[OZAPRS] TT3 works on HF but not on VHF

[Ray Wells]

David,

The first thing I would check is if you are overdriving the input to the 
tx on vhf. Note, this is different to deviation.

On HF we can assess drive quite readily by PA collector current, or ALC, 
but on VHF we don't have that facility.

If you overdrive a VHF tx you can defeat pre-emphasis by driving the tx 
so hard that both tones are transmitted with the same level instead of 
the high tone being 6dB (twice the level) greater than the low tone, 
because of the limiter/clipper. When such a signal is received and 
passed through de-emphasis in the rx, the high tone ends up 6dB lower 
than the low tone and FSK demodulators based on the Exar chip (TNC2 
compatibles) will not decode the signal.

This effect is quite independent of deviation as indicated on a 
deviation meter, which only shows peak deviation. To "see" the ratio of 
the tones you need a modulation oscilloscope.

The best advice I can give for someone without that test equipment is to 
monitor their transmission on another rx, and adjust the audio drive 
until they sound as loud as other stations, and then back off a bit. 
Given that incorrect deviation, and incorrect tone ratios probably cause 
the majority of problems with VHF/UHF packet, that method is a bit of 
fly by your pants but it's better than nothing.

If you have access to a CRO (can be software) and you can monitor audio 
on another rx, you have the tools needed to set things up properly.

Ray vk2tv

David and Justine Olsen wrote:
> I got my replacement TT3 which now shows a valid GPS light, and I 
> tested it on 40m and it worked fine. I then reconfigured it for VHF 
> and tested it on my mobile antenna but no success. It transmits fine 
> and I receive it on my base.
> Thinking my mobile which is very immobile at present (repairs) had a 
> dicky antenna or was located in a bad spot, I then set up the same 
> radio and TT# on my 2 meter J beam which I normally use to access the 
> repeater- still no luck.
>
> The TT3 is configured.
>
> Callsign VK4MDX-9
> Digi Path WIDE1-1, WIDE2-1
> SYMBOL J
> TABLE OVERLAY /
> Auto TX delay 300
> Auto Tx Rate 120
> Manual Tx Delay 133
> Quiet Time 526
> Calibration 128
> Nothing else enabled.
>
> The TT3 is connected to GPS and show valid GPS and transmits- 
> confirmed receive signal on my Kenwood 2m rig.
> My radio is an Icom706MkIIG
> The TT3 triggers transmit via the mic socket
> The rig dial is set to 145.175 MHz
>
> Why is my signal not decoded and not appear on the internet?
>
> David
>
> VK4MDX
>
>
>
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> Ozaprs at aprs.net.au
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