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Old November 28th 06, 10:24 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

Since things are a little slow here...
As I continue to experiment with my 28 foot vertical I attempted to
try a shunt feed today. It totally escapes me how I might model it
with EZNEC 4 but I do have an MFJ-269 to work with.

I set up the radials and tied them to the base of the vertical. I
moved up the vertical from the bottom 55 inches. No real reason, that
is the length of wire I had handy I fed the wire with the MFJ 269 with
the coax shield on the ground side.

What I expected to see was some thing resonant between 6-8 mhz with a
lot of reactance and who knows resistance. It did not happen....

What I did see was a resonance at 23 mhz with Rs= 48 and Xs=5 for a
1.1 SWR!

Dabbling with EZNEC seems to suggest there may be something happening
like a 5/8 wave radiator but many factors had to be forced into place.
So that leads to my question: How do you shunt feed a 28 foot
vertical?

John Ferrell W8CCW
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Old November 28th 06, 11:25 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote:


What I expected to see was some thing resonant between 6-8 mhz with a
lot of reactance and who knows resistance. It did not happen....


I don't really understand that, what is resonance, and why would there
then be lots of reactance?

Owen
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Old November 28th 06, 11:33 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

John Ferrell wrote:
Dabbling with EZNEC seems to suggest there may be something happening
like a 5/8 wave radiator but many factors had to be forced into place.
So that leads to my question: How do you shunt feed a 28 foot
vertical?


If you feed it at zero feet, you get a feedpoint impedance
of zero. At the frequency at which the radiator is resonant,
i.e. about 234/28 = ~8.4 MHz, the feedpoint resistance will
increase as you raise the feedpoint. Somewhere on the way up,
it will be 50 ohms with some inductive reactance so you need
a series capacitor. It's a Gamma feed. It is easy to model
with EZNEC.
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73, Cecil http://www.w5dxp.com
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Old November 28th 06, 11:47 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote:

What I expected to see was some thing resonant between 6-8 mhz with a
lot of reactance and who knows resistance. It did not happen....


Hi John,

It is hard to imagine your antenna showed a strictly resistive,
non-reactive load over that interval where the radiator is only a
quarter wave at best. Fed at the base it should have some capacitive
reactance until resonance somewhere near the 40M band. Fed with a
wire at the height you indicate should show an inductive reactance
over the entire span.

What I did see was a resonance at 23 mhz with Rs= 48 and Xs=5 for a
1.1 SWR!


Sounds like your antenna base was not connected to the radials (you
were driving the antenna through the length of the gamma wire, and the
structure above it).

So that leads to my question: How do you shunt feed a 28 foot
vertical?


For 80M? About a 1000pF in series with the added wire. This still
says nothing of the resistance being sub 1 Ohm IFF your gamma wire is
on par with the diameter of the vertical element. On the other hand,
if it is very much thinner (and using half the capacitance), it stands
to elevate the resistive portion (to 30ish Ohms) into a match.

A more complete specification would tighten up the variability in
this.

73's
Richard Clark, KB7QHC
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Old November 29th 06, 05:06 AM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 22:25:45 GMT, Owen Duffy wrote:

On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote:


What I expected to see was some thing resonant between 6-8 mhz with a
lot of reactance and who knows resistance. It did not happen....


I don't really understand that, what is resonance, and why would there
then be lots of reactance?

Owen

I expected a dip in the swr at the 1/4 wave length but not a 50 ohm
match.
John Ferrell W8CCW


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Old November 29th 06, 05:10 AM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 22:33:43 GMT, Cecil Moore
wrote:

John Ferrell wrote:
Dabbling with EZNEC seems to suggest there may be something happening
like a 5/8 wave radiator but many factors had to be forced into place.
So that leads to my question: How do you shunt feed a 28 foot
vertical?


If you feed it at zero feet, you get a feedpoint impedance
of zero. At the frequency at which the radiator is resonant,
i.e. about 234/28 = ~8.4 MHz, the feedpoint resistance will
increase as you raise the feedpoint. Somewhere on the way up,
it will be 50 ohms with some inductive reactance so you need
a series capacitor. It's a Gamma feed. It is easy to model
with EZNEC.

