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

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

One of the problems, if you want to use the attenuator to get close to
a strong transmitter, is that typical ham receivers aren't shielded all
that well, and you'll find that the attenuator is unable to give you as
much signal attenuation as you'd like. (Disconnect the antenna from the
receiver entirely, and the signal's still strong!) A way around that
is to build a mixer in a well shielded box, mixing the signal with some
fairly low frequency to offset it from the transmitted frequency. You
listen on the mixer output freq. For example, if you mix with 10MHz,
you'd listen to 455MHz at 465 (or 445). Then you have a knob to
control the efficiency of the mixer in some way, and you get an output
that can be attenuated smoothly. We used to use such things here, and
they seemed to work well for the small effort involved in putting them
together. I don't think I still have a schematic around, though.

If you build a step attenuator, be aware that at 500MHz, it's tough to
keep signals from leaking around your attenuator stages. Be sure to
keep leads extremely short. Use switches with low inductance...tiny
DPDT slide switches can be pretty good. And don't try to do more than
about 20dB in any one stage. Unless you have an exceptional receiver,
you'll probably find that you can't use more than about 80dB total
attenuation effectively, if that much.

Another useful tool if you're looking for a nearby well-hidden
transmitter is a field strength meter. It's possible to build one with
logarithmic response, so you can see 60dB or more range easily on a
meter scale. Some of the Analog Devices, or Linear Technology, or
National Semiconductor dB-linear RF detectors should work well. You
can detect inputs below a millivolt with them. Add a tuned circuit on
the RF input to keep other signals out. If you build the FSM so it's
well shielded and you can swap between say a quarter-wave whip and a
tiny stub for antenna, you can cover quite a range of field strengths.

Too bad you're not closer...I'd give you my old doppler RDF. But you
have some very good RDF teams in Australia, I know, and maybe some of
them could help you out with ideas and construction help if you need
it.

Cheers,
Tom

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
--

"K7ITM" wrote in message
ups.com...
One of the problems, if you want to use the attenuator to get close to
a strong transmitter, is that typical ham receivers aren't shielded all
that well, and you'll find that the attenuator is unable to give you as
much signal attenuation as you'd like. (Disconnect the antenna from the
receiver entirely, and the signal's still strong!) A way around that
is to build a mixer in a well shielded box, mixing the signal with some
fairly low frequency to offset it from the transmitted frequency. You
listen on the mixer output freq. For example, if you mix with 10MHz,
you'd listen to 455MHz at 465 (or 445). Then you have a knob to
control the efficiency of the mixer in some way, and you get an output
that can be attenuated smoothly. We used to use such things here, and
they seemed to work well for the small effort involved in putting them
together. I don't think I still have a schematic around, though.

If you build a step attenuator, be aware that at 500MHz, it's tough to
keep signals from leaking around your attenuator stages. Be sure to
keep leads extremely short. Use switches with low inductance...tiny
DPDT slide switches can be pretty good. And don't try to do more than
about 20dB in any one stage. Unless you have an exceptional receiver,
you'll probably find that you can't use more than about 80dB total
attenuation effectively, if that much.


Generally, when people build these, they put a shield down the middle of
each switch. Note that one of the resistors of each section and the bypass
strap have to pass through holes in the shield.

Tam


Another useful tool if you're looking for a nearby well-hidden
transmitter is a field strength meter. It's possible to build one with
logarithmic response, so you can see 60dB or more range easily on a
meter scale. Some of the Analog Devices, or Linear Technology, or
National Semiconductor dB-linear RF detectors should work well. You
can detect inputs below a millivolt with them. Add a tuned circuit on
the RF input to keep other signals out. If you build the FSM so it's
well shielded and you can swap between say a quarter-wave whip and a
tiny stub for antenna, you can cover quite a range of field strengths.

Too bad you're not closer...I'd give you my old doppler RDF. But you
have some very good RDF teams in Australia, I know, and maybe some of
them could help you out with ideas and construction help if you need
it.

Cheers,
Tom

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.

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