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

[Gary Slagel]

Thanks for the feedback John,

I haven't decided on the feedline yet.  I'd like to
just use commercial 450ohm window line.. not sure how
it would react at a KW but I've used it with good
success at 100W.  I expect that my 88' on a side
doublets will not have the very high impedances that
Cebick has with his 67' doublets.  He's at a full wave
on 40M so naturally has a high impedance there.  With
my 88' I'm trying to avoid the huge impedances by
avoiding hitting a full wave on the lower freqs.  I'm
still not sure how much luck I'll have tuning a KW but
will cross my fingers and if I have problems on a
particular band I thought maybe I could fool with
adding some extra feedline in on a bad band. 

The relays are really my biggest question cuz I've
never played with them before.  It sounds like what
I'm after is a relay with the highest voltage rating? 
Of the ones I've looked at the RNF100-SP open frame
relay is rated at 2kv and costs me $8, the
RF1E-26N1137 vacuum relay is rated at 2kv and costs me
$22.50,  medium amp power relay costs me $17.21 and
doesn't show a voltage rating but someone said I could
spread the contacts to 1/2" and effectively achieve a
very high voltage rating... I wonder how high?  Based
on that it looks like, costwise, I should go with the
open frame?  Of course for more money I can get higher
rated vacuum relays but I wonder if its worth it? 

Again, thanks for the feedback... any input is
welcome!

Gary

--- JMLTINC at aol.com wrote:

> 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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Gary Slagel/N0SXX
Lakewood, CO
http://marina.fortunecity.com/sanpedro/351


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