I will go back to EZNEC 4 and see if I can determine what I am missing
on the Gamma match.
John Ferrell W8CCW
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Old November 29th 06, 05:43 AM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 23:06:21 -0500, John Ferrell
wrote:

On Tue, 28 Nov 2006 22:25:45 GMT, Owen Duffy wrote:

On Tue, 28 Nov 2006 16:24:07 -0500, John Ferrell
wrote:


What I expected to see was some thing resonant between 6-8 mhz with a
lot of reactance and who knows resistance. It did not happen....


I don't really understand that, what is resonance, and why would there
then be lots of reactance?

Owen

I expected a dip in the swr at the 1/4 wave length but not a 50 ohm
match.


John,

Your terminology has me confused.

When you say you expect to see resonance between 6-8MHz, I understand
resonance to mean zero reactance at that frequency. Then you tell us
you expect to see lots of reactance. They are inconsistent.

Then you tell me that you expect to see a dip in SWR at the 1/4 wave
length. I am guessing, but I think you probably mean the frequency
where the vertical is a quarter wavelength long electrically.

If you drive the vertical at the base in series, you would expect to
observe resonance (ie no reactance) somewhere around the quarter wave
frequency.

If you shunt feed it further up, the shunt feeding arrangement acts as
an impedance transformer, and it is unlikely that you will see zero
reactance (ie resonance) at the same frequency as if you series fed
the vertical at the base. A feature of shunt feeding typically is that
it is reactive, and you have to tune out the reactance, adjusting the
tap point up or down to get the desired feed impedance.

Owen
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Old November 29th 06, 02:36 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

Hey John... Glad to see you are still experimenting with the
vertical...
One answer is that you are shunt feeding a vertical monopole near it's
third harmonic tapped at about 1/8 wave above ground... Depending upon
shunt spacing and the diameter ratios it looks like you found the magic
spot for that frequency to transform to nearly a 1:1 match...
As one fella said to me long ago, I would rather be lucky than good,
any day...

denny - k8do

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Old November 29th 06, 05:20 PM posted to rec.radio.amateur.antenna
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Default Shunt fed vertical?

On Tue, 28 Nov 2006 14:47:29 -0800, Richard Clark
wrote:


So that leads to my question: How do you shunt feed a 28 foot
vertical?


For 80M? About a 1000pF in series with the added wire. This still
says nothing of the resistance being sub 1 Ohm IFF your gamma wire is
on par with the diameter of the vertical element. On the other hand,
if it is very much thinner (and using half the capacitance), it stands
to elevate the resistive portion (to 30ish Ohms) into a match.

A more complete specification would tighten up the variability in
this.

73's
Richard Clark, KB7QHC

I will pursue your suggestions.
John Ferrell W8CCW
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Old November 29th 06, 05:31 PM posted to rec.radio.amateur.antenna
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Posts: 199
Default Shunt fed vertical?

On Wed, 29 Nov 2006 04:43:14 GMT, Owen Duffy wrote:


John,

Your terminology has me confused.

When you say you expect to see resonance between 6-8MHz, I understand
resonance to mean zero reactance at that frequency. Then you tell us
you expect to see lots of reactance. They are inconsistent.

Then you tell me that you expect to see a dip in SWR at the 1/4 wave
length. I am guessing, but I think you probably mean the frequency
where the vertical is a quarter wavelength long electrically.

If you drive the vertical at the base in series, you would expect to
observe resonance (ie no reactance) somewhere around the quarter wave
frequency.

If you shunt feed it further up, the shunt feeding arrangement acts as
an impedance transformer, and it is unlikely that you will see zero
reactance (ie resonance) at the same frequency as if you series fed
the vertical at the base. A feature of shunt feeding typically is that
it is reactive, and you have to tune out the reactance, adjusting the
tap point up or down to get the desired feed impedance.

Owen

Sorry for the confusing terminology but I am confused!
The testing I have done so far with this 28 foot antenna has been
series fed. The results obtained have been consistant with the Eznec
models.

I believe my current direction requires I puzzle out the Gamma match
in Eznec.
John Ferrell W8CCW

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