[Elecraft] Home made Sigma-GT5 & KRC2 or SGC?

Sun Mar 21 14:21:01 2004

Ron, the usual endfed vertical halfwave antenna uses a matching network -
either a parallel-resonant circuit connected between the end of the antenna
and "ground" with the coax tapped up on the coil at the appropriate point or
an L network, also connected to ground.

This being the case, the coax braid would not be at any r-f potential other
than what is might acquire from being in the near field of the antenna. A
suitable choke balun - a coil of coax or some ferrite beads - would reduce
or eliminated any r-f voltage at the station end.

I think, in any event, that he is talking about a center fed vertical
dipole, not end feeding. The presence of lossy ground near the
high-potential end of the antenna would indeed create a loss mechanism due
to current being induced in the ground and causing I^2R loss. For this
reason, it is not uncommon to place either a radial field or metal mats
under the antenna to reduce this loss.

These remarks apply of course to the situation where the end of the antenna
is only a few feet above the ground. Such a vertical mounted 30 feet high
would incur little if any round loss, as you suggest.

73, George W5YR
Fairview, TX
[email protected]
http://www.w5yr.com

----- Original Message ----- 
From: "Ron D'Eau Claire" <[email protected]>
To: "'Elecraft'" <[email protected]>
Sent: Sunday, March 21, 2004 1:02 PM
Subject: RE: [Elecraft] Home made Sigma-GT5 & KRC2 or SGC?

Mike, the problem comes from induced currents in the ground by the radiator.
That isn't affected by where the antenna is  fed but by to other things: the
polarization (horizontal or vertical) and the distance from the ground in
wavelengths.

Actually, an end fed half wave vertical is virtually identical in its
"ground independence" with a center fed half wave. The difference is that
the rig needs a good RF ground to keep it from floating up to the high RF
voltage found at the end feed point and so becoming "hot" with RF. But
whether or not there is a good ground doesn't effect the efficiency of the
system as an antenna much.

Ron AC7AC

-----Original Message-----

I'm sure you're right but I'm referring to a center fed vertical *dipole*
(ala Sigma GT5) and not an end fed vertical.

Mike K5PU

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