[aprssig] 30 Meter VARA Tests

Sun Dec 17 12:17:56 EST 2023

On 12/16/2023 11:06 AM, Michael Ford wrote:
>
> You bring up an interesting use case: the ability to use two or more modes 
> from one radio set on one frequency.  This obviously allows someone to enjoy 
> multiple modes without going to the expense or complications of running 
> multiple radios.
>
> The suggestion you passed along is to have the 500Hz VARA signal on a 1500Hz 
> center and the ~200Hz Packet signal on a 1700Hz center, which has overlap.  
> In my experience, VARA doesn't detect Packet well before transmitting and 
> will step on it.  So where the two protocols do not share a frequency space 
> well together, it may be better to separate them to avoid adverse impact on 
> the packet community.
>
> That being said, there's plenty of room in the passband to accommodate 
> Stephen's use case.  So why not shift packet off of its 1700Hz center 
> frequency to be out of the way of VARA?  Both Soundmodem and Direwolf (if not 
> others) support such a configuration.
>

Back from my 60 meters APRS-over-VARA road trip Saturday.   It was a 
spectacular success.  Nearly constant tracking over the 220 mile drive both day 
and night.

An added unexpected bonus for the test was Denis WB8SKP near Paducah, Kentucky 
sending me APRS messages to the 60M mobile.  His QTH is about 450 miles to the 
southeast of my home, and farther from the trip route.     His first message 
popped up on my mobile Panasonic Toughbook CF-51 mid-day when I didn't think 
60M NVIS propagation would support that long a haul. We exchanged several 
messages back and forth.    Later, hours after sunset, he successfully 
contacted me again with messaging both ways.  He was tracking me directly off 
RF - not from the Internet via my igate -- for nearly the whole trip.

My thinking on the tone/frequency choices for VARA APRS is as follows:

 1.   IBM once referred to  "the tyranny of the installed base" when their PS/2
    family of PCs with a new kind of expansion slots failed to redefine PC
    architecture away from their wildly-successful PC/AT "ISA" ("Industry
    Standard Architecture") expansion slot designs that everyone cloned and
    made millions of expansion cards for.   HF APRS has a similar problem.

    HF APRS on 30 meters has been done on standard AX.25 200-hz-shift HF packet
    for nearly FORTY years now.  A lot of the igates and gateways on 30M still
    use classic TNC hardware like TNC-2s, MFJ-1270s, Kantronics KAMs, etc who's
    audio tones for 200-hz shift are fixed at 1600/1800 Hz; i.e. 1700 Hz center
    frequency.

    The actual RF HF mark/space frequencies for 30M packet APRS have always
    been 10.149.200 / 10.149.400 MHz.  This has required tuning an HF/SSB
    transceiver to 10.147.60 MHz USB to yield the correct audio tones.

    [In the early days, when many assumed HF/SSB below 20 meters should always
    be on LSB, many actually set the radio dial frequency to 10.151 LSB (i.e.
    outside the ham band) and trusted to God or something that their carrier
    suppression was really good in order to place the 1600/1800 Hz tones on the
    correct RF frequencies INSIDE the band limits.  Many transceivers that
    wouldn't transmit when the dial frequency was outside the ham bands had to
    be hacked with the "general-coverage-transmit" mod to work.]

    Bottom line:  The 10.147.600 USB dial frequency is totally and unchangeably
    embedded in HF APRS operating conventions.

 2. Further re-reinforcing the current RF frequencies vs audio tones convention
    is that the TinyTrack III (still the cheapest and simplest way to generate
    APRS transmissions) emits fixed 1600/1800 Hz tones in HF mode.

 3. The VARA-HF "soft TNC" generates an unchangeable tone spread centered
    around 1500 Hz.

 4. If VARA is going to become part of a multi-mode igate, piggybacking on the
    existing packet APRS, using the same radio, unavoidably it will be 200 Hz
    "off frequency" relative to packet APRS.

 5. In my experience, the carrier detect/TX inhibit of VARA is actually quite
    good. In fact, it has a good implementation of random delay before transmit
    when a detected signal goes away.

    During my 60m test, wobbly unstable harmonics of switching-mode DC-DC power
    converters somewhere in my car periodically drifted across the passband of
    my 60m receiver. Whenever they became audible, the VARA transmit was disabled.

    On my *30*-meter combined packet/VARA igate, VARA shows a carrier-detect
    (CD) whenever either on-freq packet APRS bursts or the
    "just-below-but-audible" robust packet bursts happen.

 6. The total volume of traffic on HF APRS is low enough that collisions
    between packets,  either of the same mode or of different modes, is
    relatively rare. Further, given the repetitive nature of APRS beaconing, an
    occasional lost packet or two is not a major issue.

 7. Further, a large portion of the current ham population simply doesn't
    understand the relationship between audio frequencies and the resulting RF
    frequencies on SSB. Expecting them to offset the variable generated packet
    audio tones in applications like the UZ7HO  Soundmodem or DireWoif to 1500
    Hz center freqs, and then offset the radio dial frequency correspondingly
    is unrealistic.  Not to mention that the classic hardware TNCs, Tiny
    Tracks, etc CAN"T BE CHANGED!    Trying to get uptake for a new audio tones
    and RF dial setting convention would be hopeless....

In light of all these issues, I concluded we must continue to live with the 
1600/1800 packet tone pairing!     The simplest compromise was to JUST LIVE 
with the VARA center frequency being 200 Hz lower than the packet center 
frequency.

For classic packet-only operation on both 30 and 60 meters, for years I had 
been receiving through 500 Hz Collins mechanical filters on Yaesu FT-857s with 
their IF shifts moved from the usual 700 Hz CW pass-band center to around 1700.

Now the "multi-mode" igates are receiving through FT-891s with their DSP 
continuously-variable IFs set to around 650 Hz bandwidth and IF-shifted 
slightly low.  It works perfectly and dramatically reduces AGC activation and 
receiver sensitivity reduction from ambient noise, compared to the usual 
default 2.3 Khz USB bandwidth.

As far as I know, the FT-891 is the least-expensive 100-watt HF radio to have a 
fully-variable DSP IF system - on periodic promotion specials from Yaesu they 
can be had for USD $550-600. The FT-891 has a HORRIBLE multi-layer menu 
structure, but they are excellent performers for digi-modes projects, SSTV, etc.

-------------------------------------------------------------------------------
Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net

"Studio B" Ham Shack on Wheels
<http://WA8LMF.net/Aliner>

-- APRS over VARA  --
<http://wa8lmf.net/VARAforAPRS.htm>

60-Meter APRS!   HF NVIS APRS Igate Now Operating
<http://WA8LMF.net/map/>

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<http://WA8LMF.net/FlyingDigi>

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