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From PA0SE to All
When last year I had completed my LF-station I
measured the total series resistance of the aerial/earth-system at 61
ohm,
using a Wayne Kerr Radio Frequency Bridge type B601. The value
was confirmed by measuring the loaded Q of the system.
For a check I measured again yesterday and found
a surprising low 36.8 ohm! What could have made the change? The
earth
resistance? In case of a small change it could have been. But almost
halved? No. And then the penny dropped.
My aerial is a 2 x 20 m dipole with 11 m open
line feeder. The feeder wires are strapped for LF. The bare wires enter
the
shack through holes in the hardwood window-frame. Outside above the
window is a panel made of some kind of pressed wood. The
bare wires touched
the edge of the panel but I never worried about that. In the HF bands the
voltage on the wires is never
very high and I considered the insulation
resistance of wood high enough. In fact I never had any trouble on the HF
bands
or on 160 m.
One evening last winter I was beaconing on
136.550 kHz when my wife came into the house in a state of alarm.
"There is fire
outside the window of your shack!"
I went outside and indeed in the rhythm of the morse code bright white fireworks
could
be seen at the feeder entry point.
An investigation next morning
showed that were the wires touched the panel, slots of several centimetres long
had been
burned into the pressed wood. Also the holes in the window-frame
were charred.
To improve the situation I enlarged the holes in
the window-frame and inserted paxolin tubes that reach well outside
the
pressed wood panel. I also replaced the bare wires by PVC covered wire
where the feeders pass trough the paxolin tubes.
Since then no sparking or
charring has occurred.
Obviously the poor insulation had increased the
total series resistance to 61 ohm.
The capacitance of the aerial is 370 pF
(reactance -j3.160 kohm at 136 kHz). So the reactance of the loading coil in
parallel with a vacuum capacitor (set at a small value) is j3.160 kohm. The
measured series resistance is 9.5 ohm so the unloaded Q is 333. The calculated
series resistance of the aerial is 0.2 ohm. So the resistance of the earth is 36.8 - 9.3 - 0.2 = 27.3 ohm. Not too bad I
think.
The aerial current had increased from 1.37 A at
the beginning to 2.39 A now which means an increase of radiated power by
3.8 dB. The power fed to the aerial/earth-system
is 2.12A * 2.12A * 36.8 ohm = 162 watt. This is not too different from
the
output of the transmitter which I measured at 175 W. DC input is 259 W so
the efficiency of the class B amplifier is 68%.
It all goes to show how important the insulation
of the aerial at LF is. It has been said and written before over and over again
but
it seems that an experience like I had is needed to really burn it
in!
73, Dick, PA0SE
JO22GD
D.W.
Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel.
+31 71 589 27 34
E-mail: [email protected] From PA0SE to
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