[CQ-Contest] 75 to 50 ohm balun

[David A. Pruett]

Up until a few years ago, I would have offered the same advice.  However, a 
couple of caveats:

- Make sure you avoid lengths that are odd multiples of 1/2 
wavelength.  This causes the line to act like a "Q" section and can make 
the impedance seen on the input up to 100 ohms and a 2:1 SWR.  I saw this 
on my 80M four-square fed with ~ 270' (5/4 wavelength) of CATV 
hardline.  SWR at the antenna was flat (as expected with a ComTek 
box).  However, in the shack it was 1:1 @ 3800 and 2:1 @ 3500.  I put an 
Amidon 50:75 un-un at each end and now it's 1:1 flat in the shack too at 
both frequencies.

- The "cut to 1/2 wavelength" trick works for moderate feedline lengths of 
a couple half wavelengths or so.  My friend W8MJ did this on a long run of 
CATV hardline to a TH6 on his back tower; I would guess maybe 350'.  On 10M 
this is something like 27 half-wavelengths and his SWR bandwidth was VERY 
narrow.  Again, some 50:75 transformers made the problem go away.

Here at K8CC, the entire station is built around 75 ohm CATV hardline.  I 
have used the "asynchronous transformer" (the design using 1/12 wavelength 
coax lengths) on all my antennas.  These are monoband antennas tuned to 75 
ohms, then I use an asynch in the shack. These have been inplace since 1986 
with no SWR issues and no failures.  With multi-band antennas, the Amidon 
50:75 un-uns work well.

Dave/K8CC



At 08:32 AM 6/14/04 -0400, k1ttt at arrl.net wrote:
>in most cases this isn't necessary.  the swr caused by the mismatch is
>only about 1.4:1.  unless there are specific reasons why that is
>unacceptable just hook it up and go for it.  if you are picky you can cut
>it to a multiple of 1/2 wavelength.
>
> > I have hardline CATV 75 ohm feedline running up to my tower and need to
> > terminate both ends.  Can anyone suggest a method of making the 50 ohm
> > connections while keeping 1:1 SWR?  The outdoor connection must be
> > weatherproof.
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