Daniel:
Don't let the "75 ohms" dipole impedance fool you. That is an ideal taken from
free space, and certain heights above ground.
The first thing you need to establish is "What is the actual feedpoint impedance
where you want the antenna to hang?" Once you know this, then you can worry
about matching the antenna to the line.
You can reduce the center feedpoint impedance to 50 ohms by several means--
lowering the dipole to .166 wavelength above ground, a good ground, that is.
For poor ground, you might have to go lower.
Another approach is to turn the dipole into an inverted vee. As the included
angle becomes less than 180 degrees, but greater than 90 degrees, you will find
a 50 ohm impedance point.
Another approach is to create a very fat dipole, either with a cage structure or
widely spaced parallel wires. This will lower the feedpoint impedance also, but
I'm not sure how far. There are formulae to describe this, but I couldn't find
one in a quick scan of the ARRL Antenna Book. It could be modeled with EZNEC
and the impedance found this way.
If your antenna really is 72-75 ohms when in place, you could use the 1/12
wavelength feedline transformer approach. For an antenna Z of 72 ohms and a
feeder of 50 ohms, connect a 1/12 wavelength 50-ohm line to the antenna, then a
1/12 wavelength 72-ohm line to the 50-ohm line. Now connect your 50-ohm line to
the 72-ohm line and run it to the shack. It looks like this:
ANT-->1/12 wl @ 50 ohms-->1/12 wl @ 72 ohms-->50 ohms line to shack.
Remember to account for the velocity factor when measuring and cutting the coax
sections.
As a last resort, you could accept the 1.5:1 SWR and live with it. Many hams do
and get along just fine.
I hope this gives you some ideas, Daniel.
72 and Happy New Year,
Bob N6WG
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