Already you got the answer by Serge, if into the line there
is an High SWR (like in the well known 1:4 coaxial balun)
and if the line is long this can be a considerable amount
of extra losses added.
I can also add that much less than 1:12 SWR is enough to
rise coaxial lines losses while open wires are much less
subject to this effect.
73,
Mauri I4JMY
> 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
But SWR in any point of this line will be 600/50=12:1.
And attenuation in line will be bigger then when SWR 1:1.
SWR never depends of line length.
(It will be only better on trx side because of losses in line).
73 de Serge UT2IO.
-----Messaggio Originale-----
Da: K4SB <k4sb@mindspring.com>
A: Maurizio Panicara <i4jmy@iol.it>
Cc: Stefan Samu <isamu@netsoft.ro>; <antennaware@contesting.com>
Data invio: martedì 3 agosto 1999 23.25
Oggetto: Re: R: [antennaware] Feeding cable lenght
>
> 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
>
> 73
> Ed
>
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