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

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
--

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.
--
73, Cecil http://www.w5dxp.com

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

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

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

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
--

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

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

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

Shunt fed vertical?
 

On 29 Nov 2006 05:36:12 -0800, "Denny" wrote:

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
Stumbling across that condition has added to my confusion!

My dumb questions should show that the 28 foot stick still has lots to
teach me. I am beginning to get a little impatient to move on to a
permanent radial system and a taller antenna. Shunt feeding would
simplify lightning protection and physical mounting considerations of
a taller stick.

John Ferrell W8CCW

Shunt fed vertical?
 

On Wed, 29 Nov 2006 11:31:01 -0500, John Ferrell
wrote:


Sorry for the confusing terminology but I am confused!

John,

For the moment, forget the term resonance, what it means, what is
resonant, and the importance or not of resonance.

Just make you objective a feedpoint Z = 50+j0.

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.

Ok, I assume that means YOU have created and run the NEC models.

So now model the gamma match, initially with zero capacitance. The
diameters of the gamma arm and and the driven element, and spacing are
critical parameters, change them until you have the desired feedpoint
R. Then insert a capacitor at the feedpoint to offset the inductive
reactance in the feedpoint Z.

Then build it, and see if you can't make it work.


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

That is probably a lot easier than suck it and see experiments.

Owen

PS: When you have done all that and it works, go back to thinking
about resonance, and take a system view. In EZNEC you could explore a
solution for the same operating frequency with a shorter vertical, say
25', design the gamma match for a 50+j0 feedpoint, compare the gain to
the longer model and answer the question of whether natural resonance
of the driven element alone (ie without the gamma arm) is critically
important to performance of the antenna system.
--

Shunt fed vertical? - DipoleGammaM.ez (0/1)
 

On Wed, 29 Nov 2006 19:33:40 GMT, Owen Duffy wrote:

On Wed, 29 Nov 2006 11:31:01 -0500, John Ferrell
wrote:


Sorry for the confusing terminology but I am confused!

John,

For the moment, forget the term resonance, what it means, what is
resonant, and the importance or not of resonance.

Just make you objective a feedpoint Z = 50+j0.

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.

Ok, I assume that means YOU have created and run the NEC models.

So now model the gamma match, initially with zero capacitance. The
diameters of the gamma arm and and the driven element, and spacing are
critical parameters, change them until you have the desired feedpoint
R. Then insert a capacitor at the feedpoint to offset the inductive
reactance in the feedpoint Z.

Then build it, and see if you can't make it work.


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

That is probably a lot easier than suck it and see experiments.

Owen

PS: When you have done all that and it works, go back to thinking
about resonance, and take a system view. In EZNEC you could explore a
solution for the same operating frequency with a shorter vertical, say
25', design the gamma match for a 50+j0 feedpoint, compare the gain to
the longer model and answer the question of whether natural resonance
of the driven element alone (ie without the gamma arm) is critically
important to performance of the antenna system.
A progress report:
I have not adapted your advice yet and I am going to have to break for
a while to do some real work but here is where I am...
For ease in modeling I have a 2*28 foot (56 ft overall) free space
dipole: This frees me from ground effects. I used #12 wire to simplify
wire intersections. I have NOT experimented with spacing and wire
diameters yet.

I will attempt to attach the Eznec file here in case anyone is
interested.

TIP: to get a better view of the matching setup temporarily change the
lengths of the +28 and -28 wires to 10 feet.
John Ferrell W8CCW

Shunt fed vertical? - DipoleGammaM.ez (0/1)
 

On Wed, 29 Nov 2006 15:24:37 -0500, John Ferrell
wrote:


For ease in modeling I have a 2*28 foot (56 ft overall) free space
dipole: This frees me from ground effects. I used #12 wire to simplify
wire intersections. I have NOT experimented with spacing and wire
diameters yet.

Keep in mind that the less your model resembles the real antenna, then
the less the results are applicable.

I am not sure that your dipole in free space is better or even as good
as modelling the vertical over perfect ground as a first step.

If you are using a 28' #12 wire vertical, then it makes sense to model
that, but if you are using a self supporting tube or mast of much
larger diameter, then you are modelling something else.

Gamma matches become impractical when they call for extremely small
tuning capacitors, so your challenge is to find a solution that uses a
practical tuning capacitor.

Start the search with a low tap point, equal diameter conductors, and
wider spacing. Move the tap point up to increase R. Check the tuning
capacitor required. Explore the sensitivity to changes in all three +
driven element length.

Owen
--

Shunt fed vertical? - DipoleGammaM.ez (0/1) - DipoleGammaM2.ez (0/1)
 

On Wed, 29 Nov 2006 21:20:16 GMT, Owen Duffy wrote:

On Wed, 29 Nov 2006 15:24:37 -0500, John Ferrell
wrote:


For ease in modeling I have a 2*28 foot (56 ft overall) free space
dipole: This frees me from ground effects. I used #12 wire to simplify
wire intersections. I have NOT experimented with spacing and wire
diameters yet.

Keep in mind that the less your model resembles the real antenna, then
the less the results are applicable.

I am not sure that your dipole in free space is better or even as good
as modelling the vertical over perfect ground as a first step.

If you are using a 28' #12 wire vertical, then it makes sense to model
that, but if you are using a self supporting tube or mast of much
larger diameter, then you are modelling something else.

Gamma matches become impractical when they call for extremely small
tuning capacitors, so your challenge is to find a solution that uses a
practical tuning capacitor.

Start the search with a low tap point, equal diameter conductors, and
wider spacing. Move the tap point up to increase R. Check the tuning
capacitor required. Explore the sensitivity to changes in all three +
driven element length.

Owen

Here is a better set of numbers. The antenna(s) involved are strictly
lab experiments. I have not worked out the details of putting the
source where I want it with a vertical, it would be in the ground
plane. The objective was to get a "feel" for the shunt match problem.
With your guidance I think I am on the right path.

My early conclusions (subject to change!) is that I don't want to put
too many resources into a grounded vertical antenna. It will be easier
in the future to match an insulated antenna. If one already has a
grounded tower available it might make good sense to shunt feed it.
John Ferrell W8CCW

Shunt fed vertical? - DipoleGammaM.ez (0/1) - DipoleGammaM2.ez (0/1)
 

I have a grounded, 130 foot tower... Not being a fan of shunt feed I
use leaning-L dipoles that use the tower for a reflector - or an open
sleeve coupled radiator - depends upon your point of view...

denny