Slant polarisation - FM receiving aerial
ChelmsfordNick
Posts: 196 Forum Member
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It has struck me that during discussions about which external FM aerials are `best', there isn't really an answer. It could be argued that systems 1 and 2 described below are both `best', but for different reasons.
(1) Optimum, multipath free, fully quiet FM stereo signals are usually best supplied by a horizontally polarised multi element Yagi aerial mounted at sufficient height. However, hardly any FM signals are HP only any more and these aerials (when not on a rotator) will not provide quality reception for off beam signals.
(2) On the other hand, if one desires a greater quantity of stations (but probably at the expense of the higher quality of a few) one may opt for a half wave or folded dipole suitable for the FM band, mounted vertically.
One option which I have never tried is to slant polarise an FM antenna. I wonder if, say, a 3 element Yagi were to be installed at a 45 degree slant, whether it would offer a combination of the reception properties of systems 1 and 2 above? It would surely maintain some directivity and gain for the transmitter it were facing, but also pick up some off beam and VP signals? Given that most FM signals are mixed polarisation anyway, I wonder if this system might work rather well?
Thanks for reading. I would be very interested to hear anybody's thoughts on this matter. I will also happily be corrected if any of the above is nonsense.
(1) Optimum, multipath free, fully quiet FM stereo signals are usually best supplied by a horizontally polarised multi element Yagi aerial mounted at sufficient height. However, hardly any FM signals are HP only any more and these aerials (when not on a rotator) will not provide quality reception for off beam signals.
(2) On the other hand, if one desires a greater quantity of stations (but probably at the expense of the higher quality of a few) one may opt for a half wave or folded dipole suitable for the FM band, mounted vertically.
One option which I have never tried is to slant polarise an FM antenna. I wonder if, say, a 3 element Yagi were to be installed at a 45 degree slant, whether it would offer a combination of the reception properties of systems 1 and 2 above? It would surely maintain some directivity and gain for the transmitter it were facing, but also pick up some off beam and VP signals? Given that most FM signals are mixed polarisation anyway, I wonder if this system might work rather well?
Thanks for reading. I would be very interested to hear anybody's thoughts on this matter. I will also happily be corrected if any of the above is nonsense.
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... until you're trying to receive a linearly polarised signal and your aerial is slanted the other way, nulling it out !
I don't completely understand what you mean Gerry1, (bit of a novice here) but I think what you're saying is `it won't work', so thanks for letting me know! You will have saved me buying and installing equipment under a misapprehension.
I'm going to renew the whole aerial stack soon, and I'm just planning what to do in terms of radio reception. It's hard to hit on the perfect solution.
The smaller relays are usually vertical
You'll see a similar effect if you rotate an LCD watch while wearing polarised sunglasses.
As to the perfect solution, perhaps it would be vertical dipole for general use plus a multi-element yagi with a rotator and a device to rotate it from vertical to horizontal and vertical the other way. Or even crossed yagis so that you could switch between horizontal, vertical, slant left, slant right. Add some phase shift and you could then choose right hand circular polarisation (used by the original ILRs) and null them out by using LHCP.
I'll get my coat...
Crystal Palace nationals are vertical only. Doesn't seem stop all communal aerials using those grotty haloes.
Columns O and P in this Ofcom spreadsheet show the H & V powers.
If this is only a problem with slant polarised transmissions, however, and for example I don't wish to receive such stations, I wonder if my idea might still work? I was mainly hoping to receive broadcasts from mixed or vertical transmissions (ie most stations in the UK? )
I must admit, until your `Rinse FM' example I was unaware that any UK stations transmitted in this `slant' manner. Do any other major stations? I guess Rinse's sister station, (Spin Dry FM) would use circular polarisation?
Now I guess it's me getting my coat.
VP is becoming more common because more people listen in cars and on portables with telescopic aerials than with fixed receivers and rooftop aerials. HP is often lower power or missing entirely; it's just a nod to legacy systems. Polarity of UK and European stations can be checked at fmscan.org
A vertical aerial is probably all you need, but with multi-element arrays the drawbacks can be that they are more unsightly and the wind loading is greater.
