RadioBanter - View Single Post

December 25th 05, 06:44 PM posted to rec.radio.amateur.antenna

On Sun, 25 Dec 2005 04:05:07 GMT,
wrote:

On Sun, 25 Dec 2005 01:51:01 -0000, (Dave Platt)
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.

True, but that's not the situation we're dealing with here.

If you place a shorted quarter-wave section directly at the
transmitter's terminals, in parallel with the antenna feedline, then
the transmitter "sees" the impedance of the feedline (its usual load)
in parallel with the impedance of the shorted stub (very high). The
net impedance is that of the load (the admittance of the shorted stub
is nearly zero) and the transmitter does not "notice" the presence of
the shorted stub.

That's not the situation which occurs if the feedline itself is
shorted 1/4 waveline towards the load

Only if the short is perfect or the frequency is low enough.
Even for shorted stubs doing it right requires a bit of effort
as even a loop at the end causes interesting effects.

In that situation, the *only* thing that the transmitter will see is
the shorted quarter-wavelength "stub" between itself and the short.
The impedance at the point of the short is nearly zero - it's the

Nearly zero is not zero and it's not a constant with frequency.
At 3.5mhz I will agree with you. At 350mhz it's going to show
something different. At 3500mhz, who knows?

impedance of the short itself, in parallel with the impedance of the
antenna as seen when looking up the remainder of the feedline. No

However is it parallel or is it some complex reflection of the length
to the antenna.

matter what the antenna's impedance is, the very low impedance of the
short itself is going to dominate the parallel combination. The
resulting near-zero-ohm combination will be transformed, by the
quarter-wavelength distance back to the transmitter, so that it
appears as an open circuit to the transmitter.

The transmitter cannot, in effect, "see past the short circuit" to the
antenna itself.

The same is true no matter how far up the feedline from the
transmitter the short/pin happens to be. At the point of the short,
the impedance is going to be nearly zero, and this near-zero impedance
will be transformed to some other value on the same very-high-SWR
circle (neglecting consideration of feedline loss, of course).

No matter where you pin the coax, the transmitter is going to be
unhappy.

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

Different situation, I'm afraid.

If you have an antenna analyzer, try it out for yourself. Take an
arbitrary-length section of RG58 with a 50-ohm load at one end and a
BNC at the other. Run it into a BNC "T". Out the other leg of the T,
run an adjustable length of RG-58 to the antenna analyzer. You ought
to measure 50 ohms in this situation.

Now, stick a short directly across the third branch of the T connector
("pinning" the coax, so to speak), and see what your analyzer tells
you. It may read high-Z, or low-Z, or intermediate-Z with a lot of
reactance... but it'll be a high indicated SWR, and it won't be
anywhere near 50+j0.

Will it be greater tha say 500+janything? Depends on frequency.

Actually using a tee and doing that creates a working stub that is
tuned in the low microwave region and something reactive below
that. In fact the open Tee at high uhf is also a trouble maker.

Then, disconnect the antenna from the "T". The impedance and
indicated SWR won't change significantly.

But t does, depending on frequency and line length on the other side
of the short. Try it for an exact 1/2wave from source to load and pin
at 1/4wave point. the reflected impedences at both ends will be
in play. The pin is only a perfect short at very low frequencies
as you go up the "short" gets "longer" and behaves as a reactance.
It's a complex circuit and there are lumped analogs.

Try changing the length of the RG58 between the "T" and the analyzer.
You'll get a different Z value with the short in place (whether the
antenna is or is not attached) but it'll still have a really high SWR,
no matter what coax length you choose.

Its easier to vary F than change length. But yes you will see varying
impedences (R+J) all over the map.

I keep saying a perfect short would behave as stated. Most real
world components like that pin would be a pain at 3.5mhz and
something else at 432mhz.

Allison
KB!gmx

Try it and you will be surprised.

73
Gary K4FMX