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

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

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

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

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

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

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

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

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.

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