Notching out one frequency
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Greetings all,
I have on station locally that is wiping out the BCB (1240 khz). I do have a Clifton Labs filter in line. It attenuates the BCB from 550-1600 khz. This allows below MW (NDBs, LW to be easily heard. Of course, it doesn't help when one to listen in the BCB itself. The station is only 1000 watts, but is 2 miles as the crow flies from me. This is a recent problem. Previously I have not had it before with this station. The receiver is an Elad S2. I do not have this issue with my Perseus. With SDRC is there a way to notch 1240 without affecting the rest of the BCB? Stay safe everyone. Regards, George, NJ3H Redmond, Oregon USA What suggestions do you all have that could help me out? |
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Well,
If it’s wiping out the band then it sounds like signal overload. Post a screenshot so we can see the ELAD settings. Maybe your option is just a big, homemade notch filter.
Simon Brown, G4ELI https://www.sdr-radio.com
From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of George Stein - NJ3H via Groups.Io
Greetings all, |
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Simon,
Attached are two pics. In the Elad picture the gap in the waterfall is the time I had my bcb filter engaged. The SdrC picture shows station signals behind the "cloud" of QRM from the local station. I have tried calling the station, but no answer. The station is mostly automated. with the covid thing going on, they must not be at the office. As I mentioned in my first message, this is a new situation. You can see the station is a measly 60 over S9 here. What a weak reception. lol lol Thanks, George |
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jdow
Any notching will have to take place in the analog realm if reducing the gain or introducing an attenuator does not fix the problem. You have the attenuator off. Turn it on. Increase it if you can until a weak signals SNR gets worse. Then go back up that last step for best sensitivity and dynamic range.
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{^_^} On 20200327 14:46:06, George Stein - NJ3H via Groups.Io wrote:
Simon, |
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Thanks so much for the hints, Joanne,
Testing two frequencies, the following SNR (db) readings were observed with the offending station on 1240 KHz: 298 KHz Station Clifton Lab Filter On, SDRC 20 db Attenuator On 27 Clifton Lab Filter On, SDRC 20 db Attenuator Off 31 Clifton Lab Filter Off, SDRC 20 db Attenuator On 24 Clifton Lab Filter Off, SDRC 20 db Attenuator Off 15-31* * fluctuates as the 1240 khz BCB station rises and falls in music intensity 1430 KHz Station Clifton Lab Filter On, SDRC 20 db Attenuator On 7 Clifton Lab Filter On, SDRC 20 db Attenuator Off 18 Clifton Lab Filter Off, SDRC 20 db Attenuator On 40 Clifton Lab Filter Off, SDRC 20 db Attenuator Off 45 Hopefully your can interpret the data for a recommendation as to the next steps. Is it possible we could talk off-line? I am good at QRZ Regards, George, NJ3H Redmond, Oregon |
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George Try making a simple bandstop filter. See my LPF I made the other day using a couple of on-line calculators. One for the filter and another for coil construction. Scroll down to the bottom of this page http://www.radiomuseum.co.uk/SDR.html Use simple coils, nothing fancy. My results were just about spot on and using your SDR itself to fine tune the filter you should get 50dB attenuation with a 7 order elliptic bandstop filter. Try 1220-1260KHz. Best results will be using the setting Exact rather than Standard values. Allan G3PIY From:
main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of George Stein - NJ3H via
Groups.Io
Simon, |
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jdow
I'd trust your ears more than SDRC's SNR reading. I can spoof SDRC's SNR calculation by 40 dB or more. For those readings you gave 20 dB is maybe a touch more than needed. For 298 kHz the attenuators make essentially no predictable trend; but, the 1430 kHz station the 20 dB within the front end (SDRC attenuator) can be on with little sacrifice.
