SignalFerret wrote:
"Reg Edwards" wrote in message
...
What you need is a standard Piston attenuator whose performance is
largely calculable. Forget about dinky wire-ended resistors whose
performance in attenuators is a matter of guesswork.
----
Reg.


Just curious, what is a piston attenuator? For the life of me I can't seem
to picture it. I know how a slab of resistive material inserted in to a
wave guide works as an attenuator, but how is it accomplished in a coax
transmission line? If someone has a photo, or diagram of the inner workings
that would be great!

Robert
N3LGC

Resistive material is not involved. The piston attenuator works by
varying the length of an empty metal tube, down which the signal has to
propagate as an EM wave. The tube is a waveguide operating below its
cutoff frequency, so the attenuation depends on the length and can be
calculated from first principles.

Attenuators using small wire-ended resistors would certainly be good
enough for this particular application, where accurate attenuation
values are not required. The performance of such attenuators has often
been measured, so it's far from being guesswork. If they are well
constructed, with attention to short leads, layout and shielding, they
can be quite accurate up to about 400-500MHz. However, that still
leaves the problem of poorly shielded rigs, which allow RF to leak
straight in past the attenuator.

A simple way to de-sensitize a handheld rig for close-in RDF purposes is
to lower the whole rig (antenna and all) into a metal pipe, on the end
of a piece of string. The further you lower the rig inside the pipe, the
less sensitive it becomes. It may look crude, but this is Reg's piston
attenuator in action!

This system has no directional properties, but at short range you can
often "DF" on signal strength alone.



--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek