Tom Donaly wrote:
Still using the Tesla coil fella's ideas, are you?
The title of the article is "RF Coils, ..." The block
diagram of a Tesla coil with a top hat is identical
to a 160m mobile antenna with top hat.
A frequency dependent Z0 is a good trick.
It's no trick - just based on empirical measurements
as explained in the IEEE paper. Measurements proved
that the Z0 of a coil varies with wavelength so
wavelength is included in the empirical formula.
I observed that phenomenon during my own experiments.
What happens when you double the length of the coil?
Same thing as doubling the length of a stub. At a fixed
frequency, the delay through the coil is (roughly) doubled.
Does the Z0 stay the same? What if the coil is infinite?
Length of the coil does not appear in the empirical
formula for Z0 of a coil. Coil diameter, TPI, and
wavelength are the variables. Wire diameter would
obviously have some effect but is not included in
the empirical formula.
Can you make a quarter wave shorted stub with it?
Yes, but you need a ground plane close by. Mininec ground will do.
Here's a 75m Texas Bugcatcher coil loaded with its Z0 impedance
modeled over Mininec ground.
http://www.w5dxp.com/coil505u.EZ
The current phase shift through the coil is clearly visible
by displaying "Load Dat". The delay through the coil (EZNEC)
is roughly proportional to the phase shift, i.e. about 38
degrees. The coil is 0.5 feet long with a calculated VF
of 0.02 so the calculated phase shift (without EZNEC) is
about 36 degrees. That's pretty close agreement.
If you make it a half wavelength long - keeping in mind the velocity
factor -
will the impedance looking into the coil equal the impedance of the load?
How do you attach a load to it?
Here's the Texas Bugcatcher coil modeled at the first (1/4WL)
self-resonant frequency of 7.96 MHz:
http://www.w5dxp.com/coil505s.EZ
I have not experimented with 1/2WL self-resonance. The above
file seems to be 1/2WL self-resonant at about 19.2 MHz but
the 75m Texas Bugcatcher coil, at 19.2 MHz, does not meet
the guidelines for validity given in the IEEE article.
Reference:
http://www.w5dxp.com/current2.htm
http://www.w5dxp.com/current.htm
http://www.w5dxp.com/CoilZ0VF.xls
http://www.ttr.com/TELSIKS2001-MASTER-1.pdf
--
73, Cecil, IEEE, OOTC,
http://www.w5dxp.com
"Government 'help' to business is just as disastrous as government
persecution..." Ayn Rand