[Elecraft] open wire feeders

[Geoffrey Mackenzie-Kennedy]

Hi Guy,

OK I'll own up.  I have used "isolation transformers" as recently as this 
year before leaving Scotland, in the feed system of a Laport Rhombic (dual 
rhomboid).  I used this antenna, aimed towards your side of the pond, to 
listen for VHF signals between 88 MHz and 148 MHz in the pious hope that a 
2m contact (terrestrial) could be made across the Atlantic.

To return to the topic of Open Wire Feeders.  The 4 wire cross-connected 
type of open wire feeder is a viable and less costly alternative to low loss 
coax, especially when used at VHF and the runs are long.  I have used this 
type of line during the past decades.

Without going into the maths or the results of controlled measurements, in 
terms of "noise pickup" and radiation from the feeder the 4 wire line 
provides much better performance than its 2 wire cousin. Also it is 
practical to dimension and build a 4 wire line (wire spacing etc) whose Zo 
is 200 ohms - a useful value for transforming to 50 ohms at the Tx/ Rx end 
of the line.

However on the downside this type of line should be kept under tension, 
which obviously allows the use of fewer spacers, and it does require some 
"housekeeping" to remove any wind blown twigs if there are trees nearby.

Happy New Year.

73,

Geoff
LX2AO


On December 31, 2011 at 8:45 PM, Guy Olinger K2AV wrote:

<snip>


 > But then, who talks about isolation transformers any more.