[OZAPRS] UHF APRS freqs in VK

[Matthew Cook]

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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