[OZAPRS] UHF APRS freqs in VK

[Andrew Rich]

Go 19k2 

Sent from my iPhone

> On 15 Nov 2017, at 1:24 pm, Matthew Cook <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> 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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