[OZAPRS] HF to VHF Confused

[Ray Wells]

Tony Hunt wrote:

snip, snip


> As Andrew said we messed about with 40m APRS a while back and every so often
> it seems to raise its head again and someone has another look at it. To me
> it was not as good as 30m ( as many have noted) but if we have stations
> willing to support it then lets play a while and measure it again. Its
> certainly popular in ZL as opposed to 30m. I remember trying 80m from here
> to VK7 which was a dead loss. I got a decode every couple of days. This is
> the whole issue when you start going down in frequency like this. Its
> inheritly noisy.
>
> What I tended to find was that 30m didnt kick in till after a 500km distance
> from the Gateway and then proved pretty relaiable to about 1800km and even
> 3000+km at times. 40m on the other hand seemed to get its act together from
> about 50km out to about 800km and then lost the plot although at times it
> could certainly make the 2000km mark the S/N ratio was struggling to make a
> reliable decode.
>
>   
Those distances, that many of us who use/have used 30m can verify, may 
very well explain why ZL uses 40m. I just quickly measured ZL on Google 
Earth - the north island is just over 800km from tip to tip and the 
south island is just under 800km. Looking at the width, the widest part 
is in the north island and it's about 250km, unless you start taking 
diagonal measurements. There simply isn't enough distance for a reliable 
30m network, yet the distances really do suit 40m.

As for 40m, I'm lucky in that my rural location is exceptionally quiet 
and this facilitates copying signals at S0. Of course, when Mother 
Nature gets all fired up with electrical activity, that all goes down 
the gurgler. Phase Distortion (selective fading) seems to present 
another problem on 40m, compared with 30m. The signal can be rock solid 
S9 but it won't decode. For HF packet, in general, shorter paclen 
greatly improves throughput. The often used value of 80 is too long. 64 
returned more reliable result - less retries - in empirical tests over 
considerable time. The shorter packet stands a better chance of not 
being assaulted by noise spikes. The human ear can fill in missing bits 
but packet can't.

When 40m was so useless a few months back we considered 80m for daytime 
forwarding of mail. In previous use of 80m during the day for voice, 
quite some years back, we found daytime 80m very useful out to about 
300-400km. Signal levels were generally low but the band noise was 
almost none existent (except for thunderstorms).

Ray vk2tv
> Lets see if you here anything from VK5 anyway. Will leave it running a
> while. Its certainly busy with a few different sigs on there tonight PSK31
> RTTY but no AX25 as yet.
>
> Tony Hunt VK5AH
>
>
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>   

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