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