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

Sun Mar 21 15:59:01 2004

Absolutely right, George. I use the setup you describe with half wave
end-fed antennas. That's when I use a feedline at all. Currently, my end =
fed
half wave "Inverted L" terminates at the window next to the operating =
desk
so it goes directly to a wide-range ATU on the operating desk..

The only function of a good RF ground in that setup is to keep the =
chassis
of the ATU and rig at RF ground. I've read that some beads on the coax =
will
do the same, but I haven't had occasion to experiment with them.=20

What I was referring to was that there are currents induced in the =
ground by
the antenna even if there is no electrical connection between the =
antenna
and the earth, and these currents add up to losses that have nothing to =
do
with the efficiency of the antenna by itself. Typically, ground current
losses are worse with vertically-polarized waves than with
horizontally-polarized waves, although the ground currents are =
significant
when the antenna is very close to the ground in either case.=20

The greatest earth losses are within a quarter wave or so of the antenna
where induction produces the strongest currents, although there are
significant losses much farther away, especially with a =
vertically-polarized
wave.

Putting a lot of copper in the ground helps a lot, even if it is not
connected to anything. It's better ON the ground, since only a few =
inches of
earth will intercept a lot of the RF and induce currents before it gets =
to
the copper.=20

That's why the Force 12 guys like to show off their verticals at the
seashore as they explain in their literature. That salt water is a great
relatively low-loss ground plane even though it has no direct connection =
to
the antenna - especially for a vertical which works best when it has a =
good
low-loss ground plane extending many wavelengths away from the antenna.=20

I don't have space to put a lot of copper in or on the ground, nor do I =
live
at the seashore (yet)! I have a tuned elevated counterpoise for an RF
ground. It's very much a "compromise" ground, and not really effective =
for
reducing induction losses in the soil. I'm limited to about 30 feet of
height for my antennas. At that height the vertical  consistently
out-performs a horizontal for contacts >1000 miles on the bands below 20
meters even with the earth losses.=20

Ron AC7AC

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

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 -----=20
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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