#248

[@FXGreggan.]

You always explain everything so even slow learning people like me understand perfectly :)

[@dougb3699]

We used to tune band pass filters using a sweep generator, directional coupler, detector and a dual trace scope. This allowed us to see return loss and pass band at the same time.

[@BobHolowenko]

Just wanted you to know that I did this demo at a club meeting and it was well received (/pun) by the technical members. The lightbulbs came on with the bandpass characteristics test, but the return loss into the cavity and load really made it click for them. Thanks for all you do for us.

[@pcwrangler09]

Old video but still helping people. Thank you!

[@TheRadioShop]

Another excellent tip Alan. My boss taught me to do this in the early 90's, we had four UHF commercial repeaters at the time and a14 channel 800 MHz trunked system on the local tower. This really helped narrowing down the center frequency. Had all but completely forgotten about this. Thanks again.

[@w2aew]

Thanks - it certainly makes it easier to zero in on the passband center frequency. I don't mess with these bandpass cavities much - I was surprised that the Q is as low as it is. But, it is just one cavity, so I guess it is normal.

[@billz4071]

Very informative. Just purchased an Instek GSP-810 Spectrum Analyzer with Tracking Generator. Haven't used it yet. Been tuning using RF generator, return-loss by Mini-Circuits, and my Avcom spectrum analyzer. Watching your videos are very good. Thank You for taking your time to share these!!

[@BobHolowenko]

I would love to see more filter/duplexer/diplexer content. Specifically, what you mentioned in here about adjusting the loops to change the shape of the BP for RL or Q? I suppose Q would be most important when you have a nearby transmitter to keep out (such as a duplexer or busy site). This is one of my fav channels!

[@davideisenberger6197]

Thank you for taking your time to make this video! You are an "Excellent " instructor. So it goes to really prove that you "CAN" teach an old Dawg new tricks! 73's David. K8KEM

[@PixelSchnitzel]

Hooray! Another W2AEW video! I'm extremely interested in the topic of cavity filters, but there is precious little information that I've been able to find about them. This video is a big help as I tinker with a set that I have in my lab. I would certainly welcome more on the topic. If you have an old beater cavity, a teardown would be fascinating. I'm on the verge of opening mine up, but it may be a one-way street. Thank you for your fantastic videos and willingness to share!

[@mikesradiorepair]

Excellent explanation of a better way to tune a filter. Will have to give this a try this evening. Thanks for sharing. Mike

[@w2aew]

Keep in mind that this is a great way to find center frequency. If you want "accurate" return loss measurement, particularly outside of the filter's center frequency, you'll want to include an attenuator between the tracking generator and the coupler, so that the reflected energy doesn't adversely affect the TG output.

[@mikesradiorepair]

w2aew Thanks for the extra tip. Mike

[@niteshbushan]

Thanks a lot for providing such a great educational material. All your videos really help me a lot at my workplace 😊 please continue this great work..

[@silvergmc]

Thanks so much for taking the time to make these videos. I always learn a lot!

[@DonDegidio]

Alan, Great explanation and visual. Happy Holidays. WB3BJU

[@Usul]

I have a lot to learn. That was interesting. Thank you for sharing.

[@TheRangerDale]

Yes Very Good Allen and like that Idea very much and thanks for the extra Knowledge.. And very Happy Holidays to you and your Family 73's/GB Dale

[@danishnative9555]

Alan, please do Bp Br cavity tuning soon. They would love it. Thanks for all you do for us nerds.

[@barryg41]

Merry Christmas to you and your family Alan!

[@robertmray]

Wow! Great technique..one is never too old to learn..73s..KB4GO

[@BASANTABHATTARAI]

wow!! thanks for this neat lil technique.

[@mateuszstyczkowski139]

Thanks Alan, and more videos please:)

[@amciaapple1654]

I loved it! Thumbs up.

[@Sam-lc1lo]

Wonderful!!!

[@HansVanIngelgom]

So nice to see another video of you! Is such a shallow filter characteristic typical for cavity filters? Wouldn't mind seeing another video on the topic...

[@w2aew]

I don't have a ton of experience on these types of filters - so I'll have to look into that. I was a bit surprised at the low-Q myself. You can adjust the coupling loops to trade off insertion loss for better Q. In this case, the loops are adjusted for lowest insertion loss...

