Passive Diplexer Makes One Antenna Act Like Two

Stay in the amateur radio hobby long enough and you might end up with quite a collection of antennas. With privileges that almost extend from DC to daylight, one antenna will rarely do everything, and pretty soon your roof starts to get hard to see through the forest of antennas. It may be hell on curb appeal, but what’s a ham to do?

One answer could be making one antenna do the work of two, as [Guido] did with this diplexer for dual APRS setups. Automatic Packet Reporting System is a packet radio system used by hams to transmit telemetry and other low-bandwidth digital data. It’s most closely associated with the 2-meter ham band, but [Guido] has both 2-meter (144.8-MHz) and 70-cm LoRa (433.775-MHz) APRS IGates, or Internet gateway receivers. His goal was to use a single broadband discone antenna for both APRS receivers, and this would require sorting the proper signals from the antenna to the proper receiver with a diplexer.

Note that [Guido] refers to his design as a “duplexer,” which is a device to isolate and protect a receiver from a transmitter when they share the same antenna — very similar to a diplexer but different. His diplexer is basically a pair of filters in parallel — a high-pass filter tuned to just below the 70-cm band, and a low-pass filter tuned just above the top of the 2-m band. The filters were designed using a handy online tool and simulated in LTSpice, and then constructed in classic “ugly” style. The diplexer is all-passive and uses air-core inductors, all hand-wound and tweaked by adjusting the spacing of the turns.

[Guido]’s diplexer performs quite well — only a fraction of a dB of insertion loss, but 45 to 50 dB attenuation of unwanted frequencies — pretty impressive for a box full of caps and coils. We love these quick and dirty tactical builds, and it’s always a treat to see RF wizardry in action.

24 thoughts on “Passive Diplexer Makes One Antenna Act Like Two

  1. What I like about this is that it might keep “older” modes alive, such as the original APRS via Packet-Radio.
    Because if hams have to choose, they’ll likely fall for the latest hype and go for something such as LoRa.
    I experience this in my home town nearly every day.

    People on local repeater are interested in the latest tech (Win10, FT8, DMR/C4FM), no matter if the previous one has certain advantages sometimes, still.
    So I’m really grateful for things like this.
    It’s always good if certain things can co-exist, so diversity is being preserved. 😎👍

      1. Way to twist words.
        It’s not about avoiding new things, it’s avoiding ‘We need to make a new, all encompassing standard’ and then just end up with N+1 standards to choose from, and that you have to learn to recognize by sound so you can actually download the appropriate software for what you’re trying to receive etc etc.

      2. My bad, I wasn’t being clear enough. It’s not about new/old but rather about immature/mature and/or low-quality/high-quality.

        Packet-Radio for example, was built upon absolute reliability, just like early internet protocols.

        That’s why a bit-error is such an issue for PR.
        Normally, a defect packet would be discarded and then same information would be freshly requested from the sender again.

        That works in a two-way communication, but not in a broadcast.

        Because APRS uses UI frames, there’s no connection. Broken packets are left as is.

        That’s why classic APRS seems so inferior to LoRa APRS, which has less lost packets due to the medium.

        Anyway, there had been progress, but it’s not widely known.
        FX.25 adds error-correction information, FEC, to compensate for bit-errors. While being completely backwards compatible to normal PR hardware.

        Soundmodem and Direwolf can support it, for example.

        But again, the majority falls for the LoRa hype and doesn’t seem care about improvements of classic APRS.

        Probably because the average ham has become so indifferent that he/she doesn’t care about underlying technology.

        Who on your average repeater can explain how Packet-Radio and LoRa works, if only in principle ?

        So yeah, please spare me the “living in the past” line.
        It’s not about new or old, but about nature of certain technology.

        However, generally speaking, older technology was more being thought through than modern technology.

        People weren’t constantly consuming information, but had times in which they were all alone with their own thoughts.

        In addition, most things had to be built by hand. Either completely or by using construction kits.
        This was different to simply stacking ready-made modules.

        By interacting directly with the technology, people, hams, had learnt many things they used in their later life.

        That’s also applicable to computer programming and development of TNCs.
        Hams kept improving their own modes, created their own equipment and installed quality filters, took care of proper shielding etc.

