[NLRS] ANDREWS HELIAX QUESTIONS

[Dr. Gerald N. Johnson]

On Thu, 2006-05-11 at 23:09 -0500, christenson at charter.net wrote:
> 
> I recently had another impulse buy with the help of my mentor N0VZJ. I picked up 2 rolls of what looks to be Andrews 7/8 inch hard line. Both are approximately 120' long.
> 
> I am a bit nervous and think that the cable may be 75 ohm. The cable does not show any information on it. The connectors are labeled 75AU. On one cable both ends are N connectors on the other there are UHF connectors. The 75 in the 75AU is concerning me. Does this mean 75 ohm? According to Andrews web site the 75AU is used on 50 ohm 7/8 cable.
> 
It Andrews says the 75AU fits 50 ohm cable its unlikely its 75 ohm cable
because the center conductor of the 75 ohm cable is significantly
smaller.

> Well did I screw up again? 

75 ohm works just fine for all ham setups other than repeaters and can
be tuned to work there. I've been using 75 ohm CATV cable for years 2m
to 1296 with very fine results. I've tried tuning it but running it
mismatched hasn't yet cost me a contact I don't think. I know tuning it
didn't allow me to copy any signals I couldn't already copy. And I've
not blown up any finals yet.
> 
> The ham who owned it before me used it on hf. Had a 2 120' towers and used it to run up the tower.
> 
> 73'
> 
> P.S. 
> At AU 2007 N0VZJ Vince's next presentation should be
>  "How to get people to spend there money on Ham radio gear"  
> 
> Thank Vince! : )
> 
> Kris KC0REO

I'm sure there many here who would take it off your hands if it is 75.
Is it air or foam heliax?

It is possible to measure the velocity of propagation and the
characteristic impedance with a SWR bridge or MFG antenna analyzer. With
one end open tune the frequency for a minimum impedance. Then its some
odd multiple of a quarter wave long. Load it with 50 ohms in place of
the open. Check the input Z. If its 50 ohms its 50 ohm coax. If its 100
ohms its 75 ohm coax. Measure the outside length. Figure up the
wavelength based on the frequency of that minimum impedance, figure a
velocity factor between .67 (solid polyethylene coax) and .95 (air
heliax) and see what velocity of propagation pops out. You should find
several frequencies with minimum impedance during the open check.
Compute base on the lowest one or two to make the computation a little
easier.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer