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

Roy Lewallen wrote:
And modeling a vehicle can be challenging ...

Here's how I modeled my pickup:

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

On 13-Feb-2009, Cecil Moore wrote:

Roy Lewallen wrote:
And modeling a vehicle can be challenging ...

Here's how I modeled my pickup:

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

Nice. I'l like to model this:

http://www.qsl.net/nb6gc/

Ken Fowler, KO6NO
President, USS Hornet Amateur Radio Club
--

Roy Lewallen wrote:
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.

I'm assuming that there is a capacitor formed by the car body being some
few inches away from the physical ground also?


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.

Given my limited experience, it's gotta be very difficult to model. My
setup was worst case, as far a sensitivity to bandwidth goes, a
bugcatcher. Best of a bad lot, I guess, but that makes the tuning very
sharp and sensitive. I'm assuming that the antennas that have fixed
elements "work" and tune by being pretty inefficient.

Which makes me suspect that we won't find any Hi-Q HF antennas that
aren't manually tuned in some fashion.

- 73 de Mike N3LI -

Michael Coslo wrote:
Roy Lewallen wrote:

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.

I'm assuming that there is a capacitor formed by the car body being some
few inches away from the physical ground also?

Yes. This alters the current distribution on the vehicle, and can make
it an even more effective radiator than the "antenna".

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.

Given my limited experience, it's gotta be very difficult to model. My
setup was worst case, as far a sensitivity to bandwidth goes, a
bugcatcher. Best of a bad lot, I guess, but that makes the tuning very
sharp and sensitive. I'm assuming that the antennas that have fixed
elements "work" and tune by being pretty inefficient.

Which makes me suspect that we won't find any Hi-Q HF antennas that
aren't manually tuned in some fashion.

Yes again. Manufacturers discovered long ago that hams like antennas
that are small, broadband and quiet. No problem -- small coils, small
wire, and bingo -- the ideal antenna. Rotten efficiency, but I've heard
countless hams over the years fussing and bragging about low SWR, and
nary a one who said a word about efficiency. Fortunately most hams don't
realize how many QSOs you can have with a watt or two of radiated power,
otherwise they'd be more concerned that that's all they're getting with
their 100 watt rig.

Roy Lewallen, W7EL

"JB" wrote in message
...

snip

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.

Ah! The most fervent counterpoise argument I can recall.

JB wrote:
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.


HAR! Good delivery JB, you had me until the last sentence.

- 73 de Mike N3LI -