[TowerTalk] 50:75 ohm transformation

[DavidC]

> If a 50 ohm load is terminating a 75 ohm line, SWR at the 
> source end (when referenced to 50 ohms) will vary from 1:1 to 2:1 as 
> line length is varied. That is a very noticeable change.

He claims 1.5:1 with some exceptions mostly outside of his area of
interest.

> > He also says that he has discovered that many antennas are closer to
> > 60 ohms than 50, so 50 ohm cable also sometimes introduces a mismatch.
> That sounds profound, but how was that opinion reached? I'm not 
> aware of any "impedance survey" that has demonstrated the majority of 
> antennas in use in the the world to be 60 ohms! 

You said "majority" I only said "many".

> My verticals are all around 35 ohms, my dipoles 50 to 70. My yagis 
> are about 50 nominal, with an almost equal spread lower than 50 and 
> higher than 50 ohms.  When I average out all my antennas, the 
> impedance comes out 48.2 ohms. It certainly doesn't apply here!

Guess you have different antennas than he does.

> I prefer for ALL my antennas to present a 50 ohm load to the rig. 
> That way I can tune on a dummy load, and change from antenna to 
> antenna without touching a thing. That requires adjusting the system 
> so 50 ohms appears at the rig, no matter what goes on elsewhere.

He likes to use free and extremely low loss CATV with the fewest 
points of possible failure between the rig and antenna.  He uses a 
tuner only as necessary.

I was just sharing KM1H's experience with his antenna system (he
has positive results in contesting).  His results suggest that there is 
something worth looking at in his proposition (he is an engineer and
I am not, so I tend to accept that he has planned things out).

I'd be more likely to use UNUNs than not, but I am a more traditional
and conservative sort than he.  Since I am moving to Florida I will have 
to be designing for maximum lightning protection, so I will have to 
emphasize that factor big time!

I shared the experience of KM1H so that curious folks might ask him
directly why he made the choices he did and what the trade-offs have
been ... your mileage may vary, etc.

73, DavidC  K1YP

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