[OZAPRS] UHF APRS freqs in VK

[vk2tv]

And how does that solve the basic RF problems of tx noise and rx 
overload for a shared site?

Ray vk2tv

On 15/11/17 14:29, Andrew Rich wrote:
> Go 19k2
>
> Sent from my iPhone
>
> On 15 Nov 2017, at 1:24 pm, Matthew Cook <vk5zm at bistre.net 
> <mailto:vk5zm at bistre.net>> wrote:
>
>> As I mentioned not easy, but not impossible either :)
>>
>> In liu of going around in circles then the question is where to move 
>> it that will gain the support of the WIA TAC.
>>
>> There is no where that I can see within 430-440MHz where this could 
>> go and co-site happily it's just too crowded.... so really the only 
>> logical place is where you've already suggested between 441.000 & 
>> 442.975 in the ALL MODE section...  The complication here being it 
>> won't work well co-sited with a DATV TX and at the other end you're 
>> up against the top of the repeater links....
>>
>> Suggestions ?
>>
>> Matthew
>> VK5ZM
>>
>> On 15 November 2017 at 11:18, Glen English VK1XX 
>> <glenlist at pacificmedia.com.au <mailto:glenlist at pacificmedia.com.au>> 
>> wrote:
>>
>>     Hi Matthew
>>
>>     You are right about TX combining, but your argument does not
>>     correctly
>>     consider TX noise. You will need a cubic meter of filters to do
>>     it with
>>     a reasonable loss of  < 3dB....
>>
>>     You have to look at the percentages...
>>
>>     800kHz on 2m (your 145.175 with 1,6meg 2m repester offsets )  is OK.
>>
>>     But  800kHz on 2m is NOT 800kHz on UHF, it is 2.4 MHz on UHF.
>>
>>     300kHz on UHF  (what you proposed) is  100kHz on 2m.
>>
>>     anyone care to build a 100kHz offset 2m duplexor ?
>>
>>     at 300kHz away on UHF, most radios will be ~ 80dB down in a 16kHz
>>     bandwidth .
>>     so if we have +43dBm TX, noise is -37dBm.
>>
>>     if we are on the same coax ,  we need the difference of -127 -
>>     -37 = 90dB
>>
>>     if we are on a split antenna system (all TX in ANT1, all RX in
>>     antenna
>>     2) then the isolation between the two antennas subtracts from the
>>     filter
>>     requirement call it 40dB on 70cm, so now you require 50dB .
>>
>>     300kHz at UHF  is 0.06% frequency spacing. This wont be practically
>>     realisable with a bandpass-notch configuration because the return
>>     loss
>>     will suffer. Back to a bandpass configuration, 10dB isolation is
>>     about
>>     all you'll get so you'll need 5 x 6" 3/4 wave filters for about 7 dB
>>     loss.  and god awful return loss
>>
>>     For fixing the return loss you could use a balanced hybrid
>>     combiner, and
>>     2x the filters, so 10 x 6"  filters and 2 hybrids will yield good
>>     return
>>     loss and perhaps 6-7dB loss for your 50dB....
>>
>>     It's clearly not too easy.
>>
>>     -glen
>>
>>
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