Trying to design a 13' vertical with a loading coil at 3'. It will be
mounted on an aluminum tool box on a pickup. I modeled this on EZNEC
and came up with loads to resonate on 14.2 and 7.2 mhz. Wind the
coils with calculated inductance and it is way lower in frequency than
predicted.

Tapped up the coils and eventially got matches, but was wondering why
the design and real world are so far apart?

Gary N4AST

On Thu, 12 Feb 2009 17:00:38 -0800 (PST), wrote:

was wondering why
the design and real world are so far apart?

Hi Gary,

The devil is in the details. A more full description of the model,
like its file available from a web site, would be more productive than
a lot of guessing.

73's
Richard Clark, KB7QHC

wrote in message
...
Trying to design a 13' vertical with a loading coil at 3'. It will be
mounted on an aluminum tool box on a pickup. I modeled this on EZNEC
and came up with loads to resonate on 14.2 and 7.2 mhz. Wind the
coils with calculated inductance and it is way lower in frequency than
predicted.

Tapped up the coils and eventially got matches, but was wondering why
the design and real world are so far apart?

Gary N4AST

the real ground on the pickup truck is much more complex than you probably
modeled. what did you include for your ground model?

wrote:
Trying to design a 13' vertical with a loading coil at 3'. It will be
mounted on an aluminum tool box on a pickup. I modeled this on EZNEC
and came up with loads to resonate on 14.2 and 7.2 mhz. Wind the
coils with calculated inductance and it is way lower in frequency than
predicted.

Tapped up the coils and eventially got matches, but was wondering why
the design and real world are so far apart?

Gary N4AST

It sounds like the model of the pickup was inadequate -- it's at least
as important as the vertical. How did you model it?

Roy Lewallen, W7EL

wrote:
Tapped up the coils and eventially got matches, but was wondering why
the design and real world are so far apart?

You must have used the lumped inductance available in
EZNEC which assumes that the coil is zero degrees long.
I've found the helix option to be much more accurate.
--
73, Cecil http://www.w5dxp.com

wrote:
I modeled this on EZNEC
and came up with loads to resonate on 14.2 and 7.2 mhz.

Maybe you need to tell EZNEC a frequency in megahertz, not millihertz ;-)


(joke!)

In article tonline, Roy
Lewallen wrote:

It sounds like the model of the pickup was inadequate -- it's at least
as important as the vertical. How did you model it?

Roy Lewallen, W7EL

Hello, and that could well be it. After using NEC-4 to model USN HF
antennas in their intended operating environment one finds that the local
environment often requires beaucoup more wire segments (I never completely
trusted patches) to model than the antenna itself. If conductive objects
in close proximity can be excited by antenna currents then they are part
of the antenna. A USN example would be the 2-6 MHz twin fan-type antenna
that relies heavily on induced currents in the ship's stack for its
feedpoint impedance and radiation characteristics. Sincerely, and 73s
from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

wrote:
Trying to design a 13' vertical with a loading coil at 3'. It will be
mounted on an aluminum tool box on a pickup. I modeled this on EZNEC
and came up with loads to resonate on 14.2 and 7.2 mhz. Wind the
coils with calculated inductance and it is way lower in frequency than
predicted.

Tapped up the coils and eventially got matches, but was wondering why
the design and real world are so far apart?


The real world and modeling are a lot different with vehicles than they
are with land mounted antennas. Your ground on your truck is very complex.

side question: Did you bond the bejabbers out of the truck? You
really should have hood, doors, tailgate, exhaust system in several
places, frame in several places, radiator, engine block, and any other
place of interest you can think of.

Rule of thumb is that you need at least one more bond than the maximum
amount you'd dare to place on the vehicle.

The vehicle bonding and grounding is more important than the aerial part
IMO.

- 73 de Mike N3LI -

The real world and modeling are a lot different with vehicles than they
are with land mounted antennas. Your ground on your truck is very complex.

side question: Did you bond the bejabbers out of the truck? You
really should have hood, doors, tailgate, exhaust system in several
places, frame in several places, radiator, engine block, and any other
place of interest you can think of.

Rule of thumb is that you need at least one more bond than the maximum
amount you'd dare to place on the vehicle.

The vehicle bonding and grounding is more important than the aerial part
IMO.

- 73 de Mike N3LI -

The frame is the only substantial ground and certainly most effective for 40
meters. Use of the corners of the vehicle might actually get you a
counterpoise on 20. What you really need is a trailing wire, dragging a
cast iron stove.

JB wrote:

The frame is the only substantial ground and certainly most effective for 40
meters. Use of the corners of the vehicle might actually get you a
counterpoise on 20. What you really need is a trailing wire, dragging a
cast iron stove.

At HF, a vehicle isn't "ground" or a "counterpoise", but the bottom half
of an asymmetric dipole. It radiates at least as much as the "antenna"
due to currents flowing downward along the outside. Calling a vehicle
"ground" or "counterpoise" doesn't impart magical properties -- it's a
conductor carrying currents whose fields don't cancel. In other words,
it's an integral, radiating portion of the antenna. You can't leave this
significant part of the antenna out of a model and expect the model to
give correct results.

And modeling a vehicle can be challenging because of the proximity of
conductors, particularly the whip and vehicle. You have to follow the
rules for closely spaced parallel conductors, and watch the average
gain. You might need considerably more segments than normal where
conductors are very close.

Roy Lewallen, W7EL