[Elecraft] Elecraft: RFI Problems

[G3VVT at aol.com]

In a message dated 30/01/05 01:32:53 GMT Standard Time,  
rondec at easystreet.com writes:

All of  the 75 ohm stuff that I've seen wasn't rated for high voltages
anyway, so  it's likely to melt at the voltage loop when hit with high power.
AT the  very least, it'll be rather lossy. 



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There is more robust USA sourced 1kW rated 75 ohm twin available in the  
market though sadly not as easy to obtain now as in the past. Believe red/black  
power cable makes an effective 75 ohm twin feeder and this is available for  
quite high currents though not with high voltages. Am using commercial 75 ohm  
twin feeder cable rated at 300W at present which is very adequate for the 100W  
maximum I use. In any case we have in the UK a maximum license power of  400W 
pep which restricts our exposure to high power effects. Guess would be  
rather different in the USA with your maximum of 2kW pep.
 
QRO can have it's downside!
 
Not sure that the statement that it would be rather lossy is correct as I  
have been led to believe that twin feeder is less lossy than coax and  
particularly the 50 ohm version. Even the 75 ohm hard-line coax as used by cable  TV 
companies was always better than the 50 ohm coax in the loss figures for a  
given diameter cable.
 
I do use a resonant 1/2 wave trap dipole on 40/80m only at present  with the 
feeder cut to the length recommended by the antenna designer of 70 ft  (or 
multiples of this). Working towards adding a two extra pairs of traps for  80/20 
to give added bands of 160m and 20m. With always using resonant antennas  the 
problems that could arise on the twin feeder are minimised and hopefully any  
interference problems as a result. So far telephones seem the only problem  
area.
 
The wider spaced twin feeder would be impossible in my case as the  operating 
position is on the opposite side of the room from the window  and built into 
a fitted wardrobe. At a previous house the shack was  just that, a wood hut 
under the centre of the dipole and wide spaced twin feeder  came direct to a 
balanced ATU.
 
As always it is a matter of "cutting the cloth to suit your circumstances".  
What will be suitable for one situation may be hostile in another.
 
Bob, G3VVT