Freeview reception - all about aerials
Brian Butterworth published on UK Free TV Updated 8th January 2014.
Your ability of receive all the Freeview transmissions depends on the suitability of aerial
- the design style,
- the "group", and
- its physical location.
Standard type - Yagi aerial
The standard type of TV aerial is known as the Yagi aerial. It is mounted on a pole, and consists of a rod with a reflector (shown green) at the back and many spiky elements (in grey) at the front. The connecting cable connects to the element nearest the reflector, known as the driver (shown in blue).
These Yagi aerials are directional and so pick up signals best from a transmitter that the rod points towards. The more elements the aerial has, the better it picks up a signal and becomes more directional.
A standard-type aerial is all that is required for digital TV reception in most places. These antennae have between 10 and 18 elements and a single reflector. These are recommended for new installations for good digital television reception, but will more often than not function perfectly in good reception areas.
Typically these aerials are designed to receive only some transmission frequencies - see "groups" below.
High Gain aerials
These aerials are designed for poor digital reception areas, and have two reflectors. For maximum signal strength, some digital high gain aerials have up to 100 elements. Since the switchover to digital-only transmissions back in October 2012, most UK households now have good quality digital TV signals.
A more expensive aerial is only required where the signal strength is low, but can often provide the whole Freeview reception where it might otherwise be impossible.
The CAI (that represents aerial installers) has four standards for digital TV aerials. The highest standard "1" is for homes on the fringes of coverage areas, intermediate standard "2" is suitable for use within the coverage area; minimum standard "3" is for good coverage conditions.
These aerials can be either wideband, or receive only selected frequencies - see "groups" below.
Grid
You may haved used a 'Grid aerial' for analogue reception, but as they are generally unsuitable for Freeview reception, they have now generally been replaced by the Yagi type. However in some places a Grid aerial installation may work for Freeview: otherwise replace with a standard Yagi aerial.
Indoor
Indoor aerials are generally not suitable for Freeview reception. In areas of good signal strength it is often possible to receive some transmissions. Even where an aerial works, people often find that may get interruptions to their viewing (or recording).Loft mounted
Loft mounted arrivals are not generally recommended for Freeview reception, as the roof tiles and plumbing will degrade the signal. Some compensation for this loss of signal can be made by using satellite-grade cable to connect the set top box to the aerial.Positioning
The best position for a TV aerial is mounted outdoors, as high from the ground as possible, pointing directly at the transmitter. The signal can be blocked by hills and tall buildings. It should be positioned away from any other aerials.Horizontal or vertical?
The transmitter will either use vertical mode which requires the elements of your aerial to be up-down, or horizontal mode which requires them to be level with the ground.Groups
Both analogue and digital television is transmitted the same group of transmission frequencies (known as channel 21 through to 60). A coloured marking on the aerial shows the group.
To create the best possible analogue picture, TV transmissions from adjacent transmitters have been designated to several different groups of frequencies. By using an aerial that receives only the channels in the correct group, the analogue picture can be kept free from interference.
To receive Freeview transmissions from the same transmitter it has been sometimes necessary to use frequencies that are not part of the transmitter's normal group. When this has occurred, the aerial will need to be replaced with a "wideband" aerial (also known as group W) - one that covers every group.
As Ofcom is planning to move the TV frequencies again - perhaps as soon as 2018 - it may be wise to use a wideband aerial if you can to ensure you can keep viewing Freeview for many years to come.
Help with Television sets?
Friday, 18 May 2012
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jb3812:14 AM
Mike O'Pray: If I could just further add for purposes of explaining / emphasising why the diplexer presently installed will be detrimental to your reception and likewise has to be removed.
Diplexers are simply twin input dual band filters that only allows two sets of channels within a defined frequency range to pass through, and the type that's installed in your aerial system will be allowing channels from Ch21 - 37 to pass through on its No1 input and with anything above that range being attenuated, whereas the diplexers No2 input will only be allowing channels from Ch39 upwards to pass through and will attenuate anything under that.
As you will probably gather, that as your Sandy aerial is on the diplexers No1 input then anything above Ch37 will be attenuated, which of course includes all of Sandy's commercials, namely Ch48 / Ch51 / Ch52 and the very ones you are having trouble with.
