[NLRS] 2m Pager? noise

Sun Oct 29 17:41:07 EDT 2017

Kirk - The front end in the Funcube Dongle has a pretty bad input intercept point. I suspect that the other SDRs aren’t much better. I know that you can turn off the LNA in the FunCube Dongle in software. Try turning the LNA off and see if the interference returns.  You won’t have very sensitive receiver at this point, but if you still get the interference, there isn’t much you can do.  If the interference goes away, then you can put a preamp before the Funcube Dongle with a better intercept point. That shouldn’t be too hard to find.  You may already be using an external preamp with the Funcube dongle, in which case you probably have too much gain and just turning off the Dongle LNA might help. An external preamp may have more gain than is needed, in which case you can use an attenuator between the receiver and the preamp.

You may be able to turn down the FunCube Dongle gain in software, you can try that too. The Funcube Dongle has a noise figure that is pretty high for EME, although OK for terrestrial use, so you would probably need a preamp anyway.  Just reduce the gain of the Dongle before you add one.

Let us know how it all turns out.  - Duffey KK6MC

James Duffey KK6MC
Cedar Crest NM

> On Oct 29, 2017, at 12:54, Dr. Gerald N. Johnson <geraldj at netins.net> wrote:
> 
> 
> 
> 
> 
>> On 10/28/2017 7:21 PM, Kirk P wrote:
>> 
>> 
>> I sure appreciate all the information and ideas. Yes, Im sure its
>> APRS 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
> That filter will be difficult and not perform well attenuating the APRS signal without killing the EME signal from filter loss.
> 
> You could try a coaxial notch filter. Start with a coax T in the receive line and on the side of the T connect 1/8th wave of good coax and terminate the open end with a variable capacitor having reactance equal to the characteristic impedance of the coax. E.G. 50 ohms reactance at 144.390 and about 8 inches of RG8 or RG213 or LMR 400. The capacitor would be about 23 pf at resonance. Tune the capacitor to minimize the APRS signal. Enclose the capacitor in a shield because its a high impedance and any exposed conductor will be a receive antenna countering the desired notch. I sued such a scheme once to take down a FM broadcast signal from a mile away that was upsetting my rig. It worked. The notch may not be narrow enough for your situation.
>> 
>> 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.
> 
> The RF bandwidth of the dongle may be more like at GHz so it has to digitize the super strong signal and runs out of digitial bits for the super weak EME signal.
> 
> Since you have modified the 746, I'd consider (though I'm not a fan of Icom radios due to bad experiences long ago with an IC-211) modifying the 746 to put out an IF signal after the roofing filter of the first IF for the dongle SDR. That filter is probably 15 or 20 kHz wide and would protect the dongle very well. Then I would add attenuation after then preamp but no more than will allow the full gain system NF or MDS to be the same. Could measure by sun noise or moon noise also. Point in my paper I referenced is that for maximum dynamic range the front end gain needs to only be the minimum that gets the best system NF or MDS. More gain than that just hurts strong signal handling.
> http://geraldj.networkiowa.com/papers/EME/Intermod2.pdf
> 
> The FM FSK bandwidth probably isn't over modulated for FM, just the FM bandwidth is more like 15 kHz double side band where AM for the same audio would be under 5 kHz double side band. And SSB would be under 2.5 kHz.
>> 
>> 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.
> 
> A high IP3 LNA means it can handle stronger signals without intermod of those signals.
> 
> When APRS first came along I suggested to the world that their frequency should be 145.790, just outside the satellite band and the world agreed. That kept it away from weak signal and EME and from repeater inputs. Unfortunately hams on the space station didn't understand FM bandwidth and insisted on using 146.800 the band edge for satellite signals and complained about QRM from APRS down 10 kHz. Had they understood that half their bandwidth was outside the satellite band and moved up 10 or 20 kHz there would not have been a problem. But they wouldn't move so APRS moved to 144.390 and you are experiencing the bad results of that move.
