"Hasan Schiers" wrote,

I am a bit surprised at the small effect on losses in the number of
radials
once one gets above 8 radials of 20m length.
===========================================

Hasan, why should you be surprised? Have you been reading the
plagiarised old-wives tales in the magazines? ;o)

There's another more accurate program, RADIALS2, which may be of
interest, which deals with shallow-buried radials of any length and
number, in conjunction with a simple vertical antenna of variable
height just to illustrate the practical effects.

Download program RADIALS2 from website below.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg Edwards wrote:
For single-band operation, an autotransformer comes to mind.
Wind a coil with about 12 microhenries of inductance. Transmitter
output goes to the top of the coil. Ground goes to the bottom.
Tap down from the top on the coil for the antenna feedpoint.
The transformation ratio is roughly equal to the square of the
turns ratio.

Cec, it seldom works. You forgot to mention the most important
parameter is the input impedance to the antenna.

I didn't forget, Reg. The input impedance was given in the
original posting:

"Frequency = 3.71 MHz Impedance = 29.15 + J 0.308 ohms"

Assuming that is the input impedance, I think my advice was
reasonable. If I had to match 50 ohms to 29 ohms on a single
band, I would just wind an autotransformer.
--
73, Cecil http://www.qsl.net/w5dxp

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Thanks, Cecil, and yes, that would work just fine and I have some B+W coil
stock sitting on a shelf, should work fine. (I used part of it before for a
parallel tuned circuit to tap feed my half-square.)

....hasan, N0AN

"Cecil Moore" wrote in message
...
Reg Edwards wrote:
For single-band operation, an autotransformer comes to mind.
Wind a coil with about 12 microhenries of inductance. Transmitter
output goes to the top of the coil. Ground goes to the bottom.
Tap down from the top on the coil for the antenna feedpoint.
The transformation ratio is roughly equal to the square of the
turns ratio.

Cec, it seldom works. You forgot to mention the most important
parameter is the input impedance to the antenna.

I didn't forget, Reg. The input impedance was given in the
original posting:

"Frequency = 3.71 MHz Impedance = 29.15 + J 0.308 ohms"

Assuming that is the input impedance, I think my advice was
reasonable. If I had to match 50 ohms to 29 ohms on a single
band, I would just wind an autotransformer.
--
73, Cecil http://www.qsl.net/w5dxp

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Assuming that is the input impedance, I think my advice was
reasonable. If I had to match 50 ohms to 29 ohms on a single
band, I would just wind an autotransformer.
--
73, Cecil http://www.qsl.net/w5dxp

==================================
Cec, to prevent the transformer inductance from completely detuning
the antenna, the transformer would have to be wound with a lot of
turns on something like an E and I dust-iron core. It's not worth the
trouble when there are other, more simple ways of doing the job.

The only advantage of a transformer or auto-transformer is that it
will cover several bands or octaves with widely different antenna
impedances.

Nice to bump into each other again.
----
Reg.

Reg Edwards wrote:
Cec, to prevent the transformer inductance from completely detuning
the antenna, the transformer would have to be wound with a lot of
turns on something like an E and I dust-iron core.

Yep, somewhere around here I have a tapped autotransformer
wound on a #2 powdered iron toroid designed for 75m use.
I wound it to be installed at the twinlead/coax junction
on a G5RV to obtain a 1:1 SWR on the coax on 75m. It worked
but I found that a parallel 1000 pf cap works just as well
and is easier to clip on and off.
--
73, Cecil http://www.qsl.net/w5dxp

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hasan schiers wrote:

What would be the simplest matching network to use to produce a flat match.
My primary use would be 80m low angle work at 3.7 and 3.5 Mhz, but also want
to be able to use it (with a tuner, of course) for a local net at 3.97 megs.


EZNEC ver. 3.0

Vertical over real ground 5/28/2005 6:39:29 AM

--------------- SOURCE DATA ---------------

Frequency = 3.71 MHz

Source 1 Voltage = 29.15 V. at 0.61 deg.
Current = 1 A. at 0.0 deg.
Impedance = 29.15 + J 0.308 ohms
Power = 29.15 watts
SWR (50 ohm system) = 1.715 (75 ohm system) = 2.573

Hi hasan, If you don't want to use capacitors in your matching
circuit, just shorten the antenna a couple of feet. That will add some
capacitive reactance at the frequency of interest. Then you can use a
shunt inductor to get a 50 ohm match. What you will really have is an
"L" network with a series cap and shunt inductor. You tune the
capacitor by changing the length of the radiator, and tune the inductor
by adding or removing turns.
An example, if you shorten the antenna until the impedance is around
29-j25 at 3.71mhz, a 2.5uH shunt inductor will give a 50 ohm match.
Gary N4AST

On Sat, 28 May 2005 06:49:15 -0500, "hasan schiers"
wrote:

This is a source run for an inverted L antenna for 80m that is 43 feet
vertical and 23 feet horizontal.

What would be the simplest matching network to use to produce a flat match.
My primary use would be 80m low angle work at 3.7 and 3.5 Mhz, but also want
to be able to use it (with a tuner, of course) for a local net at 3.97 megs.

First of all, changing the match doesn't make the antenna change from
a "low angle" radiator to a radiator for local work.


While a 1.7:1 vswr at resonance is usable, I would like to get my starting
point as low as possible to maintain some sort of reasonable vswr at the
band edges, for those times when I need to check into the net.

By simple I mean no caps...they are not all that easy to find (in a hurry)
any longer. I need power handling of 500w. My two thoughts are some sort of
1/4 wave transformer (although that's a lot of coax), or a simple toroidal
transformer.

You can use a 1/4 transformer of two parallel RG-59 to get a nearly
perfect match at 3.7 MHz. But, you must realize that by doing this
you actually *increase* the SWR at the band edges. Any reactive
solution will do the same.

If you have a tuner anyway, I fail to see why you want to do this.