That said, these channels will of course be allowed to pass through the diplexers No2 input which starts at Ch39, this being why Dave Lindsay had mentioned about turning the Waltham facing aerial around to face Sandy, which of course would improve the situation but only in a second best fashion, as diplexers have insertion loses plus a signal level tapering off effect near to the crossover points between the two frequency ranges covered, and so its always better without anything being installed except possibly a booster, but only should this prove as being necessary.
Taking the explanation of how a diplexer works into consideration you can maybe understand why this device is totally unsuitable for receiving signals from Sandy because of its channel span range of Ch21 - Ch52.
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jb3812:33 AM
Mike O'Pray: Just noticed that you had replied.
What you have said with regards to your plan of action is correct, insomuch try and check the colour of the square plug on the end of the aerial cross member and if its red then another aerial of a wideband type is called for.
All you require to do in a diplexer is prise off its plastic cover and disconnect the coax from the Waltham facing aerial and tape the ends up out of the road, then take the coax connection from the Sandy facing aerial off the the diplexers input tags and join it onto the diplexers output tags where the feed to the house will already be joined onto, in other words the two coax leads joined in parallel.
By the way the only reason for you receiving more channels via the scart from the DVD will be because of either (1) the Panasonic has a more sensitive tuner, or (2) because when the signal is looping through the Panasonic then into the TV its being attenuated slightly, this effect always noticed more in weak signal areas.
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Mike O'Pray: TV is broadcast on (UHF) channel numbers and these go from 21 to 69. They equate to frequencies. So 21 is a lower frequency than 41, for example.
The channel numbers are divided up into three Groups "A", "B" and "C/D" which are roughly the first third, middle third and top third of the band used for TV. For a table of which channel ranges relate to which Group, see here:
Aerials, TV Aerial and Digital Aerial
Back in the day of four-channel analogue, each transmitter broadcast on four channels that were all within the same Group. Sandy Heath used four channels in Group A (21, 24, 27, 31) and Waltham used four channels in Group C/D (54, 58, 61, 64).
In order to join the two leads from your aerials together a diplexer was needed; they can't just be connected together. A diplexer filters the feed from each aerial. See here for some examples:
Online TV Splitters, Amps & Diplexers sales
Suppose that your diplexer "splits" at channel 36 (C36); therefore it allows C21 to C35 through from your Sandy aerial and C37 to C69 from your Waltham aerial. This worked fine with the former analogue channels mentioned above.
Due to the channel allocations of Sandy Heath and Waltham now, such a setup will mean that some are unreceivable.
Sandy Heath now uses channels 27, 24, 21, 51, 52, 48
Waltham now uses channels 61, 54, 58, 29, 56, 57
I'll come on to which carries what in a moment. But hopefully you can now appreciate that if your diplexer is as suggested, you will be unable to receive 51, 52 and 48 from Sandy and 29 from Waltham. All the other channels would be expected to come through.
For some transmitters, including these two, "out of group" channels have had to be used for Freeview. That is, some channels are outside that of the Group used by the former analogue. This is because there aren't enough spare ones, now that six are needed instead of four.
Digital channels carry a number of services whereas previously one analogue service occupied one UHF channel. See the graphic under the heading "After switchover configuration" on this page (you may need to refresh by pressing F5 to get the graphic to show):
Freeview multiplexes | ukfree.tv - independent free digital TV advice
You will see that there are six groups or multiplexes and each occupies one UHF channel:
PSB1 = "BBCA" (BBC One, BBC Two, BBC Three etc)
PSB2 = "D3&4" (ITV1, ITV2, Channel 4, Channel 5 etc)
PSB3 = "BBCB" (BBC One HD, BBC HD, ITV1 HD, Channel 4 HD)
COM4 = "SDN" (ITV3, ITV2+1, 5* etc)
COM5 = "ArqA" (Pick TV, Dave, Really etc)
COM6 = "ArqB" (Yesterday, Film 4, 4Music etc)
The six UHF channel numbers that I gave above for Sandy and Waltham those which these six multiplexes broadcast on in that respective order.
If your aerial diplexer is as I suggested, then you could theoretically receive the three PSBs from Sandy and the three PSBs from Waltham. You could only receive COM5 and COM6 from Waltham, but you won't be able to receive COM4 from either because they are outside of the range of channels that the diplexer allows through for each respective aerial.
