Reg Edwards wrote:
I was trying to think of ways to raise the radiation resistance. It is

about 40

ohms on 75 meters but only 22 ohms on 40 meters. In fact, the VSWR is

around

1.5:1 or less on all bands except 40 meters. I thought 75 would have the

worst

match but for some reason it's 40.

So far I have nine 10 ft. radials, one of which is connected to a power
distribution box near my antenna. It has underground power lines going to

my

neighbor's houses. I will try adding another radial to a near by water

pipe

next, although it sounds like any one radial is not that important.

Fortunately

I have an Autek VA1 impedance bridge so I can see what's happening. Of

coarse,

understanding it is another matter :').


-------------------------------------------------------

Ron, I came back to make sure you have the right idea of what your Autek
measurements mean. Except for the SWR range it is a very useful instrument.
I have one.

I assume you are measuring directly between the base of the 'screwdriver'
and the radial system.

Yes, through a 6 ft. piece of RG 8 cable. I am measuring VSWR as well as R +jXs.

At that point the resistance measured is the sum of the radiation
resistance, plus screwdriver coil loss resistance, plus ground-loss
resistance. On 75m and 40m radiation resistance is so small, less than one
ohm, it is completely swamped by the sum of the other two. The coil loss
resistance is what came with antenna and nothing can be done about it. But a
screwdriver is about the best of the bunch.

And ground loss ohms is much greater than coil loss ohms. So for practical
purposes the Autek on the Z range is reading just the ground loss, which of
course is what you are primarily interested in. REGARDLESS OF SWR the
objective is to reduce the Z-range resistance at resonance to the lowest
possible value.

The lower the Z-range resistance, the further it gets away from 50 ohms and
the higher the SWR. Don't look at the SWR. The only reason the SWR range
is on the meter is because some people would feel unhappy if there wasn't
one and sales volume would be affected.

So maximum radiating efficiency coincides with lowest Z-range meter
readings. And to reduce Z-range readings increase the number of radials. A
damp, salty soil helps. Bury the radial wires 8 or 10 inches deep if you
can and stamp well in. And they need not be straight to get into awkward
corners of your back yard.

To transform a low feedpoint resistance up to the 30 - 50 ohms required by
the transmitter is another matter.
----
Reg, G4FGQ

Reg,

I have been thinking the radiation resistance should be about 36 ohms without
realizing that applies only to a full length quarter wave vertical. In effect,
when the screwdriver antenna manufacturer brags about having a low VSWR, he is
bragging about how lossy the antenna it is! This brings up another question. I
substituted a fat whip for the thin one that came with my antenna and discovered
the fat one would resonate at the same frequency with the total antenna length
about 10 inches shorter. I assumed this was because of the increased capacity of
the fat whip. If the fat whip resonates at a shorter length, does this mean it
will have less radiation resistance? Also, if I use a "capacity hat" to achieve
resonance at a shorter length, does this mean I will have a lower radiation
resistance, and probably a less efficient antenna?

Thanks for your help.


Ron, W4TQT