Band-Pass Filters for QRM

In this video, I explain how band-pass filters can reduce or eliminate QRM (interference). I use a spectrum analyzer to demonstrate a band-pass filter that I built based on an article found in the September 1988 edition of QST magazine. The article can be read here: www.arrl.org/files/file/Techno...

[@wilsonlittle4833]

Cliff, I always enjoy your videos. Thank you for being out there advocating QRP. I personally do power levels from 800 watts to 6 watts SSB. 20 watts seems to be the sweet spot for my SSB, and 5 watts for CW. Currently doing POTA and need to find a solution to prevent interference from other activators in the parks.

[@QRPSchool]

These band-pass filters will help UNLESS the other stations are using the same band as you. And, as shown in the video, a nearby band (like 20m vs. 17m) will not be very attenuated as this filter design is not very narrow. Good luck with your POTA activations! That’s a very fun thing to do!!! -Cliff

[@Nico_83]

Thanks Cliff for your great videos. Keep 'em coming please. 73

[@DeeegerD]

Thanks and nice spectrum analyser. Now I just have to wait a few years until I can get one used in my budget ;) Until then FFT on the scope and a frequency sweep from my signal generator will have to suffice. Amazing what a handful of components can achieve.

[@WC6Q]

Great video! You have inspired me to build one, or two. ;)

[@stephenwhite7627]

You make great videos. Thanks for all the info. You are the Dave Ramsey of amateur radio (with more hair).

[@QRPSchool]

Thank you, Stephen. Dave has helped a lot of people. I appreciate the compliment! - Cliff

[@TheGmr140]

very nice video. good stuff

[@hosseinpersian6444]

great record . thank you .73!

[@hangfire5005]

I need to build a good trap for a nearby broadcast station that's wreaking havoc on me. Nice demo!

[@hanshendriks9209]

good info, thank you.

[@joblessalex]

Can we get a video on good antenna practices and just good antennas for qrp in general? Where to put it, how high, what type, what direction? Those kinds of things.

[@brianhall137]

It's also worth mentioning that you could have placed the bandpass filter on the mobile transmitter (w/ a 15m bandpass filter) instead of the receiver with similar results...and that the combined attenuation is additive if you had used filters of BOTH the transmitting and receiving rigs, since they both attenuate signals outside their passband. 73 de AC7NA

[@g00glian0]

Excellent!

[@northbetrue]

You are good at this, keep it up! Thanks and 73 KC1ANR

[@phillipneal8194]

Nice. Thank you

[@stationvictormike3415]

Just built one. Happened to have the parts in my parts supply

[@dalemyers5096]

I know you have little wants or need for high power filters. But what makes a high power or 1.5kw or higher filter? I'd hate my filter to burn up, because it's made wrong or undersized.

[@justinelliott3529]

Do you have anything on frequency mixers? I’m building a homebrew direct conversion and I’m looking for an easy circuit

[@user-uv4xe3cq2y]

Excellent! Thanks for sharing. BTW, what is the power rating on this filter?

[@sirowan9646]

Could you help me in making my own bandpass for 2 meter band? Thanks, 73!

[@alchemistTi]

love your channel! maybe I missed it but I didn't catch how you'd build one for other bands like 10m or 80m...is it just a change in capacitors and toroid configuration?

[@QRPSchool]

+alchemistTi That's right. The QST article that I used as a reference contains a table showing the various torroid and capacitor values for each of the bands. Check it out here: www.arrl.org/files/file/Technology/tis/info/pdf/8809017.pdf

[@srs26]

Hi Cliff, Do you have a recommendation for mobile HF? I am torn between the Kenwood ts480 and the Yaesu ft857 and I noticed you have both! What are your recommendations and also what antenna would you recommend? Kind regards, Simon M0S

[@QRPSchool]

