Is this interference i see at 137MHz?

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Re: Is this interference i see at 137MHz?

Post by LAH » Wed Jul 22, 2020 6:14 pm

Took me a while but I'm back with some updates, I finally received the LNA and the BCFM trap (don't work with Fedex if you have a choice). Been doing some testing and getting used to for the last couple of days.

My first conclusion is the cheap 137Mhz bandpass from aliexpress is useless, at least with my setup. If anyone is considering, I suggest thinking twice. I quickly found out that it will not pass through bias tee so unless you have other means of powering the RTL-SDR blog LNA, it is not possible to put the filter after the LNA. Which seems to be the only way to get reasonable results with it. Even then, I think the insertion loss at 12dB is simply too much, eating up nearly half of the LNA boost. On the other hand I have no prior experience with bandpass filters. It might simply be how they work, maybe someone with more experience might confirm this.

The RTL-SDR blog FM trap works like a charm. It does a much better job suppressing the BCFM band than the 137Mhz bandpass filter and the insertion loss is unnoticable to me. I tried both with the filter before and after the LNA. Although the general advice is to place any filters after the LNA, I found if I place the filter before, the interference was seemingly less prominent.

Overall I did not get the improvement I was hoping for with the LNA and the filter added but don't get me wrong both the LNA and the filter is absolutely performing as advertised. Maybe I was expecting too much. My reception hovers right at the edge of SNR 20dB throughout most of the lower NOAA elevations so I thought a couple dB of improvement would have significantly cleaned up and extended my images but this was not the case. I noticed the APT signal is prone to clipping with the LNA. It can get pretty loud, easily reaching SNR 36dB with an overhead pass. So when receiving APT I tend to keep the dongle gain relatively low.

Meteor signal on the contrary seem to perform better with lots of gain. I found the longest images were possible when almost maxing out the dongle gain. I am still experimenting with the gain levels, I only got a rough idea for now. When the LNA is on, It changes the "feel" of the gain and I had to readjust and explore once again to find the sweet spots.

Anyhow, I will keep trying. Once I find a reasonable way to power the LNA externally (thinking batteries) I will try the chinese bandpass filter after the LNA and see how it behaves. I have high hopes for this config.

Posts: 15
Joined: Tue May 26, 2020 5:27 pm

Re: Is this interference i see at 137MHz?

Post by LAH » Wed Aug 19, 2020 6:30 pm

The good news is i finally managed to put together a usable setup with the band pass filter from china. Bad news is it was not the answer to my problem but yielded other benefits.

I decided to power the LNA behind the bias-tee-blocking band pass filter with one of my half dead lipo batteries. The nominal voltage is 3.7v for one cell and they charge up to 4.2v so it's well within the limits of the LNA. The one i used is a 3S with a dead cell, so i have 2 healthy cells that i can use through the balance port. I soldered a couple of arduino jumper cables inside the LNA with needle tips on the open end, managed to slip them through one of the screw holes of the LNA housing so no drilling needed. The needle tips of the jumper cable fits neatly into the balance port slots of the lipo battery.

For anyone considering to power the LNA via a USB powerbank, I have tried this but it produced significant and strange noise patterns all over the place and was completely unusable. I tried 4 different powerbanks each with their own unique noise patterns. A battery was the only way.

This was still not enough, the output was rather low so i purchased another LNA and connected it after the band pass filter making the setup LNA>BP filter>LNA and powered it using the bias tee from the RTL-SDR blog dongle. This is when i came to realise why all other commercially available bandpass blocks have a similar cascading setup inside. Mine obviously is much larger in size and came to be slightly costlier than a purpose built block but gives me the flexibility of 2 seperate LNA's for any future projects.

