mobile antenna ground loss
 

I just started using the ARRL provided program, mobile.exe , for some
mobile antenna design work. I see that one of the figures required to be
input is ground loss.

Which raises the issue for me, how does one measure, ( or calculate),
the ground loss in a mobile antenna.

I am working primarily with 80M mobile antennas on medium sized
motorhomes.



Ed

mobile antenna ground loss
 

On 05 Mar 2007 17:30:06 GMT, Ed
wrote:

Which raises the issue for me, how does one measure, ( or calculate),
the ground loss in a mobile antenna.

I am working primarily with 80M mobile antennas on medium sized
motorhomes.

Hi Ed,

Work it in reverse. Put in various values and note their loss.
Compare to your own situation. Choose the value that comes closest to
that.

How do you compare? Measure against a reference antenna, while trying
to maintain the same conditions. THAT is the hard part. The
reference antenna's loss should be known (and even that is difficult).

Another way is build a software model that is the best of all worlds
(no Ohmic losses ...or at least the fewest). Compare drive conditions
of the model to the one on your motorhome. The difference between the
two is the loss, take this value and put it into your software.

73's
Richard Clark, KB7QHC

mobile antenna ground loss
 

Ed wrote in
. 192.196:



I just started using the ARRL provided program, mobile.exe , for
some
mobile antenna design work. I see that one of the figures required
to be input is ground loss.

Which raises the issue for me, how does one measure, ( or
calculate),
the ground loss in a mobile antenna.

Ed, I assume you mean ground loss stated as an equivalent series
resistance.

If you measure the feedpoint impedance or the VSWR of a short loaded whip
at resonance, you have a figure total resistance either directly reading
or from VSWR, R=50/VSWR. (Of course, those measurements need to be made
without any impedance transformation even if you might use such in
service.) That feedpoint resistance comprises equivalent ground
resistance and the equivalent of the antenna losses and radiation
resistance. If you can estimate the second component from models, the
equivalent ground resistance can be estimated by deduction of the
equivalent radiator R from the measured feedpoint R.

Owen

mobile antenna ground loss
 

Owen Duffy wrote:
Ed wrote in
. 192.196:



I just started using the ARRL provided program, mobile.exe , for
some
mobile antenna design work. I see that one of the figures required
to be input is ground loss.

Which raises the issue for me, how does one measure, ( or
calculate),
the ground loss in a mobile antenna.


Ed, I assume you mean ground loss stated as an equivalent series
resistance.

If you measure the feedpoint impedance or the VSWR of a short loaded whip
at resonance, you have a figure total resistance either directly reading
or from VSWR, R=50/VSWR. (Of course, those measurements need to be made
without any impedance transformation even if you might use such in
service.) That feedpoint resistance comprises equivalent ground
resistance and the equivalent of the antenna losses and radiation
resistance. If you can estimate the second component from models, the
equivalent ground resistance can be estimated by deduction of the
equivalent radiator R from the measured feedpoint R.

I would suggest that for the purposes of rough and ready estimation, and
considering the measurement uncertainty from most ham measuring gear,
you could estimate the radiation resistance of an 8ft long mobile
antenna without a tophat at being about 25-30 ohms for 10m, 5.5 ohms for
20, 1.5 ohms for 40, and 0.4 ohms for 75/80. Probably close enough with
about 1.5 significant digits accuracy.

Jefferies has similar numbers at:
http://www.ee.surrey.ac.uk/Personal/...es/radimp.html


Jim, W6RMK

mobile antenna ground loss
 

Jim Lux wrote in
:

I would suggest that for the purposes of rough and ready estimation,
and considering the measurement uncertainty from most ham measuring
gear, you could estimate the radiation resistance of an 8ft long
mobile antenna without a tophat at being about 25-30 ohms for 10m, 5.5
ohms for 20, 1.5 ohms for 40, and 0.4 ohms for 75/80. Probably close
enough with about 1.5 significant digits accuracy.

Are those figures for an unloaded vertical?

Ed was talking 80m, and if your 0.4 is for an unloaded vertical, it is
probably about 50% higher for a centre loaded vertical, so ~0.6ohms, and
that sounds reasonable.

Ed still needs to quantify the equivalent loss resistance of the stuff
above the feedpoint, and that will depend principally on the loading
coil, but likely to be in the range 5 to 20 ohms. Measurement in situ of
total feedpoint R (which is mainly loss resistances above and below the
feedpoint in this case) isn't so hard (as I described) and probably
better than an estimate.

Ed, it seems to me that in comparing similar resonant (80m) loaded
verticals, the one with the worst VSWR (direct fed at the base) is
probably the most efficient one. This implies that the antennas that are
advertised with a low direct feed VSWR are less efficient, they pad the
antenna with lossy loading to improve the VSWR.

