Message-ID: <000801bf803e$233d2280$b6d499d4@w8k3f0>
From: "Dick Rollema" <d.w.rollema@freeler.nl>
To: rsgb_lf_group@blacksheep.org
References: <000e01bf7fc0$25daae00$5ca201d5@default>
Subject: LF: Re: "Low loss inductors" ..EW&WW
Date: Sat, 26 Feb 2000 10:44:02 +0100
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To All from PA0SE

Alan Melia wrote:

> Hi All, a little technical conundrum . I was musing about some of the text
> in the article flagged up by Christer in Electronics World (Wireless World
> to all us older ones!) It gives a design for a vertical with
counterpoises.
> Unfortunately when you scale it to 136 it is about 450feet high. Sort of
end
> of story......but hang on a minute it said something about feeding this
> inflatable vertical via a low loss 'coaxial' inductor. A short-circuited
> length of coaxial transmission line whose length is less than a quarter
wave
> looks like an inductance at the end remote from the short. Oh yes whose
got
> a mile or two of UR67 then!!   Er .....then I thought again..... with a
> velocity factor of 0.67 for solid polythene cable, a quarter wave section
of
> line is just 370 metres.  I keyed some numbers into the formula (Radio
> Handbook Terman pg 192) and found that 300m of UR67 looks like about 2mH
at
> 136kHz . Yes, its an expensive coil, with UR67 at its best price of about
40
> quid a 100m drum, but not totally out-of-court if the loss is
significantly
> less than a traditional solenoid coil. The other plus point is that UR67
> will stand 15kV....yes I know the spec says 5kV but I used 300m of it as a
> delay line in a pulse generator I built in the 1960s for zapping submerged
> repeater amplifiers with a 15kV square pulse (simulates a thick trawlerman
> with a BIG axe). So I know there is no problem in that quarter. Its even
> better if you use 75 ohm cable as then just 200m will give you 1mH, 300m
is
> nearly 3mH
>
> Now the problem ....I have no idea how to calculate the likely loss of
this
> 'inductance' and my old (1942 edition) of Terman is no help here. It does
> say that the reactance changes more rapidly with frequency that a normal
> inductor, which might mean that a variometer would still be necessary.
> Surely one cannot just use the normal coax loss figures ( which are
> negligable at these frequencies ) as
> the cable has effectively an infinite SWR on it.
>
> Right....over to you....cos I don't know the answer.
>
> Cheers de Alan G3NYK
> Alan.Melia@btinternet.com

With the ARRL "Antenna Book"  comes a floppy disk with a Transmission Line
computer program called TLA, written by N6BV. I used version 1.0 .

The program does not know the British type UR65 but from another source I
understand it can be compared to RG-213.

For 300 m (984 ft) of RG-213, short circuited at one end, the program
produces the foillowing results::

Frequency: 0.137 MHz
Transmission line characteristic impedance: 50.0 - j 2.30 Ohms
Matched -line loss, dB per 100 ft: 0.053 dB
Velocity factor of transmission line: 0.660
Maximum voltage rating of transmission line: 3700.0 V
Matched-line attenuation = 0.522 dB
Resistive part of impedance at load: 0.0001
(I typed 0 Ohms, but the program apparently changes that into 0.0001 -
PA0SE)
Reactive part of impedance: 0
SWR at load: 4793489.50
SWR at line input: 16.67
Additional line loss due to SWR: 60.281 dB
Total line loss: 60.803 dB (100.0%)

At line input, Zin = 49.42 + j 172.52
At 1500 W, max. rms voltage on line: 988.6 V
Distance from load for peak voltage = 984 ft

So as a dummy load it would not be too bad ....

73, Dick, PA0SE
JO22GD
D.W. Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel. +31 589 27 34
E-mail: d.w.rollema@freeler.nl
or
pa0se@amsat.org