Frequency Change for 30-Meter
APRS-over-VARA Operation
HF APRS operation using the new VARA high-performance
soundcard software modem has been demonstrated to have an enormous advantage
over classic 300-baud AX.25 packet on HF.
For over 40 years, traditional packet APRS using
two-tone FSK has used the actual RF "mark" and "space" frequencies of 10.149.200
and 10.149.400 MHz. This is normally done by generating 1600 and 1800 Hz AFSK
audio tones. This yields an approximately 500 Hz-bandwidth signal centered
on an audio frequency of 1700 Hz.
This audio tone pair is generated by many hardware devices
such as TNC-2 & MFJ 127x-series TNCs in 300-baud HF mode, TinyTracks in 300-baud
HF mode, and the default setting of the HF port of the original Kantronics "KAM"
dual-port TNC. It is also the default settings of the DireWolf and UZ7HO
Soundmodem software sound-card TNCs.
These tones are inserted into an HF/SSB transceiver set
to 10.147.600 USB a.k.a. "dial frequency" to yield the 10.149.200 and 10.149.400
MHz RF FSK frequencies.
Until now, to allow side-by-side operation of APRS-over-VARA
on the same radio, the VARA-generated audio tones have just been fed into the
same radio set to the same dial frequency.
Since the VARA audio center
frequency is lower (1500 Hz) than the standard AX.25 AFSK center frequency (1700
Hz), the resulting RF signal is also centered about 200 Hz lower. It
partially overlaps the AX.25 channel.
In North America, this has not been much of a problem.
In Europe, the VARA signal also overlaps the channel being used for "robust
packet" just below the traditional AX.25 packet APRS channel used world-wide.
Robust packet radio a.k.a. "RPR" is an advanced form of
error-correcting packet using 8 OFDM parallel carriers in a 500 Hz bandwidth. It
is a proprietary mode generated exclusively by hardware or software from the
German company SCS
Communications. This mode is still very active in Europe, with an
extensive network of digipeaters and igates operating. The frequency overlap of
APRS-over-VARA and RPR makes the new APRS-over-VARA mode almost unusable in
large parts of the world outside North America.
To avoid this problem, APRS-over-VARA operation is being
urged to move to a new channel ABOVE the standard packet APRS
channel: 10.149.700 Mhz center frequency. This is
actually the frequency used during the 2000s by G4HYG's now-defunct "APRS
Messenger" program that did APRS over PSK63 and MFSK.
Effective 0000 Hrs UTC 1 January 2025, all WA8LMF
30M HF APRS-over-VARA igates and beacons will switch to the new frequency.
| To setup for the new
APRS-over-VARA channel:
Set radio to 10.148.200 Mhz USB .
Operate VARA normally with 1500 Hz audio
center frequency.
To minimize intermodulation distortion of
the complex VARA waveform (and a needlessly wide signal on the air),
set the VARA audio level to yield no ALC
indication on HF transceivers - this is similar to the TX
recommendations for PSK63, etc. A standard 100-watt HF transceiver
should be yielding 50-75 watts peak when set properly.
This will yield a very tightly-formed 500-Hz-wide signal that
will span 10.149.450 to 10.149.950 Mhz. allowing a 50 Hz guard
band at the top of the 30-meter amateur band. (Upper edge of 30
meters is 10.150.000 MHz.)
|
| To also operate classic AX.25 packet
APRS simultaneously on the existing "classic" channel, with the same radio:
Set audio tone
frequencies of DireWolf or UZ7HO AX.25 soft TNCs to 1000 and
1200 Hz frequencies (1100 Hz center frequency).
Note that this VARA frequency change
will make classic
hardware TNCs with unchangeable audio tone frequencies for HF
packet unusable for simultaneous AX.25/VARA operation.
|
