I sent a message directly to Micheal the originator of this thread but I
see from many comments that one solution that always works was left out.
I have tried to do the ingore the SWR approach of Steve LXC, and also
this approach of multiples of 1/2 WL with CATV. Neither has proved
satisfactory,
The SWR approach as Dave K6LL points out is too gross, and as Tom W8JI
points out can cause high SWR from random transformations of sychronous
line mixes of 50 and 75 ohms. Your baluns will contain lengths of 50
ohm coaxes as well that you cannot change to 75 ohms. So even trying to
convert the whole system to 75 ohms is difficult.
The 1/2 WL approach narrows the bandwidths even more on the beam since
we add a tuned transformer into the mix. The transformer being your
CATV line.
The other approach that always works is to use a properly designed 50:75
ohm wideband RF transformer on each end. Loss if these wideband
transformers is negligible, and they can be built to handle high power
levels, (no heating, no owls, no lawyers needed either).
The transformers can be constructed to handle 1.8 to 60+ Mhz in one
shot. Many are being used by folks on this reflector.
To see an example of one goto my website and look at the baluns page
http://www.arraysolutions.com/Products/baluns.htm
Surf to the bottom of the page for the UN-UN transformers
Jay, WX0B
Pete Smith wrote:
>
> At 05:11 AM 3/31/01 -0500, Tom Rauch wrote:
> >> Is there a solution for matching 75 ohm hard-line to a triband
> >> antenna?...
>
> >1.) Pick a length of 75 ohm cable that does not result in the worse
> >impedance showing up at the 50 ohm cable connection point at the
> >transmitter end.
> >
>
> am I right in thinking that the general answer would be to make the 75-ohm
> feedline a multiple of a half-wavelength at the lowest operating frequency,
> and that the worst situation would be if the 75-ohm were an odd number of
> quarter-wavelengths?
>
> 73, Pete N4ZR
> Contesting is!
>
> The World Contest Station Database
> is waiting for your input at
> http://www.qsl.net/n4zr
--
Jay Terleski
Array Solutions - RME Filters
Phased Arrays - Horizontal and Vertical
RF Switches, Antennas & Towers
www.arraysolutions.com
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