[OZAPRS] TT3 works on HF but not on VHF

Ray Wells vk2tv at exemail.com.au
Sat Apr 25 19:10:20 EST 2009


David,

No supplier can know the input sensitivity/input impedence of your tx, 
there are so many possibilities.

Now that you know what you are looking for you'll get there.

Holler for help.


Ray vk2tv

David and Justine Olsen wrote:
> 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
>>>
>>>
>>>
>>> _______________________________________________
>>> Ozaprs mailing list
>>> Ozaprs at aprs.net.au
>>> http://aprs.net.au/mailman/listinfo/ozaprs
>>>
>>
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