[Lowfer] Question on TX loop cap placement

[Chris Trask]

>
> I would like to build a HF band TX loop. These loops are .1 wavelength
> of wire or copper pipe. Do you guys see any reason why the HF guys put
> the series resonating cap in the loop opposite of the feedpoint coupling
> transformer? In the Ashlock loop, the cap is placed right next to the
> coupling transformer. All I read about the HF loops is the issues of the
> cap arcing and the need for 5kv plus caps which are expensive and
> physically big. Is the voltage node at the opposite side of the loop
> from the feedpoint or at the feedpoint? I can't see any reason why the
> cap can't be inserted anywhere in the loop. It's a series resonant tank.
> What do you guys think? The HF loop would be tiny compared to anything
> we have used on the MF or LF bands. I have read that these HF loops work
> excellent even mounted inside a attic or inside a concrete & steel
> apartment building running 5w output on 40 meters...hi hi
>

    The loop needs to loaded in such a way as to keep it balanced in order
to retain the symmetrical radiation pattern.  Having a single cap at the far
end of the loop accomplishes this fairly easily, but then the capacitor is
at a point in the loop where the voltage gradient is highest.

    Placing two capacitors to either side of the coupling transformer also
keeps it balanced, but the two capacitors need to be somewhat matched,
though it's not critical.  In the design that I posted a few months ago:

    http://www.home.earthlink.net/~christrask/Paper012.html

I used a dual variable at a point in the matching network that made that
matching unnecessary.  The coupling transformer between the loop and the
tuning section takes care of the balancing by virtue of the grounded centre
tap, and the input current balun takes care of any mismatch of the dual
variable.

Chris

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