[OZAPRS] UHF APRS freqs in VK

Glen English VK1XX glenlist at pacificmedia.com.au
Wed Nov 15 11:48:09 AEDT 2017


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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