Owen

mobile antenna ground loss
 

Owen Duffy wrote:
. . .
Ed, it seems to me that in comparing similar resonant (80m) loaded
verticals, the one with the worst VSWR (direct fed at the base) is
probably the most efficient one. This implies that the antennas that are
advertised with a low direct feed VSWR are less efficient, they pad the
antenna with lossy loading to improve the VSWR.

This is often the case, especially with small mobile antennas. It has
the added advantages of making an antenna more broadband and quieter,
both considered virtues by amateurs. The closer an antenna resembles a
dummy load, the better it's perceived to be.

Roy Lewallen, W7EL

mobile antenna ground loss
 

Owen Duffy wrote:
Jim Lux wrote in
:


I would suggest that for the purposes of rough and ready estimation,
and considering the measurement uncertainty from most ham measuring
gear, you could estimate the radiation resistance of an 8ft long
mobile antenna without a tophat at being about 25-30 ohms for 10m, 5.5
ohms for 20, 1.5 ohms for 40, and 0.4 ohms for 75/80. Probably close
enough with about 1.5 significant digits accuracy.


Are those figures for an unloaded vertical?

Short monopole, linear current distribution, rough approximations.

capacitive end loading would tend to make the Rrad higher (by as much as
a factor of 4 in the idealized case, since the general relation goes as L^2)


Ed was talking 80m, and if your 0.4 is for an unloaded vertical, it is
probably about 50% higher for a centre loaded vertical, so ~0.6ohms, and
that sounds reasonable.
Yep.


Ed still needs to quantify the equivalent loss resistance of the stuff
above the feedpoint, and that will depend principally on the loading
coil, but likely to be in the range 5 to 20 ohms. Measurement in situ of
total feedpoint R (which is mainly loss resistances above and below the
feedpoint in this case) isn't so hard (as I described) and probably
better than an estimate.

You can measure the feedpoint Z with probably 10% accuracy, but it's the
Rrad that's the tricky thing to determine, and without that, you'll not
get the ability to infer Rloss from Rfeedpoint. Of course, if the rough
and ready approximation shows Rrad to be 0.5 ohms, and you measure 25
ohms at the feedpoint, then you can just ignore Rrad, and assume the
loss resistance is 25 ohms (since the uncertainty in the measurement is
2.5 ohms)

Ed, it seems to me that in comparing similar resonant (80m) loaded
verticals, the one with the worst VSWR (direct fed at the base) is
probably the most efficient one. This implies that the antennas that are
advertised with a low direct feed VSWR are less efficient, they pad the
antenna with lossy loading to improve the VSWR.

I'd agree.. Unless the loading scheme is some sort of tapped
autotransformer at the base, in which case it's a bit tricky to figure out.

Owen

mobile antenna ground loss
 

Jim Lux wrote:
. . .
You can measure the feedpoint Z with probably 10% accuracy, but it's the
Rrad that's the tricky thing to determine, and without that, you'll not
get the ability to infer Rloss from Rfeedpoint. Of course, if the rough
and ready approximation shows Rrad to be 0.5 ohms, and you measure 25
ohms at the feedpoint, then you can just ignore Rrad, and assume the
loss resistance is 25 ohms (since the uncertainty in the measurement is
2.5 ohms)
. . .

And of course that's just where modeling becomes valuable. Even a crude
model will probably predict Rrad better than a "rough and ready
approximation". And a decent model will very often give a value for the
feedpoint Z that's more accurate than a casual or, sometimes, even a
careful measurement.

Roy Lewallen, W7EL

mobile antenna ground loss
 

I just started using the ARRL provided program, mobile.exe , for
some
mobile antenna design work. I see that one of the figures required
to be input is ground loss.

Which raises the issue for me, how does one measure, ( or
calculate),
the ground loss in a mobile antenna.

I am working primarily with 80M mobile antennas on medium sized
motorhomes.


My thanks to those who have already replied to the above question. To
re-iterate, I was using ARRL's mobile.exe program to model some
proposed antennas for my RV. I don't yet have a design built. Ground
loss resistance is one of the parameters to enter.... and entry
difficult to figure without a physical antenna, it appears.

My proposed antenna will be primarily for 80M. It will be center
loaded, approximately 8 to 12 feet tall, with a 4" diameter air wound
coil ( coil parameters can change ), and will be mounted such that the
FEEDPOINT/base will be at the top of my rear ladder, which is about 10
feet off the ground. The ladder is well bonded to the under chassis of
the vehicle. I may use a capacity hat at the TOP of the antenna... or
maybe not. 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.

Ed


This is what I am proposing to build.

mobile antenna ground loss
 

On 06 Mar 2007 05:40:40 GMT, Ed
wrote:

I wonder how to calculate or guess what it is at all.

