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Author Topic: "portable RF sniffer"  (Read 28343 times)
N0ZYC
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Posts: 20




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« on: March 24, 2013, 09:41:17 PM »

Has anyone tried this rf sniffer on ebay?  http://www.ebay.com/itm/111031819387   I bought a pile of cheap VCOs recently and will be playing with this when it arrives, to make a cheap audible sniffer. (schematic for anyone interested in the audible circuit, it's very easy to make, one easily obtained/subbed IC, four caps, three resistors, one pot)
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KB3QGV
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Posts: 4




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« Reply #1 on: May 15, 2013, 07:55:39 PM »

At the risk of showing my ignorance, what IS  a sniffer and what is it used for?
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WB6BYU
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Posts: 13174




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« Reply #2 on: May 16, 2013, 07:04:21 AM »

A "sniffer" is some sort of RF detector.  They range from a simple diode detector and
meter or headphones up to fairly complex ones with synthesized frequency and an
audio output that varies with the input signal strength (so you don't have to watch
the meter when adjusting something for maximum output.)

One common use is in transmitter hunting when one gets close to the transmitter.
If the desired signal is much stronger than anything else, a simple circuit will
allow you to home in on it by signal strength.
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N0ZYC
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Posts: 20




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« Reply #3 on: May 25, 2013, 08:03:34 AM »

At the risk of showing my ignorance, what IS  a sniffer and what is it used for?

Sniffers are needed because radios tend to be too sensitive to provide useful signal strength readings when you get very close to a transmitter.  For example, using a small 4 element handheld beam at 2m may result in full-scale reading in any direction because there's so much RF hitting the beam.  Even loops with very deep nulls can become useless when the RF is so strong that it's bleeding right in through the radio's case, and can continue to pin the meter even with no antenna or a dummy load attached.
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N6JSX
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Posts: 217




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« Reply #4 on: June 21, 2014, 04:34:05 PM »

Have you ever considered using a dual-band (2m/70cm) HT and going to 439.695 for the 3rd harmonic (of 146.565), I've seen as much as 45db attenuation)? Then +/- the lowest FM step (on 440) to go off harmonic to add attenuation. Even using a 2m tape-yagi it works.

ThiMk simplicity....
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N0ZYC
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Posts: 20




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« Reply #5 on: June 21, 2014, 07:37:42 PM »

I actually have been throwing together offset attenuators recently also.  Just get a cheap 2 or 4 mhz oscillator module. Get a small box (plastic even, radio shack's smallest plastic one works well) and a few bnc bulkheads, and a bnc m-m coupler.  Run the bnc straight through the box.  Use a 50k variable resistor to control the amount of the 2/4mhz you are adding into the line as it passes through.

Then tune your receiver to +/- 2/4 mhz and follow that.  4mhz seems to work a little better, but some HTs have problems getting 4mhz away.

You can achieve at least 70db of attenuation with this setup. Unfortunately, those oscillators create a lot of power, (5v p-p typical) and it's not needed.  It'll bleed around and through the pot and air-mix even when you have the mixing resistor turned all the way down.  If you need more attenuation, you'll need to put the oscillator in a shielded area and have a carefully shielded way to switch in ~50db of attenuation on the 2/4mhz before it leaves the shielded area to go feed the pot.
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