Resonant condition
 

In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field
strength than antenna and field strength. This comes as no surprise,
since the vehicle is usually a comparable or even greater part of the
radiating system than the titular antenna, and its coupling to ground
has a large impact on the efficiency.

Roy Lewallen, W7EL

Resonant condition
 

On Jun 7, 11:36*am, Jim Lux wrote:
* Have you a link to the data and test methodology?

I summarized the data from three CA 75m mobile shootouts at:

http://www.w5dxp.com/shootout.htm

I don't recall a test methodology being published. The test receiver
consisted of a ferrite loop antenna in the far field feeding a lab-
grade RF voltmeter. The power incident upon the 75m mobile antenna
system was assumed to be forward power minus reflected power on the
coax to the antenna system, measured using two Birds. The receive
results were normalized accordingly. I may have left out a detail or
two.

The SG-230 plus 11.5 whip at -12 dB was equal to a 75m hamstick. I
entered both the top-rated (0 dB reference) antenna and the (-12 dB)
autotuner+whip on the same vehicle. When I "superposed" all of the
three results, I assumed 0 dB for each top-rated antenna and let the
rest fall where they might. That may or may not have been a reasonable
assumption.

I suspect the SG-230 is designed to dissipate 100 watts (using large
#2 material powdered-iron toroids). During one shootout episode, I
forgot to attach the antenna to the mobile mount. The SG-230
faithfully tuned to close to a 1:1 match on the input - with a near-
infinite SWR on the output. It was a damp foggy day and the mobile
mount arced. That taught me not to mount the SG-230 unobserved in the
attic. :-)
--
73, Cecil, w5dxp.com

Resonant condition
 

On Jun 7, 1:23*pm, Roy Lewallen wrote:
In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field
strength than antenna and field strength. This comes as no surprise,
since the vehicle is usually a comparable or even greater part of the
radiating system than the titular antenna, and its coupling to ground
has a large impact on the efficiency.

Which is why, in this case, it is well to note that the 0 dB top-rated
antenna and the -12 dB antenna were mounted on the same vehicle
(mine).
--
73, Cecil, w5dxp.com

Resonant condition
 

Roy Lewallen wrote:
In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field
strength than antenna and field strength. This comes as no surprise,
since the vehicle is usually a comparable or even greater part of the
radiating system than the titular antenna, and its coupling to ground
has a large impact on the efficiency.

Roy Lewallen, W7EL

that seems quite plausible. A bigger vehicle essentially means a
physically larger antenna (think of the whole system as a dipole fed off
center, and a fan on one side but not the other.

Resonant condition
 

Cecil Moore wrote:
On Jun 7, 11:36 am, Jim Lux wrote:
Have you a link to the data and test methodology?

I summarized the data from three CA 75m mobile shootouts at:

http://www.w5dxp.com/shootout.htm

I don't recall a test methodology being published. The test receiver
consisted of a ferrite loop antenna in the far field feeding a lab-
grade RF voltmeter. The power incident upon the 75m mobile antenna
system was assumed to be forward power minus reflected power on the
coax to the antenna system, measured using two Birds. The receive
results were normalized accordingly. I may have left out a detail or
two.

Were those all mounted in the same place on the same vehicle, e.g. the
license plate bracket?

Resonant condition
 

On Jun 7, 11:48*am, Cecil Moore wrote:
For instance, the following shortened stub has a resonant frequency at
which it is electrically 1/4WL long even though it is only 1/8WL long
physically because of the 45 degree phase shift between the two
sections. ...

Are you stating that such a radiator with a physical length of 1/8-
lambda has ALL of the electrical characteristics of a self-resonant,
1/4-wave radiator -- including its radiation resistance, radiation
pattern, and peak gain in dBi?

Zero reactance at the input terminals of an electrically short
radiator does not mean that such a radiator is the electrical equal of
every other radiator with zero reactance at its input terminals.

Zero input reactance for short radiators can be attained by various
means, but the intrinsic, real radiation resistance/pattern/gain of an
antenna is a function of the electrical length, configuration, and
installation environment of the conductor(s) exposed to free space,
regardless of the reactance at the feedpoint -- whether that reactance
is zero or not.

RF