On the 9th May there was a change to the channel used by one multiplex at Sandy Heath and that was COM4 which was on C31 up to that date. This was within the channel range allowed from your Sandy aerial which probably explains why you could pick it up then.
Receivers usually give the UHF channel number that they are tuned to on the signal strength screen. You may be able to ascertain what transmitter(s) the Panasonic is tuned to for which multiplexes. I suggest that you focus on the first service from each multiplex, i.e. BBC One, ITV1, BBC One HD (if applicable), ITV3, Pick TV and Yesterday.
My guess is that Pick TV and Yesterday will be coming in on 56 and 57 respectively, which means that you are receiving them from Waltham.
If there are duplicates, then you will find them in the 800s. So you will probably have duplicate PSBx services. The signal strength screen should allow you to identify which is which.
Perhaps you are receiving the PSBs from Sandy and COM5 and COM6 from Waltham (with Waltham's PSBs being in the 800s). Therefore your regional programming will be BBC East/ITV Anglia.
As for why the Goodmans hasn't tuned-in as many services, perhaps it doesn't cope too well with signals from different transmitters or perhaps during its automatic tuning sequence it stores channels from only one transmitter. This is speculation on my part and a clearer picture of what's happened may be gained through some investigation. If the Goodmans gives the tuned UHF channel number on the signal strength screen you will be able to piece together what it is picking up. Perhaps it only has the two (standard definition) PSBs from Sandy tuned.
If it has manual tuning, you may be able to add missing channels. However, before you do this, I suggest that you use the Panasonic to find out what it is that you are receiving; are COM5 and COM6 coming from Waltham? If they are and the Goodmans has a manual tune function, then you need to try and manually tune C56 and C57.
I suggested in an earlier posting that a possible solution may be to turn your Waltham aerial to face Sandy Heath. Obviously you will no longer be able to receive BBC East Midlands/ITV Central from Waltham, but the UHF channel spacings (if you wish to receive COM4) have probably put paid to that anyway.
My reason for making this suggestion is down to the likelihood that your aerials will be "Group" ones rather than widebands. That is, they will be designed with maximum sensitivity on channels within their respective Groups only. I think that we can safely say that this is so as they are 20 years old.
This is why I think that using one or the other may not work satisfactorily. For example, if removed the diplexer (and therefore Waltham aerial) from the equation by connecting the Sandy aerial directly to your downlead, then being a Group A aerial, it may not be sensitive enough for the high Group C/D channels that Sandy's COMs operate at.
There will be many people who will only have a single Group A aerial and who will now have to replace it with a wideband one in order to receive the COMs.
However, as you have another aerial that works at the desired channels (48, 51 and 52), I suggest that the cheapest solution may be to turn the C/D (Waltham) aerial to face Sandy.
Whether this will work hinges on what channel the diplexer splits at. If it is as I suggested above (36), then it should work. If, however, it splits at C51 (for example), then obviously it's not going to work. It needs to split below C48.
It is also to be hoped that the securing bolts can be undone!
I respect jb38's comments as he's a professional and I'm not. However, I suggest this as being a possibility, primarily because it seems to be the cheapest option.
Whilst there is signal loss with using the diplexer (and I couldn't say to what degree), it would appear that you picked up COM4 (via your diplexer) prior to 9th May when it was on C31 and radiated at a transmission power of 20kW. Now it is at 170kW, so you have a much stronger signal to play with.
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jb387:50 AM
Mike O'Pray: If I could just add a qualification to the plan of action insomuch that if its found that you do have an original group A (red end) aerial, then even without changing it you would most likely find that you could receive the missing channels by simply disconnecting the Waltham facing aerial and by-passing the diplexer, as that device alone will be killing off the reception of the commercial channels far more than the fact of the aerial operating out of band, as aerials out of band will still work but with their efficiency dropping off the further out of band they are required to operating in, although with the commercial channels upping their power to 170Kw that would more than compensate for the out of band deficiency.
Should I have known that your two aerials were linked together I would have mentioned right from the start that the system had to be changed, albeit of course that you were unaware of why it would be having such a detrimental effect on reception of the commercials.
In other words, to make life easy if you know someone with a ladder that can access the diplexer then maybe you could get them to by-pass it, even if its just done by taking the coax connections off the terminals (usually always screwed) and joining the coax from the Sandy facing aerial onto the coax feeding the house using a small terminal block, because as long as the terminal block is inside the casing for protection against water then its perfectly OK.