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Other data that will indicate if a attenuators do anything is looking at the spectrum with and without the attenuation. When you go in and out of overload the spectrum changes dramatically. All that said, looking at your two displays you show I see an apples and left-hand threaded nuts comparison. (Ditch the S-units display for dBM to get back to comparing pineapples and oranges. At least both are fruits.) It looks like the amplitude range of the SDRC display is vastly different from what the other thing's display showed by as much as a factor of two. THAT will change the picture dramatically, too. (A 78 dB range on SDRC and a 140 dB range on the other thing. Make the scales equal to make a reasonable visual comparison.) By the way - what 1430 kHz station? I don't see one on either display. 1120 kHz might work. 680 kHz might work within the broadcast band. Note that the visual gain control can move the spectrum up and down on the display but does not change anything in the front end. So you can juggle the display high and low settings as well as visual gain to get the display's to be on roughly the same display range. Speaking of which, they are running at different sample rates, too. Or at least they are not showing the same range. Finally - are those two on the same antenna? It looks more like the SDRC picture is on an antenna draped over your computer. But that might be the crazy scales differences as much as anything else. Then we are into comparing types of grapes. {^_^} On 20200327 17:14:13, George Stein - NJ3H via Groups.Io wrote:
Thanks so much for the hints, Joanne, |
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Hi,
Remember to use attenuation as below. Try this…
Simon Brown, G4ELI https://www.sdr-radio.com
From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of George Stein - NJ3H via Groups.Io
Simon, |
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Tom Crosbie G6PZZ
Allan, What an extremely good read. I’ve had it in my mind to use varicaps for tuning a MW frame antenna and you’ve set me on a path! Many thanks Tom G6PZZ
From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io>
On Behalf Of Allan Isaacs
Sent: 28 March 2020 00:14 To: main@SDR-Radio.groups.io Subject: Re: [SDR-Radio] Notching out one frequency
George Try making a simple bandstop filter. See my LPF I made the other day using a couple of on-line calculators. One for the filter and another for coil construction. Scroll down to the bottom of this page http://www.radiomuseum.co.uk/SDR.html Use simple coils, nothing fancy. My results were just about spot on and using your SDR itself to fine tune the filter you should get 50dB attenuation with a 7 order elliptic bandstop filter. Try 1220-1260KHz. Best results will be using the setting Exact rather than Standard values. Allan G3PIY From:
main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io]
On Behalf Of George Stein - NJ3H via Groups.Io
Simon, |
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Thanks Tom My own aerial stopped working recently because rain got into the diecast box and is now horizontal (in a queue) awaiting attention. Alas.. it was still at the experimental stage! 500pF Varicap diodes are on ebay unless you have an old medium wave set you can cannibalise? When it was working it completely got rid of local interference so I must resurrect it because my neighbour has started using mains Ethernet adaptors for his smart TV. There’s a U-tube video which deals with filters to eliminate pick up on feeders which I’d like to try when back in business. 73 Allan G3PIY From:
main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of Tom Crosbie G6PZZ
Allan, What an extremely good read. I’ve had it in my mind to use varicaps for tuning a MW frame antenna and you’ve set me on a path! Many thanks Tom G6PZZ
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Hi Simon,
I have used the 20db attenuation. Apparently that is all the Elad allows. The Perseus is different in that the values on the Perseus GUI are replicated in your program (0, 10, 20, 30 db). It would be nice to be able to "deeply" attenuate just a small range of frequencies for example 1220-1260 kHz, which would take care of my 1240 problem. I did talk to the station owner today. He informed me that there have been no changes to the transmitter and the antenna system in a very long time. They do not have a full time station engineer. They contract out to a guy who roves around the state of Oregon. Since my setup here has remained unchanged (SDRs and antenaa), he had no idea why signal strength has gone up so dramatically (40 db over s9 to 60 db over s9). The broadcast band filter works fine to allow usage of the frequencies above and below the bcb. My issue is to get 1240 zapped so I can dx within the bcb itself. Stay safe over there and all the best. George |
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George, have you tried a series tuned notch filter for the offending station frequency ? With reasonably good capacitor and coil components, that has worked for me in attenuating a very strong nearby local BC signal.
Denny - K0LGI - |
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Hi Denny,
I have sent you an email to your QRZ address. Regards, George |
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Just sent it to you George :) Sorry has been really busy right now with tornadoes in the vicinity around here too ! Denny On Sat, Mar 28, 2020 at 5:38 PM George Stein - NJ3H via Groups.Io <nj3h=yahoo.com@groups.io> wrote: Hi Denny, |
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jdow
George, please show screen shots for no attenuation and for 20 dB attenuation. Change them from s units to dBm as it's easier to compare the program that you say works and the program you say does not work if the display units are approximately the same.
