[NLRS] 902 filter help

[Dr. Gerald N. Johnson]

For a trial you can try some coax T's in the transmission line. On the 
side arm of the T connect about 1/8th wave of coax (allowing for the 
velocity of propagation), and tune the open end (but shielded) with a 
capacitor. The handy part about 1/8th wave is that its easy to compute 
the capacitor, it will have reactance equal to the Z0 of the coax. With 
the spectrum analyzer tune for a dip of the unwanted signal. The bigger 
the coax, the less that dip will affect 902.1 MHz. It wouldn't be out of 
line to make that 1/8th wave from 1-1/2" copper water pipe with a 1/2" 
OD center conductor and minimum insulation at the high impedance end. If 
you want you can make the length a hair under a quarter wave, then tune 
it with a screw rather than a variable capacitor. And achieve a higher Q 
and so narrower notch. It may take several stagger tuned to kill off all 
the cell trash.

Then its time to work over your receiver. You need to enhance its 
dynamic range by cutting the gain of each stage before the mixer to what 
just keeps the system NF and no more. So you need to be able to measure 
MDS or NF and you add attenuation after the preamp until it affects the 
system NF and then reduce the attenuation 1 dB until the system NF is 
what it was with all the gains high. Then you do that for every stage 
after that, including at the IF. When you get done you should be able to 
stand a lot more junk outside the band without overload but you won't 
have as great a rise in the S-meter or the receiver noise when 
connecting the transverter. If there are two stages in the preamp, you 
may find improvement by taking out the second or by adding attenuation 
between them. There might also be benefit of adding a narrow bandpass 
filter at the IF. I'm sure you need to tune from 902.05 to 902.150 and 
maybe to 903.1 so the IF filter can be sharp to protect the IF receiver.

73, Jerry, K0CQ

On 9/16/2010 6:09 PM, Bill Davis Jr wrote:
>
>
>
>    Hi Guys
>
>    I'm in need of a filter for 902. I have a fairly new cell site 2.3miles from me and I have mixing issues on receive. When I turn to 135deg +/- 5deg the noise floor comes up 40db of garbage. Looking with a spectrum analyzer 902 is clean it is just the 4Mhz of cell stuff 16Mhz lower in freq.
>
>    Are there filters out there that are sharp enough out there that won't kill 902 too much? Anyone have a source? This was a real killer in the last 2 contests on 902.
>
> Any help much appreciated
>
> 73  Bill  K0AWU  EN37ed
>
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