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Old November 16th 06, 12:26 AM posted to rec.radio.amateur.antenna
Dave Dave is offline
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First recorded activity by RadioBanter: Jul 2006
Posts: 444
Default matching 50 ohm line to 75 ohm antenna

N4aeq wrote:


Im using a double boom LPA, 50 coax connected to the front with shield
to one boom and
cc to the other. The back of the boom is shorted with a bar, it is a
monoband 50-54mhz
5 element. Swr nowhere near 2:1 (i wish) and i have checked the line
so its ok so maybe
i should have ask how do you adjust the impeadance on a double boom lpa?


OK, It's a monoband LP.

I am assuming that the inter-element crossover connections are correct.

The feedpoint impedance is determined primarily by the dimensions and spacing of
the boom elements. They make a two element open wire transmission line. This is
fixed by the design and is not readily tunable [adjustable]. However, secondary
tuning adjustments are possible.

The "shorted with a bar" is a tuning stub that provides some degree of
adjustment. As stated in another post you may have a 200 ohm or possibly 300 ohm
feedpoint impedance [VSWR 4:1]. Please tell us what VSWR value you are seeing.
A VSWR of 4:1 indicates a 200 ohms design; 6:1 indicates a 300 ohm design, etc.

Check the dimensions of the tuning stub for length as described by the
manufacturer.

A LP is a balanced driven array. It should NOT be fed directly with coax [as you
describe]. Direct feed causes a current imbalance and currents on the outside
braid of the coax. You need either a choke balun or a ferrite based 6 meter balun.