[NLRS] 2m Pager? noise

[Chris Elmquist]

You might consider a 1/4-wave open-end or 1/2-wave shorted stub notch,
tuned for 144.390.  The idea would be to try to knock down the levels
you see on 144.390 so that they are no longer "slamming" you.

This is basically a short piece of coax, carefully tuned in length and
put in parallel with your Rx path with a T-connector.

They can be tricky to get on frequency and the Q (sharpness of the
notch) is a function of the choice of coax.  A piece of RG58 will be very
broad and not too good of a notch while building it with a piece of 7/8"
heliax might do pretty good.

I found some information here,

http://www.radagast.org/~dplatt/hamradio/K6OIK-filters.pdf

There is also a discussion in some of the older (cc. 1980's) ARRL
handbooks I believe.

The length is a function of the velocity factor of the coax used
and includes all the connectors and the T, etc in the calculation.
Some people start with a longer length coax "tail" and keep trimming
and testing until they have achieved the desired notch.  This is more
difficult if it's a piece of 7/8" heliax :-)

If you put this ahead of your first stage pre-amp, that might have a
negative affect on your noise figure-- although, hearing nothing due to
APRS packets also has a negative affect on your noise figure ;-)

You might be able to put it after the preamp but then that's where the
high IP3 LNA mentioned may be important so that it isn't being crunched
first.  Dr. Gerry can correct me here but basically high IP3 means
the preamp can tolerate much higher RF levels at the input before it
saturates and starts to distort the output signal.  A distorted output
signal will contain mixing products that appear as intermod or ghost
signals in your receiver.

PAR's "no way" response was to the question of building a 144.390 notch
or ??

Chris NØJCF

On Sunday (10/29/2017 at 12:21AM +0000), Kirk P wrote:
> 
> 
> I sure appreciate all the information and ideas. Yes, Im sure its APRA as thats its freq.
> 
> 
> Not sure there is anything now that I can do as they are operating within their rights though some seem awfully over modualted.
> 
> 
> It would appear that my only hope would be to find some type of filter I can set for a freq up higher in band that can just catch .380 on its lower edge..a cavity that I can adjust perhaps? I dont know..so much of this VHF stuff is newer to and above my knowledge..HF Id have it solved already HI HI
> 
> 
> I modified my 746PRO to accept a SDR RX as its RX...I use both the Funcube PRO+ as well as RTL dongles for 160m-2m utilizing the 746 as the TX radio. Amazed at how well the RX is now that Ive done this..but it appears to come at a price..the dongles have a bandwidth of up to 1mhz of RX and the FC at 196khz and fold under such strong local signals when coupled with WSJTX for EME.
> 
> 
> If I use just th746PRO as the RX in its original configuration (Yes, I can switch back and forth the way I did the mod) it handles this noise up the band much better..though doesnt hear weak signal as well as the SDR using SDR console software.
> 
> 
> Anyone that give me filtering ideas Im open to suggestions.
> 
> 
> PAR electronics got back to me and said no way.
> 
> 
> They did mention a HIGH IP3 LNA as an option though Im not familiar with those. I currently use the ARR 144 LNA.
> 
> 
> Kirk, N0KK

-- 
Chris Elmquist