[NLRS] Thoughts on VHF-UHF beacon hardware.

Fri Jul 17 11:08:22 EDT 2015

I thought this project was interesting,

https://github.com/JamesP6000/WsprryPi

where a Raspberry Pi is used as a WSPR beacon and the Pi itself generates
the RF on a GPIO pin.  That part did not impress me but I see guys
building filters to put after it to clean up such mess.  But the code
would give the basic frame work I think to generate WSPR at baseband
which you could then feed into whatever "radio" you wish to follow it.

Once you have the RPi as the beacon brains, now you can do a lot of
cool things with remote access and remote control from a network
interface.

Chris NØJCF

On Friday (07/17/2015 at 09:46AM -0500), Doug Reed wrote:
> 
> 
> Hi Jerry, David, Lloyd.
> 
> Yup, many of the newer (1980's & 90's & up) crop of surplus commercial
> 2-way radios are synthesized. But in most of them the minimum step is
> set by the intended channel spacing. Many are limited to 5 or 6.25KHz
> spacing. Depending how old or new the radio is, they usually had no
> better than 20PPM frequency stability, although newer radios that have
> digital modes might be better than that, down to maybe 2PPM.
> 
> The problem is that those radios are often more highly prized because
> they ARE synthesized. It is the crystal radios that are too expensive
> to use and they are probably free for hauling. I either have such
> radios or know where to look, and the only expense is hauling them
> home. It is the synthesized radios I want to keep and use, mostly for
> APRS, but many of them are too old to use as well.....
> 
> I suggested using the RX front-end as a RX pre-filter and mixer for
> the SDR RX to reduce out-of-band signals or get down to the HF bands.
> I didn't think of taking the signal off after the IF filter because I
> wasn't thinking of reducing the RX bandwidth that far. But there is no
> reason not to, just have to stay ahead of the IF amp stages because
> the SDR doesn't need the extra gain or the limiting.
> 
> I was going to say you couldn't step the original synthesizer output
> frequency in small enough steps for the beacon modulation, but that
> would be wrong. It might be more difficult, but I expect something
> could be done, possibly via the CTCSS input. Same with CW keying the
> middle multiplier and amplifier stages. And if you want to improve the
> PPM, I'm sure you could program an Arduino, PIC, or Ras-Pi to adjust
> the synthesizer LO to keep it on frequency against GPS timing pulses.
> That is what the Ultimate 3S does.
> 
> But all of that seems like a lot of work compared to using a
> controller and replacement frequency source like the Ultimate 3S to
> generate the beacons and drive the transmitter. The PI4 beacon mode is
> a full power beacon mode where information is sent by frequency
> hopping (FSK). The PTT line from the controller just activates the PTT
> on the transmitter. There is no modification beyond figuring out where
> you want to inject the RF. The built-in software in the Ultimate 3S
> can handle various types of CW, QRSS, WSPR, and other modes. The hard
> part is figuring out how to program it to send the data you want.....
> 
> I keep thinking in terms of feeding the original crystal oscillator
> stage as a buffer, or perhaps feeding into one of the later multiplier
> stages so the SI5351A doesn't have to go above 50MHz to work, but that
> depends if I can adjust the PI4 frequency hops to meet specs after
> multiplying up to the final frequency..... I'm not on-off CW keying
> any of the stages, so no key clicks.
> 
> But you do have to worry about raising the noise floor near the beacon
> frequency due to phase noise. I'm sure a lot of the old-timers in the
> metro could tell you how the 6M noise floor rose after KTCA channel 2
> replaced their tube transmitter with a solid-state transmitter.... I
> don't remember if all of these oscillator modules are phase continuous
> during frequency changes, but that is another potential source of
> transmitted interference.
> 
> If you use this method, any of the common oscillator modules will
> work, assuming you can improve their frequency stability and their
> phase noise doesn't trash the output signal. If you are going to write
> your own beacon controller program, that means you will be programming
> the oscillator too. If you read the hanssummers.com web site articles
> about the "ovenized SI5351A oscillator" and the "GPS stabilized
> Ultimate 3/3S transmitter" you will get some good information and
> clues to how you can achieve 10e-9 accuracy with your beacon while
> using a (relatively) garden variety AD9850/SI570/SI5351A
> oscillator..... And the various web pages and links often include
> sample code or at least flow diagrams to help you write your own code
> if you used a different CPU for the controller.
> 
> Using a commercial radio as the basis for the beacon works if they
> made one for that band. So 2M and 432 bands are covered, 6M is a
> little harder. 222MHz, 902MHz, and 1296MHz are all harder because
> equipment is limited, but there are RF power modules available for all
> those bands and you can use the multiplier stages from a commercial
> rig to get to up to the band of interest.
> 
> That's about all I've got. I tend to think of the beacon as something
> at a remote unattended site, so ideas about control are important. I
> didn't care for the beacon-at-home idea because they usually get shut
> off during contests due to local overload, just about the time people
> want to listen to them....
> .
> 73, Doug Reed, N0NAS.
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-- 
Chris Elmquist