Thanks a lot for your explanations. I am an electric engineer in the high voltage power transmission field (gas insulated high voltage switchgears) and while I have learned this more than two decades ago, I have not used it ever since and as we all know: use it or lose it. Now I had to dig into this again, because when we commission a new switchgear, we do a partial discharge measurement and we measure the PD signals by detecting their UHF signals. Sometimes there are background noises making the measurement in the time domain impossible, although this is required to check for particular patterns. So now I start using spec analyzers to determine the disturbances and build specific stub filters which then allow me to measure in the time domain without any interference. Your video was really helpful to bring the stuff back to mind.
That was one of the most clearly explained examples of a radio related topic that I’m always on the verge of sorta understanding. Thanks for the focused instruction!
I'm a returning radio ham after a 4 decade plus break and am watching your videos in my 70s to refresh and relearn. Thank-you for producing and pitching them so well. Your videos explain basic and advanced information without talking down or going over the student's head. From my own teaching experience, that alone, is quite a skill. Thank-you.
i did this way back when. When i talked on 70cm repeater our tv would go bonkers, i made this band reject filter on the repeater freq and placed it on the tv/ant feedline. (i made one for 2m too just in case.) Worked a treat, i could talk on the repeater while the rest watched tv.
Wow! You are fantastic! Such clear presentations and no wasted time. I just subscribed and I'm certainly going to share your channel with all my buddies. I don't understand why you don't have hundreds of thousands of subscribers. We need to get the word out. Keep up the great work!
Thank you very much for this thorough tutorial. There’s much more to (coaxial) cables than the average user knows. I somehow feel tempted to call you Mr Cable…
I remember years ago I used an inductor and a trim capacitor , tuned it to the 7Mhz sound subcarrier on a tv signal,, put it in the aerial line with a T piece , this confused the hell out of a tv engineer when I said I’m have trouble with my tv , I don’t have sound but good pictures .
I just built a 7 MHz version using your design with Westflex 103 and I am getting 60dB of attenuation on 7 MHz and 21 MHz. Exactly what I need for running on 2 bands next weekend.
Fortunately, what really matters is the electrical length, not the physical length of the coax. Your filter could be made up of coax with a variety of velocity factors. No one piece of coax really cares about the velocity factor of the others. So, you determine the physical length of coax A with the velocity factor of coax A to give you the electrical length you need for coax A. The same for all of the others. Hope I made sense. 🙂
I made some stubs for our WFD HF ops coming up this weekend. This video gave me a much greater understanding & ideas. That variable cap is awesome! I'm going to make one. K0ATV
You are right, I didn't explicitly state how I connected stuff up (oops). My apologies. I measured the rejection using, S21, through/response with source on port 1 going to the "input" side of the filter assy and measurement on port 2 going to the "output" side of the filter assy. I used log magnitude display after having set up the frequency limits for the frequencies of interest and performed the appropriate calibration. I hope this answers your questions.
Any info on the variable capacitor? Did you make it or buy it? Any part numbers would be great. I want to create some stubs to reject my CW on field day. Great video!
