Ian White, G3SEK wrote:
Cecil Moore wrote:
For the feedpoint impedance to be purely resistive in an electrical 1/4WL
shortened vertical, the current must undergo a round trip phase shift of
360 degrees and the voltage must undergo a round trip phase shift of 180
degrees. The loading coil must provide the phase shift that the antenna
doesn't provide. For a typical 75m 8 ft mobile antenna, the coil must
provide approximately 80 degrees of phase shift.
I'm sure you'll find the answer in Balanis, if you read it with an open
mind instead of trying to force him to agree with you.
Ian, I learned the above in the class I took from Balanis at ASU in 1995
while I worked for Intel in Chandler, AZ. I asked him a lot of
questions about center-loaded antennas as they are not covered well
in his book. The fact that none of the resident gurus on this newsgroup
will touch the above simple question with a ten foot pole speaks volumes.
A very bright engineer has been in email contact with me over this deductive
reasoning problem. So far, he has not attempted to resolve the conflict
between a lumped inductor and the absolutely necessary phase shift.
Here is the answer to the question presented by Devoldere in "ON4UN's
Low Band DXing". For convenience sake, I will draw the vertical mobile
antenna as one half of a dipole but the same logic applies. Note the
coil is drawn in electrical degrees, not in proportional physical length.
wire coil wire
-----------////////////////////////----------
22.5 deg 45 deg 22.5 deg
We know from the end result that this is what has to happen. An electrical
1/4WL antenna simply must cause a 90 degree phase shift in the current
from end to end. Otherwise, it wouldn't be an electrical 1/4WL antenna.
Until someone can explain exactly how a lumped inductor causes a 45 degree
phase shift in the current, I am going to assume that a lumped inductor
is incapable of that feat.
--
73, Cecil
http://www.qsl.net/w5dxp
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