[TowerTalk] If you had a choice

[Jim Lux]

On 10/17/12 6:33 AM, Joe Subich, W4TV wrote:
>
>> near field measurements are full of problems also. besides the
>> problems of projecting the near field pattern to something that is
>> useful in comparing in the far field there are all the little
>> distortions caused by coupling to nearby objects, feedlines, trees,
>> etc...


I was thinking of a near field range, not just making measurements 
in-situ in the near field.  I have to admit I've never *seen* a HF near 
field range, but theoretically it should be possible to do it.  (there's 
that Master's thesis thing...).    Point the antenna up on top of a 
flattened off mountain top, figure out how to correct for the partially 
conducting lossy ground plane, etc.

We actually do almost all of our antenna pattern work at JPL on a 
variety of nearfield ranges these days, often with scale models.  Our 60 
foot anechoic chamber now has a huge planar scanner in it.  The outdoor 
ranges mostly get used for doing things like evaluating multipath and 
scattering from "stuff" on the deck of a rover and confirming what we 
get from a numerical model of that.

However, "believing" the results of a near field measurement requires 
that you trust the math, and all those folks out there who don't trust 
NEC probably wouldn't trust the near field to far field transformation 
either.

A far field measurement has the advantage of conceptual simplicity, 
after all.


>
> Ground ranges have been used for years with good accuracy compared to
> free space.  With horizontal antennas but the direct wave and the
> first lobe have good correlation with the free space pattern.  Tests
> like those conducted by Steve and Ward use a consistent range and
> single reference so any range distortions fall out when making
> comparative measurements.


I don't know about "fall out", but certainly "knowable" and "can be 
calculated out"..  Consider comparing a dipole against something with a 
fair amount of gain.  The "bounce" signal on the dipole is pretty 
strong, but on the gain antenna, it's suppressed.    The classic 
approach is to move one end of the range up and down at least a 
wavelength, so you can characterise the bounce.  I suspect there are 
other clever ways to do it by changing the relative heights to a few key 
points or changing the spacing.  Somewhere around I have an IEEE 
Proceedings from the 70s that has the whole process in it from the folks 
at NIST (nee NBS)


Mountain top to mountaintop helps, too.. you're less likely to get a 
specular reflection from something "down in the valley", and even if you 
do, it's hopefully not in a sidelobe or on the edge of the main lobe. 
(that's the way the JPL outdoor ranges are set up.  Big valley in 
between source and receiver, and some attention to make sure there's 
nothing creating a specular reflection.

>
>> A fun thing might be to use one of those little quad-copter things
>> to fly an orbit around an antenna at various altitudes while making
>> measurements.
>
> I doubt that those devices have sufficient operating height to make
> practical far field elevation plots - even at one mile which is still
> not completely in the "far field" on some bands.

I have a 1/3 scale R/C powered parachute  that I was going to use for 
this kind of thing: GPS and 3 axis short dipole antennas with a receiver 
(like RELEDOP from SRI), but it has a whole lot of other practical 
problems... For instance, the little quad copters (and my powered 
parachute) can't move fast enough to overcome a decent wind.

Some folks in the UK have done this with flying a probe on a tethered 
balloon.  (and of course, SRI's RELEDOP is a probe towed by a helicopter)


>