RE: [TowerTalk] Rotator for MA550

[Jim Lux <jimlux@earthlink.net>]

At 05:56 PM 6/15/2004 -0700, Rick Karlquist wrote:
> > I find that for casual hamming the SteppIR's seem to work pretty well at
> > 20-30 feet.  For a contest I might crank it up if it were mild out.
> >
>
> I did a bunch of A/B testing of inverted vee's at 30 and 60
> ft on the same mast.  The increase in height helped on the
> average of a few dB on 20 (not a huge difference) but helped
> substantially as the frequency went up, such as on 15 meters.
>
> Assuming you could extrapolate the results to Yagis (and I
> don't see why not), then it gives you an idea of when it
> is worth cranking up.

One reason the effect might not be as pronounced for a directive antenna 
(like a Yagi) is that the pattern is already more "horizontally" oriented, 
so the effect of the ground in the close proximity is less, particularly 
for paths that are at high elevation angles.

One other factor that I've noticed is that antennas tend to be built to a 
certain physical size (based on structural limits), so a higher frequency 
antennas, as a general class, are more directive than lower frequency 
antennas (don't see many 5 element 80m beams out there, but there's lots at 
10m).  Even a multiband beam with notionally 3 elements per band (either by 
trapping or interleaving elements) might have more gain than just 3 
elements hanging in the wind.

when you look at published patterns, it seems that the plots are almost 
always of the horizontal pattern (E-plane cut), not the vertical pattern 
(H-plane cut), and of course, height above ground effects are in the 
vertical plane. It's not clear whether they're optimizing things at the 
elevation angle of maximum response, or at some fixed elevation angle, or 
at zero elevation.

N6BV's HFTA is a great start at providing a basis for a better "figure of 
merit" that takes into account vertical patterns (primarily height above 
the ground), but, HFTA assumes some standard patterns for the antennas.

Greg Ordy (W8WWV) has been using the RDF metric described by Tom Rauch 
(W8JI), which is another attempt to integrate the entire pattern into a 
figure of merit.



> I also notice that operating mobile, I do a lot better on
> 20 meters than the higher bands.  This seems to be in line
> with the inverted vee tests.


I've noticed this too.  In Southern California, among the hills where I 
drive, you're only really going to be looking at fairly high angle signals 
(because the mountains block everything low).  Raising the antenna tends to 
push the main lobe down. For the lower frequencies, a mobile antenna is so 
sort and so close to the ground that the pattern is probably pretty 
isotropic, and you've got some decent high angle radiation. At 10meters, 
it's starting to look more like a classic dipole donut shaped pattern on 
its side, and you might not be getting as good high angle radiation.


There's also the current propagation conditions to deal with too.  Even if 
your 10,12,15 m antenna DOES radiate at high angles (to get over the 
mountains), there's no ionosphere to reflect it back down.

Over the past 6 months or so, I've been occasionally trying to work my 
cousin in Boulder, CO while I drive to work (along the 118 and 210 freeways 
in Los Angeles) along a route that is basically right up against the San 
Gabriel mountains. I've also been monitoring WWV on the same route (handy 
having a high power beacon next to the person you're trying to work!).  The 
15 MHz signal is sometimes there (depending on the time relative to 
sunrise/sunset), the 20MHz signal is usually non-existent, and the 10MHz 
signal is definitely there (but,then, WWV radiates a bit more power than I 
or my cousin do).





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