[aprssig] On the Road with PSK63 APRS Mobile -- Day 4
Stephen H. Smith
wa8lmf2 at aol.com
Sat May 8 22:39:21 EDT 2010
Joe Della Barba wrote:
> I have a SSB on my boat set up for PSK31.
> What do I need to do PSK83 APRS?
>
> 73 de N3HGB
>
>
Full details here:
<http://wa8lmf.net/APRS_PSK63/index.htm>
>
> [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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