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Old March 15th 05, 03:53 AM
Dave Platt
 
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In article ,
john doe wrote:

No I didn't account for the PVC ...... but I made it longer than my design
and then trimmed past my design length and it never seemed to make a
difference. Considering my inductor was also formed around the PVC that was
probably way off too.....

I modeled the whole thing with nec first. This is what my input file looked
like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-dat.txt

and this is what my output looked like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-out.txt

Construction was a radiator 58.5 inches long fed along a 1/2" sched-40,
inductor was 19 turns around the same sched-40 2 inches long. The were 4
radials at 90 deg each 19.38"


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.

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.

The PDF to which I posted a link earlier today uses a coil of only
10.5 turns, spread out over a distance of about 2.5" on a 3/4" form.
That's about .6 uH or j550 ohms... and it's a shunt-fed design so the
actual series inductance (above the tap point) is even lower.

--
Dave Platt AE6EO
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