To: <topband@contesting.com>
>Date: Wed, 31 Dec 1997 06:00:22 -0500
>From: "Robert g. Flory" <RobnPam@compuserve.com>
>
>Hello Topbanders,
>
>I am not really sure how to analyze small shielded RX loop, but
>it seems like a very low impedance antenna. Has anyone done
>anything with matching them? Since the signal levels are very
>small, I would think you could GAIN something by matching them.
>
>Rob K2WI
>
Rob,
It really depends on which kind of loop you are talking about.
If you mean the tuned loops shown in the handbook and the antenna
books, then the impedance is quite high (several thousand ohms).
I think that most of the schematic representations of them must
be incorrect because I can't determine the "return" path for the
signal current in most of them. If I assume that the end of the
loop NOT connected to the coax center conductor is grounded even
though this isn't shown grounded, then a capacitve divider might
(if properly designed) present a reasonable match to 50 ohms.
But since you will probably need gain anyhow, it is very easy to
let a high input impedance preamp do the match to the cable.
If you are talking about an untuned wideband loop, then the
impedance is indeed quite low (a few ohms at the highest). For
these loops to work properly they _MUST_ be terminated in a low
impedance load. Since it is difficult to design a matching
network that will maintain a uniformly low input impedance for a
50:1 impedance ratio over 5 octaves or more of frequency
coverage, and these loops require quite a bit of gain as well,
they are usually combined with a preamp with a common base input
stage (with quite a bit of standing current) that does the
matching to the cable while maintaining a low impedance for the
loop to drive.
73, Eric N7CL
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