Cavity

[radium98]

I ask if in a day in my life i can do a little project of a supported 100w fm cavity filter , but i can not find enough infos about , just for 2m 144 mhz etc...and some pictures , if any detailed article available is welcome to study , later .

P1000001 (2).jpg

cav-can.jpg

[garada]

In this old but very good article from the "nuova elettronica" magazine you have data to build filters at the desired frequency, among other cavity filters.

[radium98]

garada thank you a lot . i will have a look at it .

[rigmo]

In this old but very good article from the "nuova elettronica" magazine you have data to build filters at the desired frequency, among other cavity filters.

I Know for his sine 1990 when I build first ones... tnx bro!

[Albert H]

It doesn't matter what kind of filter you're making - you'll ALWAYS need a Spectrum Analyser!

[rigmo]

It doesn't matter what kind of filter you're making - you'll ALWAYS need a Spectrum Analyser!

Do you aske me? you mean VNA?

[yellowbeard]

changpuak has a calculator for this:
https://www.changpuak.ch/electronics/Co ... signer.php
Provided you have a way of measuring the response, it should get you in the ballpark for a design. I suppose like an antenna, make it bigger and home in on the desired frequency by trimming it with a grinder. Using a 25mm central rod (1 Inch copper pipe) the sucker comes out at 2 foot long - looks about right compared to the scale of commercial filters I've seen. You won't be putting that in the box with the transmitter...

[3metrejim]

It doesn't matter what kind of filter you're making - you'll ALWAYS need a Spectrum Analyser!

Do you aske me? you mean VNA?

You can use a vna to check filters too, and it's probably a lot cheaper option these days with devices like the nano vna about.

If you think about what a spectrum analyser is: It's just a swept receiver showing frequency vs amplitude. A vna does almost the same thing but outputs the swept frequency into the circuit it is measuring. You'll need a sweep generator if using a spectrum analyser (or one with it built in). A bit simplified, as a spectrum analyser is a precision instrument, but the same general idea. A spectrum analyser also has other uses, like measuring harmonic output, spurious emissions and more, that a vna doesn't do.

--Begin RANT --

On another note (and I am disappointed to say this), if you go through the entire site and read Albert's responses, there are many claims with precious little (original) substance to back them up - I had to reverse engineer a cmos pll from a photograph to post a schematic here (on the previous version of radionecks), but I understand that Reverand has now posted another similar version (and on another site). I fail to understand why Albert just couldn't post it (it's ancient technology as it is). This is reminiscent of a radio amateur that doesn't like pirates , trying to throw spanners in the works and sow confusion (I have met a few). This is only my opinion, but it seems to fit. I haven't bothered to check every comment user id to ensure there isn't an impostor (and I shouldn't have to).

--End RANT--

[Albert H]

Jim - get off my back. I've built more rigs than you've had hot dinners - I started in 1969 and only stopped in the mid-90s when the people involved became unpleasant to deal with.....

Stephen Moss at NRG was a personal friend and collaborator, and I helped him out during the legal nonsense with P*** H*******. To that end, I redesigned a couple of his products to move them well away from the "competition"'s products - in fact we moved well ahead. The mods to the little square stereo coder (including the mono switching) were mine for example - for the first time it didn't start with random channels! I also designed the Pro IV coder for him, and still have the very first one that was built on a home-etched PCB (the first two were built on Veroboard). The modifications to the PCB for the Pro III (to include OLPD) and the use of the 4059 were mine. We were very good friends, and I spent a lot of time up at Queensbury, and he used to visit me at my North London place when he was down there.

As far as designing and constructing cavity filters is concerned - there's every chance of getting a reasonable result with a Micro VNA (I've been using one for a while). However, cavity filters are pretty pointless at the kind of power levels we're talking about here - a competently designed and properly screened lowpass filter will be sufficient - especially with the modern choice of dual FETs for finals (where they inherently cancel the second harmonic).

Incidently - I did have a couple of amateur licences (a PA one and a G3), and I've also held T&D Licences, broadcast licences, and been the "Responsible Engineer" for a group of FM stations in the USA..... I've been a Chartered Engineer for 40 years, so have some professional credilbility. Still, I have no axe to grind with pirates - having been one myself for over 50 years!