In article ,
john doe wrote:

Is that 58.5 inch figure a typo? Your NEC model says 51 inches, and
my quickie spreadsheet calculation says 50.9 inches for a 145 MHz
center of band.

Actually, the 51 you're seeing in the NEC model is probably the number of
segments. One end is at 0,0,36 and the other is at 0,0,94.5; so yes it's
really 58.5 inches.

Whups... my bad.

I came to this number by playing with the model until
the REAL component of the impedance got as close to 50 as I could get it.

Hmmm. What did that do to the pattern? You no longer have a 5/8-wave
antenna. Adding about 8 inches has brought it very close to being a
3/4-wave radiator. As such, it's going to have a substantially lower
amount of towards-the-horizon energy in its pattern, and a big lobe
aiming upwards at roughly 45 degrees above the horizon.

This is the classic problem with running a 2-meter J-pole on 440 -
it'll load up and radiate, but a lot of its radiation is aimed at
airplanes rather than repeaters :-(

I also wonder about the coil - it calculates out to be just over 1
microHenry, or about j910 ohms at 145 MHz. That seems like quite a
bit too much, based on jgboyles's posting earlier today indicating a
feedpoint Z of about 80-j300.

My model comes up with a feedpoint impedance of 5.4485E+01-j2.8560E+03

So I tried to build a coil with an inductive reactance to cancel that .. I
came up with 3.13 microhenries.

Is my model way off?????

I think you might want to take two looks at it:

- Check the radiation pattern. By lengthening it to get a 50-ohm
resistive component in the feedpoint, I suspect you've given up
much of the gain benefit of a true 5/8-wave radiator. You may
actually have less towards-the-horizon power and sensitivity than
you'd get with a 1/4-wave groundplane or a 1/2-wave J-pole.

- Check the formula and actual inductance for your coil.

With so much capacitive reactance from the radiator to cancel out with
the coil, I suspect that you may also find that you've calculated out
an antenna which is going to be rather narrow-banded. Even slight
frequency shifts, or errors in the coil winding (a fraction of a turn)
could leave you with a lot of residual reactance and an unacceptable
SWR.

--
Dave Platt AE6EO
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