[K6BW] Measuring Unknown Coax Impedance

[Bill Smith]

Here are a couple of suggestions purloined from the QRP-L newsgroup
regarding measuring the impedance of unknown coax.  Makes good reading.  One
note- SWR meters are notoriously inaccurate at SWR values above 2:1 or so.
Most of them are really "null" meters, that is they indicate minimum SWR
fairly well, but really can't be relied upon to indicate true SWR
accurately.  At any rate, you may enjoy the following:

Hi Dale,

If you have an Antenna Analyzer, just solder a 50 ohm carbon or metal-film
resistor to one end of a 10-foot piece of coax, and measure the SWR on the
Antenna Analyzer.  The impedance of the coax will be either 50*SWR ohms, or
50/SWR ohms.  Repeat the measurement with a 100 ohm resistor.  Make the
calculation again.  The number that occurs in both pairs of calculations
will be the impedance of the coax.  The 10 feet isn't critical, you just
don't want it to be so long that the attenuation in the cable starts to mask
the true impedance of the line.  Anywhere from 10 - 20 feet would be fine.

No Antenna Analyzer?  No friend with one?  Make up a 50 ohm 10db pad using
non-inductive resistors whose values you can find in the ARRL Handbook.  Put
that between your transmitter and the coax, and solder a 50 ohm carbon or
metal-film resistor at the far end of the 10-foot piece of coax.  Measure
SWR on your station SWR meter, and make the same set of calculations
described above.  Don't use a tuner.  The tuner's job is to make the SWR as
low as possible, so take it out of circuit, or put it in bypass.  Repeat
using a 100 ohm resistor.  Look for the number that occurs in both sets of
calculations.  That's the impedance of your coax.  Keep your transmitter
power low (1 watt is a nice level), and make sure the resistors you use can
handle the power.

Hope this is helpful.

ed - k9ew

Dale - If you have a MFJ-259B antenna analyzer or access to one, it will do
this measurement automatically. The instructions are in the manual. I don't
think that this is a feature of the 259 or 259A though.

You can measure the impedance of an unknown transmission line by using 2
properties of a transmission line.

1. A line that is a half wavelength long terminated in an impedance at the
output end will show that impedance at the input end. This is independent of
the transmission line length or termination impedance.

2. A line that is any length that is terminated by its characteristic
impedance will reflect that impedance at the input, regardless of its
length.

Now both of these assume a lossless line, but if you keep the lengths short
and frequencies low, this isn't a problem.

I assume that you have or have access to an MFJ antenna analyzer, or a noise
bridge, or other signal generator/bridge combination  If you don't, I
suspect that you can borrow one locally, or even build one. Now for the
measurements:

1. Terminate the line with a resistance that isn't likely to be the
characteristic line impedance, say 150 Ohms.

2. Vary the frequency of the signal generator and look for a pure resistance
on the bridge. The coax will be an electrical half wavelength or integral
multiple of an electrical half wave at this frequency. Drop the frequency of
the signal generator to insure that you have found the lowest half
wavelength. From this you can find the velocity factor of the line, which is
useful for some uses. The velocity factor of the line is just the length of
the line divided by the half wavelength in free space corresponding to the
frequency you measured.

3. Now go to a frequency that is far removed from the half wavelength; a
quarter wavelength will do fine. Vary the termination resistance until you
see a pure resistance equal to the termination resistance at the input end.
Vary the frequency of the generator to be sure that the impedance does not
change with frequency. This will be the characteristic impedance of the
line.

There are lots of other techniques for measuring the impedance of a line.

Most coax has an impedance of 50 Ohms or 70 Ohms. You will see the
occasional 90 Ohm coax, but other values are extremely rare. The velocity
factor is likely to be 0.85 for foamed polyethylene and 0.66 for solid
polyethylene. If you get values much different from these, you probably did
something wrong or don't have coax. Teflon is also used as a dielectric, but
it is not as common as polyethylene, and it is expensive as well.

You may not have coax at all, but shielded line, such as used for
microphones. The fact that your line is not labeled tends to make me think
that it is shielded line rather than coax. Coax manufacturers are usually
proud of their product and label it.

Your transmission line is not a place to cut costs in building a shack. If
you have doubts about it, I suggest that you not use it in critical
applications.

As you can see, an antenna analyzer is a very useful accessory. - Dr.
Megacycle KK6MC/5

--
James R. Duffey KK6MC/5
Cedar Crest NM 87008 DM65