MikeN wrote:
Back on 25/04/03 re "Choke balun impedance recommmendations" Roy
Llewellan wrote:

"By all means, the slide rule is fine.

To measure the common mode impedance of a choke balun, simply wind a
piece of wire on the core with the same number of turns as you'll use
for the actual balun. Connect one end of the wire to the center
conductor of the antenna analyzer connector and the other to the
connector shell. Read the impedance at the frequency of interest. Or
if you prefer, you can wind it with a short piece of the actual coax
you'll be using. Connect the two coax conductors together on each end
of the winding, and measure as you would with a single wire. If the
impedance is out of the analyzer's range, you can use a different
number of turns and the relationship that the impedance is
proportional to the square of the number of turns.

Use the same method to measure a W2DU type balun (ferrite cores
slipped over a coax line). If the measurement is out of range, measure
a different number of cores and extrapolate -- the impedance is
directly proportional to the number of cores.

Roy Lewallen, W7EL

My queries:-

Would this method give meaningful results if used at 70 cms using the
UHF range on something like a MFJ antenna analyser?

I could pop various beads onto a short piece of coax and determine the
relative effectiveness, but how good would this be for absolute
results?

Thanks

The MFJ antenna analyzer only reports SWR at 70 cm, and any value 5 is
just reported as "5". So I don't think it would be useful for this
measurement, except maybe as described below. Even at 2 meters, you'd
have to be careful to keep the length of the wire containing the beads
at a small fraction of a wavelength. That might not be possible with
something like a complete W2DU balun -- you'd probably have to measure a
smaller number of beads and extrapolate.

One thing you might try is this:

Make up a test jig from a female connector (BNC would be best), a piece
of copper clad board, a piece of small diameter coax two or three inches
long, and a small 47 or 51 ohm resistor. (You'll probably have to use
something smaller than RG-58 because you'll have to bend it into a loop.
But it's important that it be considerably shorter than a quarter
wavelength long.) Solder the BNC connector directly to the ground plane,
and connect the coax to it with extremely short leads. Call this end 1
of the coax. Solder the resistor across the other end of the coax, end
2, with extremely short leads. Hook the MFJ to the test connector with
an ordinary coax patch cable and measure the SWR at the frequency of
interest. Unless you can get a reasonable SWR (well below 5:1) with this
setup, there's no point in proceeding. If you can, then try touching the
shield side of the short coax to the ground plane. The SWR should remain
reasonable.

If you got a reasonable SWR with this setup, the next step is to touch
the *center* conductor of end 2 and attached resistor to the ground
plane at the same point as the connector is soldered. Notice that the
SWR skyrockets -- this is simulating a worst case situation for a balun.
(If the SWR doesn't skyrocket, your coax is too long to do a valid
test.) Now add beads to the outside of the short coax and repeat the
test, that is, touch the center conductor of end 2 to the ground plane
at the connector. If you can find a combination of beads that'll drop
the SWR back down to within the range of the meter, you have enough
impedance for a balun. You could probably get by with less, but any
combination of beads that passes this test will surely be enough.

Roy Lewallen, W7EL