```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 [email protected] ``````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: [email protected] or [email protected] ```