Ham Radio Fox Hunt Attenuator [FAIL]

 

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One of the many aspects of the ham radio hobby is finding a hidden transmitter; also called a fox hunt.  These are often set up by local clubs as a fun social activity, but they can also be competitive athletic events.  We had done a few fox hunts for fun, but were unsuccessful in finding the transmitter.  What we learned is when you get close to a transmitter (regardless of the power it is outputting) the signal becomes so strong there is no way to reliably get the direction information needed to track it down.   It's basically like a bright light blinding you and sunglasses are needed to reduce the input getting into your eyes.  To reduce the radio frequency (RF) signals picked up by your receiver (from the fox) an attenuator is used. 

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We saw this HMC472 Digital RF Attenuator Module on sale for $10US and thought it would be a cheap and simple project to solve our fox hunt problems.  We were wrong.....

It's not the module's fault; it works as spec'd.  Here are our measurement results from the bench:

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Wow, ten bucks buys a lot!   So....   what's the issue?  Well, after 3D printing a box for the rig we took it out into the field for a test drive and learned that 32dB attenuation is not nearly enough.   You need at least 100dB from what we later discovered.   Still, the final product came out nice, it did work as advertised, and things were learned.  We're gonna call that a win.  We could cascade a few modules to get more attenuation, but there are better solutions.  Here are some details on the simple build:

 

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XR2206 Function Signal Generator DIY Kit

   

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We have seen this signal generator come up as "suggested" in our Amazon account from time to time.  Finally, the Amazon marketing engine combined with COVID cabin fever persuaded us to shell out the $10US and give the kit a try.  

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$10 gets you a lot and not so much both at the same time.   First off, the directions are not very well written, but the kit is simply enough that they don't need to be.  We did have to ohm out the PCB just to verify that the NEG terminal for the electrolytic caps went into the "hashed" marked hole.   After about 20 minutes the kit was ready test out.  The results.....

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We took measurements with an oscilloscope for each setting.   Instead of putting up a bunch of boring sin, triangle, and square wave pics we will just state that the rig works as expected.   ONE CAVEAT:  THIS ONLY SEEMS TO BE TRUE WHEN YOU POWER THE RIG WITH 9VDC.  Also, adjusting the "Amp" knob can really effect the quality of the output signal.  There is a sweet spot on the "Amp" setting, but without a scope I'm not sure how you could find it.   Another odd behavior; a square wave will produce an almost 9V output.  The sin and triangle waves max out at about half that. 

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Here are our findings:

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So..... if you need a low end sig gen this kit does work.  It's cheap to buy.  It's easy and fun to build up.  But, be aware that without a scope you will not have a way to know the exact output frequency and output voltage or if the output signal is being clipped or otherwise deformed.  

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