[AMPS] Propeties of plastics at 10 GHz

[Jon Ogden]

As having worked in an antenna laboratory in the range of 2 to 16 GHz and 
as having designed microwave circuits up to 23 GHz, I think I am 
qualified to tackle this one:

>This issue came up during the "grate vhf parasitics debate", so I went to 
>the local plastics distributor to get some info. on dissipation factor - 
>a.k.a. D-factor  Nylon has a high D-factor at 1MHz and an even higher 
>D-factor at 10MHz.   The various vinyls and Delrin have a lower D-factor, 
>however, not nearly as low as the stryrenes, polyethelynes or Teflon..  .

I am not sure what you mean by "D" factor.  What we are interested in is 
what the material "looks" like to microwave energy.  Styrofoam has a 
dielectric constant VERY similar to free space.  In fact in the antenna 
range, we would set our antennas atop styrofoam blocks becuase the blocks 
were "invisible."  Invisible means they pass all the energy.  Also, we 
would use styrofoam as support elements for antennas as well.  No 
designer in his right mind would use an abosrptive material as a 
supporting element for an antenna.
 
>  Does it seem likely 
>that nylon would have high absorption at 7MHz and poor absorption at 
>10GHz.? 

Yes.  VERY likely.  Different materials have different atomic propoerties 
that are affected by different frequencies of radiation.  At HF, one can 
simulate a ground plane with a set of wires running from an antenna.  At 
VHF, one can simulate a ground plane with something that looks like the 
grate from a charcoal grill.  At 10 GHz, solid sheeting is needed.  There 
are also water absorption bands around 30 GHz and Oxygen absorption band 
around 60 GHz.  These absorption bands look like spikes.  Above and below 
them, energy will propagate, but not at that frequency.  So it is very 
possible that materials do different things over frequency.

Rich, if styrofoam or teflon are such good absorbers, please explain how 
coaxial cable works.   In a coaxial cable wave propagation takes place in 
the TEM propagation mode.  The electric field exists IN the dielectric 
between the surface of the center conductor and the inner surface of the 
outer conductor.  Any dielectric material (including air) causes loss, 
but this is different than pure absorbtion.  An absorptive material would 
completely attenuate all energy coming out the other side.  The closer 
one can get to the dielectric of air, the better.  This is why foam 
dielectric coax has lower loss.  The foam has a dielectric much closer to 
that of air.

If the materials that you claim are absorbers were, then when you put 
your 1.5 KW into your coax, your coax would burn up.  I would say that 
your fires or melting were caused by other issues such as voltage 
breakdown across the material and not due to RF absorption.  High 
voltages are created due to band standing wave ratios.

Also, how in the heck did the parasitic debate deal with plastic antenna 
mounts???

>- - - To check it out, put a styrene cup half full of water, 
>along with a sample of the plastic you want test, in a microwave oven and 
>blast away for a dozen or so seconds.

Ever notice that when you put a styrene cup of water in the microwave 
that the water gets hot but the styrene doesn't.  It certainly can't be 
because the styrene is "absorbing" the energy and then by conduction 
heating the water.  No, the styrene doesn't get hot because it PASSES all 
the energy that goes through it.  The heating that takes place is the 
interaction of the microwave energy with the water molecules making the 
molecules vibrate and thereby producing heat.  With a lot of energy 
applied to it an abosrptive material will get hot just like your dummy 
load.  So if the styrene material absorbed enough energy to make a cup of 
water boil, it would melt.  It would certainly have to be much hotter 
than the boiling point of water even if it DID absorb energy.  Heat 
transfer to the water would be very inefficient as the water would give 
off heat to the air as well as the styrofoam.  So in order to get a high 
enough temperature like that you'd heat the styrene past melting point.

Rich, I hate to say it but you are incorrect on this one.

73,

Jon
KE9NA



--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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