Hello, Simon. In my opinion, the TS-480 is an excellent mobile rig. It has a fairly large control head with a wonderful display that is MUCH easier to read and interact with than the small display of the FT-857. Because of the size of the head, the TS-480 can include a good number of buttons on the panel - so you don't have to go into the menu system to do the things you'd want to do while operating. It's a more directly accessible experience than having to dip into the rotating three-choice menu system of the Yaesu radios. (I bought the FT-857 to put into a Go-Kit.) My antenna is a Little Tarheel II antenna. That's a "screwdriver" antenna. www.tarheelantennas.com/little_tarheel_ii I have it mounted up at the top of my SUV's back door. Having it up higher, above the ground plane of the vehicle, allows it to work very well. It's small enough that I can use a lip-mount. I've got this one: www.diamondantenna.net/k400-38c.html I have worked a lot of DX from my car with that antenna. I would definitely buy the same antenna again, based on my good experience with it. I've had it for several years. The only problem I've experienced is the control cable eventually became crushed by the back door. I got a replacement cable for $20 and it only took minutes to disconnect the old one and plug in the new one. Nice. You can see a photo of my car antenna in this video at the 4:35 mark: kzbin.info/www/bejne/g5K4nKd4a9Kpm7s

[@frequencywatchers]

Anyway, SUBSCRIBED, Super Nice Channel You Have There :)

[@mistereearly1141]

Would a BPF work well with LoRaWAN antennas ? 915mhz?

[@hamradiostation_M0UGE]

Would using a low pass filter for 0-30MHz help to stop QRM /noise from further up the spectrum entering the transceiver and raising the noise floor on the HF bands ? Thanks

[@FFdo.]

it will certainly help, will not remove all but some.

[@rhettscal]

Great videos! Where did you find the metal housing for the filter?

[@QRPSchool]

+rhettscal I bought the metal box from Digi-Key where I also bought the capacitors. It's an aluminum box made by Hammond. The size of the box is based on how big your circuit will be. The author of the QST article used a 2"x4" board to mount his parts. Mine was probably the same. - Cliff

[@frequencywatchers]

I NEED THIS, I Hate That All My 20 Meter Band Is Full With AM Stations That I Hear Thru Upper Side band And LLower Side Band For 20+ Meter Band

[@DirkN6UNH]

I just recently watched this video and found that I had the same birdies on the same frequencies. The source was an AirTV OTA television tuner for Sling TV. I unplugged it and the noise went away

[@kamaledirisinghe]

Interesting Video - 73 DE 4S7AB

[@FoneStar78]

Does it help in areas with lots of power lines and AC noise?

[@QRPSchool]

Unfortunately, it wouldn't be very helpful for that. With a broadband noise source, the noise is going to be found on your frequency... not just adjacent to your frequency. These filters are designed to reject signals on either side of your frequency so if the noise is also being transmitted ON your frequency, there's not much you can do except reduce the RF Gain to try to lower the noise floor. - Cliff

[@Thejohnnyoshow]

Ok i'm completely confused, You put this on the 817nd right? or did you put this on the radio in the truck so that its transmissions could not overlap onto the band the 817nd was on? It looked like you put this on the 817 but i don't see how that would help. I mean it clearly does from watching the video, but if his radio in the truck is spilling over onto a band your on, on the 817nd and you add the filter to the 817nd to filter out everything other then the wanted band, how does that prevent the spill over from the truck not still being received on the 817nd. Beings the spill over is coming in on the band your filter is designed to pass through. Help did i missing something

[@QRPSchool]

Hello, Johnny. Okay, so the FT-817 is receiving on the 20m band. The radio in my SUV is transmitting on a different band... the 15m band. The 20m bandpass filter connected to the FT-817 attenuates the signals outside of the 20m band. So, the 15m transmission comes through the antenna but is then knocked down by the bandpass filter before the signal flows into the radio. Does that help? - Cliff

[@Thejohnnyoshow]

Ok so the interface is not because of signal coming in on a different harmonic from 15 meters that is being received on the 20 meter band, it's just because his radio is so close its simply overloading the 817nd and the overload is only really coming from the 15 meter band. So guess the more i think about it now there would be no harmonic from the 15 meter that would bleed over to the 20 meter anyway. right? am i close at least haha

[@QRPSchool]

You've got it! -Cliff

[@Thejohnnyoshow]

cool thanks

[@pghhdvideo]

Did you coat the toroids with Q-Dope or anything like that after tuning them?

[@QRPSchool]

I did. I used my wife's clear fingernail polish to hold the windings in place. - Cliff

[@pghhdvideo]

I started building these exact filters based on that same ARRL document about a week ago for 80, 40, 20, 15 & 10m. I have the enclosures drilled out for the connectors, the islands made in the boards and the 3 LC circuits for each filter coarsely tuned on my grid dip meter. I just need to compare where the dip is on the grid dip meter with my IC-7300 being that I don't have a spectrum analyzer. Did you tune them only with a grid dip meter or did you use the spectrum analyzer to tune each of the LC pairs? Just wondering how close to the exact frequency shown on the chart for Fr that you need to be? I realize that right on would be best but I suspect that if I tune them on the grid dip meter until the frequency shows as close as I can be on my IC-7300, even after coating the windings, just moving the leads into place for soldering tot he board will detune them somewhat? What did you find with that?

[@paulvizard6858]

Where do you buy your toroids?

[@QRPSchool]

+Paul Vizard Hey, Paul. JPM Supply in Texas. Here's a link: www.jpmsupply.com/category-s/1840.htm

[@AA-nm8fi]

Hello, could you explain the difference between a high pass filter and a low pass filter?

[@QRPSchool]

Sure! A High-Pass filter allows signals above a specified frequency to pass through the filter. Signals below the specified frequency are attenuated. A Low-Pass filter does the opposite... it allows signals below a specified frequency to pass through the filter. Signals above the specified frequency are attenuated. A Band-Pass filter allows signals on each side of a specified frequency to pass through... but the filter's "width" or "sharpness" determines how far the signal can be from the specified frequency before it is attenuated. I hope this helps! - Cliff

[@AA-nm8fi]

@@QRPSchool Hello, yes i think i get the idea, i have a HPF for the AM broadcast band connected to my Kiwisdr, which has reduced noise level significantly from the sloper dipole. Although, i did not have this problem previously it only occurred when i changed the coax cable. So from your explanation of HPF/LPF are a more general or broader type of filters and band pass filters are more band/frequency specific. am i understanding this correctly? Thank you

[@QRPSchool]

@@AA-nm8fi Yes, you are understanding it correctly. I would also add that all radios have built-in Low Pass Filters that are used to reduce harmonics that would naturally appear on bands higher than the transmit frequency. For regulatory compliance, radio manufacturers must include LPF's. You don't want someone to be transmitting on a frequency and also having that same signal being broadcast on multiple harmonic frequencies. My use of Band Pass Filters in this video is a common use of BPF's when multiple stations are transmitting on adjacent bands within close proximity to each other... such as in a group Field Day or a multi-station contest situation. - Cliff

[@AA-nm8fi]

@@QRPSchool Thank you sir for the info, great videos keep them coming cheers

[@williamcolvin3609]

How do you figure how much wattage the filter will handle?

[@QRPSchool]

I don't know. The toroid cores and wires would have to be larger but I don't know how to calculate it. But, it's not really a problem for me because I don't own an amplifier. 100 watts is QRO for me! :-)

[@williamcolvin3609]

That is big enough for me - I just did not know how to tell what wattage rating would be. Thank you.

[@smallick653]

8:18 Which Honda car model is that ?

[@QRPSchool]

That was a 2005 Honda CR-V... which is still going strong in 2021.

[@mikemcdonald5147]

I know most people use the bandpass filters for field day or for multi station ham shacks. But would it help reduce noise and interference for a single radio user?

[@QRPSchool]

Hey, Mike. The Bandpass Filter would only help if there's another ham nearby who is transmitting on a band other than the one you're on and "bleeding in" on your frequency. The filter isn't sharp enough to help with other station on the same band. Really good for multi-station Field Day use or when you and your buddies are trying to work portable on separate bands in the same vicinity. - Cliff

[@mikemcdonald5147]

Thanks so much for that. I was looking at buying or building one for each band thinking it would help but I guess it won't unless I'm field day or multi operator. Thanks for saving me money :)

[@fm71450]

Thank You! Excellent information, well explained. KQ4IXD

[@WadeAbout]

Love your videos. You got me into qrp and now SOTA (16 summits under my belt - check my channel and blog VK1FWBD.com). Would this be useful in say in the suburbs where I have a s9 noise floor and basically don't play radio at home because of it.

[@QRPSchool]

Hello, Wade. Unfortunately, it won't help for S9 noise... only for reducing/eliminating the QRM of another ham transmitting nearby on an adjacent ham band.

[@eknaap8800]

"Bandpass filters for QRM" ---> So these are designed to give QRM??? Or did you mean BP filters against or counteract QRM, for HAM operators?? English is sooooo difficult.... 😢

[@keesvanoosbree5911]

Watch the video. It's pretty obvious