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