VHF / FM aerial: a polarisation question

[Skywave]

Update, Oct. 10, 2015.

This thread is associated with
https://www.vintage-radio.net/forum/...d.php?t=120164,
to which reference should be made.

After considerable experiment, I finally settled for North Hessary Tor, Devon, as my target transmitter. It became quite clear that I could have chosen Wenvoe, but I chose North Hessary in order to receive BBC Radio Devon on 103.4 MHz plus the 'usual stations'. By carefully measuring the associated freqs., I was able to discriminate to which TX I was listening to and obtained satisfactory signal levels (i.e. giving a stereo signal with very little 'background mush' - if any) from North Hessary and Wenvoe. (That result puts Wenvoe broadside to the aerial main beam, but a 3-ele. aerial does have a fairly wide azimuthal bandwidth, of course.)

So it would seem that the hard work has been done, just a few other matters arising to sort out.
1. The tuner - a Sony tuner / amp. - has a balanced 300 Ohm aerial input impedance, so a balun / Z-match network would be nice. Without that, my arithmetic indicates a power loss of about 2 dB.
2. A general tidy-up of cabling - inside the workshop and out.
3. Wait for the scaffolding to be removed! I didn't mention that, did I? (Reason: I didn't want to complicate matters and encourage off-topic posts). This Edwardian-style house had a very tall - and disused - chimney, which upon inspection, was in a dangerous condition. Its height has been reduced: for that, scaffolding was required. Naturally, I took simple advantage of the opportunity!
4. Finally, get out of the habit of choosing only Classic FM (with its all-too-frequent adverts) as my default station for music (on the radio) and listen to BBC Radio 3 more often. (That station was always weak on my impromptu indoor 'metal tape' aerial: it isn't now.)

And that concludes this update. My thanks to everyone who so constructively contributed to this thread.

TTFN,
Al.

[paulsherwin]

Once you get the aerial up high and clear of buildings you may well get a clean off beam signal, though there is a much greater risk of multipath.

2-3dB loss is indeed right for a 75-300 mismatch (and the other way around). You shouldn't notice this unless you need every dB of signal you can get, though baluns are cheap and easy to find - lots of portable tellies used them to match the loop aerial to a 75 ohm socket.

[emeritus]

In the late 1970's, when I was living in Chadwell Heath (near Romford in North-East Greater London) at an elevation of abut 70' above sea level, I used to be able to receive French and Dutch FM stations in stereo most evenings, as well as Radio Oxford, using nothing more complex than a simple horizontal dipole cut to 100MHz made from some 15mm copper pipe, suspended from the rafters in the loft with some nylon cord. The deep end null was useful for tuning out strong local transmissions for better foreign reception. Foreign reception eventually became impractical when the FM band filled up with local radio stations broadcasting 24/7. I was using a budget Alba UA700/UA800 tuner + Amp set up from Laskys. While I never suffered from multipath, FM stereo reception of BBC broadcasts later became impractical because the tuner could not cope with interference caused by the BBC's RDS signals, and the BBC could offer no solution.

[Hybrid tellies]

Something else to throw into the mix. Just discovered that Chard has a BBC low powered national FM station, BBC R's 1-4 only,which is a relay of Wenvoe. But looking at the pictures on the MB21 site it is only VP.

[Skywave]

Hi Simon: thanks for that report. I think you have now identified Stockland Hill, from where we get a very strong UHF TV signal. As for VHF FM, I am told that for Stockland Hill, BBC R2, R3 and R4 are relays from Wenvoe, whereas R1 is a relay from North Hessary Tor (Not that I ever listen to BBC R1 anyway! )

As for my new installation - as I mentioned above - although the aerial is nominally pointing to North Hessary Tor, I am sure I am also receiving signals from Wenvoe and it is also highly likely that Stockland Hill is in the mix as well. At the end of the day, it doesn't really matter to me: all the stations that I want to receive are coming in at a good strength (adequate to produce clean stereo). There's also quite a few - yet unidentified sources - that are below that signal strength level, so overall, I'm quite pleased with the end result. And the overall cost? About £25: new chimney lashings and assorted other bits 'n' pieces, mainly 'left-overs' and salvaged items.

As for TX data from the 'Net, it seems that some of it is wrong, conflicting with other Internet sites or simply out-of-date.

Again, thanks for your help in this matter.

Al. / Oct. 11, 2015. //

[paulsherwin]

MB21 usually has accurate frequency data: http://tx.mb21.co.uk/gallery/gallerypage.php?txid=449