Filters 101 : Improving reception with your SDR or radio - a look at RF filters 

Thanks Joao. Appreciate your kind words.

@@FrugalRadio 🙏🙏🙏🙏

Ha ha, you're welcome!

Yes, the broadcast transmitters throw out a lot of power. My local FM transmitters are pumping out over 100kW. Haven't looked up the 3 local AM transmitters yet.

Welcome back to the radio hobby Marty. Certainly a lot has changed in 40 years! Glad the video was helpful 🙂

You're welcome. There's always so much to discover it seems!!!

Thanks for your comment. Wishing you a happy new year!

Thanks @Tommy Boy. Glad it was helpful!

Thanks.

Great. Welcome to the channel Dave!

You're welcome, and thanks for taking the time to comment.

I think there is an example showing the SDR being unusable in 1 or 2 of the videos. You don't hear the difference so much as see it on the waterfall display & FFT.

I'm glad to be of help, and appreciate your thoughtful comments.

Thanks Lane.

Sounds like you've nailed it when it comes to the filters. Good job! Nothing beats a nice clean signal.

Thanks.

Glad to be able to help!

@@FrugalRadio I've subbed, lots to learn on here. It's great that you go to this effort to make some quality videos to teach others. Very appreciated. :D

Welcome to the channel @Sarah Cartwright!

Hi Ron, good to hear from you again. Thanks for the comments. Yes, these SDRs are certainly widening the hobby which is great. The Nooelec Sawbird NOAA filter linked in the video description actually includes a bias t powered LNA, so I expect you would receive quite favorable results with it!

Thanks for your kind comments, and for taking the time to get in touch. Right now I'm using a simple home-made dipole for civil air monitoring. In Ireland I was using a bow tie antenna, which I hope to make a video on someday...

Glad you've identified the issues. I didn't get into the issue of filters not being perfect, rolloff, and insertion loss. I wonder if their filtering in the RSPdx is tighter?

@@FrugalRadio Sadly it is the sdrRSPdx I'm referring to. I'm lucky in the respect that if I use my dedicated airband antenna the FM breakthrough is non existent. If I use my J pole however I get a little breakthrough like you at around 118 - 120MHz and that's when I noticed the distinct attenuation to the frequencies within the airband spectrum. I guess it's a tradeoff and that said the function of the software defined reject filter does do what it says on the tin.

Oh, interesting. That's one thing I like about the Airspy HF+ Discovery. The bandpass filter starts at 118 MHz and seems to be rock solid. It's the highest SNR I've seen on 119.5 & my local tower freq of 118.3. But having these filters built in certainly does help. Even if there is some loss, you do get signal coming through.

Hi Paul. Yes, the Ham It Up actually starts with a low pass filter, although it is quite wide. It attentuates 65 MHz and up when activated. You can see the charts on the datasheet for more detail. www.nooelec.com/store/downloads/dl/file/id/98/product/16/ham_it_up_v1_3_barebones_datasheet_revision_2.pdf

@@FrugalRadio You are a mine of information, sir.

If you ask the wife, it's useless information 😮 But we know better, don't we!

Hello. There is a switch for the Broadcast & DAB filters. There is a graphic on the following link that shows it. www.sdrplay.com/explaining-filters-for-newcomers-to-sdr/ The notch filters also work, but in a different way. They are better for dealing with noise on the audio side, and are used for processing the audio signal. The preselect (automatic) and MW/FM & DAB filters deal with the RF side of things.

It depends. The correct method of installing an LNA is to position it as close to the antenna as possible. Generally when you do this you will experience an improvement in signal strength, as the signal is boosted before being sent through the coax. You may received mixed results if the LNA is located closer to your SDRs.

No, haven't played with a HackRF. Looks like it could be a fun device.

My advice is really contained in the first few episodes of the 2020 / 2021 SDR Guide: ru-vid.com/group/PLe5ZKeM2hRBJ2G_Gvt1JnBxqtjHMMej3q

www.parelectronics.com/index.php will make any filter you need, if they haven't got a standard model that will work for you.

@@FrugalRadio Thanks!

Thanks for your kind comments Dan. I use one of the FA dongles too.

@@FrugalRadio Yeah, I've been in an out of ham radio for awhile. Started in 2001 or so and had lots of life "things on my plate", and starting to get back into it again. Lots has changed and for the better. JS8CALL, digital modes [D-STAR, DMR, etc] and of course, SDR. Came across your channel a few weeks ago and getting right back into it, so I will be monitoring your channel. My QTH is at CYWG. I used to live at CYEG in 04-06 when I was going to NAIT, so we could have been neighbours....

Yes, there have been loads of developments in radio over the last few years. I love what is possible now, and would have bitten someone's arm off to have virtual radar and inexpensive SDRs when I was a teen! I enjoy some HAM activities as well, and use DMR through reoeaters and a hotspot. We might indeed have been neighbors in 05/06! Hope you're keeping safe out there in YWG.

If you tune to signals in the regular FM Broadcast band you can expect to still find the stations. Their signal strength should be lower than without the filter. Although it creates a notch, many are still strong enough to cover through. However where you see the real improvement is in the areas the signals were breaking through before - e.g. other VHF / UHF frequencies. But if you are finding that you are still being breakthrough from FM Broadcast on other frequencies, then there is a problem. You can test by lowering the gain on your SDR. At the lowest setting, tune to an FM station, and note the SNR. Next insert the filter and take the SNR reading again. If it is not significantly lower, then there is a problem.

