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