[Elecraft] [KX1,K1,KX3] QRP Portable Antenna

Thu Aug 23 18:53:13 EDT 2012

The other way to increase efficiency of an end-fed wire is to reduce the
current that needs to flow in the ground return. That's done by raising the
impedance of the radiator at the feed point, the maximum being when it's 1/2
wavelength long. Most ATU's can't handle the impedance of an exactly 1/2
wave radiator, but the closer you can get to it, the less current flows into
the radiator and so the less current flows into the "ground", ergo, lower
ground losses due to the ground resistance. 

All sorts of intermediate lengths can provide good results, especially
compared to a shortened "loaded" antenna, as long as you stay well above 1/4
wavelength in radiator length. 

The major purpose of a "ground" return then becomes keeping the RF potential
at the rig low enough to avoid problems with "RF in the shack" since the
tuner and rig will tend to "float" up to the RF potential at the feed point
of the radiator. 

73, Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of FredJensen
Sent: Thursday, August 23, 2012 1:45 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] [KX1,K1,KX3] QRP Portable Antenna

On 8/23/2012 6:51 PM, Phil Hystad wrote:
> I have operated my KX1 out in various parks and fields by hanging one end
of a wire up in a tree that I would raise using fishing line and a sling
shot.
My usual method too.  I use the internal KXAT1 tuner, works fine.
> A while back, Wayne said that he would use just a single wire up in the
tree and then another wire laying on the ground as sort of a ground plane.
My version of Wayne's antenna is a length of RG-58.  I pulled off all but a
few inches of the outer jacket, spread the shield wires, and pulled the
center conductor through to yield a ~26 ft antenna conductor [center
conductor and dielectric], and a ~26 ft length of uninsulated "wire" [the
shield braid].  A male BNC goes on the end where the coax is still intact,
and plugs directly into the KX1.  I run the shield wire out on the ground.

To radiate, you have to have RF current flowing in your antenna conductor.
Maxwellians will tell you it involves both the "conduction" 
and "displacement" current.  The displacement current effectively represents
your radiated signal.  In order to have a current, you need a closed circuit
so the conduction current has a path back to the TX.  
Resistance in that path lowers the current [both conduction and displacement
and hence your signal] just as Ohm's Law would suggest.  
The path can be through the earth [higher resistance], or you can provide a
lower resistance path with a conductor(s) ... your choice.
> 1.  Is more then a single ground wire (wire laying along the ground) good
enough or would two or three be better?
EZNEC4 "suggests" that one is around 85% as effective as a perfect ground.
I say "suggests" because EZNEC4 won't model wires on the ground so I had to
elevate it about 1/2 inch, at which point, its length becomes a minor
confusion factor.  Two seems to gain you another 5% or so, and the benefit
per additional wire decreases while the complexity of the system increases
rapidly.  As you add wires on the ground, the probability that a little
human or dog passing by your position will trip on your wires becomes 1.0
fairly quickly.  I've compromised efficiency vs complexity to one wire [see
above].
> 2.  My wire antenna has usually been on the order of 50 feet but it also
depends on how close I get for reasonable operating position (e.g. picnic
table)?  So, would a ground wire need to be the same length or would half
the length be as reasonable.
The antenna length needs to be such that the impedance at the TX end can be
matched by the tuner on the band(s) you want to operate.  32 ft is a
1/4 wave on 40 and will work well, it has a fairly low impedance [30-50
ohms].  At 20m, it's a half-wave with a high impedance and may be a hard
match for the tuner.  My KX1 does 40, 30, and 20 [I figure life is too short
for 3W on 80].  The KXAT1 seems pretty happy on all three bands with my ~26
ft length, which is sort of non-resonant on my 3 bands.

On the ground, length is not critical, more is generally better, but you're
in the field.  Do what mechanically works for you.  Mine made from RG-58
winds onto two little crank reels a friend gave me.
> 3.  How does this compare to a Buddipole antenna?
I just sold my Deluxe BP because of weight.  BP's can be set up in multiple
configurations, comparison is very difficult.  On any band but possibly 10m,
a BP is a loaded antenna in any config and loading introduces losses
however.  It is not "hard" to set up, but, so long as you have some sort of
"organic towers" for your wire, it is quite a bit more work than tossing the
line over a branch.

73,

Fred K6DGW

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