On Nov 12, 11:22?am, John Ferrell wrote:
On Sun, 11 Nov 2007 15:57:33 -0800, wrote:
Hi, Designing a 33' portable 80M vertical. Modelled it, and said I
needed a 37 uH loading coil around halfway up. Wound one using a 2"
cardboard form and #18 teflon insulated wire. My Z meter said it was
37 uH, upon placing in service, the resonant point was fairly close to
the design point on the antenna.

My question: I was hesitant to to put the coil in service because my
MFJ259B would not confirm the inductance value. It said the Z of the
coil was than 650 ohms or that L(X=0), all dependent on frequency.
I could never get a good indication of the Inductance. I suspect this
is due to self resonance of the coil, but don't know. I calculated
the Inductance of the coil given the F and Z reading on the 259B and
it came close to 37uH, but still (inductive reactance) X=0. Anyone
know what is going on?

Thanks,
Gary N4AST

Did you have the test frequency set to a point in the 80M band on the
MFJ 259? The operating frequency does affect the measured inductance.

If it is not too much of a hassle, put the antenna into a temporary
operating position and put the analyzer at the feed point to determine
if things are close.

The Q of the coil is going to be important if are to achieve an SWR
below 2:1.

A quick run of the numbers indicated you need 43 turns of #18 wire on
a 2 inch form that is 4 inches long.

Since you did not specify the diameter of the mast, I choose 1.5
inches which computed to a resonant point of 3.375 mhz.

You may find my experiment athttp://dixienc.us/28FtVert/28FtVertical.htm
interesting...

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"- Hide quoted text -

- Show quoted text -

Hi John, thanks for the reply. My coil was about what you calculated,
but the vertical was of the portable variety, made from #18 as well
supported by a 33' fiberglass mast. I was trying to hit 3.8 mhz, I
just tried it and it was close. SWR was around 1.7 at resonance.

Since the Xl of a 37 uH coil is 650 ohms at 3.8 mhz, I had to tune
the 259 to a lower F to get an L reading. When I could get an
Inductance reading, it said X=0, which must mean the coil impedance
must be purely resistance?? Since I have tried it and it worked OK on
the antenna I guess that is what matters.

Gary N4AST