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Locked AM frontend overload?

 

?? (Recording of my terrible RFI)
Is this AM frontend overload?? What could I do?
 

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It could be receiver overload, but a very severe case. For sure there's intermodulation (sounds like at least two signals). Does the garbage appear no matter where you tune the receiver? If you had an attenuator you could switch in between the antenna feedline and the radio antenna port you should be able to increase attenuation and at some point see a dramatic drop in the garbage if it's being generated in the receiver. If the stuff just gets weaker but doesn't go away, then it's likely some kind of intermod being generated externally.

73...
Randy, W8FN

On 1/21/2022 5:45 PM, Kun N7DMR wrote:

?? (Recording of my terrible RFI)
Is this AM frontend overload?? What could I do?
 

It appears today (It could happened earlier but I didn't check). I need to try to turn my loop and see if I could notch them out. It's possible that the wind may have blow my loop into a position that's being overloaded by AM broadcast. I don't know.? Do you think an AM Broadcast filter would help?
 

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Check your loop. Maybe there's something mechanically damaged that's acting as a diode and creating the RF trouble. If the loop has a preamp can you turn it off and see what happens. A broadcast filter would help only if the crud is being generated in your receiver. If it's being made by a mechanical problem in the loop or an external intermodulation source such as a semiconducting joint in something like a chain-link fence or another electronic device that's coupling into the power lines it won't fix anything.

73...
Randy, W8FN

On 1/21/2022 7:16 PM, Kun N7DMR wrote:

It appears today (It could happened earlier but I didn't check). I need to try to turn my loop and see if I could notch them out. It's possible that the wind may have blow my loop into a position that's being overloaded by AM broadcast. I don't know.? Do you think an AM Broadcast filter would help?
 

And it's my local time 4pm, that AM overload magically "almost" disappeared now. Not sure why.
 

And I didn't touch my SDR or receiver at all. The RFI just magically "disappeared" at around 4pm today.
 

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Probably one or both of the AM broadcast stations that are generating the spurious signal decreased power at "sundown" as required by their licenses.

73...
Randy, W8FN

On 1/21/2022 7:35 PM, Kun N7DMR wrote:

And it's my local time 4pm, that AM overload magically "almost" disappeared now. Not sure why.
 

I guess that's likely. I just looked up. There are 2 broadcasting station with less than 1 mile from my location. They are broadcasting at 1KW during the day time. Only one station is required to reduce power to 60W at night.? I guess the SDR could handle 1KW+60W but not 2KW.
 

I observed strong intermod spurs from AM BCB stations caused by oxidized contact in my antenna (although not a loop). Copper oxide is a semiconductor.
Cleaned the contacts, and the spurs were gone.

73, Mike AF7KR
 

Hi Kun,

You have three very strong broadcast stations operating on 1320, 1420 & 1490KHz which are overloading the receiver.

I posted a notch attenuator circuit for you to try on the KiWi forum website.

It's a simple series tuned circuit that is connected across the receiver input, and should provide at least 10 dB attenuation at 1320 & 1490KHz and 20dB attenuation at 1400KHz.

3dB attenuation frequencies are at 1000 & 1900KHz.

The inductor and capacitor should have a decent Q in order to achieve a decent notch depth.

The 4.7uH inductor can be made by winding 30 turns on a T50-2 (red iron powder) core and the 2.7nF capacitor should be a good quality polystyrene or similar.

Adjust the exact number of turns to tune the circuit to the required frequency.

Regards,

Martin
 

Thanks. I have an existing RTL-SDR Blog AM BCB filter. I'll try to get it installed to see if it helps. I originally though it won't work because the signal already amplified by the Loop amplifier.?
 

Instal the BCB stopband filter in front of the loop preamp!

Dave - W?LEV


On Sun, Jan 23, 2022 at 7:21 PM Kun N7DMR <sa4591@...> wrote:
Thanks. I have an existing RTL-SDR Blog AM BCB filter. I'll try to get it installed to see if it helps. I originally though it won't work because the signal already amplified by the Loop amplifier.?



--
Dave - W?LEV
Just Let Darwin Work
 

I can't install it in front of loop preamp. There is no place for me to install it. The preamp is on the antenna itself.?

But filter in front of my kiwi does solve most of the problem.
 

On Mon, Jan 24, 2022 at 12:36 AM, Kun N7DMR wrote:

But filter in front of my kiwi does solve most of the problem.
Yes now you have the filter fitted, it looks like the loop seems to be performing OK with regard to overload.

The rest of the interference looks like it's general noise and hash from switched mode power supplies, computers and other IT equipment or similar.

You may not have noticed this before, but the ability to 'see' 30MHz of spectrum with the KiWi, makes it much easier to spot wideband swathes of noise, that may previously not been so apparent when observed in a narrower bandwidth.

It was quite an eye opener when I got my first KiWi, as I thought my site was electrically quiet, but the waterfall and spectrum display told me otherwise.

Regards,

Martin
 

There is another tool the amateur can use to view large swaths of spectrum:? The SDRPlay.? I was recently made aware of a spectrum analyzer application made for the SDRPlays by Steve Andrews.? In the past, the SDRPlay line-up of products has not necessarily been my favorite, but this app has finally moved my RSP1A off the shelf.? Steve's SA application is really outstanding and well beyond the capabilities of the SpectrumSpy for the AirSpy products.? Possibly that is why SDR# no longer offers SpectrumSpy?? Believe me, it's spiffy!!? I'd encourage anyone requiring more than 8 MHz of on-screen spectrum display to give it a try.? And it covers all the way down to the submarine comm band and below to almost "DC" without an external converter.?

????

Dave - W?LEV?


On Mon, Jan 24, 2022 at 11:24 AM Martin via <martin_ehrenfried=[email protected]> wrote:
On Mon, Jan 24, 2022 at 12:36 AM, Kun N7DMR wrote:

But filter in front of my kiwi does solve most of the problem.
Yes now you have the filter fitted, it looks like the loop seems to be performing OK with regard to overload.

The rest of the interference looks like it's general noise and hash from switched mode power supplies, computers and other IT equipment or similar.

You may not have noticed this before, but the ability to 'see' 30MHz of spectrum with the KiWi, makes it much easier to spot wideband swathes of noise, that may previously not been so apparent when observed in a narrower bandwidth.

It was quite an eye opener when I got my first KiWi, as I thought my site was electrically quiet, but the waterfall and spectrum display told me otherwise.

Regards,

Martin



--
Dave - W?LEV
Just Let Darwin Work
 

Yes. Once I start running Kiwi, I suddenly aware of noises. I have replaced all the switching power supply in the building and taking some other approach to reduce noises. My Kiwi is in a grounded metal shelves. I changed USB cables for leaking noises. I do have to still house my MMDVM board and Raspberry pi in the same shelves. So I think that's a noise source. Also, the location is still close to power transmission line. That I can't solve.

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