3dB attenuator?

Discussion in 'General Technical Questions and Answers' started by Guest, Apr 9, 2004.

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  1. Guest

    Guest Guest

    I have an amp that calls for no more than 10W of drive.  I have a rig that puts out up to 25W.  How can I build or, where can I purchase a 3dB attenuator for this application.  If you can give me advice on building, it can be a purely resistive circuit, right?
  2. K7FE

    K7FE QRZ Lifetime Member #1 Life Member Platinum Subscriber QRZ Page

    1. Turn your mike gain down so that your xcvr output does not exceed ten watts, (if SSB).  
    2.  If FM or AM modes are desired, then you are right and need an attenuator that is resistive.  A 3db splitter with one side going to a dummy load will work with your amp on the other side. (A Wilkinson combiner/splitter would work as a 3db splitter and can be made with two 1/4 wave length sections of coax in a "Y" configuration and a 100 ohm resistor across the top of the Y)
    3.  Many xcvrs have an adjustment for output power.
    4.  Attenuators can be designed in many ways such as resistors in a "T" configuration or an "PI" configuration.  The ARRL handbook has info on how to determine the resistor values.  You will need to parallel a number of carbon resistors so that they will be able to handle the power level desired.  Heat sinking them will increase the wattage capacity of the resistors.

    Try this site for resistive attenuator calculations:


    5.  you could also use "PIN" diodes to make a variable attenuator and adjust your xcvr output precisely.  Most diodes will perform somewhat as a pin diode.  Ordinary power rectifiers can be used at HF quite effectively in an RF attenuator or SS relay circuit and will handle considerable power.

    A 3db pad will reduce your 25 watt radio to 12.5 watts output, so if 10 watts is the max desired, then design a 3.5 db pad. Also consider that your radio may put out more than 25 watts and need an even greater attenuation. Measure it before connecting to your amp.

    Terry, K7FE
  3. AG3Y

    AG3Y Guest

    Before everybody goes offering all sorts of advice on splitters, attenuators, etc. we should find out more specific information on the type of setup he is attempting.

    Please let us know if this is AM,FM,SSB,CW ?

    Is it HF,VHF/UHF, Microwave ?

    Normally an attenuator would be designed to present a non-reactive load between the rig and the amplifier, but depending on the band you are attempting to work, would be as simple as a resistive pad design, or as complicated as a directional coupler . Let us know a bit more about what you are attempting to do.

    TNX and 73 from Jim AG3Y
  4. Guest

    Guest Guest

    I found a website that described in detail the formulas for making a "Pi" and "T" attenuators. Sounds like a fun project for me. The only problem is finding resistors that are rated for at least 8 watts. Guess I can alwayse just solder a bunch in a series/parrallel configuration. There are plenty of higher power rated wire wound resistors (which can't be used) but power resistors with low inductance are rare. OHMITE does make some heat-sinkable TO-220 type, but they are spendy.

    Can I use heat sink paste on the barrel (round?) type carbon resistors to aid in heat conduction to the sink, or will there be some adverse effects?
  5. Guest

    Guest Guest

    Also. . .
    I'll mostly be using for SSB on HF, but it is a mobile installation and I want to avoid accidently applying more than ten watts to the input. I'll probably be operating the gain about 3/4 of the way toward full. With the additional voltage applied (about 14.4V) while the motor is running I figured I need more like a 3.9 dB attenuator. Yeah, the simple solution is to just ease off on the gain, but that don't account for human error. . ya know what I mean?
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Why not ditch the attenuator idea and add an ALC detector and output port to the amplifier? That doesn't take much circuitry, and it's all real low-power stuff. If your transmitter/transceiver has an ALC input, an adjustable ALC in the amp would take care of everything.

    You don't need 8W resistors to build a 3dB 25W pad. You'll be losing 12.5W in a 3dB pad. The simplest pad is an L-pad (as opposed to T or pi), which will work perfectly in your application since it needn't be bi-directional: You only need 3dB looking in one direction.

    Heatsinking the molded bodies of carbon or metal film resistors doesn't help them much, since the bodies aren't the greatest thermal conductors and a lot of the heat escapes via the leads, except in surface-mount components.

  7. W4CGP

    W4CGP Ham Member QRZ Page

    The ALC is a good idea if you can do it; it keeps the amp finals from being damaged from being driven too hot, from what I understand.
  8. Guest

    Guest Guest

    WB2WIK & W4CGP-
    The L network sounds simple. Say like a 50 ohm in series with the load, then both those in parallel with a 100 ohm. That should still keep the input Z seeing 50 ohms. I need to research though to figure out how to get the -dB that I need. Can you help?

    The level controll thing sounds good too, but seems like it might be a bunch more complicated. I'll do some research on that also.

    I wish there was some company out there that sold carbon resistive material that couild be tuned to the correct resistance by simply grinding away at the material. Maybe include a high resistance backing for ease of mounting to a heat sink, along with solder points.
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page


    -An ALC detector with adjustable output so you can make it compatible with your exciter isn't "complicated." It's a diode detector and filter followed by a variable voltage divider made of resistors. Including an adjustable resistor like a small pot, the circuit might use six or seven components, none of which cost more than ten cents except possibly the pot, which might cost $2.

    -If the input impedance of your amplifier is about 50 Ohms (as hopefully it is), all you need to do to "swamp" it with 3dB loss is to add 50 Ohms in series with the input of the amp. That takes just a 50 Ohm resistance wired in series between the amplifier's T/R relay and input tuned circuit. That resistance will dissipate 12.5W peak if you're running 25W PEP output power from your exciter. Since we're talking "HF" here (below 30 MHz), this is not a critical application at all. I'd just use six 300 Ohm, 2W carbon resistors in parallel to make a 12W 50 Ohm resistor. Granted, these take up space, so hopefully that space is available inside your amplifier.

  10. Guest

    Guest Guest

    Doesn't seem that hard. After all the good advice and some research, I think I'll change my project. Thanks ya'll.
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