I have been mobilling for years but never on 80/75m. Getting ready to
build something to mount on my Toyota 4Runner. Given the following
choices I would appreciate some advice. Assume that both my roof mount
and rear trailer mount are "perfect" and will not be the determining factor.

Choice #1:
Mount a 1" or so mast starting at the trailer hitch going vertical for
4.5' and then have the loading coil (which clears the roof line) and
finally a 6.5' whip. Base height is about 2' off the ground and the top
is 13+' off the ground.

Choice #2:
Mount a 1" mast 4' high from the top of the roof, then the loading coil,
and then a 5' whip which is vertical for 2' and then horizontal for 3'.
Base height about 6' and top height about 12.5'.

Choice #2 will have a lower ground loss (good) than choice #1 but choice
#1 being taller will have a higher radiation resistance (good) than
choice #1. For an 80m antenna which of the tradeoffs generally is more
important for an antenna this size?

Thanks,
Larry Benko, W0QE

Larry Benko wrote:

Slight ERROR!

Should be:
Choice #2 will have a lower ground loss (good) than choice #1 but choice
#1 being taller will have a higher radiation resistance (good) than
choice #2.


Choice #2 will have a lower ground loss (good) than choice #1 but choice
#1 being taller will have a higher radiation resistance (good) than
choice #1. For an 80m antenna which of the tradeoffs generally is more
important for an antenna this size?

Thanks,
Larry Benko, W0QE

I can't prove this but I suspect there will be very little difference.
The wavelength on 80/75 meters is so much longer than the car body that
going from a low mount to a high one will be almost unnoticeable.
Ground loss will be about the same because the capacitance between the
car body and ground is the important factor and does not depend on
where the whip is mounted.

What will matter greatly is the Q of the coil. Make it inherently as
high as you can and keep it away from metal parts of the car body.
Resonate it and match it and you will have lots of fun. 80/75 is a
great band for mobile and much underused.

Bill, W6WRT

Bill Turner wrote:
I can't prove this but I suspect there will be very little difference.
The wavelength on 80/75 meters is so much longer than the car body that
going from a low mount to a high one will be almost unnoticeable.
Ground loss will be about the same because the capacitance between the
car body and ground is the important factor and does not depend on
where the whip is mounted.

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. Of
course, roof mounting creates a different set of problems.
--
73, Cecil http://www.qsl.net/w5dxp

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. It's hard to decide which would be better on
paper. But...On my "play" truck, I decided to go whole hog. I
mounted the base of the antenna on the rear pillar of my cab,
back behind my head. The base of the antenna is appx 64 inches
off the ground. Yes, it kicks butt... But I sometimes wonder how
it would do with the bumper mount, and longer lower mast. The
problem is I have campers on both of my trucks, and have always
been afraid to have the lower mast right up against the back tailgate,
and camper. It's hard to decide of the longer antenna would outweigh
the higher mount, and shorter antenna. I think really the only way to
know for sure is to actually try and compare both.
But in the past, and present, I'm a "high mounter" as far as mobile
whips.
BTW, I was out camping in Utopia TX about 2 months ago, and had
the chance to really give that truck and antenna a good workout.
It was browning the food. I was S9 plus to all TX stations, and even
S 9 to a Salt Lake City puter receiver listened to on the internet.
That was 80m...On 40m, it's even better. Course, that antenna when
parked is 14 ft tall, and has the coil at 8 ft from the base. It's 11
ft
tall in the driving mode. Even the short version is tall, when mounted
on the cab of that truck. The radio was the 706 barefoot. I use no
amp when mobile. I do know it's really bad news to have the coil
near body metal. But I've never had that problem yet on my various
vehicles.
MK

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

"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

ORIGINAL MESSAGE:

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.....


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No doubt that is correct. So how about this: I have a '95 Thunderbird
which I dearly love and don't want to cut holes in. I've been think of
going to a welding shop and having a metal piece made which I could
bolt to the frame in the back and which would stick out about six
inches or so behind the rear bumper, and installing a ball mount on it.
This will keep the lower part of the antenna about a foot away from the
body and allow a nice, long whip overall. The loading coil would be in
the center, homebrew of course. :-)

And not a hole in sight.

Comments?

Bill, W6WRT

Bill Turner wrote:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No doubt that is correct. So how about this: I have a '95 Thunderbird
which I dearly love and don't want to cut holes in. I've been think of
going to a welding shop and having a metal piece made which I could
bolt to the frame in the back and which would stick out about six
inches or so behind the rear bumper, and installing a ball mount on it.
This will keep the lower part of the antenna about a foot away from the
body and allow a nice, long whip overall. The loading coil would be in
the center, homebrew of course. :-)

And not a hole in sight.

Comments?

Bill, W6WRT

I'm not sure why, but most amateurs don't seem to realize that the whip
isn't an "antenna" and the car "ground", but each is half of a
dipole-like antenna. The car part is often much more important with
regard to radiation characteristics and efficiency than the whip part.
With the arrangement you suggest, the antenna consists of a vertical
wire -- the whip -- and a fat, horizontal "wire" -- the car. Whatever
current flows into the whip, an equal current flows over the outside of
the car, originating at the base of the whip.

Any antenna with a low horizontal wire will be quite lossy, because the
wire's current will induce a heavy current in the lossy ground beneath
the wire, or car.

The best arrangement, as others have pointed out, is to mount the
antenna right at the center of the top of the car. This makes the car
"wire" vertical, a much more efficient arrangement, which the
"shootouts" consistently show. You'll also find that larger trucks,
which effectively form a longer vertical "wire" for the car part, outdo
smaller ones for the same whip.

Of course, sometimes you don't have any choice, and you just have to do
the best you can. I once had a bumper mounted antenna consisting of a CB
whip base loaded with an inductor wound on a powdered iron core to
resonate on 40 meters. The car was a VW Squareback, so the antenna had
the increased disadvantage of proximity between the square back and the
antenna. As others have pointed out, this can reduce efficiency farther.
Yet I had a successful QSO with JA while driving down Highway 101,
running 8 watts, CW. So you can still communicate and have lots of fun
even with a very sub-optimal system. But anyone wanting to improve his
system has a much better chance of doing it if he has a basic
understanding of how the antenna really works.

Roy Lewallen, W7EL

Bill Turner wrote:
So how about this: I have a '95 Thunderbird
which I dearly love and don't want to cut holes in. I've been think of
going to a welding shop and having a metal piece made which I could
bolt to the frame in the back and which would stick out about six
inches or so behind the rear bumper, and installing a ball mount on it.
This will keep the lower part of the antenna about a foot away from the
body and allow a nice, long whip overall. The loading coil would be in
the center, homebrew of course. :-)

The only way to improve on that on 75m would be to mount
a piece of sheet metal on fiberglass poles connected at
the ends of both bumpers. The piece of horizontal sheet
metal, located 13.5 feet from the ground, would have the
same footprint as the T-bird and would be used as the top
hat. You do want optimum performance don't you? :-)
--
73, Cecil http://www.qsl.net/w5dxp