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Forum Index : Microcontroller and PC projects : Need a bug-finder...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9897 |
Hello all.  The system failure I had the other week, where a transmitter module was being held on 100% duty cycle with dead-air, has revealed the need for me to have a tool, to help me track this kind of fault if it ever happens again. The guy from Radio Spectrum Management who came out to help me, had a rather exotic(read EXPENSIVE) HP branded signal tracing thing, but they cost the best part of ten grand, and I really don't want to spend THAT much. I've had various thoughts, including using the Radiometrix receiver module itself as the basis, by employing it's RSSI pin and a MM2 and an LED bar-graph. Then I started thinking about bug-finders on ebay or AE, but they seem to be hit and miss, and reviews tend to be more negative then positive on the couple I liked the look of. Then I started looking at cheaper hand-held spectrum analysers or SDR's. You can get some quite impressive looking spectrum analysers on AE etc these days, for only a few hundred bucks, but I again, question their accuracy and reliability etc - you get what you pay for, if you know what I mean. Does anyone have any other suggestions they would like to throw into the mix? Assume the following fault condition: A Radiometrix UHF transmitter module on 100% duty-cycle @ 434.650MHz, 25mW output power, dead-air carrier. I would want to walk around, and track down where it was. In my initial testing - before I called in the guy from Radio Spectrum Manangement to help me - I just used a scanner radio, set to that frequency, and drove around, till I could localize the dead-air carrier to within a few hundred meters of the source of the problem. Basically, dead-air silence means strong RF signal, any kind of white-noise radio hiss means I was getting further away from the source. Once localized, I then just needed the Radio Inspector guy with his fancy equipment, to pin-point where the signal was coming from, so I could disable it. In years gone by, people used to have a game similiar to this, where someone would purposely setup a bug transmitter like that, and to win the game, you had to be the first one to find it and shut it off.  My objective is the same, but I don't refer to this kind of fault as a game.  Edited 2026-03-06 16:21 by Grogster Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6487 |
A receiver with a signal strength meter and a directional antenna such as a high gain yagi. An adjustable gain control can also help. Jim VK7JH MMedit |
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| ville56 Guru Joined: 08/06/2022 Location: AustriaPosts: 426 |
Have a look at TinySA, affordable and accurate. I possess one. You may also need a directional antenna (yagi) and an attenuator to avoid overloading. Only downside is, the inputs are not protected against heavy overloading so you should always know what you are doing. But the device is unbeatable for its price. Not as sensitive and accurate as HP but doing very well. And beware of the chinese clones. There is a list of authorized dealers on the website www.tinysa.org. Btw, the community is very active and helpful. Edited 2026-03-06 15:51 by ville56 73 de OE1HGA, Gerald |
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panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1118 |
Hi G, I have the small version of this SA tinySA and it works a treat. I got mine from aliExpress. I used it to track a weather station output. panky ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 3054 |
When you get close that may be important, as the signal gets stronger the Yagi may become less directional. A couple of co-ax attenuators may do the job, perhaps 12dB and 24dB. When the signal strength goes up plug the 12dB one between the scanner and the Yagi. Then the 24 and finally both. Even just a basic whip may work with the attenuators, though it would take longer as you hunt back and forth. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9897 |
Thanks a bunch, chums.  I have ordered a TinySA.  My thread also had it's title truncated to "I need".....which is odd, but I do recall other members also saying the forum software altered THEIR thread titles. I have edited my OP, to put the correct title back, but I appreciate that this is something most members don't have the power to do. Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9897 |
Yes, I have ordered a directional high-gain antenna for the band I want to hunt in. LINK... 430-440MHz directional Yagi - right in the sweet spot I need, and with 10dB gain. @ phil99: Good point on the attenuators. I might get some, but the TinySA seems to incorporate them as part of its design, so perhaps not required? Edited 2026-03-06 16:19 by Grogster Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9897 |
