Calling W8ji and G3tQX

[M6AWG]

How do you feel re paralleling 2 equal lengths of 75 ohm coax to create an impedance of 37 ohms feedpoint to feed a 20 metre monoband ground-Plane ?

[G3TXQ]

As G3tQX hasn't responded, I'll take a shot

I presume you would want to do that if you knew the ground-plane impedance was close to 37 Ohms and you felt that the extra loss due to the mismatch on 50 Ohm coax was significant.

Be aware that using two parallel lengths creates a 37.5 Ohm line with exactly the same matched loss as one length of the 75 Ohm line. I don't know what type of 75 Ohm coax you would be using, but you could easily find that its matched loss was no better - perhaps even worse - than the loss in mismatched 50 Ohm coax. For example, on 20m the increase in loss of 100ft of RG213 due to a mis-termination of 37+j0 is only 0.05dB. That doesn't sound like enough to warrant going to the trouble of creating a "special" line.

73,
Steve G3TXQ

[N3OX]

For example, on 20m the increase in loss of 100ft of RG213 due to a mis-termination of 37+j0 is only 0.05dB. That doesn't sound like enough to warrant going to the trouble of creating a "special" line.

I thought perhaps the idea was to make a 1/4 wave transformer to match ~25 ohms to 50 ohms.

Unless I had good solderable 75 ohm cable like RG-59 or RG-11 I would probably build a twelfth wave transformer like the one shown in the last picture on this page.... it needs ONLY 50 ohm cable, though it does need a section of paralleled cable. When I installed a 40m quarter wave ground mounted vertical here it had something like a 22 ohm impedance and I built such a transformer for it.

[G3TXQ]

I thought perhaps the idea was to make a 1/4 wave transformer to match ~25 ohms to 50 ohms.

Dan,

That's possible - he really needs to define the requirement a bit more tightly.

73,
Steve G3TXQ

[M6AWG]

Dan,

That's possible - he really needs to define the requirement a bit more tightly.

73,
Steve G3TXQ

Im sorry Steve re the callsign,

I have shelved the whole idea now due to being more realistic about my lot (balcony) 8x8 feet and crammed radials etc and will pursue the EFHW with 30 guage wire to render the element more or less invisible at 20 feet to the human eye.
Its almost as much of a challenge to get a signal out of a small lot homebrewed as it is to buy a commercial monoband yagi and work easy DX. The challenge has gone in that case.

Best wishes gents and thank you, M6AWG.