This setup gave me a significant increase in antenna sensitivity and consequently SNR, allowing to register usable images as early as 5 sometimes even 3 degrees elevation. I also observed the chinese band pass filter to pass 134 to 146 MHz with a soft roll off at each end as advertised. However the reception is rather unpredictable. Sometimes it gives great results and sometimes not, producing images with several artifacts and/or banding. The worst of these problems is the fluctuation of the signal level. This is more pronounced with the Meteor signals for some reason. I decided to experiment with different type of antennas next. The potential is great, when everyhing works out, i can get a very long and very clear image from both NOAA and Meteor with the V dipole but it's hit or miss.

The repeating interference i posted initially is still there but much less prominent. I can say it is no longer an issue for me. Having said that, i think i also managed to find the actual signal that might be the culprit and is a total destroyer. This one i call the airband monster. It is a single strong signal moving the same erratic way and seemingly have no connection to the initally reflection-like repeating interferance i posted in the beginning. I am attaching a couple of sample waterfall images. The one on the left is at moderate gain and the other at max. I also made an audio recording of the noise but could not attach here, it sounds like a helicopter, i measured the pulses to be repeating at a rate of around 6.5hz if that means anyhting.
Pulsating meteor.png
Pulsating meteor
Pulsating meteor.png (630.48 KiB) Viewed 1592 times
Airband monster
sidebyside_.png (1.31 MiB) Viewed 1592 times

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Re: Is this interference i see at 137MHz?

Post by JEL » Fri Aug 28, 2020 2:45 pm

LAH wrote:
Wed Aug 19, 2020 6:30 pm
This one i call the airband monster.
I get those too.

I am using the Airspy HF+ Discovery, but with that radio they are dependent on the gain (Or attenuation, as it is on the discovery)

If I change the attenuation they will either show up or go away, at the frequency I am on. So that obviously makes it easy to get rid of them on the discovery, but may not be of much help to you.

On some frequencies it is a certain gain that brings them in. On other frequencies it is a different gain-setting that brings them in.
(Or again; attenuation-setting, as it is on the discovery)

I don't know what causes them though, but I'm guessing it has something to do with how these SDR radios work in general.

To me they sound like howling snow-storms :)
Like a hurricane in antarctica or something.
Basically a loud screaming noise that floods the frequency completely as it wobbles up and down in frequency.

I have had those on all SDR radios I have used.

I even think I remember hearing similar noises when I used analog radios in the 1980s, but I could remember wrong.

I have no idea how to get rid of them (Other than via the gain-trick I mentioned)

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Joined: Tue May 26, 2020 5:27 pm

Re: Is this interference i see at 137MHz?

Post by LAH » Sun Oct 04, 2020 6:14 pm

I like that HF+ discovery has filter banks onboard, i don't know how good they work but sounds appealing to me. I was considering airspy mini as an upgrade, if i can justify the cost, i might go for the HF+. I've been reading mix reviews for upgrading from the RTL-SDR blog dongle, most say if for reception purposes only, there is little change. I could use a wider dymamic range though, which i think both the airspy mini and HF+ will provide not to mention a wider waterfall.

Unfortunately the noise won't budge with gain adjustments for me. If it's not on my signal or if my signal is strong enough (which it usually is with the double LNA's) then it's not much of an issue. Would be nice if i can figure out the source.

I witnessed the imprtance of the antenna setup and also following your advise, the placement. I figured i was also suffering from what i think are reflections. I have plenty of straight concrete surfaces around. These are not apperent on the waterfall but probably mess up the polarized signal, which you can hear in the audio and see as equally spaced banding on certain positions on the images with passes at certain elevations. Some elevations come in spotless while some have these bands with varying intensity. I tried new locations with reflections in mind and found a quite ok place, not perfect but i have no choice but to compromise. I'm quite happy with this setup for now. This might even be close to the optimal which can be achieved with what i have at hand.
JEL wrote:
Fri Aug 28, 2020 2:45 pm
when I used analog radios in the 1980s
I can barely remember the 80's if at all :) but it must have been very interesting, especially when internet was not around and all the cold war stuff going on. Can't imagine listening to the woodpecker live on radio :D

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