[Cu-wireless] Summary of antenna test results

[Paul Wefel]

On Mon, Nov 12, 2001 at 04:26:06PM -0600, Paul Riismandel wrote:
> At 03:58 PM 11/12/2001 -0600, Dave wrote:
> >I had a hunch that we'd get better reception with all antennas at a height
> >off the ground.  I only tried the Pringles. At 1.3km, we got 25dB when
> >I stood on top of the van with the Pringles can antenna, 20dB with the
> >same antenna when I stood on the ground beside the van. I suspect that
> >the ground plane of the earth affects reception (there appears to be a
> >lot of discussion of some such phenomenon in the ARRL Antenna Handbook),
> >but there's no telling. It was not a very scientific test, climbing on
> >the roof of the van.
> 
> Yes, I'd suspect that the Pringles can provides a ground plane of some sort 
> that is lacking in the NCSA antenna, and unnecessary with the parabolic 
> (which is well tuned and focused to the frequency range in question).  I

The helical antennas have a metal plate inside the PVC cap.  This plate is
connected to the outside of the N connector and acts as the ground plane.
The parabolic also has a ground plane which is the dish grid itself.  I
believe all antennas need a ground plane of some sort in order to operate.

The performance difference you saw from height was probably due to the
fresnel effect.  See: http://www.ydi.com/fresnel-zone.php for a picture of
the fresnel zone.  Also http://www.qsl.net/n9zia/wireless/page06.html for a
good description of the effect.

The pringle can antennas rock! Jon and I have the parts to build a couple
more to add to the collection.  We just need to find some time to do it.

Good work on the antenna tests.  Its a start. 
 
-paul (the other Paul)
 
> can only guess at what this specific parabolic antenna looks like, but I'd 
> guess you could probably make similar antennas with sheet metal, metal 
> screen or some other conductor, since it appears to me that the foil in the 
> pringles can acts as both a collector/reflector, like a parabolic dish, and 
> as a ground plane radial, like a ground plane antenna.  My antenna 
> knowledge at this wavelength is minimal, but I'm pretty sure that the 
> ground plane effect is not terrifically useful for line-of-sight 
> applications.  At least in the VHF band it's mostly useful for 
> omni-directional broadcasting (many FM antenna designs utilize ground 
> planes). Ground planes become much more important at lower frequencies, 
> especially around the AM broadcast band.
> 
> --Paul
> 
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|Paul Wefel            NCSA                 Network Engineer    |
|phone: 217-244-2193   cell: 217-840-9929   pwefel at ncsa.uiuc.edu|
|CAB 269, 605 E Springfield, Champaign IL 61820                 |
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