> 
> 73, Jerry, K0CQ
>> 
>> 
>> Kirk, N0KK
>> 
>> 
>> Sent from Outlook<http://aka.ms/weboutlook>
>> 
>> 
>> ________________________________ From:
>> nlrs-bounces at mailman.qth.net<nlrs-bounces at mailman.qth.net>  on behalf
>> of Mary Brown<maryalanab at gmail.com> Sent: Saturday, October 28, 2017
>> 2:38 PM To: geraldj at netins.net; nlrs at mailman.qth.net Subject: Re:
>> [NLRS] 2m Pager? noise
>> 
>> 
>> 
>> You have KE0CRW-10 RX I-Gate in New Hope, KD0TLS-7 also on APRS,
>> either could be the source...
>> https://aprs.fi/#!addr=New%20Hope%2C%20MN
>> 
>> Mary W0AAT
>> 
>> -----Original Message----- From: nlrs-bounces at mailman.qth.net
>> [mailto:nlrs-bounces at mailman.qth.net] On Behalf Of Dr. Gerald N.
>> Johnson Sent: Saturday, October 28, 2017 1:52 PM To:
>> nlrs at mailman.qth.net Subject: Re: [NLRS] 2m Pager? noise
>> 
>> 
>> 
>> While intermod of out of band signals isn't impossible, its almost
>> certain the signal at 144.390 is APRS, 1200 baud packet.
>> http://www.twinslan.net/packet.html says listen on 145.01, 145.67
>> and 144.390 to hear packet signals. At least a couple should be 1200
>> baud AFSK, 145.67 might be 9600 baud FSK which sounds like noise.
>> Around here 144.390 is still 1200 baud packet according to my ears.
>> Other than APRS I think there is very little VHF packet in Iowa now.
>> And the APRS graphic locator page shows several APRS stations close
>> to N0KK.
>> 
>> 73, Jerry, K0CQ
>> 
>>> On 10/28/2017 11:40 AM, Chris Elmquist wrote:
>>> 
>>> 
>>> On Saturday (10/28/2017 at 10:56AM -0500), Dr. Gerald N. Johnson
>>> wrote:
>>> 
>>> [... snip ...]
>>> 
>>>> I have it on line at:
>>>> http://geraldj.networkiowa.com/papers/EME/Intermod2.pdf Intermod
>>>> from other band activity is not a new EME problem.
>>> 
>>> If you determine it is actually intermod, I have had good success
>>> with a notch filter from PAR Electronics,
>>> 
>>> http://www.parelectronics.com/amateur.php
>>> 
>>> His approach is to notch one or more of the out of band signals
>>> enough that intermod is no longer an issue in your front end.
>>> 
>>> I was playing with one of the super cheepy Chinese 2m FM rigs (a
>>> TYT-9000D) and found that two signals in the 152 to 157 range were
>>> mixing and ending up on 146.4'ish, plain as day.  I wasn't sure
>>> why W0GHZ was dispatching ambulances on our "intercom" frequency
>>> ;-)
>>> 
>>> After I determined the exact frequency of the real ambulance
>>> service, PAR built the filter to notch it out and the problem was
>>> solved.
>>> 
>>> Now, these won't take 1500W EME transmit power so you would have
>>> to put this in your receive-only path somehow if you are
>>> transmitting with more than about 50W I think.
>>> 
>>> PAR (aka W4OP) do a nice job and give you an insertion-loss&   SWR
>>> plot for the filter they build, showing in-band and out-of-band
>>> losses. Well done, IMO.
>>> 
>>> Chris NØJCF
>>> 
>>>>> On 10/28/2017 7:31 AM, Kirk P wrote:
>>>>> 
>>>>> 
>>>>> Good morning,
>>>>> 
>>>>> I recently installed a 4x7 array for EME and a SDR RX for all
>>>>> bands.
>>>>> 
>>>>> 
>>>>> I have some very loud noise that sounds like a pager system at
>>>>> 144.390.
>>>>> 
>>>>> 
>>>>> I have tried all measures I can think of to determine if it is
>>>>> my
>> network or something from within my home. I have disabled network,
>> unplugged wifi and router, inserted a 2m bandpass filter etc. No
>> luck.
>>>>> 
>>>>> 
>>>>> If I move antennas there is no change in signal ...
>>>>> 
>>>>> 
>>>>> Stumped here looking for ideas. Can anyone else hear this
>>>>> locally or has
>> anyone experienced this at all?
>>>>> 
>>>>> 
>>>>> My main concern here is that it does at time seem to desense
>>>>> WSJTX
>> ability to hear and decode which of course isn't good if I want to
>> succeed at EME.
>>>>> 
>>>>> 
>>>>> Kirk N0KK
>>>>> 
>>>>> 
>>>>> Thank you
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