means – after what, exactly? "Final prediction"??). H means the aerial polarisation needs to be horizontal, and V for vertical (the aerial is mounted so the director and reflector elements – ie the spiky bits – are either horizontal or vertical). So, for Wenvoe, the table suggests a Group B or a Group K aerial (even though Group B might not be good enough to pick up COM7 on UHF21 – see below).
Once you have determined your transmitter, further down the table you will find more detailed information for each of the transmitters. I prefer to take the West region rather than Wales, so I have selected out my transmitter (Mendip):
So, what does this tell us? First of all, the top line says that to receive all muxes (multiplexes – BBC A, D3&4, etc) I currently need to pick up channels UHF32 to UHF58 inclusive. Comparing those with the group table at the top of this post, that means a Group T aerial*. The green bars marked "100" indicate I should get very good reception of all muxes.
* I had my aerial changed from Group C/D to Group T to accommodate the downward shift, but the installer just called it "wideband" (and apparently failed to comprehend my references to Group T) even though the spec sheet clearly says "CH21...CH60" - ie Group T not Group W. Strange, because I chose an experienced firm for the job... but not unreasonable in as much as Group W is no longer needed and ultimately neither will Group T (see later). Whatever, I now have the muxes that had gone missing with the old Group C/D aerial. For reference: £80 got my aerial changed and saved me multiple trips up a ladder (with the attendant risk).
This is still not necessarily the final story though. Regardless of the term "final prediction", those in the industry have this to say:
and:www.aerialsandtv.com** said:
www.aerialsandtv.com** said:
** https://www.aerialsandtv.com is a mine of useful information, but can take some digging through! The website is written by an aerial company operating around Sheffield, and they appear to know their stuff and are happy to share it - obviously real enthusiasts and engineers.
So is that it then? Take the aerial group recommendation from the Coverage Checker, or match your (best guess) selection of mux channel numbers against the aerial group chart? That might well be what a fly-by-night installer would do, but we can do better.
Aerialsandtv.com provides a lot of advice on aerial siting and such like which shows the lengths they are prepared to go to as competent installers, and which are probably impractical for a DIYer (this may explain why they are willing to share). However, if it comes to simply selecting a suitable aerial, they have provided performance data for a wide range of aerials here:
https://www.aerialsandtv.com/gaincurves.html
An important thing to note is that aerials designed for a specific group (A, B, C/D, etc) do not simply receive that range of frequencies (as illustrated in the groups diagram above) - they have variations in their efficiency within the band, and reception continues (to some extent) outside their nominal band. So, for example, an 18-element Yagi design aerial for Group E still receives passably down to channel 35 or below! The next thing to note is that you do not need a high gain aerial if you are in a high signal area (which could give you signal overload), and if there is too great a difference in the gain performance curve for the frequencies of your particular collection of muxes it could be that making sure the strongly-received muxes do not overload your receivers results in the weaker-received muxes dropping out.
Possibly the most important thing to note is: don't buy cheap (AKA "contract"). Your TV aerial is (typically) inaccessible and out in all weathers, and the last thing you need is to have to replace it when (for just a few pounds more) you could have had a sturdy one that's good for 30 years. It's not the purchase cost of the aerial that's important - what's important is the cost/effort/inconvenience of fitting it.
Footnote: Calculating Frequencies and UHF Channel Numbers
Digital TV combines multiple data streams on a spread-spectrum signal occupying the whole 8 MHz channel bandwidth. The centre frequency for a particular channel number, or the channel number for a particular frequency, can be calculated as follows.
For F = Digital TV Carrier Frequency (MHz),
& C = UHF Channel Number:
F = 306 + 8C
C = Round( ( F - 306 ) / 8 )
PAL analogue TV (no longer relevant) used separate vision and sound carriers displaced from the channel centre frequency by -2.75 MHz and +3.25 MHz respectively.
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