Thread title has been again edited by the forum software. It seems if you include anything inside double-quotes, the forum software deletes that. I will contact Gizmo. Smoke makes things work. When the smoke gets out, it stops! |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 3054 |
Attenuators are cheap and combined with the Yagi and your scanner may be all you need. |
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| ville56 Guru Joined: 08/06/2022 Location: AustriaPosts: 426 |
Yes and no. YES, The TinySA has a attenuator BUT NO, the input is not protected. So you are better off to put an attenuator with, say, 20dB in front of the TinySA and use the internal LNA to get 0dB attenuation over all, if you need that sensitivity at all for your task. The 20dB in front allow you to have a 20dB stronger signal on the input before you blow out the frontend. It's just to be ont safer side. 73 de OE1HGA, Gerald |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 8642 |
I've never tried it, but a simple DF loop, a receiver with a signal strength meter and a map might be all that's needed. Pick a location. Swing the loop for minimum strength. Record the direction - there will be nulls in both directions. Put a line on the map. Go somewhere else and do the same thing. The lines will cross at the transmitter point. A third location will give you a check on the other two. Looking for a null is more accurate than looking for maximum strength, particularly at frequencies where signals can bounce. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5746 |
For the hobbyist on this forum... A yagi antenna is a nice tool to find direction, but there is a simpler way to locate a source. This method is used professionally as well. Take 2 antenna's (standard whip) and mount these at 1/2 lambda distance. Use below circuit to connect the 2 to one radio receiver. Put the radio receiver on the desired frequency and FM demodulation. When the source is perpendicular to the axis on the 2 antenna's, there is no audio tone from the radio. When it is in line with the axis on the 2 antenna's the sound from the radio is loud. The system has makes no difference between forward, and backward attenuation, so you always need 2 measurements to do a cross-pole. The "dip" in sound when perpendicular is very sharp, so your cross-pole measurement is very accurate. This works better than a yagi. It is used a lot in amateur radio "fox hunting". But you need 2 (whip) antenna's and a FM radio (i.e. scanner) for the desired frequency.  Look here... https://www.handi-finder.com/ This is the professional application.. (i.e. Rohde&Schwarz). You do not need to "rotate" the antenna. It is fixed.  Example of use of such a system at airports.  Volhout Edited 2026-03-06 19:57 by Volhout PicomiteVGA PETSCII ROBOTS |
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| ville56 Guru Joined: 08/06/2022 Location: AustriaPosts: 426 |
I would say this is the good old Adcock locator, just no moving parts. You get a phase shift whenever the antenna position is switched to/from the signal source. This phase shift gives the direction relative to the antenna square. Or am I completely wrong? 73 de OE1HGA, Gerald |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5746 |
Yes, the phase shift of the RF between 2 antenna pairs (or antenna's if 2) is demodulated in the FM demod of the radio. And as mentioned in the handi-finder data: it does not work with "noise", only with carriers. Volhout PicomiteVGA PETSCII ROBOTS |
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| ve2yag Newbie  Joined: 07/02/2026 Location: CanadaPosts: 2 |
Hello, I play with Lora module since few year, with ESP32 and C. Now I wrote for the fun a driver in Basic using Picomite. I use SX1278 or SX1262 module in the 433MHz range. If I want to track a dead carrier of a specific frequency, I use these Lora module, tune them to frequency you want to monitor and use special register to reas current RSSI. I use them to read background noise floor to select antenna location far from computer interference. Great noise floor around -102dbm. Maybe you can use these reading to put a trigger, by example if current RSSI do not drop under -30dbm for a minute, trig a error. This can be written on a Picomite or any microcontroller using Lora modude. I don't use any library on the net, I wrote my own. Remi VE2YAG |
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| ve2yag Newbie Joined: 07/02/2026 Location: CanadaPosts: 2 |
Just additional note, Currently I made a small basic program with Lora driver, to receive and send Lora APRS packet(ham radio thing). I can add RSSI reading function too if this can be usefull. I compress my Lora driver written in C into a just few basic line, talking to module using SPI port. Remi |
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