Message-ID: <001e01bec01f$ebc79f80$46218cd4@default>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: "rsgb_lf_group" <rsgb_lf_group@blacksheep.org>
Subject: LF: Field strength measurement....Rik
Date: Sat, 26 Jun 1999 22:53:30 +0100
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Hi Rik,
    I have been grappling with the idea that I need to know how good (or how
bad!) my antenna is on reception. After several months of listening and
slight changes I know it isn't good enough...but how do I know if I have
built a better one or improved it? My searchings brought me up to a page in
Terman (mine's a 1948 copy!) in Ch13 pg 974 (but I think it will vary with
the editions) which advises on how to inject a known field strength into a
loop with a signal generator. It uses a coil across the sig gen output with
a reactance at least three times the output impedance of the generator, and
placed about twice the largest dimension of the loop aerial, away from the
loop. There is an equation for the field strength generated. It occurs to me
that a coil of 150ohms reactance at 136kHz is about 300uH which is quite a
sizeable solenoid. I guess that an emitter follower to reduce the output
impedance to about 1 ohm, would bring the requirement down to about 3uH
which is a lot easier to wind, and a lot more compact. Then your 1-turn
loop aerial would be a possible way of doing it??

The field strength is given by
                            e= (18.85*N*E*(r)^2)/X*d^3
    where N is the number of turns on the exciter coil
             E is the sig-gen voltage in mV
              r  is radius of the exciter coil; in cm
              d is the distance between the exciter coil and loop aerial in
metres
              X is the reactance of the exciter coil
        and e is in uV/metre
There is not a lot more than that in the text but I am willing to send a
photocopy to anyone who wants it. I believe that there is a similar
explanation in a 1956 copy of
The Radio Laboratory Handbook by the well known Mr Scroggie. I can't check
the
above formula against that because I loaned my copy to someone a few years
ago and didn't get it back.
Hope that's useful....there is a lot of interesting gen about loop aerials
in the latest Radcomm, probably similar to a lot that has been published
before, but an interesting practical design incorporating a 'rejector' tuned
circuit that might help Wolf with his strong out of band QRM.
73 de Alan G3NYK
Alan.Melia@btinternet.com