[aprssig] On the Road with PSK63 APRS Mobile -- Day 4
Stephen H. Smith
wa8lmf2 at aol.com
Fri May 7 02:22:40 EDT 2010
[Posted from Iowa City, IA Motel 6 via WiFi from another motel about
1/3rd mile away, using my "WiFi Super Blaster" DX antenna.]
Stephen H. Smith wrote:
> I'm now on the fourth day of my cross-country trip, experimenting with
> 30-meters HF APRS over PSK63.
Today I have noticed a number of STRONG PSK63 signals that simply
don't decode at all. These signals appear rather wide, and lack the
characteristic vertical double-line "railroad track" in the middle of
the waterfall spectrum display. They appear to be PSK transmissions
with excessive TX audio levels (or attempted RF power output), that
creates intermodulation distortion in the transmitter.
I have just added the paragraphs below to my PSK63 APRS page at:
<http://wa8lmf.net/APRS_PSK63>
------------------------------------------------------------------------
IMPORTANT: PSK63 is a simultaneous two-frequency transmission, unlike
conventional FSK packet or RTTY which are single-tone-at-a-time
constant power transmissions similar to key-down CW.
*It is essential to keep the _average_ output power of the SSB
transmitter far below maximum in the PSK mode, to avoid severe
inter-modulation distortion (IMD) of the transmitted signal. *
High IMD will make your signal occupy excessive bandwidth on the air,
and worse make it unreadable to others, even if the RF level is strong.
If the transmit level is PROPERLY SET, the PSK63 signal will be received
without errors, even on signals too weak to move the S-meter.
1. Disable any speech processors, limiters or compressors in the
transmitter audio chain.
2. If you are reading transmitter output power on an averaging-type
wattmeter, you must adjust the soundcard audio level (or
transmitter mic gain) for an indicated power of about 1/3rd of the
keydown maximum power on CW or FM. You should NOT be showing any
ALC (automatic level control) voltage which would indicate that
you are at or trying to exceed maximum transmitter power output.
3. If you are reading power output on a PEAK reading wattmeter (or
monitoring the TX signal on a scope), gradually increase the
soundcard output level (or transmitter mic gain) until the power
output stops increasing. Then turn the level back down a bit
until the output drops about 20%. This will ensure that you are
not saturating the transmitter power stages.
------------------------------------------------------------------------
--
Stephen H. Smith wa8lmf (at) aol.com
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M band]
Skype: WA8LMF
Home Page: http://wa8lmf.net
NEW! *** HF APRS over PSK63 ***
http://wa8lmf.net/APRS_PSK63/index.htm
Universal HF/VHF/UHF Antenna Mounting System
http://wa8lmf.net/mobile/UniversalAntMountSystem.htm
"APRS 101" Explanation of APRS Path Selection & Digipeating
http://wa8lmf.net/DigiPaths
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