[TowerTalk] Building a W6NL Moxon 40 Meter Beam.

[L L bahr]

Most of the radiation comes from the current part of the antenna which is going to be close to the center point of the driven element. If you are using a U shaped channel 2 foot long at this area of the antenna, you will be pumping RF into the channel at the high radiation point of the antenna for 2 feet!  Looking at a whip antenna on a car, if you place the whip low on the rear bumper and have a lot of it length within near proximity of the auto body, it will in no way work as good as if the base of the antenna was "in the clear" say at the top of the trunk. A lot of the radiation is going to go right into the car body.  I'm not so concerned about the capacitive coupling.  Maybe this is part of the reason the center is 50 ohms. I'm concerned about the RF being shorted and grounded thus reducing radiation efficiency with this long 2 foot piece of aluminum channel.

Lee, w0vt 

----- Original Message -----
From: "David Gilbert" <xdavid at cis-broadband.com>
To: towertalk at contesting.com
Sent: Sunday, August 3, 2014 3:42:26 PM
Subject: Re: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.


1.  There is nothing inherently wrong with "coupling to ground" at the 
feedpoint.  As long as whatever capacitance that may be there (which I'm 
not convinced would be significant anyway) is relatively low loss it 
would just be part of the overall tuning system. Besides, coupling to 
anything is not uniform along an antenna.  The net effect of capacitive 
coupling is going to vary as a function of the voltage at that point, 
and the voltage at the center of a "dipole" is low while the current 
there is high.  That's why loading coils (which require current to be 
useful) are placed more toward the center of a typical halfwave element 
(see note below) and capacitance hats (which require voltage to be 
useful) are placed near the ends.