Loading coils are used to extend the effective length of a horizontal dipole and
the "above ground" side of a vertical. Why not add a coil in series with
the common radial node to extend the effective length of the radials?

Ron, W4TQT

Hi Ron, W4TQT,

Loading coils in surface-laid or buried radials serve no useful purpose
unless the soil has a very high resistivitity as with solid rock and dry
sand.

There are no significant resonant effects which can be tuned out in ground
radials in ordinary soils.

If the soil loss resistance is not low enough in ordinary soils nothing can
be done except to lay down more radials. Increase number of radials rather
than increase length. Make optimum use of copper and aluminium.

Even then, doubling the number of radials might make a noticeable difference
but not a great improvement in overall radiating efficiency.

In summary, forget about coil-loading of ground radials whatever your
favourite band may be.

And forget Marzipan-the-Magician's magic number of 118.5 radials in any sort
of ground. Stop at 32 or thereabouts if you think you may need more than 8
or 16. Or who cares - it may be 6 or 12? But don't forget your best
adial - the domestic incoming cold water pipe.

Check the received signal strength on a standard transmission. If there was
no increase in received signal strength the last time you increased the
number of radials by 50 percent, then stop laying more radials. You are
wasting both time and copper.

Also, in good agricultural or fertile garden soil there's little to be
gained by laying shallow-buried radials longer than 1/10 or 1/12 of the
free-space wavelength.

It's uneconomic to mine copper ore in the outposts of Empire, spend a
fortune in electrolytically refining the stuff, shipping it 1000's of miles,
hot-rolling it into bars, drawing it through progressively smaller graded
diamond dies, and then with back-breaking labor burying it back in the
ground again.

We Hams should profitably pay more responsible consideration to wasteful use
of this small Earth's rapidly diminishing mineral and energy resources,
Earth warming, and waste less time waffling about "power-wave reflections"
on antenna feedlines.

Good luck in the Contest! ;o)
----
Reg, G4FGQ

Thanks to everyone for their responses. I'm installing a screwdriver antenna in a
Rubbermaid shed at the back corner of my condo. Only about 5 feet of the whip sticks
up above the top of the shed and I'm hoping the home owners assn. won't pay any
attention to it.

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 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.

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 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