[TowerTalk] 50:75 ohm transformation

[Tom Rauch]

Hi David,

I enjoy reading what people do with their own installations, and 
like you I appreciate Carl's opinions. 

I just didn't want the topic to be confused by any impression 
that might be read to mean:

Many antennas are around 60 ohms. Antennas for our applications are, 
for the most part, targeted at 50 ohms. There should be just as many 
below 50 ohms as above. That was my only point there.

It would be pure blind luck if any one system using 50 ohm 
designed antennas wound up with many 60 ohm antennas. If that did 
appear to happen, the most likely cause would be an instrument error. 

The claim that the impedance variation caused by using 75 cable in 
a 50 ohm system isn't factual.  While the 75 ohm normalized SWR 
on the line would only be 1.5:1 with a 50 ohm load, the terminal 
impedance can reach values of 100 ohms. That produces a 2:1 SWR in a 
50 ohm system, by no means a negligible bump for today's equipment! 

No one can argue with the fact the cable is an inexpensive 
alternative to other low-loss lines. I use it myself. But we 
shouldn't "sluff- off" the fact that it does greatly affect SWR, 
and we have to be careful how to use it.  In particular we have to be 
VERY careful when it is mixed in with other cables, or if the loads 
are less than 50 ohms or have reactive parts in the impedance.  

> 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!

If you do the box containing the un-un properly, lightning shouldn't 
be a problem so far as the un-un is concerned in most systems. The 
key is maintaining good shield integrity. Lightning mostly likes to 
flow over the outside of the shield, and the bulk of the energy is 
common mode (traveling the same way at the same instant of time) 
current. 

While differential mode lightning currents can take out a front end, 
it isn't likely to damage to something that could handle kilowatts of 
power IF the shield integrity is maintained around that device.

Generally a  flaw in shield integrity is required to set up enough 
differential mode current to damage a high power component across 
the transmission line.

   

73, Tom W8JI
w8ji@contesting.com

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