> With 250mW (+24dBm) into the transmit port... antenna port is
> a quality 50 ohm load, I see -72dBM at the receiver port. (pretty
> much what I expect.. 1.8dB loss through the xmit side, & 100dB
> notch through the RX side.
Those numbers are fine.
> With it all hooked up receiving an input signal of about 0.7uV,
> application of the 250mw to the transmitter side will cause noise
> in the receiver, although not much.
That's bad. With the amount of isolation you have (assuming you have the
same amount of noise supression - did you confirm that?), you shouldn't need
any more isolation. Whatever problem you have that's manifesting as desense
is NOT due to insufficient carrier supression or noise supression (assuming
they're symmetrical - again, you should measure the latter).
How are you injecting the signal into the receiver while transmitting, by
the way? Lossy tee or some other method?
> So, it appears that if I add an additional 30dB notch (another 'can'),
> the problem at high power may go away.
Probably not.
> If I compare what 50 watts with an additional can would be (-130dB)
> to what I get with 250mW & current notch (100dB), then it
> looks like I just
> need to add an additional can.
Probably not.
> With the current notch @ 50 watts, I see a receive signal of -53dBm.
> Some have said that the Micor can handle that, while others (off-line)
> have said no way.
The others are wrong, and if they they want to defend their position, I
invite them to do it here on the list.
-53 dBm 1 MHz away is not a problematic signal by any stretch of the
imagination. Here's a simple sanity check for those that feel otherwise. I
like using real-world examples as sanity checks.
How many thousands of 2m repeaters are out there running 100 watts at 600
kHz offset without desense? Let's be generous and say they have 100 dB of
isolation in the duplexer. +50 dBm TPO - 100 dB = -50 dBm transmit carrier
hitting the receiver. No big deal. And that's on 2m. The offset on 2m is
only 0.4% (0.6 MHz / 146 MHz), whereas on 6m, it's 1.9% (1 / 53 MHz), making
isolation requirements that much more stringent on 2m.
Now let's look at a 6m example. You have a 6m repeater on a 1 MHz split?
Let's say it's on 53.99-, highest channel in the band, putting your receiver
on 52.99. Some other ham is working simplex on 52.525, using 100 watts into
a unity-gain antenna, and he's 40 miles away. His signal into your
receiver, assuming unity gain on your end too, and line-of-sight, is -53 dBm
(that's what the free-space path loss works out to: 103 dB for 40 miles on
6m, check my math). Would you expect this guy 40 miles away talking on 525
to desense your repeater? If so, then you should expect *every* ham who
transmits on 525 (or potentially any other frequency within 1 MHz of your
receiver) within a 40 mile radius of your repeater to cause you desense;
those that are closer than 40 miles are just going to desense it even worse.
Silly. Just plain silly.
> This setup appears to support the opinion that -53 is still
> way too much.
No, it doesn't. You're drawing a conclusion from incomplete data.
The only thing you know at this point is that what you have DOESN'T work,
but what you DON'T KNOW is WHY it doesn't work. And I'm telling you, with
the utmost certainty, that based on the numbers you've given, lack of
carrier supression and/or noise supression in your duplexer at your
operating frequencies is NOT the problem, you should be looking elsewhere.
There are many potential causes of desense beyond just carrier supression
and transmitter noise supression afforded by the duplexer at the Tx and Rx
frequencies proper. To say that all of your problems will be solved by
increasing either/both is not a valid conclusion to draw.
Just a few of many possible issues that could be causing your desense
problem:
- lack of mid-band isolation
- noisy components/connections in your duplexer, cabling, etc.
- spurious transmitter
- spurious LO
- component breaking down/arcing when excited by RF
- insufficient shielding (cabling, radio chassis/subassemblies, etc.)
- bad test load or antenna
- bad channel element or crystal
- corroded/oxidized connector, contact, pin, etc.
- cracked/broken/microscopically-intermittent connection, trace, conductor,
etc.
- bad/cold solder joint
- other problem in exciter or receiver
- loose, mis-installed, or otherwise faulty RF connector
- bad RF cables (LMR, bare copper braid, etc.)
- bad luck, bad karma, bad mojo, phase of the moon, alignment of the
planets, etc.
When you're duplexing, everything has to be damn near perfect otherwise you
end up with desense. On 6m, you have a few things going for you, including
a wide frequency split (percentage-wise), a high ambient noise floor (hides
some sins/duplex noise), but otherwise, all of the same issues/caveats apply
as they do on any other band when duplexing, noise is noise, dynamic range
is dynamic range, whether you're on 50 MHz or 50 GHz.
--- Jeff WN3A
