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
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