Results from Querry - How to test 50 Ohm Coax

Tue Jul 9 21:27:08 EDT 1996

Results from 50 Ohm Coax Test Question ...July, 1996 

 After 10+ years of having various sections of 50 OHM Coax on the tower,
> I figure it was time to check to see if any of this should be replaced.
> Any suggetions on a easy way to check 50 Ohm Coax?

> Would a MFJ-259 HF analyzer & Resistance Meter be of any value in checking
> coax without the antennas attached?
> Thanks, Jack, KA8D

Answers Reveived:
..........................................................................

HI,
YES IT CAN HELP.  YOU CAN ESTIMATE THE LOSS OF THE COAX WITH IT.
SHORT (OR LEAVE OPEN) THE FAR END OF THE COAX AND SWEEP THE FREQUENCY (AROUND
HIGHEST FREQUENCY OF INTEREST).  LOOK FOR THE MINIMUM VALUE OF Z YOU GET.

DB LOSS (50 OHM LINE)= 0.17* MINIMUM Z VALUE.

THIS WORKS JUST FINE AND CAN TELL YOU IN A SNAP IF THE COAX IS BAD.

GOOD LUCK.

73 DE BRIAN
K3KO
............................................................................

Without a lot of spendy test gear, it is easy. A Good wattmeter.
Take a reading at the input of transline, then another at the
output (antenna). You then know how many watts are dissapated in the
transline, and compare against the published values. If it is
20 percent higher than the published ratings, then should be replaced.
de KL7HF

Jack: I should have added that the in/out power test is only
valid with a flat line! If you have voltage nulls and peaks
on the line from vswr, then placement of the meter will affect
the readings. Probably should be done with a dummy load at the output!
de KL7HF
.............................................................................

Hi Jack,

How about transmitting into the antenna with two calibrated watt meters, one
after the rig, and the other just before the antenna?

73...Scott
............................................................................

Hello,

Name is Jim, KC4ZHQ, located in Woodbridge, VA. abt 25 S. of Washington =
D.C.

The MFJ will do a good job but I think you have to have the complete =
circuit....Antenna attached.  Use one for the first time at Field Day to =
check out the 40 meter beam.  It will tell you the resonance freq and =
resistance.

Just remember one thing..."IF is ain't broke don't fix it".  Hi Hi

But ten years is a long time.

 73s

Jim
...........................................................................

Jack,

Here's a copy of my previous posting on the subject. The MFJ analyzer
just doesn't read high enough values of swr to be useful (unless the
coax is _really_ bad). 

There were some negative comments on this method. Some folks questioned
the accuracy, especially for _very_ low loss coax, with the antennas
still
connected. Some folks questioned the accuracy of the AEA and Autec
analyzers for high values of swr. It works for me, as verified by several
tests of brand new coax, using several different antennas. Measured 
results agreed closely with the specification values for the new cable.

Of course, the procedure is even easier with the antennas disconnected.
In that case, you don't have to find a frequency where the swr peaks.
Any frequency will do, preferably toward the high end. 73.

Dave, K6LL
k6ll at juno.com
------------------------------------------------------------------------

Here's a quick method for checking
cable loss from inside the shack, with the antennas connected.
The basic concept was described somewhere in QST in the past few
years.

Plug your Autec or AEA analyzer into the feedline in the shack
and tune it to a frequency where it shows a peak SWR. At this
frequency, the antenna, whatever it is, will be a damn good
approximation (DGA) of an open or short circuit. The frequency
doesn't have to be in the ham bands. Start at 30 MHz and work
down. Plug the SWR into the following equation which is for
calculating loss in a shorted or open line:

                                     SWR + 1
Loss in dB = 10 log  ----------------------------------
                                     SWR - 1

For example, suppose you find a peak in the SWR (7 to 1)
at 25 MHz.

                             8
loss =  10 log  ------------------------
                             6

loss = 1.25 dB

All you have to do now is to figure out what the loss would be
in a brand new piece of coax of the same length at 25 MHz. 
Just look it up in the coax charts, then decide if it's worth
while to replace the old cable with a new one. 

Dave, K6LL
............................................................................

Hi Jack,

Probably the best way to do it is with a spectrum analyzer and a tracking
generator.  But, besides me, who has one to play with whenever they want?
So, assuming you can't get your hands on one, the next best way would be to
get a sinewave generator and a scope that both go up the at least the lowest
frequency of interest.  The idea is to apply the sinewave generator signal
to the scope at a freqwncy of interest and note the amplitude of the signal.
Then insert the coax cable you want to test between the scope and generator
and note how much the signal goes down.  You will need to do some math to
convert the scope voltage readings to dB.  You can graph the "loss vs
frequency" for the coax by taking several reading across the spectrum of
interest.

After you have done that, you can compare your data to data published for
new cable and decide if it is worth the cost the replace the cable.

Stan  w7ni
............................................................................

Sure. With the coax open-circuited the MFJ should show very high SWR at 
all frequencies. If it does not, you have lossy coax.

First order test, to be sure, but you use what you have.

Garry
-- 
"Alternating currents are dangerous. They are fit only for
                                powering the electric chair."
                                         -- Thomas A. Edison
Garry Shapiro, NI6T                                                      

Editor, "The DXer" 
        --monthly bulletin of the Northern California DX Club
..........................................................................

Hi Jack...If you have a wattmeter, you could check the loss through the coax
(if you have two runs of coax up the tower, connect them together at the top
temporarily and you can make the measurements at the ends in the shack). ...
Use a dummy load and measure the power out to it, then insert the coax
"loop" running up the tower and see how much less ....73/Jon AA1K

...........................................................................
Jack,

What about simply putting a 50-ohm dummy load on the far end and 
using the analyzer?  Or using the transmitter?  You could put a wattmeter
in at the transmitter end, note the power output, then relocate the wattmeter
to the far end and note the power there--needs two people, obviously.

73,
Dale, kg5u
...........................................................................
Hi Jack,
I generally use a wattmeter & "dummy" load at the highest frequency of interest.

1 - Read the power at the load.
2 - Add in the cable I wish to test (with appropriate connentors/adapters)
and read power again without retuning output.
3 - Calculate additionall power loss in dB.

Another way which may be more convient is to terminate 50 Ohm coax with 100
Ohm or 150 Ohm noninductive resistor (carbon composition should work for
most applications) and read VSWR. Difference between expected VSWR and
actual VSWR can be backed into the formula to determine cable loss.

Have fun & 73, Steve - N1SR
...........................................................................

Jack,

I don't know about the MFJ but the Autek RF-1 comes with a handy formula to
calculate losses in a piece of coax.  Good for RF losses but doesn't address
dielectric breakdown at operating voltages....only a few hams have access to
that kind of equipment.  If the RF losses check out ok I'd leave it up.

Gary K7FR
.............................................................................