I wouldn't worry about the 50/75-ohm mismatch. If you are feeding a dipole,
chances are the load is closer to 75 ohms anyway, unless the dipole is low
to the ground. The pi-network should load that impedance without any
problem.
My 160m vertical uses a simple L-network at the base of the tower. It is
tuned to present a 50-ohm nonreactive load at 1900 kHz. The SWR reads about
2.5:1 at 1800 and at 2000. I just leave it set for 1900, and match the
transmitter into the feedline (buried RG-213) with a separate tuning network
between the transmitter and feedline. I don't notice any difference in
signal reports from one end of the band to the other.
The only problem is that the Gates broadcast transmitter, which is almost
like a ricebox in that its output network is designed to take a 50 to 70 ohm
load, will not load to full power at extreme ends of the band. I fixed that
by adding an additional outboard L-network between transmitter and feedline.
One thing I found out the hard way (cost me two perfectly good 0-5 amp
thermocouple rf ammeters) was that if the load on the L-network is
accidentally lost, the circulating current between the transmitter and the
unloaded L-network runs high enough to instantly burn out the thermocouple
in the meter. The first time, I lost the load when the antenna changeover
relay failed. The second time, I simply forgot to reconnect the antenna
feedline after a thunderstorm. As soon as I figured out what was going on,
I bypassed the rf ammeter in the Gates, and if I want an rf feedline current
reading, I use an outboard meter between the output of the L-network and
antenna feedline.
It was nice being able to monitor power output directly by calculating per
ohm's law, but not worth the risk of loss of another meter. Like just about
everything else related to radio, thermocouple rf ammeters are getting
expensive and hard to find.
Those "transmatches" that use an unbalanced L- or T- network fed into a
bal-un to feed open wire tuned feeders are bogus. A bal-un is designed to
work into a purely resistive load, usually at less than 600ohms. Open wire
tuned feeders may present a much higher or much lower impedance, which
probably has a significant reactive component. Even if the toroidal bal-un
doesn't burn up, it probably inserts considerable power loss in the system.
I once tried to load a highly reactive open wire feedline on 160, using a
link-coupled balanced tuner with a BC-610 split-stator plate tank capacitor
in the tuned circuit, and the capacitor, rated at 7000 volts breakdown
voltage, arcked over on modulation peaks with only 100 watts carrier output!
I added about 60 more feet of open-wire feedline to get away from a
particularly highly reactive length, and then none of my transmitters were
capable of generating enough fully modulated carrier to arc over the tuning
capacitor of the same voltage rating.
Don, k4kyv
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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak/
http://gigliwood.com/abcd/
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