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 Feb 12, 9:02�pm, 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

From the responses I have gotten, that is the problem, I didn't model
the pickup. I wanted a simple way to determine the L I needed to get
the antenna resonant. I don't think it would be worth it to try and
model the truck just to get that small amount of data. By the cut and
try method I have determined the inductance I need is about 70% of the
value I get on EZNEC. I guess that is not too bad, gives me a
starting point. Thanks for the responses.

Gary N4AST

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

"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 -

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