Topband: Efficiently Matching Low-Impedance Antennas to 50ohms

[Tom Rauch]

>
> Shouldn't the coax lengths be electrical quarter
wavelength rather than half
> wavelength?

Indeed it would.
It would be a 1/4 wl, and the impedance required would be
the square root of the product of load impedance times
source impedance.

With a 20 ohm antenna and 50 ohm rig, it would be 20*50=1000
and Sqrt of 1000 would be 31.62 ohms.
With a 25 ohm antenna and 50 ohm rig 25*50=1250 and sqrt is
35.36 ohms in the Q section.

A pair of 50 ohm cables 1/4 wl long in parallel would be 25
ohms. With a 25 ohm load the 50 ohm SWR would be 2:1. With a
20 ohm load SWR would be 1.4:1.

You can do it this way. Take the load SWR on the 1/4wl line
you make up and multiply it times the line impedance and you
have the impedance at the rig. A 20 ohm load on a 25 ohm
line is a 25/20= 1.25:1 Q section SWR. 1.25:1 Q section SWR
times 25 (the Q section impedance) is 31.25 ohms, that would
be the rig end impedance. That would be a 50/31.25 or a
1.6:1 50 ohm SWR.

A pair of 50 ohm cables 1/4 wl long connected to a 20 ohm
antenna would produce a 1.6:1 50 ohm SWR.
A 50 and 75 ohm 1/4 wl line in parallel would be 30 ohms,
and that would be 45 ohms or a 1.1:1 SWR with a 20 ohm
antenna.
A pair of 75 ohm cables would be 37.5 ohms and that would be
70.3 ohms or a 1.41 50 ohm SWR

A pair of 75 ohm cables would be the closest match for a 25
ohm antenna, being a 1.13 SWR at 50 ohms.

You can also use a non-synchronous transmission line
transformer.
http://www.w8ji.com/transmission_line_75_ohm_to_50_ohm_transformers.htm


73 Tom




A 75 and 75 would be 37.5.