> After the the Stew Perry event I need to get some RX
> improvement on
> topband. A small-space candidate is what many refer to as
> a "magnetic loop"
> or "shielded loop" antenna. After reading lots of
> contradictory material on
> the subject, I think W8JI has it right. If I understand
> correctly this
> antenna...
Don't give me the credit! Faraday and Maxwell and the rest
learned that long before I was around.
A few Hams and SWL's might have it wrong, but all the good
engineering, physics, and science books have it right.
> So, basically the outer shield surface (a) is the antenna
> that generates a
> voltage, the inner surface (b) is the primary of a
> transformer, and the
> center conductor (c) is the secondary of the transformer,
> which is
> terminated in a capacitor. Thus, a more proper name for
> this antenna might
> be "a coax tranformer loop".
>
> If I understand correctly, the same current runs on all
> three conductors.
> Now, my question is: Why bother with all of this
> transforming hassle? Why
> not use a single wire for the loop, terminate it in a
> capacitor, tap off the
> received energy with a small balun transformer and on to a
> coax to the RX?
I tried to explain that at this place;
http://www.w8ji.com/magnetic_receiving_loops.htm
PROPERLY implemented, the "shield" can physically support
the antenna and it makes it easier to balance. So there is
some advantage to the "shield".
Improperly implemented the shield can increase common mode
and make the antenna couple to the feedline.
If the gap is not directly opposite the feedpoint or
grounding point, the loop will have significant common mode
response with the feedline. It is a bad design.
A single conductor "unshielded" loop is just as effective at
isolating the antenna if you couple into the single
conductor properly. It works just as well, if not a little
better in some cases. It just might be a little easier with
the "shield" in place because it can simplify the feedpoint
by integrating it mechanically.
73 Tom
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|