[OZAPRS] UHF APRS freqs in VK

[Matthew Cook]

It gets worse when you can generate DATV at +50dBm (egads !)...   I've got
a few plots of the output of that amp here somewhere.

73

Matthew
VK5ZM

On 15 November 2017 at 14:03, Glen English VK1XX <
glenlist at pacificmedia.com.au> wrote:

> I'll write a tech paper and send it to NTAC.
>
> plenty of spectrum even with the repeater links
>
> analog ATVers on 444.25  would still produce a bit of rubbish on the
> bottom of their VSB into the top end of 442. how much I dunno. a problem
> , dont think so.
>
> DTVers  carrier on 446.5 with 7 MHz nominal BW is actually ahh 6.65 MHz
> BW or extending to 443.175
>
> but in practice due to non linearities, it will spill over another half
> MHz at -40dB. probably 442.675 is most likely the highest you might want
> to use where there is 446.5 ADTV in use like cap cities....
>
>
>
>
>
>
> On 15/11/2017 2:24 PM, Matthew Cook 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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> >
> >
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