[OZAPRS] TT3 works on HF but not on VHF

[David and Justine Olsen]

Thanks all, the miniature screwdrivers are out and busy
:-)


David


On 25 Apr 2009, at 19:10, Ray Wells wrote:

> 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
>>>>
>>>>
>>>>
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>>>
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