[@AndrewMurphy8383]

can i use a Use an Oscilloscope and Signal Generator help tune duplexer if i cant get a anlizer

[@AndrewMurphy8383]

spectrum analyzer

[@HansVanIngelgom]

I might have a better solution for you. You can use a noise source and RTL-SDR dongle and use it as a sort of spectrum analyzer. See hansvi.be/wordpress/?p=91 (if you can manage to fix the url)

[@w2aew]

The only real problem with that is dynamic range. When tuning cavity filters, you often need 80dB or more of measurement dynamic range, which is difficult to get with a dongle and noise source. For peaking the filter response, this works fine. For fine tuning a reject / notch, it probably wouldn't be OK.

[@DM-fz3ly]

Hey Alan, Excellent as always! So I have had experienced radio techs tell me that for an in service repeater that the actual impedance of the antenna and/or the next filter (in the case of say a 4 can duplexer) will make a difference in the proper tuning. What are your thoughts on tuning individual cans using your example and then connecting them together and terminating to a real life antenna? Will the system still be at optimum tuning or will the system require final tweaking due to all the components not being exactly 50 ohms? And YES please do an video on the other parts and aspects of tuning with the capacitors and other adjustable parts of a band pass/reject system. 73, n7kbc

[@w2aew]

The old techs are right. You can start with adjusting each individually in a perfect 50 ohm environment, but you will want to tweak it when connected to the actual equipment and antenna.

[@jquintana7097]

Hi great video. Okay how is the notch done with the mini circuit directional coupler? Or not needed when doing the notch?

[@javiermoreno1612]

Thanks for a great video. A set six db-4021 “duplexer I was told” we’re donated to our club. I will be trying to tune them but I noticed these are bandpass filter cavities. They are set up 3 for RX 449.375 and 3 for TX 444.375. Is this use of bandpass filters as duplexers possible or usable.?

[@w2aew]

Bandpass only filters can be used for duplexer applications, they just won't provide as much isolation as bandpass/band-reject arrangements. The information on this filter set can be found here: www.repeater-builder.com/antenna/db/pdfs/db-band-pass-cavity-filter-installation-and-tuning-instructions.pdf You'll have to carefully examine the filter characteristics to see if they are "sharp" enough to provide the level of isolation needed in your repeater application. There is a wealth of great information on the www.repeater-builder.com website.

[@rapsod1911]

This reminds me of trick used in radiogoniometry. Radiation pattern of antenna change sharper between lobes than on lobe maximum. so instead of searching max of received signal they search for min.

[@w2aew]

As long as you precisely know the radiation pattern of the antenna, that works great. Of course, assuming a symmetrical pattern, you could search or minimum on either side of the main lobe, then split the difference.

[@dennisqwertyuiop]

great information

[@moatacemaskar7313]

Thank so much for the video and I have a question. Does cavity filter is a reciprocal element. If I make input output and vice versa.

[@w2aew]

yes

[@moatacemaskar7313]

@@w2aew Thank you very much for the awsome videos you I realy enjoy wachting and get a lot of knowledge. If you dont mind sharing a video about S matrix theoretcally and practically.

[@glenwoofit]

Top Tip, Thanks.

[@ahmadaltaweel4981]

Thank you very much

[@feicodeboer]

It seems to me you didn't put a 10 dB attenuation on the tracking generator output. That was a recommendation in your video (#158 I think) showing SWR measurement with a directional coupler. Is there a specific reason for in this case?

[@w2aew]

No real reason why I didn't use the attenuator. It wouldn't make much difference in this case, since the region we're interested in is where the match is good. Good catch though!

[@PapasDino]

Until you see it demonstrated t's hard to visualize the difference in measurement fidelity between doing a simple bandpass and the return loss methodology - can't make it any clearer than you've done here Alan. Are those double shielded cables you're using for the interconnections as you would use in a duplexer system? 73 - Dino KL0S

[@w2aew]

No, they're not double shielded in this case, but you certainly would want them when tuning a high-Q duplexer.

[@thomasmaughan4798]

I saw it as Tune *BADASS* filter! Yeah show me how to do that.

[@peterb8387]

Hello Alan, General Question, I have a trolley on wheel style bookshelf structure made of wood (mostly) with some metal dowel pins, typical IKEA or Office Depot construction. I got myself a new spectrum analyzer and I need to add to the trolley/shelf/cart to make shelves for my power supplies, scopes and the new spectrum analyzer. Can I use metal for the 4 posts to bolt the wooden shelves ? I don't know if have metal near the scopes or analyzers will have any detrimental effects on the the instruments. My work table is rather cluttered, hence the new trolley/cart project ? Any advice, Kindest regards, Peter

[@w2aew]

There should be no problem placing the instruments near any metal objects.