Thanks! The variable capacitor that I showed in the video actually came off of an old hybrid ring duplexer which sports one of them for each ring/cavity. I do not know where someone would buy one. I'm sure they are out there. But the design is simple enough, a home brew version shouldn't be too hard to come up with using similar techniques. :-)
The only place I have found them is attached to a Hybrid-Ring Duplexer. Thats where I got mine from ... an old duplexer whose cavities had developed internal mechanical issues. One could DIY them, I suppose, with some effort. 🙂
It will continue to multiple harmonics with lesser and lesser effectiveness for higher harmonic frequencies. With open stub it will be at frequencies where the stub is an odd multiples of 1/4 wavelength. With shorted stub where the stub is multiples of 1/2 wavelength. 🙂
@@clownworld7763 The very simplest form of this is putting a parallel tuned circuit in series with the signal with some finite resistance to follow. This finite resistance might be just all the circuitry that follows such as the input impedance of the circuit that follows. At resonance the parallel tuned LC circuit acts as an open circuit. I'll warn you, here, it is a pretty wide banded filter. You could also add a properly chosen series tuned LC circuit across that resistor. This one acts as a short at resonance. Remember, that resistor could be just the impedance of the circuits to follow, so an overt, physical resistor may not be needed. I threw one together using LTSpice. MUCH narrower response that the parallel tuned circuit only. I simulated it from 1MHz to 100 MHz with an fo of ~5 MHz. No harmonic interaction. If you are intending this for anything that will be handing any sort of power (e.g. transmitting), then you need to be careful to choose components that can handle the currents and voltages associated with that application. Hope this helps. 🙂
Hi: I really enjoy your work. I had two questions: 1. 15 meters is taking my antenna TV offline. Can I do the "stub" thing with RG-6? I am thinking I could filter out the 15 meters; and 2. I built an End-Fed, Half-Wave antenna. I wound the 49:1 on a T140-43 and have the wire cut for 80 meters. I guess I shouldn't complain, but I don't get why not only do I get my bands and their harmonically related bands, but it works great on 30, 17 and 12 meters. It amazes me as I don't need a tuner at all for any of these bands. Does this make sense? I bought another T140 and will be winding that one too to check if it is the toroid or something else. Thanks, Marc
Thank you! 🙂 1. Yes, you can do the stub thing with *any* transmission line you choose. 2. A lot a strangeness goes on with antennas. Of course, a good dummy load works on every band. So, the question is, "Is it all working as a dummy load or a real antenna?" ie. "What is its radiation efficiency on those 'extra bands?' " I know that I do not have a handle on a *lot* of what antennas work. I do a LOT of modelling with 4NEC2 ... virtual antenna experiments. You could try modelling it and see what it should theoretically do. 4NEC2 is **FREE** and I have some videos on how to use it starting with the one here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Zd6BT558YRA.html If you are using a 4:1 BALUN to feed an antenna, you put 200 Ohms in for the characteristic impedance. Hope this helps. 😁
Your radiated power on 30,17 and 12 is very very low. The 100-150pf capacitor is showing your radio decent vswr but your power out is probably 20w at 100w. Check out that capacitor under a thermal camera on these bands and see how much they glow. 🤩
Is it possible to create a very narrow notch filter? Either with this method or any other? I'm receiving Meteor and NOAA weather satellites, from 137.1 to 137.9 MHz, but there are strong VDL2 blips that ruin the reception due to overload, especially for Meteor since it's digital and loses sync when a strong blip happens. I've tried reducing the gain but then the reception always becomes weaker. The VDL2 interference is at around 136.7 MHz. It would be great to eliminate or greatly attenuate thr 136.7 interference without compromising thr 137.1 reception
Really good question! I had similar questions when I first started playing with these years ago. Unfortunately, by stacking these (adding more stubs) I was able to get deeper and deeper notches, but the "skirt" remained just as wide....too wide for what I was hoping to do. Maybe put a pre-amp before the filter to overcome the loss anticipated by the filter at the desired frequency(?) The over all response at the desired frequency is 0 dB, but still -60 dB at the reject frequency. It might be worth a try. There are some inexpensive duplexers out there that you could retune to do the job, too. 🙂 I cannot recommend anything, but I see several options on eBay for around $99.
@@jim9930 If I understood correctly, in this case the frequency I want to attenuate is 136.7 so I should mix in 126 MHz to have an IF of 10.7 and use the crytal filter?
@@t0nito I think what he is saying is that this mixer/filter combination is for singling out the signal you want to receive and, in doing so, attenuating the adjacent signals you do not want. 🙂
I am surprised at your cable length calculations - have you tried this directly? The theory is good, until you discover the VF changes with frequency, that frequency incidentally not specified by the manufacturer... Try it on your VNA and see - you will see the VF change dramatically away from the manufacturers spec as you go up in freq. The calculations are only useful at the manufacturers specified frequency, if you can find that anywhere... Just use the VNA in impedance mode to cut to electrical lengths, not the calcs. Top video otherwise, thanks for teaching.
