Water in Belden 9913 gives a very nice match at 440 MHz, but also makes
the air insulated coax into a very effective attenuator.
If a transmission system has both low SWR and proven low loss, then SWR
changes can indicate opens and shorts. It can be instructive to
intentionally check the SWR with the antenna end of the transmission
line left open and then shorted. Either an open or a short is a relative
term at RF. I tried that with some twinlead at 144 MHz once. I had
noticed that in coax that I could figure forward and reflected power and
thus measure feed line loss. When I added a balun, I could detect its
loss the same way. A length of twinlead (this was very large conductor
200 ohm transmitter twinlead) should have had loss too. The shorted
(with its 12 gauge conductors bent at right angles and soldered
together) twinlead had a swr indicating match, until I moved up a SHEET
of copper to short the wave that was propagating around the twinlead.
Hence my claim that twinlead at VHF does radiate. With the copper sheet
as the short, the very heavy duty twinlead did show the predicted loss
at 144 MHz.
Well DeMaw got that right (though I have a letter from him that says I
don't know what I'm talking about so I can't hold him in high regard).
Baluns will easily saturate and both create heat and harmonics. I've
seen baluns tuned with a tuner so that all the power was dumped into the
balun, and removing the antenna had no effect on the tuner input SWR.
The E.F. Johnson Matchbox is a variation on a good tuner circuit. True
its link coupled and that is good, but the load is connected through a
differential capacitor that limits its load impedances to fairly high
and not too reactive. There are many antennas, the double extended zepp
for one, that it will often not tune, not even on the design band. It is
a handy tuner for those antennas it will match but limited in range.
I prefer the open link coupled tuner where I can connect the antenna
side as serial or parallel tuned (two separate capacitors) with either
variable link or a capacitor in series with the link (ala 30's era
antenna handbooks). So far within the limits of my coil (a bit on the
large side) this tuner has tuned any balanced feed antenna I've
connected from 160 through 20 meters. And now its tuning my 80 meter
inverted V fed with RG-141 coax, though I've not tried other bands. I
expect the shack end feed Z at some bands to be unreasonable because of
the 50 characteristic Z of the coax. But my current main interest in HF
is on 80 meters so its perfect. And my Corsair II likes the load it
sees.
73, Jerry, K0CQ
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