Hi Ed,

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

The loss is going to be in frame and chassis connections where the
loss is going to predominate. Another matter is getting over the idea
that what sticks up the air is the principle part of the antenna. You
may as well get used to the idea that it is only a tuned radial for
the bigger element, your RV. And what polarization is that RV going
to present to the world? (Probably horizontal.)

73's
Richard Clark, KB7QHC

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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

mobile antenna ground loss
 

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?


mobile.exe is a little DOS based program that gives you radiation
efficiency, Q factor, etc, after you input several proposed design
measurements of a mobile antenna. When calculating these antenna
parameters for a "fixed station" type of mounting, ( not on a vehicle, I
guess), ground loss is one figure required for input. It is NOT a
required input when figuring for mounting on a vehicle, I just realized.


Ed

mobile antenna ground loss
 

Ed wrote:
It is NOT a
required input when figuring for mounting on a vehicle, I just realized.

Does it require one to input the type of vehicle?
--
73, Cecil http://www.w5dxp.com

mobile antenna ground loss
 

mobile antenna ground loss
 

Cecil Moore wrote in news:u%gIh.9028$jx3.5787
@newssvr25.news.prodigy.net:

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



A beautifully constructed 23' marine antenna on the roof with custom
fabricated lift mechanism, I believe? I think I saw that rig at Quartzite
in January. Fantastic.


Ed

mobile antenna ground loss
 

Cecil Moore wrote in news:ahiIh.6363$M65.6276
@newssvr21.news.prodigy.net:

Ed wrote:
It is NOT a
required input when figuring for mounting on a vehicle, I just
realized.

Does it require one to input the type of vehicle?


yes, input selections are pretty much based on vehicle size; largest
vehicle choice is a Van type. Last choice is "fixed station" which I
assume means no vehicle at all.. and that is the only choice requiring a
ground loss entry. Since my proposed antenna is for an RV, it appears I
really needn't have asked the ground loss question at all!

mobile.exe is a rather neat little DOS program located on the CD in
the back of newer ARRL antenna books, I believe.... maybe in the Handbook
too, I'm not sure on that.


Ed

mobile antenna ground loss
 

Ed wrote:
... as previously mentioned, I think, my
needed antenna design is for 80M, operation only while stopped, must be
vertical, and lastly, must not require me to climb up to the RV rooftop
to put it up or take it down. I have a good idea where I'm going with
this and will discuss the final product when it is completed ... probably
several weeks from now.

At one of the CA 75m shootouts, Sanford, W6YY had
a motorized 30 foot vertical mounted on top of his
huge RV. While he was traveling, it laid down against
the top of the RV. When he was stopped, he had it
motorized so he could raise it to a vertical position.
He beat the best of the best 75m mobile bugcatchers
and screwdrivers by 2 dB and beat a 75m hamstick
by about 14 dB. It's certainly worth considering.
--
73, Cecil http://www.w5dxp.com

mobile antenna ground loss
 

On 09 Mar 2007 19:32:20 GMT, Ed
wrote:

mobile.exe is a rather neat little DOS program located on the CD in
the back of newer ARRL antenna books, I believe.... maybe in the Handbook
too, I'm not sure on that.

Hi Ed,

Doesn't sound worth the clock cycles to run it when a Magic 8-Ball
would be just as accurate.

You can get better results from the free version of EZNEC because you
know what you've put in and what relationship it has to the whole. If
you belly up to the bar and buy the full-blown version, then you can
design a model of your antenna, RV, ladder and frame.

73's
Richard Clark, KB7QHC

mobile antenna ground loss
 

Richard Clark wrote in
:

On 09 Mar 2007 19:32:20 GMT, Ed
wrote:

mobile.exe is a rather neat little DOS program located on the CD in
the back of newer ARRL antenna books, I believe.... maybe in the
Handbook too, I'm not sure on that.

Hi Ed,

Doesn't sound worth the clock cycles to run it when a Magic 8-Ball
would be just as accurate.

You can get better results from the free version of EZNEC because you
know what you've put in and what relationship it has to the whole. If
you belly up to the bar and buy the full-blown version, then you can
design a model of your antenna, RV, ladder and frame.

73's
Richard Clark, KB7QHC



Didn't realize there was a free version. I had been thinking of biting
the bullet and buying it, but wasn't sure if I would like the program. A
freebie access to it will allow me to decide.

. . . off to download....



Ed

mobile antenna ground loss
 

On 10 Mar 2007 05:05:30 GMT, Ed
wrote:

. . . off to download....

Hi Ed,

You are about to discover there are more questions than answers...

That's what keeps it interesting.

73's
Richard Clark, KB7QHC