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

[FredJensen]

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