Hi Elie. Thank you for your contributions to the radio community via Live ATC. It is great to have people like yourself providing these feeds 😄 Interesting to hear you say you are still suffering from FM Broadcast issues even though you are using the AOR Bandpass filter. The first thing I would try would be removing the pre amp. Preamps often make things worse in urban environments. It will probably be amplifying whatever Broadcast signals are coming though. I would also check the coax to make sure extra signals aren't getting in. The FM notch filters are quite inexpensive, so I guess it could be worth trying as well. I would place it before your bandpass filter.

@@FrugalRadio Hello, I've just received and add to my setup the Flamingo v2. Now I don't have any problem with the FM band while receiving planes on the ground. Thank you for your help. Today I'm receiving the signal over a Uniden 9000XLT. Do you think it will help if I change it for a Malachite, knowing I'm searching for the cleanest signal possible ?

I haven't yet used a Malachite, so I'm unsure. If you decide to try one, report back and let us know if it cleaned up the signal some more!

Hi. No, a filter generally wouldn't help in that situation, because the signal causing the overload is on the same frequency you want to monitor. I'm not sure what SDR hardware / software you are using, but you could possibly try enabling the auto gain control (AGC). An SDR with a higher dynamic range would probably help in this situation as well.

@@FrugalRadio Appreciate the sound advice. Thanks for getting back with me.

You're welcome. We are all here to help each other in this hobby 😃

Good to hear the RTL-SDR fillets are working as intended, even with such strong sources nearby. As for the AC, I'm not sure what the best way to take that is. Which part of the band does the interference show on?

@@FrugalRadio I hear it pretty much everywhere, but it is a lot stronger in the lower frequency ranges from medium wave up into the 20mhz range but I can still hear it in the higher frequencies bit not as bad, I can at least get some air traffic and some VHF and UHF activity, but the noise floor is still high enough to where it's harder to hear anyone talk.

Sounds terrible! A post that might help you get started solving the AC noise is swling.com/blog/2021/03/guest-post-indoor-noise-and-ferrites-part1/ That chap's experiments seem to have paid off.

@@FrugalRadio the interference even effects my Eton Elite Executive, but it seems to be able to reject the interference lot better than the SDR. Medium wave on the EEE isn't effected by it. The interference starts on the EEE at around 1700kHz up to 30mhz, the air bands aren't effected either, the FM broadcast bands aren't effected by it either (being that they're right next to the air band and the air band isn't effected).

When you're a few blocks away from an AM site, that's going to be tough. You can try a double filter (i.e. 2 filters in series), or you could make / order a specific notch filter for the offending frequency. You could try someone like PAR Electronics who will custom make one for you.

Hi @JustSomeGuy. It would be important to note which signals are breaking through on which frequencies. For example, are you hearing Broadcast FM in all these bands? Or does Broadcast FM just impact the VHF Public Safety band? Do you have a cell tower nearby? They can also cause interference in the 700MHz band, but like you say it can also be TV transmitters. I would start with a Broadcast FM filter. The Flamingo+ filter (amzn.to/39ZJtHP) has greater attenuation than the Flamingo model I tested, and is only $1 more. I would also try to move away from a Discone. An antenna tuned closer to the bands you are interested in will also attenuate out of band signals to some degree. The antenna I use for Public Safety monitoring is designed specifically for the bands you wish to monitor. It covers the VHF, UFH & 700/800 frequencies. See amzn.to/2MDMNza - I have this antenna paired with a Ground Plane Kit (amzn.to/3aDJVuc) mounted 40ft up. I also have 2 for vehicular use that work really well. If the TV channel interference is quite strong, you would need to work out which channels are causing issues. You can then pick a filter that will help reduce those from somewhere like www.markertek.com/category/rf-channel-filters. Hope that helps!

The HF+ Discovery beats the RSP1A in terms of VLF, LF & HF performance. It is a great SDR. The RSP1A offers much greater frequency range though. And yes, the FlightAware blue dongle has done filtering built right in 🙂

@@FrugalRadio OK, so will buy HF+ next month :).

Happy days. Guess we know what you're Christmas money is going on 🙂

@@FrugalRadio rather, from the January payoff

Fair enough @MrDwadi84

That would depend on what you're trying to accomplish, and on which frequencies. A lot of single filters are band reject. Eg a typical high pass filter would only allow frequencies over 30 MHz. This is effectively an HF band reject filter.

@@FrugalRadio Yes, My RTL.SDR Is Going Up To 30MHz With Q Branch Settings, When I Switch To Disabled Then It Goes To Whatever Frequency, But With Q Branch I Can Listen To HF Bands, I Made Antenna, But The Thing is That AM Is Cutting In To My 80 Meter Receptions, Makes Me So Sad. ;D

There are 12 preselect filters that are fully automatic. So if you tune to the fairly active 11175 kHz USAF GHFS frequency, everything above 12000 kHz is automatically filtered out. You can see a list of the automatic filters at www.rtl-sdr.com/a-review-of-the-sdrplay-rsp1a/ There are 3 extra notch filters that can be accessed via SDRUno. I don't know if these are available on Mac or Linux when using Cubic SDR.

@@FrugalRadio thank you I appreciate you

I wondered the same thing Fritz. I had thought it would have been marked. In the end I tested mine in both directions, and both seemed to work. If yours is not marked, I guess trial & error is the way forward!

Ya some of my filters are marked, especially the professional ones but the Noelec filters and amplifiers are not marked.

I don't know why they aren't marked. With an amplifier, your think that it would be important to know the direction. Seems odd that Nooelec have opted not to make things clearer, especially since some of their photos show markings!

Yes, and each model builds upon the others in terms of features and components.

Yes, FM broadcast filters are designed to reduce the bleed from high power commercial stations.