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Message-ID: <5.1.0.14.0.20021128091317.00b464e8@pb623250.kuleuven.be>
Date: Thu, 28 Nov 2002 09:15:45 +0100
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
In-reply-to: <3DE5B958.5738.32FCF@localhost>
References: <5.1.0.14.0.20021125171906.00b9d028@pb623250.kuleuven.be> <001d01c29475$f77f6140$0500a8c0@charter.net>
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Subject: Re: LF: loop antenna
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Hello Dave,

A Q of 20 seems very low, it would mean that losses are as high as 5 Ohm (Q 
= XL/R).
But maybe Q is calculated based on the Rrad of 2.5 Ohm, that would explain 
the low value.

73, Rik

>On 25 Nov 2002 at 17:58, Rik Strobbe wrote:
>
> > I am about to put up a transmitting loop antenna. Dimensions will be 23m
> > long and 15m high. Thanks to Dick (PA0SE) I could achieve a wheel of 7mm
> > litz wire, so wire losses should be minimized (about 0.04 Ohm).
>
>I have just done a couple of calculations on Rik's loop using
>formulae from Mike Underhill G3LHZ (which are quoted on my web site).
>Mike says that the maximum Q is in fact limited by the size of the
>loop and has a limiting value of Q=500/D, where D is the diameter of
>the loop (or for a rectangular loop the circumference divided by pi).
>Feeding Rik's dimensions into this gives a maximum Q of 20, and it
>will in practice be lower than this due to resistive losses.  This
>formula has been derived by Mike from actual measurements on loops
>and while I would vouch nothing for its validity myself, measurements
>I did on my loop some time ago did indicate it was in the right ball
>park figure. Reducing dc resistance will not increase the Q above
>this figure and the astronomic voltages and currents rumoured will
>not arise.
>
>The second formula from Mike is his for loop inductance - it gives a
>value of 123uH which sounds about right.
>
>The third of Mike's formulae on my site is his for radiation
>resistance. As I have previously found it produces silly values of
>radiation resistance for 136 loops - in Rik's case of about 2.5 ohms,
>clearly nonsense.
>
>Regarding currents flowing in the loop (and assuming the Q stays
>within the sensible limits above) measurements on my loop done at the
>5W level and scaled indicate a loop current of 26A at 400W. This is
>probably well in the handling range of Philips 376 polycarbonate
>capacitors - GW4ALG used this power with his loop with no problems.
>These capacitors are cheap and worth experimenting with.
>
>73s Dave G3YMC
>
>dsergeant@iee.org
>dsergeant@btinternet.com
>http://www.dsergeant.btinternet.co.uk