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Re: LF: loop/update

To: [email protected]
Subject: Re: LF: loop/update
From: "Rik Strobbe" <[email protected]>
Date: Wed, 13 Mar 2002 10:12:49
In-reply-to: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
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.



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