Re: through glass coax coupling

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
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