Hi Paul

Couldn't quite understand that (word ot two missing?)

See if I can explain better..

A coax line will radiate (if its a transmitting circuit) and receive
signal (as a receiving cct) if it isnt terminated into the (same)
impedence at both ends of the cable that is the same as the cable
itself. In other words the shield wire isnt always an effective screen.
What is also an issue is if the symmetry of the source and load isnt
preserved. ie if you feed a dipole direct with coax the "balanced"
aerial and "unbalanced" coax will add up to an amount of line radiation.
A balun is often used to balance an unbalanced feeder. This can either
be a wire wound transformer or loops of coax. Feeding a vertical 1/4
wave whip is a good example of feeding an unbalanced antenna with
unbalanced cable..

You can probably imagine that this coax radiating/receiving issue is a
big problem when the cable runs through a cars engine compartment.

Would this need a transformer, inductance/capacitance circuit, static
resistor, or what??

Well that depends on the cct inside. Being a wide band device the
impedence will vary greatly and no doubt have a reactive component. I
have no idea what the norm is for FM broadcast receivers, even those
with 75 ohm or 300ohm connectors! I'd suspect a simple L/C matching
circuit would do it but might even opt for a small toroidal balun. I'd
just assume that the Z would be about 300 ohms and create a 4:1 RF
transformer (thats 2:1 turns) for the 75 ohm coax. The balun will tend
to resolve any reactance and balance problem as well.

My personal preference for an antenna would be a single quad loop fed
over the last 1/4 wave with a piece of RG62 93 ohm coax. This will match
the Z of the antenna (about 110 ohms) with the coax. I'd also coil up
about 5 turns of coax into a 3" loop where it feeds the antenna. This
will resolve the balanced/unbalanced line radiation problem. This is of
course more a transmitter config and given that you want to use over a
wide range of freqs the 1/4 matching section would be a mute point.

Bob, could you explain one mystery to me? A normal aerial arrangement,
whether fed by ribbon or coax, is effectively one continuous loop of
wire. DC resistance is at most a few ohms - ie pretty much a dead
short. Impedence is resistance to AC, ie the signal, but the fact that
impedence is present doesn't remove the dead short. So how come
anything gets around the loop at all?!!

You have to think of the antenna as a tuned AC circuit. In fact
completely throw away any thoughts of a DC circuit havning any effect
whatsoever. Same kind of logic as a power transformer not looking like a
short circuit. Think of the antenna as being feed by an instantaneous
voltage that takes a fixed time to get from the feedpoint to where there
is a "short cicruit" in the wire. The signal from one side of the coax
arrives "in phase" with the one from the other side so no current flows
between them. The trick is in the length of the wire in question. For a
folded dipole the impedence looks like about 300 ohms provided the
antenna is tuned to resonance. If you depart from resonance the
conjugate impedence (resistive plus reactive) will always rise. If you
start feeding this antenna at twice the freqency for what it was
designed you end up with a very high impedence (say 10K plus ohms). You
are now "voltage feeding" the thing.

It can and is a little more compex than this and I am not sure I have
done it justice. Try another analogy. If you are pushing a child on a
swing in sync with it, the energy you put in will be effectively used.
If however you push in the exact opposite sense (by timing 180 degrees
out of phase) you hit what look like a short circuit. Think in terms of
instantaneous voltage in different parts of the antenna that change over
time.

And another: Where does this PD occur? Across the receiving element of
the aerial, or within the element relative to earth? Does this mean an
aerial 'loop" works differently from a single long wire?

Really testing my basic theory here! grin

For most applications you would tend to think that the volts would occur
at the antenna feedpoint as an AC value 180 degrees difference from one
side to the other. You can draw the instantaneous current and voltage on
the wire if you like (and this is handy for working out the shape of the
radiated pattern) but probably isnt helpful in your case.

Also think in terms of maximum power transfer with soruce and load Z...
Reactance of course complicates that and the *real* theory involves
looking at capacitive and indictive reactance..

I hope this all means something to you!

Cheers Bob