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LF: 73kHz/Coaxial Stubs/field strength

To: [email protected]
Subject: LF: 73kHz/Coaxial Stubs/field strength
From: "James Moritz" <[email protected]>
Date: Mon, 28 Feb 2000 20:24:21 +0000
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
Dear LF Group,

Spent a couple of hours on 73kHz on Friday evening; didn't see/hear any other signals - Rugby TX was generating it's usual racket. However, the effort was worth it because my signal was seen by Markus, DF6NM, in Bavaria, as he reported earlier. Great stuff. I intend to keep doing Fridays on 73kHz; I will also try and get on the band on Saturday mornings if time permits - we've only got until June, after all! I hope to improve my signal before then, too. Unfortunately, I probably won't be on tommorow (well, it's sort-of - first-Tuesday-in-March), due to other commitments. I noticed the Rugby TX is quite often idling these days - it was Friday before last, and also over most of the weekend - under these conditions it puts out two carriers, separated by 85Hz, and although there are intermods at 85Hz intervals through the whole band, these are much less of a problem than the wideband noise usually present. Worth keeping an eye on.
Read the discussion about using a coaxial stub for a loading coil
with some interest - it just so happened that today I was doing
something which involved cutting a lot of lengths of UR67 from a
reel, so before I started I put the whole lot on a HP4275A LCR
meter, measuring at 100kHz. With the far end shorted, the
inductance of the 50m reel was 14.5uH, and the series resistance
1ohm. this amounts to a Q of only 9.
It's a bit difficult to use the text book equations, since the cable
parameters at 136kHz are not usually specified. In order that
people can work out the impedance for other lengths, I also
measured the per-unit-length parameters with open and short
circuit terminations (and assuming 50m is a small fraction of a
wavelength). These were:
L=290nH/m;      R=0.02ohm/m
C=103pF/m;      G=60nS/m

(G is too small to be measured really, or to make a lot of difference for that matter; 100kHz should be close enough to 136kHz for practical purposes. UR67 is equivalent to RG213 etc.). I used the PSpice circuit simulator to work out the impedance of different length stubs (saves me getting it wrong 98 times on paper!). The results were similar to what G0MRF stated - the maximum inductive reactance I could get was about 320ohms, or about 375uH. At this point the Q was 1, so not very helpful.
As a capacitor, the 50m of cable was much better, 5.16nF with a Q
of more than 1000 (more than the LCR meter can measure), so
using it to tune a loop looks like a good idea.
Interesting to see Christer's field strength measurement results -
from my experience, loop antennas behave quite well , so the
results should be within a few dB. In view of the discussions about
propagation, losses etc., I wonder if Christer could tell us a bit
about the environment around his QTH, and the places where he
made the measurements? I am working on a portable field-strength
measuring setup (so much to do, so little time.....), for which I have
an old Rycom 3136 selective voltmeter - if anyone knows where I
can get circuit data for this, I would be eternally grateful!
Cheers, Jim Moritz
73 de M0BMU

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