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Date: Wed, 13 Mar 2002 10:12:49
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: loop/update
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>Inductance measured = 173 uH
>Resonant F measured = 3065 khz
>DC resistance of loop wire plus coax feed 1.5 ohms.
>Loop wire 2.5 mm insulated.
>Feed 75 ohm coax, about 150 ft long, loop feed point, bottom right hand
>corner.

Hi Mal,

I don't know if you measured the loop-inductance with or without the coax.
But in case you measured with the 75 Ohm coax you'll have to take the coax
capacity (typical 67pF/m) into account, unless you matched the loop (R =
few Ohm or less) to the 75 Ohm of the cable.

If the coax length is short compared to the wavelength (for sure it is at
136kHz) and you have a sufficient mismatch (that's the case if you connect
the cable direct to the loop, at 136kHz) then the coax will mainly act as a
parallel capacitance.

150 ft (45 m) of 75 Ohm coax represents 3 nF that is in parallel with the
loop inductance. You probably have to take that into account when matching
the loop.

73, Rik  ON7YD

PS : 
Nice to see that some good old thumbrules still work, one of these say that
you need +/- 1 wavelength of wire to get a loop resonant. Your loop is 124
x 40 ft, so in total 328 ft (99 m) of wire. Fits very well with a resonance
just over 3MHz.