While in my early learning days about VHF, 1986, I built a poor man's cavity filter for 2m. Like Peter says, it works but not very efficiently. Great video Peter. 72 de KB8AMZ
A few words... The frequency is determined by the length of the inner (not mass, it can be (usually is) hollow). It should be 1/4 wavelengthll long. The Q is set by the ID of the outer (bigger is better), and the conductivity of the inner surfaces (mostly). You want the Impedance to be 50 Ohms - set by the ratio of OD / ID. The end where the connector(s) are (is) must be shorted. The other end needs to be closed only to preclude corrosion. It could be a removable plastic cap. Your milage may vary. 73 de Derek, KN6TD.
I wonder if that was what someone was doing in the cuban embassy? The cricket noises and brain damage always sounded like microwaves to me. I should think a Theremin resonator for microwave frequencies would be quite compact.
While in my early learning days about VHF, 1986, I built a poor man's cavity filter for 2m. Like Peter says, it works but not very efficiently. Great video Peter. 72 de KB8AMZ
OMG I am so going to get involved with that, this, the most fascinating information I have found in the sea of TH-cam.🤓👍
A few words...
The frequency is determined by the length of the inner (not mass, it can be (usually is) hollow). It should be 1/4 wavelengthll long. The Q is set by the ID of the outer (bigger is better), and the conductivity of the inner surfaces (mostly). You want the Impedance to be 50 Ohms - set by the ratio of OD / ID. The end where the connector(s) are (is) must be shorted. The other end needs to be closed only to preclude corrosion. It could be a removable plastic cap. Your milage may vary. 73 de Derek, KN6TD.
I've just made a Cavity Filter from an old gas bottle. Works a treat. Cheers from old George ps cost to make about a UK pound.
Your infant will love this when grown up.
I will now watch this again 👍
Are there general rules about the diameter of the cavity used? GREAT video! 73!
I believe that it gets more efficient/sharper as you increase the diameter, but there's a point where the diameter is excessive for proper operation.
Very creative thinking Peter 👏👏👏
Would adding additional mass at the end of the internal tube change the filter frequency?
Nice jobs dude.
What if the internal tube was a coil?
Longer tube coiled to fit into a smaller space.
Wow! Suffered 5 adverts to get through this, most of them mandatory.
That's not great. I didn't have to suffer one.
then install an ad blocker like most people with sense do.
@@bigpimp347 can you give me a link describing how that is done for the TH-cam app on a phone or iPad?
@@terrybondy5458 aaaaah, yes, don't use an app, get a PC.
Free tube on pc or newpipe on android
Would large bore copper pipe work?
Many thanks for the inspiration to make filters. I used discarded steel gas bottles. Work a treat. I have posted on here. George 🇬🇧
I wonder if that was what someone was doing in the cuban embassy? The cricket noises and brain damage always sounded like microwaves to me. I should think a Theremin resonator for microwave frequencies would be quite compact.
Does it matter what end you attach a filter to the antenna?
Sorry I don't understand your question.
As shown, it does not matter; the device is symmetrical and bi-directional. Either port can be the input with the other as output.
How it's work , Why Loops ? Still a lot to tell ...
Loops are needed to couple the signal to the main resonating element. See links in the description for more background.
Is it possible to build cavity filter for HF? Just curious
73 de VU3WTJ
Possible yes, practical no, especially since other types of filter are more effective at lower frequencies.
@Paweł Hadam thank you for info
You can build cavity resonators for pretty much any frequency but they become very large below VHF.