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R: [antennaware] Feeding cable lenght

To: <>
Subject: R: [antennaware] Feeding cable lenght
From: (K4SB)
Date: Tue, 03 Aug 1999 22:25:31 +0100
Maurizio Panicara wrote: ( I think )
> > Hi to all,
> >
> > I have spent a lot of time to calculate the feeding coaxial cable to my
> > 40m inv "V" antenna. I know if I get waveleght/2 or its multiplies the
> > antenna impedance comes down to the radio and I eliminate the loss.
> ....everything in your statements works until last four words.

And Stefan Samu replied
> The only loss you can eliminate is eventually that one between
> cable and RTX at transmitter edge, value that comes out when
> antenna and/or line are different, and/or different than RTX
> impedance.
> You can use what so ever cable impedance to feed a 50 Ohm
> antenna into a 50 Ohm transmitter, but inherent cable losses
> will occurr.
> To minimize coax losses match antenna to RTX and line
> impedance.
> 73,
> Mauri I4JMY

This may be a case of my not understanding Mauri's reply, but to
clarify a little...

1. An electrical 1/2 wave, or multiples thereof, of any type and
impedence will ALWAYS repeat the load impedence at the opposite
end. This occurs regardless of the charateristic impedence of the
line. For instance, if you have a 50 ohm load and a 50 ohm Tx
output, AND the coaxial line is an electircal multiple of 1/2
wave, it matters NOT what the impedence of the coax ( or whatever
else you use ) is. You can use any impedence line, and the losses
incurred will ONLY be that which is ordinary from the coax
itself. If you use 600 ohm line, and load impedence is 50 ohms to
50 ohms, the only loss will be the normal attenuation of a line
of that length. SWR is and will indicate 1:1

As others have pointed out, a 70 to 50 ohm ( or 30 to 50 )
mismatch does not introduce enough loss to even worry about.

And as a matter of determination, when cutting that line, (
assumming you
have a device such as a MFJ or similar or even just a SWR bridge
) if you will cut the line at a even multiple of the frequency
you want, the accuracy of the cut is much closer. For example,
suppose you want a 1/2
wave electrical line for 7.0 Mhz. If you set your device to 140
Mhz ( a multiple of 20 ) you will find a much narrower range of
dip. You can be
off 200 kHz at 140, which will translate into only 20 kHz at 7.0


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