UHF - 6pole cavity DVBT(T2) filter question - Page 1
Forum section: electronics / rf-microwave

author: smdbg
author_rank: Newbie
timestamp: December 09, 2024, 01:04:34 pm
content: Hi all , the model of this filter is a-cl6x30c-01 from Italian producer Com Tech . Just opened it and notice some elements i haven't seen before:Can anybody recognize the function of the elements in the red circle ?!My first though is that there are for matching impedance , but i am not 100% shore.Also i can not imagine what are fore the tuning elements in green circles.My goal is to tune the filter to different frequency sadly no manuals exist for this.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2024, 04:48:09 pm
content: You do realize this is not an 8MHz filter for your HF ham radio? Nor can you tune it to work in that frequency range. The 6-8MHz is the width of the passband. For example, you could tune it for 600MHz +/-4MHz.See datasheet:http://www.com-tech.it/datasheets/CL6X30C.pdf

author: smdbg
author_rank: Newbie
timestamp: December 09, 2024, 05:04:19 pm
content: Yes , i corrected the topic name - to be more clear to others. And yes it is not for my Ham radio - but let say for my Ham DVBT transmitter.Another idea that cross my mind is that this rotating frames are maybe for phase shifting ?!

author: IconicPCB
author_rank: Super Contributor
timestamp: December 09, 2024, 08:29:15 pm
content: The elements circled in green may be there to adjust coupling coefficients between adjacent cavities.

author: szoftveres
author_rank: Regular Contributor
timestamp: December 10, 2024, 01:54:59 am
content: Quote from: IconicPCB on December 09, 2024, 08:29:15 pmI think the loops encircled in red are also there to adjust the coupling (at least between the outermost and the 2nd transmission lines). A VNA will tell for sure.

author: coppercone2
author_rank: Super Contributor
timestamp: December 10, 2024, 04:32:13 am
content: its like a butterfly valve.Those rods are just for tuning. Cheap designs use bolts, this one did it a nicer way.This circuit really makes you wonder, the wavelength is 1/2 meter long.It looks like when you turn the apatures, there is more or less of the waveguide 'visible' to the antenna stub (waveguide to coaxial transmission)Is there a electrical contact between the apature and the stub, or is it a big wide hole without contact? Post a picture at an angleI tried to think about it in terms of waveguide E and H fields. The rectangle apature looks like how you would draw the H field of a wave guide. Does it agree with the polarization?Those rod are helping impedance match the irregular shaped cavities?Because most waveguide launcher, the wave is inserted in the wide side, or the end, this one is narrow sideis that a h-plane translation?and what do you call those shorting screws? Does the adjustment of that work like a collar that is pulled up on a screw?or is it this? capacitorswater pipe type coaxial resonance capacitor?

author: smdbg
author_rank: Newbie
timestamp: December 10, 2024, 06:47:28 am
content: Quote from: coppercone2 on December 10, 2024, 04:32:13 amYes there is - the rod sits in collet.Quote from: IconicPCB on December 09, 2024, 08:29:15 pmYou are right - on vna puling them in/out flattens the filter curve/match vswr between cavities , although i haven't been able to get lower than 1.4 to 1.9 in total.The filter is 4 pole - 4 cavities between input and output , that's why the elements circled in green are 3 . The other two poles are just to make filter slope more steep

author: coppercone2
author_rank: Super Contributor
timestamp: December 10, 2024, 07:05:27 am
content: so the center pin is electrically connected to that butterfly valve looking piece and there is a short circuit to the body of the wave guide from the center pin? Or insulator. It sounds bizzare if its all connected like that lol , the center pins are normally floatingThe shorting pin is used for a end launch transition, and it reminds me of something along the lines of a magnetic loop antenna inside of the waveguide.Quotemw101so in this case its a shorting rotating thing with a hole in it thats connected across the whole waveguide

author: szoftveres
author_rank: Regular Contributor
timestamp: December 10, 2024, 03:58:36 pm
content: The coupling loop attached below was common thing in first generation hand-built UHF TV tuners; the coupling factor could be adjusted during manufacturing by bending this wire to- or away from the stubs.It is reminescent to the rotating thing encircled in red.

author: ftg
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:35:20 pm
content: As others have said, the green screws are for adjusting the coupling between cavities.More coupling means less loss, but wider bandwidth.And the other way around.In the broadcast TV filters I have fiddled with in the past, the elements circled in red are another mechanism for adjusting the coupling and thus filter shape.As for retuning the filter to 70cm, I have had good success in retuning smaller 470-500MHz filters down to the amateur band.But the 700MHz upper band filters don't seem to gracefully tune down to 70cm or even up to 868MHz.