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+--- Thread: 6m and 2m cavity sets, building and maintenance (/thread-132.html)
6m and 2m cavity sets, building and maintenance - VK2ZIW - 06-03-2017
Hi all,
70cm sets are common and available so there is no problems here BUT:
2m and 6m with much narrower frequency split are much harder.
And, there is very little ex. commercial gear around.
With D-Star, Yaesu System Fusion and DMR being available modes on 2m,
we have a new need for 2m cavity sets.
Nationwide, who will put their hand up and be the "guru"?
Me, I've been trying to learn the 'tricks of the trade', the hard way (soldering iron).
I'm looking for someone to talk to.
And, I'm happy to help with any needs in NSW.
Keep Smiling
Alan VK2ZIW
Re: 6m and 2m cavity sets, building and maintenance - VK2ZIP - 12-03-2017
Hi Alan,
I have found that the design in an old ARRL FM and Repeaters handbook works well. I did it at UHF, but I believe that the article gives dimensions for 2 m and 6 m as well.
This is a design that uses bandpass cavities, adding a small inductance or capacitance between input and output to give a notch above or below the centre frequency.
Initially, I had a lot of difficulty getting the notches at +/- 5 MHz, because the article did not explain how to adjust that. I expect that this article may have been updated in more recent editions, but if not I can tell you how to do it. It's actually a great way to do notch filters as it gives you independent adjustment of the offset of the notch, and the depth of the notch.
In a high RF environment, I found it useful to have two notch filters and one bandpass for RX, and the same for TX, to give some extra receiver protection from transmitter spurii and strong signals in adjacent bands.
Best wishes,
Lew.
Re: 6m and 2m cavity sets, building and maintenance - VK2ZIW - 21-03-2017
Please follow my blog at:
http://www.unixservice.com.au/hamradio/repeaters/index.html
Alan VK2ZIW
Re: 6m and 2m cavity sets, building and maintenance - VK3ALB - 21-03-2017
When using the URL thingo I have found that you can make it look neat by putting an equal sign after the first occurrence of URL followed by the real URL. Stick the nice text in between the blocks. Of course, you have to remove all the spaces inside the square braces.
[ url = <!-- m --><a class="postlink" href="http://www.unixservice.com.au/hamradio/repeaters/index.html">http://www.unixservice.com.au/hamradio/ ... index.html</a><!-- m --> ] Nice text goes here [ / url ]
Nice text goes here
Re: 6m and 2m cavity sets, building and maintenance - VK2ZIP - 16-04-2017
Hi Alan,
Thanks for the post with link to your 6-m cavity filter. You've obviously done some excellent work on this. What is the rejection from pass to notch, and what is the loss at the centre of the pass-band? Is there a stability issue with the large loop? Are you happy with it?
I don't really understand how this coupling mechanism works - does it give you more-or-less independent adjustment of offset and notch depth?
It could be worth talking sometime. If you send me an offline message, I will send you my phone number and a couple of photos of my 70 cm cavities / notch filters. Like you, I tune by means of a rod that projects inside the centre element.
Best wishes,
Lew.
Re: 6m and 2m cavity sets, building and maintenance - VK2JDS - 06-05-2017
If you can come across some Radio Frequency Systems RFS VHF cans with the 4 port box on the side they are really good.
I got 4 of them and they were set for 400KHz offset.
After much experimentation I can now set the offset, the frequencies and the intended slope of the cans.
What I mean is , the cans for the transmit side for a repeater above 147MHZ want a deep notch on the high side and a peak on the low side, while the receive side wants the reverse.
This is all set by the lengths of the coax cable connecting the 4 port box into the top of the notch cavity and the flying lead dangling out of the bottom of the 4 port box.
To widen the offset to 600 KC I lengthened the cable from the can into the box, this I did on all 4 cans and adjusted them for the desired peak and notch frequencies , now to set them for the different peak and null slopes from high side to low side, its just an adjustment of the length of the cable dangling (unterminated) from the lower port on the box.
So, whats in the box ? Lets open it up . Its glued on so some persuasion was required to open the lid.
It consists of 4 N connectors joined up with teflon silverplated coax in a square. The lengths are interesting and i will get around to measuring them so people can replicate them for their own use.
Apparently these things aren't made anymore, so I am hoping the manufacturer wont mind me describing the lengths of whats inside.
Any interest in this? it would appear these can be added onto any old notch cavity and give it the peak and notch as required.
73 Dave jds
Re: 6m and 2m cavity sets, building and maintenance - VK2ZIW - 13-05-2017
Hi Dave and others,
Trix:
The offset between pass and notch.
I have a 4 can set on 70cm here 60mm OD in bad shape in that the cabling,
harness RG303 teflon coax has been floating around in a car and some coax braids have
broken. These cans have a side loop with a cover plate and a short piece of coax going to
a "T" (3 coaxes) for the signal "pass by".
I've removed the antenna "T" (N socket, two coaxes going in) and put on two connectors (for now)
Now, a single can and a set of three.
One can has a 3.5cm coax from the T to the can loop. Thus notch to pass spacing is about 3MHz.
This can has a 21cm open stub from the T. Why??
3 cans have a 7cm coax from the T to the can loop. Thus notch to pass spacing is about 1.5MHz
At 440MHz, a 1/4 wave is 12cm in this coax. Teflon, Velocity Factor 0.69
I have seen a 1/4 wave line from the can loop to the T to swap the pass and notch. I suspect
this should be semi-rigid.
The lines to the antenna T should be 1/4 wave or 3/4 wave if too short practically.
Reason: The notch of a can is when it "shorts" the signal to GND. Thus a 1/4 wave line (or 3/4) will show
an open circuit back at the antenna T so the SWR will be not affected for the signal going down the "other" chain.
The two loop cans pose a problem here: Though the SWR of a can at the notch frequency is infinite, the impedance
could be anywhere on the outer circle of the smith chart. Thus you will see two different coax lengths BUT never
have I seen any discussion on this!!
I made up a short coax with two coaxes going into an N plug. I can then check the SWR and get the coax lengths correct.
I suspect the gurus of old had a zillion coax joiners, male to male and female to female and "built" a cable to length.
GR (General Radio) made an extendible line, probably for this exact purpose. With GR connectors of course. I have one.
It's all fun.
Alan VK2ZIW
Re: 6m and 2m cavity sets, building and maintenance - VK2ZIW - 31-05-2017
You can catch up on my notes:
http://www.unixservice.com.au/hamradio/repeaters/duplexers/2Mnotes.html
80 Alan VK2ZIW