In article . com,
wrote:
Thing that is bugging/puzzling me is the amount of signal I get off
the co-ax.


This sounds as if the receiver is not matched to the feeder. Could it be
that the receiver has a balanced RF input, even though the socket is
coaxial? If that is the case it would pick up a lot of signal on the
coax, and would make all attempts to use an aerial at the other end of
the cable rather pointless.

Dunno about recent/modern FM tuners. However mumble-mumble years ago
when I was involved with working on developing an FM tuner we borrowed and
tested a few commercial tuners. Many of them had an input that was neither
'balanced' or 'unbalanced' (if that is taken to assume a properly
sheild/ground effect).

IIRC The problems seemed to be twofold.

1) Many designs hung both the '75 Ohm unbalanced' and '300 Ohm balanced'
inputs together with an open arrangement that didn't really match/suit
either. Sort of 'bodge that will work OK' approach. But in effect each
input then had some 'bare wires', etc, in the box that could upset
performance.

2) Partly due to (1) no real attempt to contain the RF system in a shielded
box so that any external currents from the outer were unable to couple
power into the RX.

Even when nominally shielded OK, the system may have been tweaked so that
various imperfections tended to cancel out when the antenna provided
suitable termination, so then didn't work as well if the antenna was
removed.

Also in some cases, affected by the earthing via mains and/or audio coax.

FWIW I've just changed the FM antenna I'm using with a Quad FM4, and before
I did so, walking around the room affected the signal on one low-power
station, despite the system having an antenna some distance away from the
room. The symptoms showed that the tuner and coax was/is coupling into its
local environment.

Problem 'cured'[1] with a decent signal level down the coax, thus
overpowering the problem. :-)

Slainte,

[1] i.e. swamped.

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

tony sayer said the following on 15/01/2005 19:28:
Yes but one of the main reasons for having mixed is the reduction in
multipath fading this brings about in built up areas due to distortion
of the polarisation due to reflections etc.

Ah, i didn't know that! - Having Horizontal polarisation would make
sense in some cases then - Having said that it might be a hinderance in
cases like Nottingham where some stations :-

Come from the North (Holme Moss Nationals)
Come from the East (Nottingham ILR)
Come from the South South West (Saga)
Come from the South (Century)

(Although if you can position it vaguly between Sutton Coldfield and
Copt Oak you'll probably be fine due to the stregth of the ILR's -
especially in the north of the city)

Kev