Create a Band Reject Filter using just Coax: Shorted Stub Version (049b) 

Thank you! 🙂

Thanks! It is a lot of fun to experiment with! 🙂

Hmmm...I never thought to try that. 🙂

Thank you very much! Meilleures salutations de l'Iowa ! 🙂 (compliments of Google translate)

Thanks, Dennis!

Thanks! 🙂

Thanks! 🙂

I suppose that if the frequency of choice was low enough that the stubs could be brought back to the shack, this could be done.

Forgot to mention. High gain antennas to be constructed in a colinear configuration each, thus achieving high gain.

Thanks, man! Of course....one problem you might run into .... just thinking ... part of the 70 cm band is a third harmonic of the 2M band (432 to 444 MHz = 3x 144 to 148). So ... your 2m filter is also going to attenuate this third harmonic! So, the 70 cm filter will be just fine, but the 2m filter will have an issue with this portion of the 70 cm band. :-(

Very good question! In fact, that was one of the things I was thinking when I first started playing with this concept. Unfortunately, the Q of this filter is not high enough to accomplish this task especially if we are talking amateur radio 600 KHz, 2 meter band separation. If the transmit and receive frequencies are far enough apart, I do not know why not. I'd use the open stub with a variable capacitor termination so you could tune it easily. The only other issue I see might be the changes in tuning with temperature(??).🙂

Ya know, that is what I thought to do with it, too. the answer is, "It depends on the separation between the transmit and receive frequency." For the 2M amateur radio band with a 600KHz separation, not at all (I discovered). The dip can be made to be deep enough for the reject, but the overall bandwidth on the "skirt" afford a lot more attenuation than you want 600 KHz away. Will it work on a 70 cm duplexer with a 5 MHz separation? Well, I actually didn't pay much attention to it, so maybe...maybe not. So, the big issue isn't the depth of the notch as much as the attenuation on the skirt affecting the receive frequency. 🙂

I am not sure I fully understand the question. Need more information.

Lol

All you'll end up with is two warm stubs.

Of course....one problem you might run into .... just thinking ... part of the 70 cm band is a third harmonic of the 2M band (432 to 444 MHz = 3x 144 to 148). So ... your 2m filter is also going to attenuate this third harmonic! So, the 70 cm filter will be just fine, but the 2m filter will have an issue with this portion of the 70 cm band. :-(

I think he is talking about a Y configuration. One side, with the 70 filter feeding the 2m antenna and the other side with the 2m filter feeding the 70cm antenna. The common point is the radio feed point.

@@eie_for_you now I see, so the 70 cm antenna will be virtually useless in this configuration :( - ty for the heads up

@@xxleite It was a pretty cool idea, though.

You are right. They are quite wide banded. With that said, I have known folks to have used something like this as band reject filters for HF operations on field day so two stations operating on different bands didn't interfere with each other. 🙂

WOW! This is *exactly* the question that I had when I first discovered this cool thing. Great question! 🙂 So I did some experiments. The problem is the bandwidth of the notch and the separation "distance" between the transmit and receive frequencies, as I discovered. I could attenuate the transmit frequency really well, check! However, the insertion loss at the receive frequency was unacceptably high for a 2m repeater, separation of 600 KHz. You **might** get away with it for a 70cm repeater where the separation is 5 MHz(???). I recently was working on a duplexer for a public works repeater that had a HUGE separation. It would have worked there. This was a VERY good question. 🙂

@@eie_for_you And one very near my heart ! Lol ! As recently I dove head first into the World of GMRS ! And understand that obviously with high Frequencies comes loss thru attenuation ! But.....I never dreamed how outrageous they are ! I've been around RF since I was a kid, and have a pretty solid Amateur Radio background, so this is nothing really new, but I had just never waded that far into the pond ! The most shocking for me was the level of attenuation at 467 Mhz. ! Then the insult to injury when you see the cost of Feed line, that even remotely offers any relief, not to mention the size ! I'm sure I'm preaching to the choir here, judging from your comment. And thank you too, for getting back, as I think your initial posting was some time ago ? But yes the frequency I am interested in is in the GMRS range ! 467.600 to be exact, as I am in the process of putting a repeater there ! Since I don't have the necessary gear for proper testing, do you offer your services and build such stubs ? Fingers crossed 🤞 !! Obviously I am willing to pay for such service ! Hopefully we could figure out how to pay, or maybe you already offer these on eBay ? Again Thank You for getting back ! And like most things RF related, I find it intriguing ! I have said since a child, if only we had a way to see RF visually ! Makes me wonder how things would change, if that were possible ? Also, I have always said, since RF signals are all around us, if we could see RF, we likely couldn't see our hand in front of our faces !

Here's an idea, since we're most interested in this for vhf/uhf applications. Firstly, the lower the loss of the cables used, the better the Q and deeper the notch, but here's the idea: have small cases at the end of the stubs containing the tuning coils with connectors on them (perfect since theyre coax) and use brass bolts going into the coils to fine tune them.

@@Dazzwidd Hmmm...interesting idea except brass is not ferromagnetic and would have no (little) affect on the inductance of the coil. You would have to get a ferrite rod that could be attached to the end of your bolt. This would change the inductance of the coil as it is inserted, much like the much smaller, low power variable inductors I used for this demonstration. Or, maybe, you could have a sliding assembly. One end of the coil is soldered to the center conductor of the connector. The other end is soldered to the end cap of the sliding assembly which is connected to the shield side. As you pull the slide out the coil lengthens and the inductance decreases. Push the slide in and the coil shortens increasing the inductance. The trick is to keep the coil from turning into a slinky and getting all deformed. The other option is an Open Stub filter which uses termination capacitors. It is a LOT easier to create a variable capacitor than it is a variable inductor. Check out this video where I show one such setup. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Fd7d6oKHr_A.html Hope this helps! 🙂

​@@eie_for_youit doesn't need to be ferromagnetic to influence the inductance. I believe the effect brass will have will be to lower the inductance of the coil, not raise it..

The good news is ... you can stop and start the video and you can also watch it more than once. 🙂