[Elecraft] field ops

[George, W5YR]

For some time back in the 40's I used a 260 ft "longwire" antenna that was
end-fed from a parallel resonant circuit on the shack wall near a window.
The coil was center link-coupled to the center swinging link on the final
tank circuit with a length of 300-ohm twinlead. (Anybody recognize those
terms thes days?)

The end of the antenna connected to one end of the antenna tuner coil while
the opposite end of the coil was floating. A single-stator capacitor took
care of the tuning adjustments once the proper coil taps had been
determined.

Tuneup procedure:

1. tune up the 813 final into a 250-watt light bulb dummy load for normal
power level.

2. pull the swinging link out to reduce the coupling.

3a. hold the key down and tune the antenna circuit for max voltage on the
antenna end - detect this by drawing an arc of that end of the coil (or the
other end worked equally well!) with a wooden "lead" pencil while adjusting
the tuning cap.

3b. at night, look out the window and adjust the tuner cap for max
brightness of the 40-watt fluorescent lamp fastened alongside the far end
of the antenna with tape.

4. when tuned, increase the loading on the final by moving the swinging
link back in until the same plate current was drawn as with the dummy load.
Re-dip the final as needed for any reactance coupled in from the antenna
circuit.

5. work DX! 

I used that antenna - it was maybe 15 feet off the ground in the center and
about 20+ at each end - to operate 75 AM phone (one full wave)and 20 CW
(four full waves). It was remarkable how well it worked on 20  -  it was
pointed right into the heart of Asia and South Africa. And, yes, that was
back in my shameful QRO days . . .

Modern rigs and antennas work so much better but somehow just aren't as
much fun or as exciting.  I haven't drawn any arcs in several years now! 
<:}

But, one thing hasn't changed: end-fed wires can be very good antennas!

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

All outgoing email virus-checked by Norton Anti-Virus 2002


Ron D' Eau Claire wrote:
> 
> > 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.