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Variable Attenuator
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 |
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 |
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 |
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