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[TowerTalk] Fwd: remote switching for balanced lines - homebrew plan

Subject: [TowerTalk] Fwd: remote switching for balanced lines - homebrew plan
Date: Sun, 13 Apr 2008 16:15:31 EDT
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Hi Gary,
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.
-John, N9RF

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