80m mobile antenna question
 

Roy Lewallen wrote:

I hope this has encouraged at least a few people to think a little
before declaring every conductor to be either an "antenna" or a
"ground plane" and assuming that by doing so they'll somehow cause it
to behave in some predetermined and only vaguely understood fashion.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~

A very good explanation, thank you Roy.

However... in your example of the giant tin can in free space, the top
of the tin can is acting like a ground plane, the side is acting like
an antenna and the bottom is again acting like a ground plane, just as
we have been saying. When this model is transferred to a car body, the
bottom of the car, in addition to the above, is also acting like one
plate of a capacitor coupling the signal to the earth below it,
commonly known as "ground". If someone disagrees with this I believe we
have a problem with semantics more than physics.

In other words, we are arguing over nothing.

Bill, W6WRT

80m mobile antenna question & "Skeleton Sleeve Antenna"
 

Howdy Dan:
Thanks for the info on "Mobile Vhf Ant.pdf", very interesting stuff.
I was checking out the rest of your site, and found an interesting pdf on the "Skeleton Sleeve Antenna",
and it's similarity of the J-pole, and you state that the distance from the 1/4 wave elements, and the 1/2 wave
element is not that critical. I have heard this before about the elements on the J-pole too, but what I don't understand
is why this dimension is not critical, it is an electrical path that should be subtracted from both elements, right?
Is it correct to shorten both elements by 1/2 of the dimension between the elements?

--
SeeYaa:) Harbin Osteen KG6URO

!sdohtem noitpyrcne devorppa-tnemnrevog troppus I




"Dan Richardson" wrote in message ...
On Thu, 2 Mar 2006 10:56:15 -0600, (Richard
Harrison) wrote:

Action of a common ground plane is different. When its balanced radials
are perpendicular to its whip, radiation from its radials zeros out
leaving the whip to do all the radiation. Ideally, a whip mounted on a
vehicle or directly on the earth behaves the same. It is the whip which
radiates.

Boy Richard, you sure missed the boat on that one!

Roy is quite correct in stating that a vehicle's body behaves as one
side of a dipole. A lopsided dipole to be sure, but one half the
antenna just the same.

A number of years ago I did a study using NEC modeling comparing VHF
mobile whips (1/4, 1/2 and 5/8-wavelength) mounted on different
vehicles. I created wire frame vehicles models for a full and mid-size
passenger cars, a small pickup truck and an SUV. The results for the
same antenna mounted top-dead-center on the different vehicles was
quite noticeable sometimes substantial.

An article I wrote on the subject can be viewed at:
http://k6mhe.com/files/mobile_vhf_ant.pdf

73,
Danny, k6MHE







email: k6mheatarrldotnet
http://www.k6mhe.com/

80m mobile antenna question
 

If I remember correctly, the higher the coil goes, doesn't its value
have to increase as well? If so, might the coil dimensions become a bit
too big to handle?

Scott
N0EDV

Jerry wrote:

"Cecil Moore" wrote in message
et...

wrote:

What we found at the CA shootouts is that when the bottom
section runs closely parallel to the vehicle body, as it
does with a trailer hitch mount on an SUV, the field
strength is much lower than if that bottom section is
in the clear, e.g. mounted on the roof of the SUV.....

I often wonder about this myself, but never get around to trying a
bumper mount. In the past, I've always preferred to have the lower
mast and coil as clear of the body as possible. But on the other
hand, if I mounted the base on the bumper, I could have a longer
mast below the coil.

What worked like a charm for me was using the trailer hitch
hole on my GMC pickup and removing the tailgate. I looked
for a fiberglass aftermarket tailgate but couldn't find one.
--
73, Cecil http://www.qsl.net/w5dxp


With that in mind, I have a friend who has a Ford Exploder--I mean, EXPLORER
:) --- with his DK3 mounted on a homebrew mount level with the rear bumper.
The bad part of it (IMHO) is the loading coil is level with the body about
where the rear window is and about 8 inches from the body. I mentioned to
him that it would be better to get the coil up in the clear above the truck,
but he is says he can't get in his carport. Well, what about this: move the
coil UP to clear the body and use a shorter whip? IOW, faced with the
lesser of two evils, which would be better. Left as is with longer whip and
putting up with the loss caused by proximity to body metal, or coil clearing
the top of the truck and a shorter whip--even it it has to be 5 feet instead
of 6 1/2? I voted for the higher coil and shorter whip. What say ye? :)


73

Jerry
K4KWH

80m mobile antenna question
 

Cecil Moore wrote:

Amos Keag wrote:

Cecil Moore wrote:

I'll bet that
if the vehicle were located 1/2WL in the air, the efficiency
would increase.


Kind of tough though going under power lines, bridges and overpasses :-)


What if the vehicle is a helicopter? :-)

Ahah!! No we have to consider the whop whop effect!! :-)

80m mobile antenna question
 

Scott wrote:
If I remember correctly, the higher the coil goes, doesn't its value
have to increase as well? If so, might the coil dimensions become a bit
too big to handle?

