Re: NEC-LIST: Passive Intermodulation in antennas

Zvi

We have had a lot of experience in the past with PIMs at microwave
frequencies in waveguide arrays and reflector structures, particularly
satcom, also some at UHF with broadband structures, both printed and
otherwise. It's difficult to generalise but the following I hope is of
interest!

Firstly low PIM specs, can be an absolute pain to measure- take care!
As far as I know there is no real accurate way of calculation- some
attempts have been made for specific cases but we are left with best
practice type of rules. Much depends on the actual spec you have to
meet, both in terms of power and the PIM level.

There is some literature available but not much and pretty specific to
particular problems. The European Space Agency run a symposium some
years ago, there is a bit in IEEE APS- but the literature search
should start over ten years ago!

As you probably know, PIM is generated due to discontinuities in the
structure disturbing the current in a non-linear way- in effect a
tunnelling process occurs. To first order apply the rules you would
for anti-corrosion issues-that is avoid dissimilar metals if at all
possible, if not then do not use materials with significant
electro-potential difference. This only gets a first order result, for
low PIM no dissimilar metals in high current regions is a golden
rule. Next, it's a good idea to ensure extremely good pressure contact
between parts of the structure carrying current , and at all costs get
very good conductivity at joints.

I've not run across the zinc/copper problem that Ron mentions , but
can well believe it.

The tighter the spec, the harder it gets. PIM can be readily generated
between the same metal at a joint with reasonable power unless care is
taken. Examples include rivets on large reflector surfaces where the
rivets , even properly fitted (that is tight) and of the same material
as the reflector surface, generate PIM. JPL did some good work on this
years ago as did the University of Sheffield in the UK a little
later.At waveguide joints, low PIM specs mean special high contact
force flanges, and other techniques. For connectors on coax, use
highest quality and those capable of high contact pressure (connectors
are good PIM generators if you are not careful)

On very high power systems, dielectric/metal and even
dielectric/dielectric interfaces in the same material have been shown
to generate PIM - but don't worry about this as by the time you get to
this level of power the printed circuit will probably vaporise!

PIM's are a difficult problem to generalise on but we believe are of
growing importance

Hope this is of interest

Tony Brown

----- Original Message -----
From: zvi frank <zvi_f_at_mti-group.co.il>
To: <nec-list_at_ee.ubc.ca>
Sent: 14 September 2000 14:02
Subject: NEC-LIST: Passive Intermodulation in antennas

> Hallo All,
>
> Has anyone information on Passive Intermodulation in antennas? What
> sort of PIM value can one expect for two signals of 10 Watts? How does
> one minimise PIM? Especially in Printed antennas? Does the solder
> joint contribute?
>
> Any help would be welcome. Thanks in advance.
>
> Zvi Frank
>
Received on Fri Sep 22 2000 - 03:42:30 EDT