Homemade Panadapter Brings Waterfall To Old Radio

Ham radio operators can be pretty selective about their gear. Some are old-school tube purists who would never think of touching a rig containing transistors, and others are perfectly happy with the small Software Defined Radio (SDR) hooked up to their PC. The vast majority, though, of us are somewhere in between — we appreciate the classic look and feel of vintage radios as well as the convenience of modern ones. Better yet, some of us even like to combine the two by adding a few modern bells and whistles to our favorite “boat anchor.”

[Scott Baker] is one such Ham. He’s only had his license for a few months now and has already jumped into some great projects, including adding a panadapter to an old Drake R-4B Receiver. What’s a panadapter, you may ask? As [Scott] explains in his excellent writeup and video, a panadapter is a circuit that grabs a wideband signal from a radio receiver that typically has a narrowband output. The idea is that rather than just listen to somebody’s 4kHz-wide transmission in the 40m band, you can listen to a huge swath of the spectrum, covering potentially hundreds of transmissions, all at the same time.

Well, you can’t actually listen to that many transmissions at once — that would be a garbed mess. What you can do with that ultrawide signal, however, is look at it. If you take an FFT of the signal to put it in the frequency domain (by using a spectrum analyzer, or in [Scott]’s case, an SDR), you can see all sorts of different signals up and down the spectrum. This makes it a heck of a lot easier to find something to listen to — rather than spinning the dial for hours, hoping to come across a transmission, you can just see where all of the interesting signals are.

This isn’t the first (or even the twentieth) time that [Scott]’s work has graced our pages, so make sure to check some of his other incredible projects in our archives!

15 thoughts on “Homemade Panadapter Brings Waterfall To Old Radio

  1. He used a HackRF transceiver which costs over $300 for the SDR, which seems overkill to me. Many cheap SDRs start at 25 MHz so they won’t reach down to IF frequencies. You can use an upconverter like the HamItUp board, or build your own upconverter which could be a simple crystal oscillator (at a random frequency which gets you into the range of the SDR) and couple of diodes for a mixer.

  2. I never comment on these hackaday articles but wanted to add to the comment above “Well, you can’t actually listen to that many transmissions at once”.

    You might not be able to listen in real time, but with most SDR software you can record the full waterfall and rewind to replay missed transmissions. An excellent upgrade to an old transceiver.

  3. There were panadaptors in WWII. Lots of construction articles since then, standalone receivers that tuned through a fixed segment of the radio spectrum. So mostly superhet receivers, the local oscillator tuned by a varactor, and a sweep oscillator tuning it one way, then the ramp drops and it starts again. The sweep generator feeding the horizontal input of a scope, the vertical fed from a detector at the IF output.

    Most were intended for use with an existing receiver, connecting to a mixer output. So mostly a small range above and below the signal frequency.

    Though someone went full hog in Ham Radio in 1972. They took a tv set, and used it as an oscilloscope. Then a superhet that tuned about 500KHz. And 5 converters for the five ham bands. So each band tuned in turn, the results multiplexed onto the “scope” so at a glance you could see what was happening on each band.

    A panadaptor is a limited range spectrum analyzer. But some actually built units that were actual spectrum analyzers, better specs and tuning a much bigger range.

    I fear that the focus on simple and older equipment is actually anti-technology, a fear if more recent advances. Hams. did build panadaptors and frequency synthesizers fifty years ago, no fear of ICs.

  4. Given that the receiver here is a vintage/antique Drake (though it is still younger than me), a swept IF would suffice for the pan adapter. There is no need for coherent FFT IF because the operator is hunting for slowly evolving single sideband voice, radio Teletype, or continuous wave Morse code signals due to the inherent design limitations of the receiver. Covering the few tens to few hundreds of KHz of amateur bands in even a leisurely 1/10 second or longer is plenty fast enough.

    Of course the signal conditioning described above is still necessary to get a decent level and appropriate filtered width IF out of the receiver. I would consider using a TinySA spectrum analyzer as the basis for the pan display. The TinySA is only about $50 US from R&L electronics and can tune down to 100 KHz without external help. The screen on the TinySA is, well, tiny. It probably wouldn’t be that much of a hack to pull out the electronics board and have it drive a more comfortably sized screen. China, Inc. probably already has a derivative of the TinySA with a larger screen on your favorite auction web site anyway. Using a $300 HackRF seems like overkill.

    TinySA info here: https://www.tinysa.org/wiki/

  5. This would be a good basis for radio astronomy especially in SETI . Scanning a wide band and looking for interesting signals such as fast burst events or a transmission from Vega.

    1. Vega? Why bother? Contact was fiction. Once the novelty of watching something from an alien planet wears off you realize that Vega is basically the planet of daytime TV. So boring! If you are going to go through the effort at least point your dish at the Trappist system. Now that’s content!

  6. A neat find to go with this Drake station would be an old Heathkit SB-620 Scanalyzer. That was Heath’s pan adapter back in the 1970s. The Scanalyzer used a small long persistence CRT with anode voltage derived from a Cockroft-Walton voltage multiplier, so no flyback noise. The IF could sweep either 10 or 50 KHz. The center frequency of the IF was chosen by the builder by selecting one of the coils supplied with the kit. Adapting a hamfest-find SB-620 to a Drake R-4B might take a small amount of work because Heath used 3.something MHz IF and the R-4B is 5.645 MHz. An SB-620 out of a Heath station probably would need a different IF coil from the original kit or to have the new owner wind a coil.

    You can find nice military surplus pan adapters, but check carefully. I have one that is late 1940s or maybe early 1950s designed for aircraft use. Something I remembered after I got my pan adapter home is that aircraft AC power is often 400 Hz to save weight because less iron is required for transformer cores and filter capacitors can be smaller too. Unfortunately there isn’t enough iron mass in the power transformer to run my pan adapter at 60 Hz. Modding the power supply is yet another project for one of these years…

    1. Aircraft? To be sure I can’t contradict but Navy ships run AC at 440 IIRC. Maybe I need coffee. I’d ask my dad, who was a flight engineer on the B29. He’d know about that. I miss him.

  7. He has a digital frequency counter on his rig.
    It correctly compensates for his IF offset and shows the correct tuned frequency.
    He’s a software guy.
    So I’m really disappointed he didn’t rig it so his panadapter display also shows the correct frequency on the display — it instead shows the IF frequency.
    HackRF probably even gives you an API to do exactly that compensation.

    Also amazed at all the images present in the spectrum: obvious when they go the ‘wrong’ way when he tunes.

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