<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Go 19k2 <br><br><div id="AppleMailSignature">Sent from my iPhone</div><div><br>On 15 Nov 2017, at 1:24 pm, Matthew Cook <<a href="mailto:vk5zm@bistre.net">vk5zm@bistre.net</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">As I mentioned not easy, but not impossible either :)<div><br></div><div>In liu of going around in circles then the question is where to move it that will gain the support of the WIA TAC.</div><div><br></div><div>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....</div><div><br></div><div>Suggestions ?</div><div><br></div><div>Matthew</div><div>VK5ZM</div></div><div class="gmail_extra"><br><div class="gmail_quote">On 15 November 2017 at 11:18, Glen English VK1XX <span dir="ltr"><<a href="mailto:glenlist@pacificmedia.com.au" target="_blank">glenlist@pacificmedia.com.au</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Matthew<br>
<br>
You are right about TX combining, but your argument does not correctly<br>
consider TX noise. You will need a cubic meter of filters to do it with<br>
a reasonable loss of < 3dB....<br>
<br>
You have to look at the percentages...<br>
<br>
800kHz on 2m (your 145.175 with 1,6meg 2m repester offsets ) is OK.<br>
<br>
But 800kHz on 2m is NOT 800kHz on UHF, it is 2.4 MHz on UHF.<br>
<br>
300kHz on UHF (what you proposed) is 100kHz on 2m.<br>
<br>
anyone care to build a 100kHz offset 2m duplexor ?<br>
<br>
at 300kHz away on UHF, most radios will be ~ 80dB down in a 16kHz<br>
bandwidth .<br>
so if we have +43dBm TX, noise is -37dBm.<br>
<br>
if we are on the same coax , we need the difference of -127 - -37 = 90dB<br>
<br>
if we are on a split antenna system (all TX in ANT1, all RX in antenna<br>
2) then the isolation between the two antennas subtracts from the filter<br>
requirement call it 40dB on 70cm, so now you require 50dB .<br>
<br>
300kHz at UHF is 0.06% frequency spacing. This wont be practically<br>
realisable with a bandpass-notch configuration because the return loss<br>
will suffer. Back to a bandpass configuration, 10dB isolation is about<br>
all you'll get so you'll need 5 x 6" 3/4 wave filters for about 7 dB<br>
loss. and god awful return loss<br>
<br>
For fixing the return loss you could use a balanced hybrid combiner, and<br>
2x the filters, so 10 x 6" filters and 2 hybrids will yield good return<br>
loss and perhaps 6-7dB loss for your 50dB....<br>
<br>
It's clearly not too easy.<br>
<span class="HOEnZb"><font color="#888888"><br>
-glen<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
<br>
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