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Let's determine if there really IS something wrong or if it is a setup problem. From the looks of things you are objecting to a setup problem. (Notably the visual gain should be reduced. And the range for the display should be set to the same as the range on the other program, at least for testing.) Maybe Simon needs to make a nominal change or maybe the controls simply have to be set better or maybe operator interpretation of what is seen needs to be changed. Let's methodically go after a solution rather than scatter shot for silly solutions (change at the station). And just for my knowledge base the two screen captures I've seen from one of your previous postings were from the same computer taken at basically the same time on the same day, right. A third screen shot with 20 dB attenuation installed would help determine what is going on. Without it and without a willingness to make adjustments I cannot (hence will not) help you. With these prerequisites we can iterate in on a solution. {^_^} On 20200328 13:43:18, George Stein - NJ3H via Groups.Io wrote:
Hi Simon, |
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Hello Joanne,
Thank you for you offer to help. We have our grandson every day, so was busy with him yesterday. He will be here shortly, so am trying to do what you need quickly and accurately. Attached are 5 files. 1. Elad No Attenuation 1 2. Elad No Attenuation 2 3. Elad 20 db Attenuation 4. Perseus No Attenuation 5. Perseus 20 db Attenuation The Elad is the radio that appears more susceptible to the strong station (1240 khz). The Perseus appears to be able to withstand the strong signal. Per google maps, I am 1.79 miles (straight line) from the transmitter. The station broadcasts 1000 watts both day and night. The screen captures were made Sunday 29 March 2020 just after 11 AM PDT. The reason for two "Elad No Attenuation" files is that as the station in question is broadcasting, the spectrum display would jump up and down. I tried to capture both the worst and best images of the jumping. The other files names are self-explanatory. I tried to make the display of both radios as equal as possible covering the same frequency range and signal strengths. You are correct. The previous screen captures were made just a few minutes apart. Kindly let me know if there are any other information I can provide you. Regards, George |
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Additional Information
I realized it might be helpful to include a couple of screen captures of the Perseus waterfall. The Perseus cannot display the spectrum and waterfall view at the same time. These were made approximately two hours after the other Perseus pictures sent in my previous message, but they are representative of the situation. The pictures show no attenuation and 20 db attenuation Regards, George |
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jdow
Um, I can't tell much from those pictures because they were not all with the ELAD FDM-S2. Different front ends MUST be presumed to behave differently in the presence of noise.
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One thing I can tell from the attenuator/no attenuator on the SDRC FDM-S2 pictures is that you really honestly truly assuredly want to run with the attenuator on. You actually show maybe a 5 or 6 dB drop in signal and about a 26 dB drop in noise. You are actually MORE sensitive with the attenuator in. That is partly due to the reduction in noise introduced by over-driving the ELAD's front end. It appears you have a fairly large antenna you are working with. That means it is at least "fairly" efficient. The Perseus results hint that this is the case when its attenuator is set to 20 dB. The distance between the peak signals and the noise line does not change. That means noise coming in from the antenna is swamping any noise the receiver front end may be producing. This is extremely likely (99.44% (old person humor)) that the Elad behaves the same. This is die to several noise sources that raise best you can do WAY above the best a radio can do. With conventional radios disconnecting the antenna is a very simple test for "too much attenuation in front of the radio?" If the noise level (meter reading or noise from the speaker) drops you can likely add more attenuation without masking any signals that the natural noise external to your receiver is not already masking. At 28 MHz external attenuation can be 6 dB or so. For 2 meters, 1 dB or so for really quiet receivers. At 1.8 MHz you can add 30 dB or even 40 dB without masking more signals. This pretty much holds all the way down to the single digit kHz if I remember the graph correctly. (It actually continues on up with noise more than 100 dB in excess of a simple resistor's noise, which is the best you can do. Antenna efficiency goes down so I suspect darned few people can add 100 dB of attenuation at 40 kHz.) Ultra low noise receivers on HF are generally a waste unless you are using something like a short whip antenna. {^_^} On 20200329 11:47:02, George Stein - NJ3H via Groups.Io wrote:
Hello Joanne, |
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Hello Joanne,
Thank you for your analysis. I will study it thoroughly. I should have mentioned the kind of antenna I have. It is a Wellbrook ALA1530-S2. This antenna is nominally 1 meter in diameter, with an active amplifier mounter at the base of the antenna. I have the antenna mounter at about 12 feet high 40 feet from any structure. Inside the house is an active Bias T that feed directly to an Elad ASA-42 amplified splitter. In an email exchange with a gentleman in Washington state, with a very similar setup, he replaced the active bias T with a passive bias T from Wellbrook. He mentioned that resulted in alleviating his issue with a 5kw station. In an email discussion with Andrew Akin, Wellbrook owner, I was told that going from the active to passive bias T would result in a 9db reduction in signal strength. The passive version is 60£. Is this a worthwhile option for me? I will look at disconnecting the antenna tomorrow and seeing the results as described in you reply. If I interpret your message correctly, I should be able to attenuate the incoming signal and not affect the week stations that I want to receive. I do not transmit on HF these days (too close to neighbors). I do monitor FT8 just to see what DXCC countries I can hear (158 to date). So my goal is not to lose out on the weak stations (of course on FT8 a lot of them are). I used to use Ivory, but my wife suggested a moisturizing soap, so I got the 99.44% reference. HI HI Thank you again, Joanne. Regards, George |
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jdow
Very likely you can add attenuation beyond the Elad's 20 dB without affecting signal to noise ratios. I think your measurements suggested you had another external 20 dB attenuator. Note the amplitude of the 1240 kHz peak and the noise with the ELAD attenuator enabled. Add the external 20 dB and repeat the measurement. That should be quite informative. If the difference in readings is 20 dB or even as low as 18 dB leave it in as that is about the best you can do.
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{^_^} On 20200329 23:04:20, George Stein - NJ3H via Groups.Io wrote:
Hello Joanne, |
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