[OZAPRS] TT3 works on HF but not on VHF

[David and Justine Olsen]

Ray

Thanks, I'll give it a go. I assumed a bought (as opposed to kit)   
TT3 would be adjusted to not overdrive.

David



On 25 Apr 2009, at 18:21, Ray Wells wrote:

> 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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>
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