Gene Fuller wrote:

Gene, I want to commend you on a rational, mostly technical,
posting. Hopefully, others will follow your lead.

I don't remember why you chose a frequency of 5.89 MHz for all of your
analysis, but in any case I believe that frequency is slightly out of
the 75 meter band. The FCC will be calling. 8-)

:-) Here's the history. I tried to model my 75m bugcatcher coil in
EZNEC. It appears to be impossible without violating EZNEC's guidelines.
So I changed the coil from 4 TPI to 2 TPI. That moved the resonant
frequency from 3.8 MHz to 5.9 MHz, close enough to the 60m band
that I thought no one would object. Do you think the FCC calls everyone
who uses EZNEC to model an antenna out of the amateur bands? :-)

I tried to adjust the frequency downward, but the segment length limit
is reached for the coil model. I redrew the coil with only six segments
per turn, rather than eight. Now the frequency can be lowered to about
3.94 MHz without EZNEC guideline check warnings.

Hey, great.

In order to get the 90 degree phase curve I extended the horizontal wire
to about 55 feet. (not sure why this matters if the antenna is purely a
terminated traveling wave antenna, but, hey, no loose ends.)

I found the same thing. Seems no matter how one changes things, it
appears to be a diverging series. The main goal is to get the reflections
reduced to a low level, maybe not to eliminate them entirely.

The result from EZNEC is that the phase shift in the coil is about 9
degrees.

Yes, but that's for a coil designed for a 6 MHz antenna. You need to
add a lot of turns to make it typical of a 4 MHz coil. Doubling the
turns would make for an 18 degree phase shift - a detail you seem to
have missed. Please expand the coil until it resonates an 8 foot
antenna on 4 MHz and repeat your findings.

You might observe that this shift is a bit smaller than the "tens of
degrees" noted below, and it is also smaller than the guru-inspired
transition point of 15 degrees.

Of course, a 60m mobile coil used on 75m is going to have a smaller
phase shift. But you dropped a bit in your logic, Gene. What you need
to do is go back and create a coil that resonates an 8 foot antenna
on 4 MHz. Then do your phase calculations. When you do that, the delay
in the larger coil will turn out to be tens of degrees. Could you email
me your EZNEC file? (Hopefully, without the worm/virus I received from
someone else recently.)

I would like to model a coil more typical of common use than the strange
beast you designed, but the segment length limits in the NEC engine seem
to preclude such models. (I have never seen a mobile coil that is 12
inches high, 6 inches in diameter, with 2 turns per inch.)

We do the best we can do with the tools we have. Actually, I
have seen such a coil at one of the CA 75m shootouts. It was
made out of half-inch copper tubing.

If I was the cynical sort I might think that your choice of coil
dimensions and frequency were picked to get a phase shift of 16 degrees,
which is just over the guru limit. But since I am a straightforward kind
of guy I won't think such thoughts.

I tried to model my 4 TPI 75m bugcatcher coil. EZNEC would have
none of that. So I modeled what I could. The phase shift of
16 degrees was a complete coincidence, but interesting, no?
--
73, Cecil http://www.qsl.net/w5dxp