It surely was. All the other 18 ILRs were horizontal polarisation, if I remember correctly.
Err, no !! they were circular or mixed, only Hallam Sheffield had Horizontal, which became mixed later
A few BBC relays too, the old Whitby site springs to mind
http://tx.mb21.co.uk/gallery/whitby/index.php
Wireless World in the 1970s carried a photograph of aerial arrays on Holme Moss, to show the new slant polarised array. I recall that some were for BBC Radio Blackburn, the first BBC local radio station to have them. But then that's nearly half a century ago, and I could be wrong in some of that detail... any BBC Blackburn experts to confirm or deny that memory ?
Reading the monthly magazines "Radio User" ( out the last Wednesday of the month ) and "EPE" ( out a few days into a new month ), they both suggest discone aerials for domestic use ( especially with SDR dongles, which can nowadays cover from long wave to UHF ). One size of discone covers up to the HF end of shortwave, and another the VHF/UHF parts of the spectrum. For anyone who wants "more than just VHF/FM" then a discone is an ideal answer.
Traditionally, unless you had a big garden and wanted a longwire as part of the setup, then a big whip aerial mounted high on low-loss coax served well. I had an ex-tank aerial, made of copper sections each a bit short of a metre long. As other forum users point out, in the UK, the gales soon move around big FM arrays, always when you are listening to a DX station at a vital moment...
Also big fat pigeons/blackbirds etc etc are good at moving horizontal arrays on landing and take-off, and starlings the vertical arrays...
I'd go for a discone... You can make your own - there are plans on the internet for them...
Maybe the RSPB would complain about a slant array - not good landing sites for anything feathered... I've already got my coat on - I live in an old cottage, and in the winter only take the coat off to go to bed...
MB21 is your friend, the slant was produced by H and V aerials
http://tx.mb21.co.uk/gallery/hameldon-hill.php
Actual slanted aerials exist at Waltham for Smooth FM
http://tx.mb21.co.uk/gallery/gallerypage.php?txid=679&pageid=1889
And at Mendip for Heart Somerset (nee Orchard FM)
http://tx.mb21.co.uk/gallery/gallerypage.php?txid=620&pageid=1607
I purchased an Antiference `radio rod' (FM half wave Omni) to see how it would compare to my current HP `vision' (three-sides-of-a-square) variant on a halo.
Mounted the FM Omni vertically on the pole on the chimney stack, same location as the `vision'.
It performs less well on BBCR3 and BBCR4 than the vision, by 3DB (41 vs 44 DB). A minor difference, but a decrease none the less. Background hiss is slightly more audible at high volumes. This was a disappointment (but almost expected) as the BBC themselves have stated that HP aerials outperform VP ones on their frequencies from Wrotham. I had insomnia one night and read all about their testing of Wrotham frequencies post mixed polarisation, and despite it being some 30 years later, my results were very similar to theirs.
What REALLY surprised me, however, is that London FM frequencies have come storming in on the VP dipole which the `vision' barely touched. For example, 95.8 barely registered a signal on the vision - 16DB and very intermittent RDS. The VP dipole gets apx 30DB, and a very listenable FM stereo signal. I am so surprised at this, as I always thought a more marginal signal would perform better on HP. I flipped the rod to HP just to test, and can confirm that the `radio rod' on HP does not get a good signal either on the London frequencies (in fact slightly worse than the `vision', which is actually not as hopeless as the bad press it gets!) so this is, for sure, an issue of polarisation. Perhaps it is one of those random things specific to this area?
I guess my next stage is to get a multi element FM and try it VP and HP, though mounting these VP is problematic (no space on the end of the boom for the clamp) so I think I'll have to remove the reflector, and use the mast as reflector. Looking at the pattern I would predict that BBC Wrotham nationals will come in far stronger (as they are supposed to, hopefully at 50DB+) but that the London commercials will be worse than on the VP omni? If an HP omni barely registers a signal, an I expect a multi element HP to do much more? I will say again, I can't get over just how well Croydon / CP FM is coming in VP.
Thanks for reading, fellow aerial anoraks - just thought this might be of interest to you. Please let me know of any thoughts or advice you have, or if I am making duff statements etc.......
Nick