[NLRS] 2m Pager? noise

Sat Oct 28 22:45:09 EDT 2017

Yes. True.  Well, maybe the preamp is getting crunched then and a newer design with high IP3 would be a solution?

Tricky.

Chris N0JCF

On October 28, 2017 8:45:06 PM CDT, Mary Brown <maryalanab at gmail.com> wrote:
>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
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-- 
Chris Elmquist