The bigger the top hat, the smaller the required reactance.
--
73, Cecil http://www.qsl.net/w5dxp

80m mobile antenna question
 

Bill Turner wrote:

David G. Nagel wrote:


Actually it is acting as one half of a dipole. It is just a
non-resonant half of a dipole. Remember "di" means two.

Dave WD9BDZ


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~

In a strict sense you are correct, but in the context here where one
half of the dipole is an eight-foot whip and the other half is four
feet of car body, we don't have much of an 80 meter antenna without the
coupling from car body to earth ground.

Bill, W6WRT


No argument here Bill. The point I guess I was trying to make is that a
dipole antenna system is two elements no matter what you make them of.
I use a Hi Sierra screwdriver antenna on my Honda Element. Even though
is is an impressive construct I don't harbor any illusions that it is an
efficient radiator. The body of the car is longer than the length of the
screwdriver, coil and whip. I have also used an Outbacker. Some say that
is a good antenna for it type, I have not had that good of a result with
it. Of course I am using my mobile for Civil Air Patrol and the
Outbacker does not fit that frequency very well on the precut tuning jacks.
I have found this thread to be interesting but I think that is had
passed that point.
I do not consider myself to be anything other than an interested amateur
and always consider your comments with great interest. Thank you for
your personal comments.

Dave WD9BDZ

80m mobile antenna question
 

Cecil, W5DXP wrote:
"The only way to improve on that on 75m would be to mount a piece of
sheet metal on fiberglas poles connected at ythe ends of both bumpers."

Kraus gives some support to that idea. Cecil has the 3rd edition of
"Antennas" In that edition, there is a "Disc antenna" on page 720 with
some similarity to cecil`s suggestion.

The "flush-disk" antenna, (d) in Figure 21-11 is said to be comparable
to a 1/4-wave vertical in performance, but has no projection. It could
be covered with a dielectric sheet, make no noise in the wind, and break
out no fluorescent tubes in parking garages. But, at 75m, the 0.3 lambda
dia. depression to contain it would measure 22.5 meters. That woud
require a vehicle that was very large indeed. At VHF and UHF it could be
very practical.

Best regards, Richard Harrison, KB5WZI

80m mobile antenna question
 

Bill Turner wrote:
Roy Lewallen wrote:

I hope this has encouraged at least a few people to think a little
before declaring every conductor to be either an "antenna" or a
"ground plane" and assuming that by doing so they'll somehow cause it
to behave in some predetermined and only vaguely understood fashion.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~

A very good explanation, thank you Roy.

However... in your example of the giant tin can in free space, the top
of the tin can is acting like a ground plane, the side is acting like
an antenna and the bottom is again acting like a ground plane, just as
we have been saying. When this model is transferred to a car body, the
bottom of the car, in addition to the above, is also acting like one
plate of a capacitor coupling the signal to the earth below it,
commonly known as "ground". If someone disagrees with this I believe we
have a problem with semantics more than physics.

In other words, we are arguing over nothing.

Bill, W6WRT

I interpreted your comments and those by some others as claiming that
radiation from the car is insignificant, and that it therefore isn't
effectively part of the antenna. I attempted to show that this isn't
generally true. I also showed that coupling to the ground actually
increases radiation from the car. So either I've convinced you by my
illustration, or I misinterpreted your earlier remarks.

Roy Lewallen, W7EL

80m mobile antenna question
 

On Thu, 2 Mar 2006 21:35:30 -0600, (Richard
Harrison) wrote:

Danny did not specify where he thought I erred in my previous posting. I
said that a whip mounted on a vehicle is not exactly like a dipole. I
meant that the whip did most of the radiating because it carried a
concentration of current in the same direction while in the car body the
current is dispersed in various directions, some of which canncel in
their effects.

That may have some validity in the VHF and higher ranges, but on HF -
particularly on 80 meters - a car body's size is a small fraction of a
wavelength (as is the whip portion). Consequently the vehicle body
acts like the one half of a dipole antenna. Slightly lop-sided yes,
but a dipole just the same - just as Roy stated.

Regards,
Danny

Question for Roy (was 80m mobile antenna question)
 

Roy, your analogy of the car body as a tin can really got me to
thinking.

With the whip mounted dead center on the top of the car, I can see how
the roof acts like a ground plane (a very short one) but I'm puzzled
about the radiation from the lower part of the car body. If one
visualizes RF flowing through the sides, hood and trunk of the car, the
currents will all be in phase with each other (roughly, of course) but
the currents are displaced in space by several feet.

How does this affect the net radiation from the car body as a whole? Is
there some addition or subtraction due to having the same current, same
phase but at a different location in space, and arranged in a more or
less 360 degree pattern?

An interesting thought.

73, Bill W6WRT