[Elecraft] Antenna

Mon Jul 7 13:41:02 2003

The "balun" won't do a thing to "keep the line cool" if you are talking
about avoiding losses in the feed line. The only way to do that is to
operate it with something close to a 50-ohm non-reactive load at the =
antenna
end of the feedline.=20

A 1/2 wave center fed wire will provide the needed match at the antenna =
end
of the feed line, but only where the antenna is 1/2 wave long. At 3 =
times
the 1/2 wave frequency it will be pretty close too, where the antenna is =
3/2
waves long. That's why half wave center-fed antennas for 40 are often =
used
very successfully on 15 meters.=20

That only works where the antenna is a multiple of 1/2 wavelength long.
Other combinations will result in very high SWR's on the feedline, and
consequently much higher feed line losses. The only effect of a balun at =
the
feedpoint of a center  fed antenna is to reduce the currents flowing on =
the
OUTSIDE of the coax shield. RF does not flow "through" a conductor. It =
flows
only along the surface. So the RF from your rig going to the antenna is
flowing along the surface of the inside of the shield and the outside of =
the
center conductor. No RF flows on the outside of the shield. At the =
antenna
end, RF can find its way to the outside where the shield ends and is =
tied to
the antenna. Will, that hurt? Not likely. But if you want to be certain =
that
you don't have any RF on the outside of the shield you can add a balun =
at
the antenna end. Still, you will have RF on the outside of the feedline
unless you do something else too. You MUST run the feedline away exactly =
at
right angles to the antenna. That is to cancel the induced RF on the =
outside
of the coax.=20

You see, RF currents are induced in any conductor near the antenna, and =
the
feedline is certainly "near" the antenna. You can prevent that from
happening if your antenna is electrically balanced and if you run the
feedline directly away at exactly right angles. That way the currents
induced on the outside of the shield by one side of the antenna will be
exactly canceled by the currents induced by the other side. The net RF =
on
the outside of the shield will be zero. If you can't run your feedline =
away
at right angles, you will have RF on the outside of the shield, Balun or =
no
Balun. Normally, RF on the outside of the coax is absolutely of no =
concern
in a simple wire antenna.=20

Some ops have problems running coax at high SWR's because the length of =
the
coax and a lack of a decent RF ground at the rig conspire to put fairly =
high
RF voltages on the outside of the coax shield and that gets back into =
the
rig. Probably the best way to kill that effect is to put a choke at the =
rig
end of the coax. Coil up the coax to form a choke or put a bunch of =
ferrite
cores over the coax to form an RF choke. (Adding length of the coax that =
way
also changes its electrical length which can bring down the impedance =
and RF
voltages at the rig end).  Also, a decent RF ground at the rig does =
wonders.

But even with all of those precautions, you still have the losses in the
coax caused by having a high SWR on the line on any band except where =
the
antenna is 1/2 or 3/2 waves long.  How much those losses will be depends
upon the frequency and the length of the coax. They can range from 9 out =
of
every 10 watts of RF being turned into heat to as little as a watt out =
of
every 10 watts or so.=20

If your feedline is short, you may not see losses that are too great. =
The
KAT2 will permit the rig to deliver full power into the high SWR (Note =
that
the KAT2 cannot reduce the losses in the feeder - you can only do that =
by
matching the antenna to the feeder at the antenna end. The only function =
of
the KAT2 is to provide the 50 ohm non-reactive termination the rig =
requires
to deliver full power). A balun will be absolutely no help in improving
radiation and may actually absorb a lot of RF in losses, depending upon =
the
balun and impedances it sees.=20

That's why low-loss feedlines are popular (such as open wire feeders) as
well as various schemes to present a decent match at the antenna end of =
the
feeders on more than one band such as trap dipoles, multiple dipoles, =
remote
tuning, etc. =20

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected] =
[mailto:[email protected]]
On Behalf Of Mike Elliott
Sent: Monday, July 07, 2003 9:56 AM
To: Elecraft
Subject: [Elecraft] Antenna

I have a K2 w/a KAT2 installed.  I'm thinking about 100' or so of=20
flattop, fed with RG-8X, with a 15T balun
at the feedpoint to keep the line cool.  Opinions,please?
Mike