Fun with cavity filters

[@MoTown44240]

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

[@markhonea2461]

OMG I am so going to get involved with that, this, the most fascinating information I have found in the sea of KZbin.🤓👍

[@MakeDoAndMend1]

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.

[@bendunselman]

Your infant will love this when grown up.

[@markhonea2461]

I will now watch this again 👍

[@dereklassen4841]

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.

[@markg6jvy135]

Very creative thinking Peter 👏👏👏

[@coblecam]

Are there general rules about the diameter of the cavity used? GREAT video! 73!

[@vk3ye]

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.

[@artemisdefcon]

Nice jobs dude.

[@MakeDoAndMend1]

Many thanks for the inspiration to make filters. I used discarded steel gas bottles. Work a treat. I have posted on here. George 🇬🇧

[@chronobot2001]

Would adding additional mass at the end of the internal tube change the filter frequency?

[@chronobot2001]

What if the internal tube was a coil? Longer tube coiled to fit into a smaller space.

[@terrybondy1948]

Wow! Suffered 5 adverts to get through this, most of them mandatory.

[@mabselves]

That's not great. I didn't have to suffer one.

[@bigpimp347]

then install an ad blocker like most people with sense do.

[@terrybondy5458]

@@bigpimp347 can you give me a link describing how that is done for the KZbin app on a phone or iPad?

[@bigpimp347]

@@terrybondy5458 aaaaah, yes, don't use an app, get a PC.

[@scimitar123]

Free tube on pc or newpipe on android

[@markhonea2461]

Would large bore copper pipe work?

[@gillywild]

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.

[@ssubaihi]

How it's work , Why Loops ? Still a lot to tell ...

[@vk3ye]

Loops are needed to couple the signal to the main resonating element. See links in the description for more background.

[@Max-kn9yi]

Does it matter what end you attach a filter to the antenna?

[@vk3ye]

Sorry I don't understand your question.

[@thomasmaughan4798]

As shown, it does not matter; the device is symmetrical and bi-directional. Either port can be the input with the other as output.

[@SirsenduGhosh1989]

Is it possible to build cavity filter for HF? Just curious 73 de VU3WTJ

[@antennafarmer7380]

Possible yes, practical no, especially since other types of filter are more effective at lower frequencies.

[@SirsenduGhosh1989]

@Paweł Hadam thank you for info

[@thomasmaughan4798]

You can build cavity resonators for pretty much any frequency but they become very large below VHF.