        This also reflected in commercial ham products, which had to met the same standards.

        That’s why, say, a “Standardctwo-way radio (walkie-talkie) in a metal chassis was rock-solid.

        Hams still had standards (pun intended) and weren’t satisfied by a cheap plastic radio (Boafeng, Wouxoun, Shing Shang etc).
        They still had a comparison back then.

        Alas, this has changed. The oldtimers also forgot a lot or don’t care anymore.
        Personally, I’ve seen many hams who threw away high quality radios in favor of the latest tech
        Despite this tech of low-production quality and missing filtering (cheap plastic radios).

        I often wonders how they ended up betraying themselves or rather, their former younger selves.
        I assume they do it, because they’re in some sort of midlife-crisis and want to feel young again.
        That’s why they throw out older high-quality equipment in a metal chassis.

        But if it was just that. The competence also has decreased.
        I know, that’s an old story and every generation blames another one.
        But really, the knowledge has gotten somewhat superficial since the 1990s or so.

        The decline of *real* homebrewing (QRO transceivers, filters, horn antennae for microwave technology, laser links, building sophisticated PC interface devices or antenna rotors from ground up, without relying on an Arduino and pre-made software etc).

        To give an example: Many of the modern hams (any age) or wannabe hams (any age) don’t know about the technology on the bit-level anymore,
        which frequency pairs are being used (in case of FSK),
        what NRZ/NRZI or Manchester encoding is, how FM works, what emphasis/de-emphasis is and how FM differentiates from PM (phase modulation).

        Heck, they cannot even read a oscillogram on an oscilloscope anymore.
        Or use a standard dip-meter to measure resonance of oscillating circuits.

        These are all things a ham should know, out of his/her own interest without being forced.
        The love for technology and experimentation alone should be a drive.

        A ham shouldn’t be a consumer, but a maker.

        Obtaining, for example, a Technician class license and a LoRa module should be a start, not the end of the journey.

        And having outdated knowledge about something is better than not trying to gain knowledge, at all.
        Outdated knowledge allowes people to derive principles, at least.

        But that being said, I’m just a CBer. What do I know. vy73s

        1. My apologies for this long comment. Maybe I overdid.
          I’ve just meant to explain my point of view in detail, to avoid misunderstandings.
          I didn’t mean to hijack this comment section, either.

          The diplexer being shown is nicely, built, also.
          It’s using a metal chassis, as it had been used back in the day for such projects. Kudos to the creator, there’s nothing to complain I think.

          1. I accept your clarification, and agree with much of it.
            (Although I may as guilty of what you wrote as any ham
            -no excuses)

  2. I have tried to mount something ugly style, and I never got the solder to stick to the box, so it has a bent wire held in contact by is own spring force and a prayer to the electromecanical gods.

    1. Usually it’s a pair of low-pass and high-pass filters, this is easier to make and the isolation between both the low and high band can be optimized so that transmitting on one band won’t damage the receiver on the other band.

  3. “tweaked by adjusting the spacing of the turns”

    This was my first job after leaving the US Air Force.

    I got a job at Motorola, adjusting the output filters of 25 watt mobile two-way radios. You had to watch not only the stop band (point of maximum attenuation as well as maximum ripple,) but also the pass band. The transmitter itself wasn’t linear – it had more or less amplification at different frequencies. You had to adjust the ripple in the pass band to level out the transmit power across the entire band. If you did it properly, you’d have the rated power +- 1 watt across the entire band. If you didn’t, you’d have +- 8 watts (from 17 to 33 watts from a 25 watt radio.)

    My job was to twist and twiddle the coils in the output filter. Once they were properly adjusted and passed the power output tests, the coils were potted with epoxy so they’d keep their shape.

  4. The diplexer is one of the parts that the Japanese military could not make by 1945. (The Nazis could not have made it either.)

    Even if they had seen the captured items, they would not have understood the importance of the part and therefore would not have imitated it.

    The Japanese did not know the importance of the Smith chart (there was a similar chart invented by a Japanese mamed MIZUHASHI), so I think it would have been extremely difficult to design this part.

    If this part had been made, Japan would have been able to lose in a better way (maybe even better).

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