"John Ferrell" wrote:

I am attempting to compare the gain of a full size (1/4 wave) vertical
to various shorter verticals at 160 meters.

I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the
full 1/4 wave and I see 1.52 dbi for the 28 foot version.

I don't believe the shorter antenna has less gain. Where am I going
wrong?

Take a look at the phase angle between the feedpoint voltage
and the feedpoint current for the 28 foot version and try to
figure out an efficient matching network. That phase angle
is ~89.95 degrees. The feedpoint voltage is 1400 volts for
only 1.2 watts input. The feedpoint impedance is about
1.2-j1400 ohms.

Try using the helix feature to install a loading coil in the center
of the 28 foot antenna. The gain will drop as expected but the
feedpoint impedance will be easier to match.
--
73, Cecil, W5DXP

I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

I was not aware that the feeding of the radiator was affected by the
driving source. My "lab models" require a base inductance to get the
radiator into a range I can feed. I have not been including that in
the models.

I added a base inductor of 100uh(r=3) and got the results you
predicted. The 28 foot radiator is now showing -3.75 dbi gain!
Lesson learned!

The matter of the phase angle and the feed point voltage is currently
over my head, I will study it further.

I am using the TLW program from the Antenna Handbook to determine an
appropriate match. I have not tried to verify those calculations with
a lab project YET!

Once again, thank you for the assistance. I was getting pretty
frustrated...
de W8CCW John

On Thu, 20 Apr 2006 18:23:34 GMT, "Cecil Moore"
wrote:



Take a look at the phase angle between the feedpoint voltage
and the feedpoint current for the 28 foot version and try to
figure out an efficient matching network. That phase angle
is ~89.95 degrees. The feedpoint voltage is 1400 volts for
only 1.2 watts input. The feedpoint impedance is about
1.2-j1400 ohms.

Try using the helix feature to install a loading coil in the center
of the 28 foot antenna. The gain will drop as expected but the
feedpoint impedance will be easier to match.
John Ferrell W8CCW

Correction: the base inductor was 75uh
On Thu, 20 Apr 2006 19:06:17 GMT, John Ferrell
wrote:

I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

I was not aware that the feeding of the radiator was affected by the
driving source. My "lab models" require a base inductance to get the
radiator into a range I can feed. I have not been including that in
the models.

I added a base inductor of 100uh(r=3) and got the results you
predicted. The 28 foot radiator is now showing -3.75 dbi gain!
Lesson learned!

The matter of the phase angle and the feed point voltage is currently
over my head, I will study it further.

I am using the TLW program from the Antenna Handbook to determine an
appropriate match. I have not tried to verify those calculations with
a lab project YET!

Once again, thank you for the assistance. I was getting pretty
frustrated...
de W8CCW John

On Thu, 20 Apr 2006 18:23:34 GMT, "Cecil Moore"
wrote:



Take a look at the phase angle between the feedpoint voltage
and the feedpoint current for the 28 foot version and try to
figure out an efficient matching network. That phase angle
is ~89.95 degrees. The feedpoint voltage is 1400 volts for
only 1.2 watts input. The feedpoint impedance is about
1.2-j1400 ohms.

Try using the helix feature to install a loading coil in the center
of the 28 foot antenna. The gain will drop as expected but the
feedpoint impedance will be easier to match.
John Ferrell W8CCW
John Ferrell W8CCW

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.
--
73, Cecil, W5DXP

On Thu, 20 Apr 2006 19:22:17 GMT, "Cecil Moore"
wrote:

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.

Not to worry. Ground loss will help with that problem.

Found the helix. I will have to puzzle it out. I have a lot to digest.
I am glad I asked the question. The answers turned on a lot lights.

On Thu, 20 Apr 2006 19:22:17 GMT, "Cecil Moore"
wrote:

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.
John Ferrell W8CCW

"Cecil Moore" wrote in news:JbR1g.62010$F_3.14225
@newssvr29.news.prodigy.net:

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.

A funny anecdote comes to mind in this connection. I was trying
desperately to load a very short helical dipole on 40M and ended up
hooking it to my old Dentron 1KW matchbox to see if that could do
anything with it. Damn antenna loaded up perfectly. SWR at on the line
to the matchbox 1.1 to 1, good power transfer at 25 watts from the
exciter. So I went to listen with it. NOTHING! NADA! Not even a dit,
even though the big vertical array just hummed with CW signals on the
band. So I leaned over to check connections and darn near fried my hand
on the case of the Dentron. THAT'S where all the watts were going. Damn
tuner was loading its own case!

--
Dave Oldridge+
ICQ 1800667
VA7CZ

John Ferrell wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.

You'll find it in the EZNEC manual index under "Helix Creation".
(There's lots of other good information in the manual, too.)

I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

It'll be less cumbersome, but you probably won't get the same results.
The amount difference will depend on the geometry of the helix. However,
a lumped load is adequate to illustrate the phenomena you're investigating.

I was not aware that the feeding of the radiator was affected by the
driving source. My "lab models" require a base inductance to get the
radiator into a range I can feed. I have not been including that in
the models.

I added a base inductor of 100uh(r=3) and got the results you
predicted. The 28 foot radiator is now showing -3.75 dbi gain!
Lesson learned!

The loss is due solely to the inductor's resistance -- you'll get the
same result by replacing the inductor with a 3 ohm resistor. The amount
of gain reduction due to the inductor will get worse and worse as the
antenna gets shorter and shorter, for two reasons. The first is that the
antenna's radiation resistance drops as it gets shorter, so the inductor
loss becomes a greater fraction of the total feedpoint resistance. The
second is that you need a larger inductor for a shorter antenna and,
assuming a constant Q, that means more inductor resistance.

. . .

Roy Lewallen, W7EL

Thank you, I get that!
On Thu, 20 Apr 2006 13:12:20 -0700, Roy Lewallen
wrote:



The loss is due solely to the inductor's resistance -- you'll get the
same result by replacing the inductor with a 3 ohm resistor. The amount
of gain reduction due to the inductor will get worse and worse as the
antenna gets shorter and shorter, for two reasons. The first is that the
antenna's radiation resistance drops as it gets shorter, so the inductor
loss becomes a greater fraction of the total feedpoint resistance. The
second is that you need a larger inductor for a shorter antenna and,
assuming a constant Q, that means more inductor resistance.

. . .

Roy Lewallen, W7EL
John Ferrell W8CCW