The Simplest Microwave Receiver

We are used to microwave receivers requiring complex chipsets and exacting PCB layouts, but as [CHZ-soft] has shown, it does not always have to be that way. With nothing more complex than a germanium point-contact diode and an oscilloscope, you can quickly, easily, and cheaply resolve microwave signals, as we are shown with a 2.4GHz wireless mouse.

Of course, there’s nothing new here, what we’re being shown is the very simplest incarnation of a crystal set. It’s a wideband device, with only the length of the wires providing any sort of resonance, but surprisingly with the addition of a very selective cavity resonator it can be turned into a useful receiver. Perhaps the most interesting take-away is that the germanium point-contact diode — once a ubiquitous component — has almost entirely disappeared. In most applications it has been supplanted by the Schottky diode, but even those usually don’t quite possess the speed in the point contact’s home ground of radio detection. This is a shame, because there are still some bench-level projects for which they are rather useful.

So if you have a point contact diode and AM radio doesn’t attract, give it a go as a microwave detector. And if the point contact diode has attracted your interest then you may want to read our piece on Rufus Turner, who brought us its archetype, the 1N34A.

Via Hacker News.

31 thoughts on “The Simplest Microwave Receiver

  1. There’s a book, I think “The Invention that changed the World” about radar development during WWII, and the impact on electronics afterwards.

    They needed to receive ever higher frequencies, but tubes weren’t up to the task. So they went back to cat’s whiskers, and the result was solid state diodes like the 1N21 that were great as mixers in UHF and higher receivers.

    The 1N34 seems to extend from that, and of course did hit the hobby worled before transistors. It’s not clear if the transistor was based on those WWII radar diodes. The path doesn’t well mapped.

      1. * NTE Electronics 1N34A Point Contact Germanium Diode (Pack of 10) $8.05 USD

        https://www.amazon.com/NTE-Electronics-1N34A-Contact-Germanium/dp/B008UG12XK

        There are cheaper sellers on Amazon, but NTE is a known brand (provided the NTE diodes aren’t fake too). Here is the NTE datasheet for download:

        https://www.nteinc.com/specs/original/1N34A.pdf

        From NTE: “1N34A Point Contact Germanium Diode Description: The 1N34A is a point contact diode in a DO7 type package employing N−form Germanium and gives an efficient and excellent linearity when used in TV image detection, FM detection, radio AM detection…”

      1. Schottky diodes are fast and they have a low barrier potential, so in that regard they’re similar to 1N34 diodes. I think the spec to look at is junction capacitance. The 1N34A from NTE is spec’d at 1pF at -1V. It’s hard to get an exact read from the graph ST Micro provides for the 1N5711, but it seems to be about 1.5 pF, so I think it would perform similarly. But the 1N34A looks far sexier… ;-)

    1. Even 20 years ago I went through some older boards and uncovered germanium diodes. I was surprised to find them on boards that dated from a more silicon era. They were distinctive, larger than the average small signal diode, and seemed more prone to not be painted. FM radio discriminators seemed to offer up germanium diodes especially.

      Analog UHF TV tuners I thought generally used germanium diodes in their mixers. 1N82s seemed to be a common number there.

      With a DMM it’s easy to find them. There is distinctive grouping between silicon, schottky and germanium diodes.use

      For hobby puposes I don’t think there’s anything special about the “1N34”. Obviously it came early, but there were lots of other small signal germanium diodes. But it became a familiar part number, in effect code for “small signal germanium diode”. Companies that sold to hobbyists would stock them because they were used in articles, and their availability reinforced their use. I suspect you may nkt have always gotten a 1N34, certainly as germanium diodesgotkess common. Germanium was they key point rather than anything else.

        1. I think it only notices about 20, because ppl are always randomly hitting it trying to scroll.

          Though last week had something weird going on, comment threads disappearing and reappearing and posts going into moderation review hours after posting. It seemed like someone was having bots hammer the report buttons.

  2. FYI:
    “RF and microwave power detection with Schottky diodes”
    https://www.infineon.com/dgdl/Infineon-AN_1807_PL32_1808_132434_RF%20and%20microwave%20power%20detection%20-AN-v01_00-EN.pdf?fileId=5546d46265f064ff0166440727be1055

    >Various Infineon low-barrier Schottky diodes are used, namely BAT15-02EL, BAT62-02V,
    BAT63-02V for single diode detector structure and BAT15-04W for double diode detector structure

    They show detection results for 2.4GHz and 5.5GHz

    1. The Schottky diode needs a small (200mW) DC bias to operate at low AC levels, so it seems that the biggest advantage of the point contact diode is that you can operate it in a few niche applications where you don’t want to use external power.

      1. I’ve used BAT15s and BAT63s for crystal radios (AM frequencies) without biasing and they are both louder than 1n34as and 1n270s. Haven’t tried them at GHz frequencies though.

        From quiet to loud: 1n34a 1n270 bat15 bat63

        No other schottky diodes tested were suitable for a crystal radio than those two. The only downside I could see of the Schottkies is that they only have a few volts reverse voltage handling so they will distort in very strong signals (maybe next to a tower).

          1. You’re not likely to get more than “very low current” in a crystal radio. I can’t think of a hobby use for a germanium diode other than crystal radiis.

            Which reminds me. Germanium diodes have high reverse bias leakage, so sometimes they are in circuits that count on that. A schottky diode may need a resistor to compensate.

      2. Most Schottky diodes are n-type silicon. However, if you don’t have a DC bias, then choose a Schottky built on p-type silicon (ref. B. L. Smith and E. H. Rhoderick, “Schottky barriers on p-type silicon,” Solid-State Electronics, vol. 14, no. 1, pp. 71–75, Jan. 1971). The following link has a table that can guide you in choosing the correct Schottky for either biased or zero-bias application. http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/dt_d004.pdf

  3. I have no real knowledge of antennas. But I wonder if this principle could be used to create a high gain wifi antenna. Like mount it to a dish and receive signals with an Alfa-Card. Would that be worth a try? I am pretty sure I have some germanium diodes in one of my junkboxes. But never used them.

  4. Can I put this in my microwave oven to detect when it’s turned on????
    Just kidding, but otherwise, this definitely sheds new light on those dusty 1N34’s.
    I wonder if I’d get to see anything on my old 60Mhz analog scope?

    1. Don’t put metal in a microwave oven.

      A simple microwave oven leakage detector is a diode with full leads across a sensitive meter. Solder the diode across the meter’s terminals and leave the leads, they act as a dipole antenna.

      I have no idea how well it works, it depends on how much the oven leaks.

      1. Don’t put metal in a microwave if you don’t know what you’re doing.

        There’s no problem keeping a metal spoon in a bowl of soup, for instance, as long as you keep it away from the walls.

    2. Older MW ovens have two visible “anti-leakage” features. Some have a perforated metal plate over the window. The dimension of these holes looks like a “short” to the RF – i.e., a solid plate. The other, which you see on most ovens is a “groove” around the window on the inside of the door: a choke flange. A choke flange is a quarter-wave resonant short-circuit stub, and has a high (ideally infinite) input impedance at its mouth. This helps to disallow MW leakage.

  5. the 1n34 was in everything in my youth, one of my first projects had 4 of them which I bought at one of the local electronics shops ( another bit of history gone) later I took to salvaging them or buying in bulk grab bags from Poly Packs. I also enjoyed the side bar piece on Rufus Turner. I still have one of his books I bought as a youth.

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