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

Ray Wells vk2tv at exemail.com.au
Wed Jan 7 19:33:03 EST 2009


Michael wrote:
> Hi,
> I am using a Tiny Track 3 plus to a Icom 706 with a Terlin antenna for 
> HF and 1/4 wave whip for VHF, have got 2 wires out of the plug going 
> to the radio which I twist together for VHF, plan on putting in a 
> switch when I get home, seems to work ok but still not quite set-up 
> right, think just the audio level needs adjusting a bit.
> Any tips on getting the audio to the radio right?
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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