I suspect the line loss when using two pieces of coax to simulate a balenced
line is closer to the loss from using shielded twinlead as opposed to
unshielded twinlead or ladderline. In this case the coax cable is not being
operated coaxially so published loss figures for coax are ,in my opinion,
not applicable. I've never seen anything published about line loss with coax
used this way. Shielded 300 ohm twinlead for TV receiver use was available
many years ago and was very low loss as compared to coax.
I personally use only a very short length of parrelel RG-8x as a jumper from
the tuner to the window feedthru plug. Beyond the window feedthru is 14ga
ladderline with a slow twist. The length is less than two feet and the
flexibility of RG-8x over the ladder line I use outside is an advantage. At
this short length loss and impedance don't really matter.
Pat, WA4TUK
> While using two pieces of coax in a physical configuration to simulate a
> balanced line may appear to be satisfactory, I point out that the loss
> in coax operating with a high SWR i.e. 10:1 is much much higher than
> open wire line with the same SWR figures. It would appear that using
> coax to obtain a balanced feed then defeats one of the purposes of a
> balanced antenna feed.......low loss.
>
> Typically 100' of RG-8 or RG-213 has 1.0 dB loss at 30MHz when matched.
> With a VSWR of 10:1 the additional loss is 2.5 dB so the overall loss is
> 3.5 dB. For a 20:1 VSWR the additional loss goes up to 4.5 dB or an
> overall loss figure of 5.5 dB. To the contrary using 1" balanced wire
> line, the loss at 30MHz per 100 ft is 0.15 dB when matched. Thus with a
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|