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

I've as often seen the shields between attenuator sections, instead of
in the middle of a section. I can assure you, there is plenty of
coupling across the switch itself where you can't place a shield, and
the coupling is both inductive and capacitive. Maybe use two separate
SPDT switches with a shield between them and mechanically ganged?

It's instructive to take apart a good microwave relay to see how they
manage high isolation and constant impedance, but check the price of
such relays before you destroy one to just have a look at it. (e.g.,
Digikey 255-1579) With such relays, SMT resistors and careful board
layout, you can make a very decent step attenuator up through UHF at
least.

Cheers,
Tom

In article . com,
K7ITM wrote:

It's instructive to take apart a good microwave relay to see how they
manage high isolation and constant impedance, but check the price of
such relays before you destroy one to just have a look at it. (e.g.,
Digikey 255-1579)

#CHOKE#

With such relays, SMT resistors and careful board
layout, you can make a very decent step attenuator up through UHF at
least.

"... and at the price you charge for drinks, I can see why!"

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"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

Gary:

Building is an option, but you can also find some nice commercial units
on eBay which may have simply failed a calibration check. There are
several brands of attenuators out there retired from shops and labs
regularly.

I purchased a few HP 355D VHF attenuators, which are 0 - 120 dB step
attenuators made for 50-ohms from 0 to 1000MHz with a rotary switch.
A couple were just out of calibration (not a problem for my application),
and the cheaper one had some problems with individual steps that could be
repairable. I think I've paid $40 to $90 each for these last year.

The cheapest I found were Kay Electronics 0 - 100 dB attenuators with
nine sections switched by toggle switches. A couple on eBay are about to
sell for less than $20 as I type this!

Jeff

In article ,
Ian White G/GM3SEK wrote:

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!

Yup. Pringles cans are often used for this. I made a nice
heavier-duty version by recycling the lower half of a water-filter
cartridge - light aluminum, easily hacksawed to the desired length.

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

You can arrange some amount of crude directionality by holding the
attenuator-tube-with-HT-in-it in front of your torso, right next to
your chest, and then spinning around in place. This "body fade"
technique can give you a reasonably directional null when you're
facing away from the signal source. If the null isn't sharp enough,
lower the HT further into the tube to increase the attenuation, and
try again.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

K7ITM wrote:
I've as often seen the shields between attenuator sections, instead of
in the middle of a section. I can assure you, there is plenty of
coupling across the switch itself where you can't place a shield, and
the coupling is both inductive and capacitive. Maybe use two separate
SPDT switches with a shield between them and mechanically ganged?

The attenuator that G4PMK published in 'The VHF/UHF DX Book' was based
on earlier ARRL publications. It used common low-cost slide switches,
and 1% 0.25W metal film resistors mounted directly on the switch tags
with the shortest possible lead lengths. There were screens between the
switches.

The following is a selection of results (which I didn't have time to
type out last night). The 'Theory' column is the attenuation expected
from using available resistor values, and the '1MHz' column shows the
actual DC/LF result. Then read along each line to see how the
attenuation changes with increasing frequency.

Best viewed with a fixed pitch font...

Nominal Theory 1MHz 30MHz 145MHz 432MHz
dB
1 0.98 0.98 0.97 1.00 1.27
3 3.02 2.88 2.81 2.92 3.45
10 10.08 10.06 10.10 10.09 10.56
20 19.9 20.0 20.0 19.9 19.7

That's really pretty good; in fact some of the apparent error may be due
to the network analyser that was used for the measurements.

It seems that the main effect is a small *rise* in attenuation with
frequency, probably caused by the increasing inductive reactance of the
resistors. There doesn't seem to be any problem with internal coupling
for these particular switches.

Your mileage will definitely vary. This is partly because of the switch
construction, though garden-variety slide switches are all pretty much
the same. Another source of variation is that metal-film resistors of
the same value coming from different manufacturers may have
significantly different values of series inductance. However, very small
1% metal-film resistors are the best wire-ended components you can get
for this application. (My article in RadCom a few months ago has more
information. Don't even think of using carbon composition!)


It's instructive to take apart a good microwave relay to see how they
manage high isolation and constant impedance, but check the price of
such relays before you destroy one to just have a look at it. (e.g.,
Digikey 255-1579) With such relays, SMT resistors and careful board
layout, you can make a very decent step attenuator up through UHF at
least.

You certainly can. Part of the solution seems to be to use a physically
separate SPDT switch at each end of the attenuator, rather than DPDT
type where the switches may be too close together. Some commercial step
attenuators use individual switch contacts operated by insulating
push-rods, and a long rotating cam-shaft selects the combinations
required.

Yet another option is to use complete individual attenuators, shielded
from one another inside a rotating 'turret' block.



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