On 10/5/2013 4:14 PM, Ralph Mowery wrote:
"Irv Finkleman" wrote in message
...
Hi Jeff, and thanks for your reply.

To begin with I run QRP (5W on a Yaesu ft-817ND).
I have limited workshop space (my desk in a senior's villa), but
lots of my old tools.

I was comparing a wood coil with an air wound coil.
I should have said ABS rather than PVC, and I am
well aware of dielectric losses although I don't worry
about them too much as even 5W minus the losses will
keep me on the air. I'm not overly concerned about
Q as long as I can get a signal out!

My main concern was with winding a loading coil on
a square form, and I was just wondering how much
effect that would have.


After this discussion I think I'll go ABS, and with
the aid of some epoxy may even slip it off the
pipe and go air core supporting the coil off to the
side of the whip mounting. Remember the old
'Gotham Vertical' antennas (1956 Handbook ads)?

Lots of sutff cut out..

I ran a quick and dirty check for you. I took about 8 turns of about #22
wire and a 100 pf capacitor. Wound the wire around a piece of 1/2 inch PVC
pipe that is about 7/8 inch outside diameter. I made a parallel circuit of
the two. Put it on a HP 8924c set as a spectrum generator and tracking
generator.

It resonated about 13.08 mhz and had a relative loss of -46.46 db. Took it
off the form and resonated about 13.14 with a loss of -47.0 db back on the
pvc and it went back to the first number.
I did not have a piece of wood that would exectally fit it,but one that was
close. Resonated at 13.05 mhz and loss of -46.94.

All in all, I doubt there would be enough loss or change to make any
differance. The small loss and frequency change could just be the spreading
of the coil slightly and the loss could be how close the leads are to each
other.

For running the 5 watts like you are, I don't see any problems with
changing the forms on the low bands. I doubt that even at 100 watts it
would have any noticable effect.

Excellent work, Ralph. This is very useful information. Many thanks!

73,
John