[NLRS] Thoughts on VHF-UHF beacon hardware.

[Doug Reed]

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.