[Elecraft] field ops

[Ron D' Eau Claire]

> My RSGB book "The Antenna File" from 2001 has an article by G3CCB on
> "Taming the End-Fed Antenna". He has made a chart of lengths that are
> favorable based on the criterion that he wanted to avoid lengths close
> to half wavelength (actually between 3/8 and 5/8) in order to avoid a
> high impedance, and simplify tuning....
> Sverre Holm, LA3ZA

The problem with adjusting the length of the end-fed wire to achieve a low
feed impedance is that you again become dependent upon a good ground for
good efficiency.

Think of the radiation resistance of the antenna and the ground resistance
as being in series. The r-f flows through both of them. The higher
resistance will consume the greater power.

So you want the radiation resistance to be many times the ground resistance.

The radiation resistance will be about 35 ohms for a 1/4 wave wire. A short
antenna less than 1/4 wavelength like many of those used for portable work
may show a radiation resistance of a fraction of one ohm.

If you use a single counterpoise wire, you might see a 'ground' resistance
of about 35 ohms. In that case, half of your r-f goes to the antenna if you
have a 1/4 wavelength radiator. Half warms the earthworms. If the antenna is
less than 1/4 wave - say 1/8 wave where the radiation resistance is about 2
ohms, then you can expect perhaps 95% of your power to be warming the
earthworms and at best 5% being radiated.

Even if you make the end fed wire longer than 1/2 wave  - say 3/4 wave where
you will again see a low impedance at the fed end, the same situation
continues. It's not as bad. A 3/4 wave wire will have a radiation resistance
well above 35 ohms, but you are still penalizing yourself by not selecting
the highest impedance point where you can to feed it. Being right on 1/2
wave, or even a multiple of 1/2 wave, will usually produce an impedance
outside the range of most ATU's to handle (even the KAT2). But the most
efficient end-fed wire system is where you are as close to it as possible.

Of course, the other way to make the end-fed wire more efficient is to
install a better ground. You can place about 60 radials at least 0.2
wavelength long on the ground around the rig and use a short wire with
rather good efficiency. But that is pretty hard for most of us to do at
home, much less at a campsite!

Usually, making the antenna closer to 1/2 wave long to raise the radiation
resistance is easier than lowering the ground resistance - at least on the
higher frequency bands.

Ron AC7AC
K2 # 1289