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. |
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How does it look like,..well see the picture at the
right. |
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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. |
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 6½ mm copper tubing. I used refrigerant tubing
because I got a spool of it. |
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Click the right picture to see the frequency responce from 130-160Mhz. |
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Here a picture of the three coils I made from the
copper tubing. |
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A 144Mhz Cavity filter |
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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. |
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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. | |
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