[@PeterWalkerHP16c]

Hmmm. I was hoping that cavity filters were sharper than that. +- 500kHz is only -5dB or did I miss something?

[@w2aew]

It depends on the design of the cavity. Some are much narrower.

[@saintfather7757]

What about 2 frequencies in a preselector?

[@pa4tim]

I once had to align a multi-cavity filter for 70cm. I used S11 and S21 and it was a real pain to get it decent. Then I read that it was better to use TDR instead of S11. I tried that, and with TDR it was indeed "easy" to get all three cavities at 50 ohm and minimun transmission loss. So besides finding broken Christmas tree lights TDR turned out to be handy for cavities too ;-)

[@w2aew]

Hmmm - interesting. I'll have to think about how a wideband signal like a TDR pulse can be used to tune a narrowband filter...

[@nathanhaltman7235]

w2aew He might be talking about the TDR function on a VNA. It is a reverse FFT. I use it on my Agilent 8753ES. The HP application note recommends using the TDR function to adjust multi polel filters

[@alanwolke6253]

Yeah, I understand. If you start with a relatively narrowband frequency domain plot, then the iFFT would be able to show you which cavity is contributing to poor return loss. Makes sense.

[@pa4tim]

Yes, I forgot to mention, I used the TDR function of my VNA

[@w2aew]

OK, makes much more sense now!

[@jimromine2917]

Does this method only work for bandpass cavities?

[@w2aew]

It will work for most non-absorptive filters (where out of band signals are reflected - i.e. have poor return loss).

[@jimromine2917]

@@w2aew I guess I should have asked if tuning a bandpass/ band reject cavity with a Rigol DSA815 spectrum analyzer is it best to use directional coupler for the pass then use spectrum analyzer to set notch. I really like your videos.

[@w2aew]

@@jimromine2917 You can use a coupler for the bandpass too - the return loss will give you a sharper view of the bandpass adjustment vs. the through response.

[@sergiomackay789]

Hello, I'm making some duplexers (first time) I want to know if the TX and RX duplexers are the same, I mean they have both in TX and RX side an capacitor? because the RX side is opposite of the TX, how to get that reverse?

[@w2aew]

Duplexers don't know or care whether a signal is from a TX or going to an RX - they're just passive filters.

[@AndrewMurphy8383]

can i use a Use an Oscilloscope and Signal Generator help tune duplexer if i cant get a anlizer

[@AndrewMurphy8383]

spectrum analyzer

[@davidkierzkowski]

What cables are those?

[@w2aew]

They are some nice Huber + Suhner Enviroflex 142 cable assemblies www.hubersuhner.com/en/documents-repository/technologies/pdf/radio-frequency/enviroflex

[@davidkierzkowski]

w2aew thank you!!!!

[@amciaapple1654]

You did not explain why S21 is so flat and S11 is so sharp.

[@VK4GHZ]

Thanks Alan. I enjoy your videos. Always a no-nonsense approach, and easy to digest. It's a shame more hams haven't embraced video as a method of sharing knowledge. In the past I have shard your videos via the (now defunct) VK Logger Discussion Forum. I'm now a co-admin of a Facebook group dedicated to VHF/UHF/microwaves: facebook.com/groups/vhfuhfmicrowave/ If you feel inclined, please join our group, along with any of the like-minded viewers also reading this. Always keen to have original content that can educate others posted in our group. Cheers, Adam, VK4GHZ

[@w2aew]

Thanks. I joined the Facebook group. A question for you... ...I don't play with these filters very often. I was expecting the Q of the filter to be higher. Is this the normal bandpass response for a single cavity 900MHz bandpass (about 20MHz), with the coupling loops adjusted for minimum insertion loss? I realize that the Q can be improved at the expense of insertion loss, but I guess I was expecting something a little tighter...

[@leonardosoto2951]

Hello, my name is Leonardo. I wrote from Chile!! first congratulations for us videos! Is very important to know work this equipment.. now I want to Ask if you can help me, if send me more information about how tune “Serie Notch” model 28-37-02A with Rigol DSA. Or any mail for contact! Leonardo Soto CE7RFS Many Tanks

[@rpbale]

Hi, over on The Current Source he's built an RF power divider and the resonance freq landed pretty far off his target. Maybe you can tell what he's doing wrong. I think this is the area where you accel. kzbin.info/www/bejne/qmHFm2t6i6d1oKM