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John6:53 PM
Chesterfield
All my bbc programmes on freeview are poor reception.
We have the Waltham transmitter and prior to switch-over very good reception, and for 3 months AFTER switch-over reception on bbc reasonable. Now hopeless, in both TV and radio channels
Scan shows 35% strength 26% quality. All other channels 100%
Ariel (type 2) on mast
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John's: mapJ's Freeview map terrainJ's terrain plot wavesJ's frequency data J's Freeview Detailed Coverage
John: Have you confirmed that it is Waltham that you are tuned to for the BBC services? The signal strength screen should tell you what UHF channel you are tuned to and for BBC One etc it should be C61.
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Stephen P11:43 PM
According to
UK digital TV reception predictor
You may have multi-path trouble.
* - where the field strength is asterisked, there is a possibility of multipath reception effects. It might be worth trying again with a greater antenna height.
and it looks like you might do better with Nottingham
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Sunday, 20 May 2012
N
nigel12:17 PM
Chichester
Hi jb38
Thanks for the info. sorry for not coming back sooner but not been able to mess about with the aerial until today.
1. i have removed 4 of the elements from my aerial but signal was poorer whether vert. or horz. Put them back one by one no improvement this was using the labgear signal strength meter.
2. signal strength at tv whether by-passing the 8 way splitter or not was the same.
3. I have a hitachi digi box recorder and the signal out of that was still two leds lit.
4. With regard to the transmitter being 27 miles away I hung a piece of aerial out the window and with it attached to tv got super pictures and sound on beb 1 and 2 but no other channels were available. when the aerial is attached to the digi box, the picture is broken up on beb 1 and 2 and no other channels available.
5. With the agro I am having would you advise getting rid of this new aerial and changing it for a XB10K or ????
Thanking you for your time
Nigel
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nigel's: mapN's Freeview map terrainN's terrain plot wavesN's frequency data N's Freeview Detailed Coverage
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jb383:09 PM
nigel: Thanks for the update / info regarding tests carry out, and in particular the result of your wire aerial test somewhat proving that a good signal does exist at your location but one that most likely requires some experimenting with the aerial position to achieve a satisfactory balance over the six muxes, as if that test had been carried out using a length of coax connected to a DM log you would most likely have discovered that moving the log to the left or right as well as at various vertical heights (not necessarily meaning higher) could well have resulted in the other muxes being picked up.
With regards to the test you made on the aerial by removing some of the elements, this should really have been carried out using the TV's signal checking facility as any differences seen in signal level when this sort of thing is done in a non line-of-site location would be so marginal that your Labgear tester would not really show it, as proven when you replaced them one by one and no difference was noticed, not of course that there would have been that much of a difference if viewed on the TV either, as its only in line-of-site (or very near) conditions that this is noticed, and only "if" the aerial has been aligned bang on the signal source in the first place.
Regarding your present aerial which as was aforementioned is not really suitable for use in situations such as yours even with some elements removed, and although I did previously mention that a Log 40 would suffice, however going by the result of your wire aerial test I now feel that even the shorter DM log would be more than adequate in your circumstances as well as being easier to move around within a loft environment, and so even although the Log 40 is still OK for the job I would be inclined to use the DM log, that also being shown in the link previously sent.
A little tip though, for successful results when anyone is trying an aerial in different positions and not in possession of a professional meter, then they really require some other form of instant feedback on any actions taken, and this to be perfectly frank is where your Labgear tester is not really good enough for that purpose, because in most cases the best "quality" signal is not achieved when accompanied by a maximum signal strength reading, and indeed can be quite the reverse! this being where your Labgear can be misleading! and so if at all possible you should try to borrow a small portable TV to use in the loft with the aerial connected directly into it and leaving the TV sitting on its signal checking screen, as this would enable you to peak the reception quality irrespective of signal strength as well testing the reception across all the muxes after every movement of the aerial position.
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Stephen P4:29 PM
Nigel I am no expert but IIUC the various elements of your aerial are intended to work as pairs at different frequencies, so unless you know what you are doing just removing some is unlikely to get results.
There is a lot to be said for paying a LOCAL installer for his local knowledge.
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