[TowerTalk] Fwd: remote switching for balanced lines - homebrew plan
JMLTINC at aol.com
JMLTINC at aol.com
Sun Apr 13 16:15:31 EDT 2008
Looks like an interesting, and perhaps time-consuming project.
I quickly perused Cebik's article. Frankly, the azimuth patterns were not
all that impressive for the amount of work you will presumably have to put into
this. As his article states, the Old Timers used a twisted trio of wire. Your
1KW of CW make toast of that feedline as there is insufficient spacing. His
article suggests a higher Z feedline. That is wider spacing. Are you going to
construct custom ladder line to achieve his targeted 600 -800 ohm
impedance? If so, you may have a problem with interaction with the tower. If you are
going to use commercial window line. Its characteristic impedance is only
about 450 ohms.
You did not say, but I presume you are looking to use this antenna on
several bands. Quoting Mr. Cebik: "...40 through 10 meters, can show feedpoint
impedances in the thousands of Ohms, with considerable reactance". This will be
the problem you will have with the relays (and possibly your tuner). If the
feedpoint at a particular frequency is of high impedance, you will certainly
have voltages proportional to the impedance and power presented.
At 1 KW, if the resultant load impedance is 2K ohms (resistive), expect
1,414 volts. Reactive loads (which you certainly will see if you are planning a
multi-band version) will drive the voltages even higher. Depending upon the
ultimate impedance, relay contact spacing, and ambient moisture; an open frame
relay may work, but at higher frequencies I would not count on it. The 30 KV
relays you were looking at are overkill, but certainly have headroom! Maybe a
lesser vacuum relay would suffice for 1/4 the cost.
Perhaps you can build the antenna without the relays and, using the feedline
of your choice, connect only two of the legs. Then play the antenna. See
first, if it will take the KW without zapping your tuner, and if you like the
pattern. If you like the results, then perhaps you could dig into the pockets
and take the next step. Personally, I'd rather climb than spend.
Remember to be careful to keep the balanced line away from the tower.
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