[OZAPRS] How do I go mobile on HF APRS ?

Michael vk6bhy at hotmail.com
Sun Jan 11 13:29:10 EST 2009

Thanks Ray,
will have a go at getting the audio right when I get home and can 
monitor the audio out

73 from Michael


> Yes, but how you go about it depends on the resources you have available.
> If you have, or have access to, a service monitor it's as easy as 123. 
> However, most amateurs don't have that facility and need to resort to 
> other means.
> If you have access to another receiver and an oscilloscope (can be sound 
> card based) you can visually compare your transmitted audio level with 
> other stations. You need to accept that those stations may not be 
> correct, but they may be close enough.
> If you have access to another receiver but no oscilloscope you can use 
> your ear. Increase the level of your transmitted audio until it "sounds" 
> as loud as other stations, and then back off a bit. This is definitely a 
> case of louder not being better!
> The last method, especially, has pitfalls because you have no way of 
> knowing if the ratio of the two tones is correct. Some TNC's, notably 
> those based on the Exar FSK demodulator (like TNC2 clones) are 
> particularly intolerant of the high tone (2200Hz) being lower in level 
> than the low (1200Hz) tone. They will simply refuse to decode under 
> those circumstances.
> It's normal for an FM transmitter to utilise 6dB per octave of 
> pre-emphasis. This means the high tone is transmitted at approximately 
> twice the level (deviation) of the low tone. If, however, you over-drive 
> the audio input of the modulator you can easily drive the tx 
> clipper/limiter into clipping/limiting, resulting in the two transmitted 
> tones being close to the same level, and this is undesireable. The first 
> two methods above will usually provide a visual display of the relative 
> level of the transmitted tones. The third method will not, hence the 
> suggestion to back off a bit.
> If you use a commercial rig such as a KenYaeIcom, the deviation is 
> probably set correctly but you can run into the over-driving the audio 
> input problem. Keep that in mind.
> Looking at the receiving end, and taking a signal that contains both 
> tones at the same level, if the receiver utilises de-emphasis, the high 
> tone will end up approximately half the level of the low tone and Exar 
> based FSK demodulators will refuse to decode it. If, on the other hand, 
> the receiver is not utilising de-emphasis, there shouldn't be a problem.
> If you set your modulation by ear and find your signal consistently not 
> being decoded, suspect either deviation is way too low, or you are 
> over-driving the tx clipper/limiter, or your deviation is WAY to high.
> Sorry, but there is no one size fits all answer, and tx audio levels 
> probably account for 99% of all difficulties with packet decoding.
> Ray vk2tv
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