"Roy Lewallen" wrote in message
news:TJ-dnV1WvLdVMWjanZ2dnUVZ_r6rnZ2d@easystreetonline...
Mike Kaliski wrote:

Haines,

The versions I used years ago relied purely on capacitive coupling. The
braid was broken and only the centre lead of the coax was connected to
the plates. The earth return was via the automobile chassis for vehicle
use, or via earth connections for domestic use. The metal frame of the
window might work if you have any means of getting a good connection,
otherwise you will need some form of reliable (low impedence) earth
connection inside and outside the building for the system to work
properly.

The coupling through the glass should be purely capacitive although some
inductance effects will inevitably be introduced due to the size of the
plates.

You'll have a considerable amount of inductance for another reason if you
follow this approach.

Consider what happens when you separate the two conductors of the coax as
Mike suggests. One conductor continues more-or-less directly through the
glass. The other conductor becomes the "ground" path from wherever the
current leaves the inside of the coax shield on one side of the glass to
where it again enters on the other side. These are the two conductors of a
transmission line which you've inserted in series with your normal line.
If you'll do a little rough calculating, you'll find that this line has a
much higher impedance than 50 ohms. So, if electrically short, its effect
will be that of a series inductance. If it's not so short, the impedance
transformation will be a lot more interesting. Another thing that happens
is that the length of the "ground" conductor is greater than the length of
the center conductor. This will probably give rise to common mode
currents, which will involve the outside of the coax in the active
feedline.

As an alternative you might consider making up two matched tuned circuits
(coil and capacitor) set for the 10 metre band and try inductively
coupling the coils through the glass. To my mind that would be smaller
and easier to set up without the need to worry about earth feeds. A kind
of through glass balun if you like. Obviously you will need to keep a
reasonable distance away from the metal window frame, but a couple of
turns of wire and a couple of capacitors may be all you need.

That might have a better chance of producing a smaller disturbance to the
whole feed system. If you use direct capacitive coupling through the
glass, however, you should remember that you have two conductors to deal
with -- calling one "ground" doesn't give it magic properties. I'd use two
coupling plates, one for each conductor, as close together as possible. A
bit of intentional shunt capacitance would help compensate for the
inevitable series inductance which would be created by even that setup.

Roy Lewallen, W7EL

Roy

Very helpful and useful additional comments as usual. Thank you.

Mike G0ULI