Roy/Jeff

This is all interesting stuff that I have always wanted to experiment on
some more. I note the comment (was it Roy?) about the positioning of a
CP antenna over a vehicle roof and the effect that had on the total
signal polarization. I slapped my forehead on that one! (ie I should
have known and tested for it)

I also used a skew planar loop. It was I guess maybe 400mm above the
vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was
the same basic construction. At the time the results of H vs V really
astounded me. In my case OTH UHF performance was maybe 12db "better"
mainly from the reduction in flutter.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

Yes of course! (slaps forehead again!)

This is something that has come out recently in some experiments I have
been doing with mobile data comms on VHF SSB. I have a badly written and
incomplete page;

http://pages.suddenlink.net/vk2yqa

Before getting into the data side of it I had noted that a lot of
intelligence could be gleaned by looking at the Doppler effect from the
moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed
to my mind that a lot of signal comes from multiplathing and even some
comes from double reflections. The level of shift over the Doppler
"bandwidth" also showed that it was fairly evenly distributed. ie
reflections off plane objects like oncoming vehicles were not really any
stronger than from other directions. Such things as large oil tanks
though are quite obvious when I correlate the trip timing with nearby
objects. This also bought sense to the experiments I did back in the
80's where I tried a 3 el quad on the vehicle with dissappointing results.

So where is this going? I wonder what the result would be Doppler
"bandpass" wise if I used a CP antenna on the vehicle and base now? (The
above mentioned results are all VP) Given the sense change reflections I
wonder if I would get a "null" in the received bandpass due to movement.
I am trying to visualize the result of this;

http://pages.suddenlink.net/vk2yqa/img1.png

during that test. Would I get a series of bands parallel to the
envelope edges that would further indicate direction of travel relative
to the base. I realize that the direct path already gives that info,
just trying to get the likely scenbario in my head.

Apologies to Art for taking his post so far OT.

Cheers Bob VK2YQA