[NLRS] 2m Pager? noise

[Mary Brown]

Problem is the frequency is so close that any filter is going to drop wanted
signals badly if it is going to be effective enough to work in so close...
you could design for a single center frequency then use an interdigital
filter(big, bulky on 2m, hard to construct...) and not stray more than 20kc
or so either side...

Mary
W0AAT

-----Original Message-----
From: nlrs-bounces at mailman.qth.net [mailto:nlrs-bounces at mailman.qth.net] On
Behalf Of Chris Elmquist
Sent: Saturday, October 28, 2017 8:09 PM
To: Kirk P
Cc: nlrs at mailman.qth.net
Subject: Re: [NLRS] 2m Pager? noise



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
______________________________________________________________
NLRS mailing list
Home: http://mailman.qth.net/mailman/listinfo/nlrs
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:NLRS at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus