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
--

On Fri, 13 Feb 2009 22:55:52 GMT, "Ken Fowler"
wrote:


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


Hi Ken,

Very nice. All those R390s, R1051s, RBBs; but only one URC-32????? I
didn't see much familiar UHF/VHF either (SRC20/21).

Does any of this gear have power? Antenna access?

73's
Richard Clark, KB7QHC

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

Roy Lewallen wrote:

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.


I haven't been able to compare my setup with one of the small systems,
but I have to think that it was worth th eeffort. It's anecdotal of
course, but signal reports have been pretty good.

I really must post a picture somewhere some time. It's a true
monstrosity on a little Suzuki Vitara. My biggest regret is that a lot
of people just *have* to come over to talk to me while I'm stopped and
operating. Law enforcement is also interested - every one who has
stopped to talk to me has been friendly but intrigued. A guy from the
Fish and Game commission is going after his license now. The regret is
that it can take away from operating time. 8^)


- 73 de Mike N3LI -

On Mon, 16 Feb 2009 10:21:56 -0800, Roy Lewallen
wrote:

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".

Maybe, maybe not. Roadway surfaces are rarely conductive. More like
static dissipative materials. While the area is significant the
opposite conductive pole plate is missing, computing the effective
capacitance may be challenging.


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

JosephKK wrote:
On Mon, 16 Feb 2009 10:21:56 -0800, Roy Lewallen
wrote:

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".

Maybe, maybe not. Roadway surfaces are rarely conductive. More like
static dissipative materials. While the area is significant the
opposite conductive pole plate is missing, computing the effective
capacitance may be challenging.

Much more than the roadway surface is involved. The skin depth in
average soil varies from about 12.6 feet at 30 MHz to 15.9 feet at 3.5
MHz. So significant current flows to depths of several tens of feet,
well below the road surface. Within this distance of the surface you'll
usually find strata with various conductivities and permittivities, as
well as possibly buried pipes, rebar, and who knows what else. Computing
an "effective capacitance" is virtually impossible, and useless if the
vehicle is to be moved even a short distance. So only generalizations
are possible. But any reasonable assumption about the characteristics of
the ground under the vehicle points to it having a significant impact on
the current distribution.

Roy Lewallen, W7EL