[aprssig] On the Road with PSK63 APRS Mobile -- Day 4

[Stephen H. Smith]

[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