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!

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

"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

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!

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

"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