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Re: Plagued with Mode J "noise" on TX




  In response to Scott Townley, Douglas Cole wrote:
 >>> What you really need to do is get a seperate antenna for each band and
use the
 >>> DCI filters as they are a real "brick wall" when it comes to mode J
desense (
 >>> well ok it is just my opinion) , or find yourself a nice bandpass
cavity for the
 >>> vhf port etc.....

Then Jim Walls wrote:
 >> Keep in mind that a cavity filter will pass the 3rd harmonic just as
well as the
 >> fundamental.  You need a low-pass filter of some sort.  That's why the
multi-band
 >> diplexer has been suggested.

Then Roy Welch responded:
 > I have found the 70cm filter works fine here.  Most of the problem was
 > caused in my radios by the 2M signal getting into the 70cm receiver front
 > end where the third harmonic was then generated in non linear devices.
The
 > 70cm banspass filter keeps out the 2M signal and then no third harmonic
 > signal is generated.  There could be some third harmonic stuff generated
 > externally and then pass through to the 70cm receiver, but in my case
that
 > part was minor.

Now back to Jim Walls:
Roy,  Douglas specifically suggested a cavity type filter for the VHF port
and I was only suggesting that this would be a bad idea because a cavity
type filter will pass the 3rd harmonic just as well as it passes the
fundamental.  I was not trying to solve the whole problem (I already did
that as private E-Mail to Scott).  I was only trying to point out that
putting a cavity filter on the 2M transmitter won't help at all in reducing
the 3rd harmonic from that transmitter.  A cavity type filter on the UHF
port will keep 2M RF out of the 70cm receiver, but the down side is that a
cavity is too narrow to be used on multiple satellites without retuning it
for each pass (way more trouble than it's worth).

73
-----------------------
Jim Walls - K6CCC
k6ccc@amsat.org
Mobile Radio Operations
Southern California Edison Co.
Ofc:   626-302-8515   -   PAX   28-515
FAX:   626-302-9999   -   PAX   29-999





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