Hi, i would like to build a variable attenuator for 450-500MHz. It will be
used for RDF assistance.
Could people please give me construction details of different designs or
different ideas on the topic.

I woould love an electronic RDF unit but havn't found many kits here in
Australia so an attenuator will be
a good start.

Thanks


Gary

On Tue, 9 Aug 2005 23:41:27 +1000, "Gary Smith"
wrote:

Hi, i would like to build a variable attenuator for 450-500MHz. It will be
used for RDF assistance.
Could people please give me construction details of different designs or
different ideas on the topic.

Hi Gary,

You don't need precision, so building your own is a good option. The
circuit would be a switched, cascaded pi-network of resistors also
known as a decade attenuator.

You need only three resistors and a DPDT switch for each section. The
traditional sections add 1, 2, 2, 5, 10, 20, 20, 50 dB of attenuation.
For RDF you can skip the first 3 or 4 sections.

73's
Richard Clark, KB7QHC

is dick smith electronics still in business?? i bought a couple rdf kits
from them many years ago, very reasonable price even with overseas shipping
and they worked nicely. there is also one on my web site at:
http://www.k1ttt.net/technote/doppler.html that is derived from their much
more complex 4 antenna version.

"Gary Smith" wrote in message
...
Hi, i would like to build a variable attenuator for 450-500MHz. It will be
used for RDF assistance.
Could people please give me construction details of different designs or
different ideas on the topic.

I woould love an electronic RDF unit but havn't found many kits here in
Australia so an attenuator will be
a good start.

Thanks


Gary

On Tue, 9 Aug 2005 16:51:36 -0000, "Dave" wrote:

is dick smith electronics still in business?? i bought a couple rdf kits

Yes, they were purchased by a grocery chain and have pretty much
abandonded amateur radio products, focussing more on consumer
electronics with electronic components available in the larger stores
(for the moment).

They also own Tandy in Oz, so Tandy stocks are near identical to Dick
Smith (less the electronic components).

Owen
--

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.

====================================

"Gary Smith" wrote in message
...
Hi, i would like to build a variable attenuator for 450-500MHz. It
will be
used for RDF assistance.
Could people please give me construction details of different
designs or
different ideas on the topic.

I woould love an electronic RDF unit but havn't found many kits here
in
Australia so an attenuator will be
a good start.

Thanks


Gary

Dear "Gary Smith" (no call sign):

Wow! I agree with Reg! Perhaps he has yet to start on evaluating this
evening's wine.

Waveguide-in-cutoff, with coupling at each end and means for moving at
least one end towards and away from the other end, will do. K7ITM's
comments are also important for success. 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"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.

====================================

"Gary Smith" wrote in message
...
Hi, i would like to build a variable attenuator for 450-500MHz. It
will be
used for RDF assistance.
Could people please give me construction details of different
designs or
different ideas on the topic.

I woould love an electronic RDF unit but havn't found many kits here
in
Australia so an attenuator will be
a good start.

Thanks


Gary

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

"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.

On Tue, 09 Aug 2005 21:48:00 GMT, "SignalFerret"
wrote:

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!

Hi Robert,

It is a "below wavelength cutoff attenuator." It is basically two
coupling loops put in a conductive tube the size of a toilet paper
roll. One loop is movable on the so-called "piston" (although the
resemblance ends there, no compression is expected). At the other end
of the "cylinder" is the other loop. As you draw away, or closer, the
attenuation is linearly variable to quite a high degree.

Of course, all other provisos still apply.

There are far more step attenuators than these, however amateurs like
doing simple things difficultly. If sealing the box of the stepped
attenuator was difficult, I can imagine the fun of circular seals.
Nothing precise nor accurate is required for RDF - but low leakage
does win at the end of the day.

73's
Richard Clark, KB7QHC

A very simple form of a piston attenuator for the OP's application is
nothing more than a metallic tube into which you can suspend the
receiver on a string. Adjust how far you drop the receiver into the
tube (held vertically) to adjust the attenuation. Seriously. You can
wrap a cardboard tube about three or four inches diameter with aluminum
foil to make the "meatllic tube." It's a trick that many locals here
have used effectively in hunting hidden transmitters.

A more usual form is a tube with a couple of coils in it, with a means
to adjust the separation of the coils. See the third paragraph of
http://www.measurement.gov.au/index....1D#attenuators
for an example.
http://ej.iop.org/links/q55/Y67w5gpi...17i12p1172.pdf is
an article about correction of small errors in (precison) piston
attenuators. This pdf has a cross-sectional diagram in it, but don't
get bogged down in the details.

Cheers,
Tom

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