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G0SYR > VHFUHF 06.02.06 11:49l 86 Lines 3677 Bytes #999 (0) @ WW
BID : 15761G0SYR
Read: GUEST
Subj: Re: Freq Selective propagation
Path: ON0AR<F6KMO<F5GI<DK0WUE<7M3TJZ<CX2ACB<ON4HU<GB7YFS<GB7CIP<GB7CIP<
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Sent: 060206/1046Z @:GB7CIP.#32.GBR.EU $:15761G0SYR
T:From: g0syr <g0syr@gb7cip.ampr.org>
T:Newsgroups: ampr.prop.vhf
T:Message-Id: <yuwm4hvxujxf.1grsh60q4d4im$.dlg@40tude.net>
Hi Andy and all,
Sorry for delay in responding I've been a bit busy :-)
On Sun, 29 Jan 2006 14:12:00 +0000, g0ftd%gb7nna.#31.gbr.eu@gb7cip.ampr.org
wrote:
> I've just been reminded of this whilst thinking about another matter,
> both of which I'll describe.
>
>
> Firstly has anyone ever put a spectrum analyser on the end of a TV antenna
> and watched the selective fading and enhancement of the channels as the
> propagation becomes frequency selective ?
>
> For example, in the UK we have a basic 4 terrestrial tv channels and a fifth
> but only in some areas.
>
> Watching a group of 4 and their signal strength I can see how (for example)
> 1 channel increases in strength, 2 go down and one stays the same !
>
> (I think each channel is about 24Mhz apart from the next in the group).
>
> Has anyone else ever seen it ?
>
> The other oddity I've noticed whilst monitoring a local 70cm repeater
> (GB3EK about 22 miles away) is how the signal strength changes on 2
> different antennas.
>
> I have here two homebrew "Slim Jim's" - one is for 2m and the other for
> 70cm. They are at the same height but approx 2 metres apart.
>
> Their feeders enter a switch box and then into my ic-706.
>
> Now what I notice is that the repeater signal can go UP on the 2m antenna
> and DOWN on the 70cm antenna. I'm a bit baffled by this as I don't see
> the effect on other 70cm repeaters.
>
> Good stuff this rf lark ;-)
>
> - Andy -
Yes I have seen similar effects. I suspect most of the variations
you are seeing are due to various forms of multipath reception.
The signals at VHF/UHF are rarely recieved over a single path,
reflections are nearly always present and are far from stable.
As the signals are made up from multiple signals if the path
length varies then the signal phases will also vary and this
will be wavelength dependent affecting each channel in a
different way. The most stiking occurence I have observed is
when trying to use a loft antenna to receive Crystal Palace
about 11 miles north of me and LOS from a properly mounted
antenna. Trying to bodge an antenna in the loft, although signals
were very strong I found good reception on 3 of the 4 channels
(this was pre channel 5) Channel 4 was almost unwatchable
sometimes losing colour sometimes line/frame lock. This was
due to a clean reflection from nearby junk. As you watched the
signal the variation was so freqency selective that you could
watch the nulled signal shift in freqency through the video
passband some times affecting sync frequencies sometimes the
higher frequency colour subcarrier part of the video spectrum.
The cause of the steady variation of the path was not obvious
until you remember the Crystal Palace tower sways back and
forth significant amounts even in relativly calm weather.
Mounting the antenna on the chimney, as I should have done
in the first place, cured the problem giving high quality
reception on ALL channels until Channel 5 came along with all
its inherent problems with keeping the French happy and
transmitting from the Croydon mast. That has built in multipath
which you can't really fix but that's another story.
I found an interesting web site the other day which gives similar
results to the old DOS based radio path plotter but this is geared
up for easy plotting for TV transmitter sites to a viewers site.
http://www.megalithia.com/elect/terrain.html
--
73 de Bryan g0syr.ampr.org [ 44.131.244.60 ]
Amprnet mail g0syr@gb7cip.ampr.org
AX25 mail G0SYR@GB7CIP.#32.GBR.EU
Internet Mail g0syr@beeb.net
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