On Sun, 25 Dec 2005 00:35:08 GMT,
wrote:

On Sat, 24 Dec 2005 15:40:25 -0800, Roy Lewallen
wrote:

wrote:
I leave only this.

At VHF and up it's common to use a shorted 1/4 wave section for
second harmonic suppression at the output. Very effective and dirt
cheap. The finals are not the least bit bothered.

If a short appeared near a 1/4 wave node at operating frequency
it might go unnoticed.

I'm afraid it wouldn't go unnoticed. The transmitter would see an open
circuit, instead of the proper load of typically 50 ohms. The effect on
the transmitter would be the same as disconnecting the feedline at the
transmitter.

Roy Lewallen, W7EL

I did use the word "might" rather than will.

Actually it depends on the real life characteristics of the short. If
it were a perfect short (in theory) yes. But if there is any varience
from that it's going to be harder to predict. Likely it world look
more like a higher impedence, but not completely. In all likelyhood
the parameter that needs to be know more than any one
its frequency. At 432 it's impact would be very different than say
7.2mhz.



Allison

A short across the transmission line will have much the same effect at
432 as it will at 7 mhz.
What you are thinking about is a shorted stub attached to the
transmission line or output of the transmitter. A shorted 1/4 wave
length stub at the operating frequency placed across the transmitter
output will present a high impedance at the operating frequency and
will not be noticed by the transmitter. But at the second harmonic of
the stub it will be a 1/2 wave shorted stub which will present a short
at the output of the transmitter at the 2nd harmonic frequency.

The shorted stub would still allow energy to flow to the antenna
normally. But shorting the transmission line would not no matter where
it was.

73
Gary K4FMX

On Sat, 24 Dec 2005 20:44:34 -0500, Gary Schafer
wrote:


I did use the word "might" rather than will.

Actually it depends on the real life characteristics of the short. If
it were a perfect short (in theory) yes. But if there is any varience
from that it's going to be harder to predict. Likely it world look
more like a higher impedence, but not completely. In all likelyhood
the parameter that needs to be know more than any one
its frequency. At 432 it's impact would be very different than say
7.2mhz.



Allison

A short across the transmission line will have much the same effect at
432 as it will at 7 mhz.
What you are thinking about is a shorted stub attached to the
transmission line or output of the transmitter. A shorted 1/4 wave
length stub at the operating frequency placed across the transmitter
output will present a high impedance at the operating frequency and
will not be noticed by the transmitter. But at the second harmonic of
the stub it will be a 1/2 wave shorted stub which will present a short
at the output of the transmitter at the 2nd harmonic frequency.

If it were a "perfect" short yes.

For real life the short have real impedence between center conductor
and shield. As frequency goes up a .2" peice of wire accumulates
enough real resistance and reactance to be a factor at high VHF and
uhf.

My favorite filter for 2m is found on the ARRL.com TIS site. it's
made with series and parallel shorted sections operating as tapped
resonant circuits. The stubs are only something like 2" and for 2m
thats about 13" short of 1/4 wave. Just shows what happens when
a transmission line stops being simply that.

The shorted stub would still allow energy to flow to the antenna
normally. But shorting the transmission line would not no matter where
it was.

Yes and No. See above.

Allison
Kb!gmx