The more practical consideration is: "What can you do about it?"

Richard,

Your point is understood, but I was asking the question because I am
playing with the ARRL program, mobile.exe and one of the parameters to
enter for mobile antenna design input is ground loss. I simply was
wondering how that entry was figured, since it plays an important roll in
the program's calculations.


Thans.

Ed K7AAT

On Mar 5, 11:40 pm, Ed wrote:
I am wondering how ground loss/resistance is effected by my
having the bottom/feedpoint of the antenna so high off the ground, not
to mention, I wonder how to calculate or guess what it is at all.

The bigger the vehicle, and the higher the mount, the lower the ground
losses. But the type mount will effect also. IE: mounting directly on
a
roof will almost always be better than mounting on a ladder, with both
at the same height. I've found that the more metal under the base of
the antenna , the better. Mounting on a ladder, thin metal rail,
angle
iron, etc will usually result in inferior performance. But..Sometimes
there is nothing you can do about it if the mounting options are
limited. I ran into this on one of my trucks. At first I mounted the
antenna on a piece of angle iron that ran across the truck bed,
just behind the back window. Both ends were well grounded.
But, I kept seeing overall lousy performance. So I decided to
move it over on top of one the tool boxes, which has a good deal
more metal area under the antenna base. Made a large difference,
and woke the antenna up. That truck has a utility bed, and the
boxes are pretty much part of the body. I'm not talking about
a toolbox thrown into the back of the truck. Often those are
not the greatest, unless very well grounded. I don't see why
you need the exact ground loss. It's not going to effect your
antenna design. But when doodling with vertical programs, I'll
just stick in about 6-8 ohms as a "very good" mobile ground
example.. A huge truck with the antenna on top could be slightly
lower, but many small cars will be much higher. Knowing the
exact loss is really not critical. Your antenna design will still
come
out the same.
MK

wrote:
On Mar 5, 11:40 pm, Ed wrote:
I am wondering how ground loss/resistance is effected by my
having the bottom/feedpoint of the antenna so high off the ground, not
to mention, I wonder how to calculate or guess what it is at all.

The bigger the vehicle, and the higher the mount, the lower the ground
losses.

The ARRL Antenna Book assumes ~10 ohms of ground loss
for mobile antennas. I've found that 12.5 ohms is the
lowest I can get for a 75m mobile antenna feedpoint.
--
73, Cecil http://www.w5dxp.com

wrote in news:1173412553.256780.306530@
8g2000cwh.googlegroups.com:

On Mar 5, 11:40 pm, Ed wrote:
I am wondering how ground loss/resistance is effected by my
having the bottom/feedpoint of the antenna so high off the ground,
not
to mention, I wonder how to calculate or guess what it is at all.

The bigger the vehicle, and the higher the mount, the lower the ground
losses. But the type mount will effect also. IE: mounting directly on
a
roof will almost always be better than mounting on a ladder, with both
at the same height. I've found that the more metal under the base of
the antenna , the better. Mounting on a ladder, thin metal rail,
angle
iron, etc will usually result in inferior performance. But..Sometimes
there is nothing you can do about it if the mounting options are
limited. I ran into this on one of my trucks. At first I mounted the
antenna on a piece of angle iron that ran across the truck bed,
just behind the back window. Both ends were well grounded.
But, I kept seeing overall lousy performance. So I decided to
move it over on top of one the tool boxes, which has a good deal
more metal area under the antenna base. Made a large difference,
and woke the antenna up. That truck has a utility bed, and the
boxes are pretty much part of the body. I'm not talking about
a toolbox thrown into the back of the truck. Often those are
not the greatest, unless very well grounded. I don't see why
you need the exact ground loss. It's not going to effect your
antenna design. But when doodling with vertical programs, I'll
just stick in about 6-8 ohms as a "very good" mobile ground
example.. A huge truck with the antenna on top could be slightly
lower, but many small cars will be much higher. Knowing the
exact loss is really not critical. Your antenna design will still
come
out the same.
MK


Thank you very much for the helpful info to my situation. The RV roof
is fibreglass. All I have to really work with is the ladder, which in
itself is bonded to the aluminum frame in the back walls, as well as very
well bonded to the chassis underneath by means of large copper straps.
Bout the best I can do. The figures you suggested should help too.
Thanks again.

Ed K7AAT

The ARRL Antenna Book assumes ~10 ohms of ground loss
for mobile antennas. I've found that 12.5 ohms is the
lowest I can get for a 75m mobile antenna feedpoint.



Yes, but most mobiles are cars or pickups. Wouldn't you think that
losses might be less on a large vehcle like an RV or truck, especially
with such a high feedpoint as previously suggested ( 10 feet ) ?



Ed

On 09 Mar 2007 07:24:38 GMT, Ed
wrote:

Thank you very much for the helpful info to my situation. The RV roof
is fibreglass. All I have to really work with is the ladder, which in
itself is bonded to the aluminum frame in the back walls, as well as very
well bonded to the chassis underneath by means of large copper straps.
Bout the best I can do. The figures you suggested should help too.
Thanks again.

Hi Ed,

You've gotten several suggestions. 6-8 Ohms is no better than 10-12
Ohms as a guess; however, just what does this program do that this
number is so important in performing what it does?

73's
Richard Clark, KB7QHC

Ed wrote:
The ARRL Antenna Book assumes ~10 ohms of ground loss
for mobile antennas. I've found that 12.5 ohms is the
lowest I can get for a 75m mobile antenna feedpoint.

Yes, but most mobiles are cars or pickups. Wouldn't you think that
losses might be less on a large vehcle like an RV or truck, especially
with such a high feedpoint as previously suggested ( 10 feet ) ?

Sorry, I've been out of town for a few days and didn't
realize that an RV was being discussed.
--
73, Cecil http://www.w5dxp.com

Ed wrote:
"Wouldn`t you think that losses might be less on a large vehicle like an
RV or Truck,---."

I earlier rederred to an article by W1DBM in QST and an ARRL Antenna
Anthology from the 1970`s. The title was: "Antennas for Trailers and
Campers".

Everything old is new again. Please see the article: "Compact 40 Meter
Loop for Your Recreational Vehicle" by W6NBC on page 41 of the March
2007 QST.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Everything old is new again.

In one of the CA 75m shootouts, the strongest signal by far
was from a large RV with a motor-erected vertical on top.
Of course, it violated the 12.5 foot height-above-ground
limit for official entries. Heck, the top of the RV was
almost 12.5 foot above ground.
--
73, Cecil, w5dxp.com

On Mar 8, 10:01 pm, Cecil Moore wrote:
wrote:
On Mar 5, 11:40 pm, Ed wrote:
I am wondering how ground loss/resistance is effected by my
having the bottom/feedpoint of the antenna so high off the ground, not
to mention, I wonder how to calculate or guess what it is at all.

The bigger the vehicle, and the higher the mount, the lower the ground
losses.

The ARRL Antenna Book assumes ~10 ohms of ground loss
for mobile antennas. I've found that 12.5 ohms is the
lowest I can get for a 75m mobile antenna feedpoint.
--
73, Cecil http://www.w5dxp.com


Well, in my book, they state it varied for the size vehicle. They
state as
low as 4-6 ohms, but I agree, thats kinda optimistic...
But it still really doesn't matter as far as designing the antenna.
It's going to be the same no matter what... And it's consideration
of current distribution, etc, etc, will be important.
My antenna design will be the same , no matter what the ground
loss. So I don't worry about it too much. I just accept what I get..
BTW, my book is getting a little old, "90's something" so maybe they
have changed that section.
MK