[Fourlanders] FW: [Mw] K2RIW: Performance Degradation on Large Apertures Due to IceBuildup

[Jim Worsham]

Wow, K2RIW never ceases to amaze me.  I thought that I would share this with
the group.  How many of you knew that water, when frozen, does not absorb RF
and you could actually make a lens out of ice if you could carve it
accurately enough?

73
Jim, W4KXY

It doesn't matter how beautiful your theory is, it doesn't matter how smart
you are.  If it doesn't agree with experiment, it's wrong.
Richard P. Feynman

-----Original Message-----
From: microwave-bounces at lists.valinet.com
[mailto:microwave-bounces at lists.valinet.com] On Behalf Of Richard Knadle
Sent: Wednesday, March 03, 2010 1:07 PM
To: Mike Feher
Cc: 'Microwave Lists'
Subject: [Mw] K2RIW: Performance Degradation on Large Apertures Due to
IceBuildup

N4FS es The Microwavers de K2RIW  3/03/10

Mike,

Water has a dielectric constant of about 81 at most RF frequencies.  It is a

very RF-absorptive substance.  That's primarily how your Microwave Oven (uw)

works.  With a proper layout, water makes a good uw dummy load.

But, when water freezes, it becomes an entirely different substance that has

almost no RF absorption characteristics, and a Dielectric Constant of about 
3.  Place an ice cube, or a snow ball in your uw oven, alongside a glass of 
water, and you'll discover that only the glass of water experiences any RF 
heating.  That gives you some idea why the uw oven has a so-called Frozen 
Food Defrost cycle mode of operation.

The exact RF properties of water, ice, and many other substances are
contained 
in an old, large format book authored by Von Hipple (I'm not sure of the 
spelling).

For a rough approximation, I think you assume that your blob of ice has a 
Dielectric Constant of 3, and a very low Loss Tangent.  You could literally 
make a double convex lens out of ice, and place it in front of your feed 
horn, if you desired to do RF focusing.

There are many parabolic dish antenna Ground Stations that seem to perform 
very well with a considerable amount of ice and snow on their surface.  I 
believe the major problem would be an ice build up on the feed horn causing
a 
lensing effect.

  73 es Good SHF/EHF DX,
  Dick, K2RIW


 ---------------
[Mw] Performance Degradation on Large Apertures Due to Ice Buildup
 From: "Mike Feher" <n4fs at eozinc.com>
 To: "'Microwave Lists'" <microwave at lists.valinet.com>
 
Can anyone recommend a reference that would address the subject for
frequencies between 7 and 30 GHz? I realize that it would have to be an
approximation as there are too many variables, but, a fairly thorough Google
search on my part did not yield any results. I am familiar with performance
degradation due to tolerance variations across the surface as a function of
wavelength, and, expect ice to be somewhat similar, but, not certain. Thanks
- Mike

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960
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