Helical resonator filter.

I am re-building a 3mtrs linear into a 2mtrs version, and on the output it will need a filter most of the time. So I looked into it for what was possible to make,.. the choise was a bandpass filter for now. I wanted to fabricate it in another manner like on a circuit boards with C`s and L`s on it. So I opted for a Helical Resonator. I didn`t have any knowledge or experiance with this subject, so I looked into my books and the internet. There`s also a nice little program to design these filters, complete with coil data and sizes, so you can go ahead when you got a design ready. Here the url to the program.

How does it look like,..well see the picture at the right.
I made it for a frequency of 144.300MHZ.
I used 3 resonators made from copper wire of 1,5mm2. One coil in each compartment, the coils can "see" eachother through a hole of 21mm wide. The first and last coil are coax fed at 48o from the start of the coil. This is the 50Ω point of the coil. It downs`t matter how you connect it, the output can also be the input or vice versa. The three bolts on top of the box are there for adjustment purposes. You can adjust the frequency and bandwide a little bit, not much.
It looks very good on my spectrum analyser, but it still can be improved to get things even better,..but you learn when you make progress in doing.

Here a more detailed drawing of how it looks like. The  feedpoints are at half a winding, not 48o, but it still can be at the 50Ω point. This is caused by the size, bandwide and other factors of the design.
Material I used is 0,75mm thick tinned metal plate, this van be bend easily and perfectly soldered with a soldering iron.

Here a screendump of the resonator..

Click both pictures for a full size printable one.

After building the first prototype above I started to build another hopefully better one. It`s also going to be a 3-pass filter but now build from 6 mm copper tubing. I used refrigerant tubing because I got a spool of it.
Here I also used the prgram "Helical" for the design, but now woth a lower coil impedance, so the coil and housing is going to be bigger. Click the screendump at the right for a bigger picture.


This time I made the drawing of the schematic from "Helical" into the program "RFsim". This because the frequency response diagram is much better in resolution so I can see much better what it is doing. (click left picture for details)

Click the right picture to see the frequency responce from 130-160Mhz.


Here a more detailed picture from 140-150Mhz. Here you can see the attenuation at 140 and 150Mhz at a level of -60dB. A nice figure. Perfect to put this filter between antenna and coax relais. This way the linear amplier`s harmonics are lowered and the crap coming in from the antenna outside the desired range is lowered before going into the LNA.

Here a picture of the three coils I made from the copper tubing.
4.3 windings, 42mm diamter, 63mm height. Wound on a 40mm PVC tube, when the tension is of the copper the diameter will be around 42mm. The 50Ω tap is at 0.0692 windings from the start of the coil, that is 360/0.0692=24.912 degrees from the start. Same for the end of the coil at 4.362 windings. The 0.362 is at 360/0.362=130.392 from the end of the 4th winding. These degrees can be drawn onto the PVC tubing circumference.

Housing coming soon.....

A 144Mhz Cavity filter


After some measurements on my 2mtr cavity amplifier I wanted to try to make a cavity filter for 144Mhz. For this design I took the measurements from the RSGB UHF/VHF manual from the 70`s. Converted the values to metric, bought some brass material and began creating the parts I needed on my lathe and soldering it together. Here at the right you see the finished cavity.

With a capacitor the frequency is adjustable between around 110-200Mhz. The bolt to adjust the C is made from 6mm thick brass and has threatsize of M6x0,5, that is extra fine threat so one revolution gives a 0,5mm adjustment. I also could have made one bigger non adjustable C and one smaller adjustable one to fine tune it.
I also put the cavity under test, and here the results in a computer plot. The downwards peak is the transmit curve, the upper peak the receiving curve. Click picture for details.

Here you have the drawing of my cavity, all sizes are in millimeters. The complete cavity is made out of 1mm thick brass sheet, brass pipe and wire. The connectors I used are N-type. Click picture for details.