Quick info
Created on :
Frequency band :
980MHz suppression :
50 dB

An interdigital filter for the 23cm band

GSM base stations are everywhere nowadays and make a lot of signal. When trying to receiving 23cm ATV signals, living near such a station can be a real pain. But since I use a 26-elements Yagi for reception, virtually every GSM station is 'nearby' to me.
If a far-away ATV signal happens to be in one line with a base station the Yagi will be pointed straight at it. The pre-amp will thus have to be able to cope with the strong unwanted 950MHz signal it receives. The pre-amp I build some time ago does have filters, but they don't really suppress enough to get rid of the GSM signals. Actually, when turning the antenna in the direction of my favorite ATV repeater, the first input stage gets nearly saturated by the strong GSM signal, and that results in lower gain of the amplifier.

So I decided to go and build a band-filter for the 23cm band. After some searching over the internet I found the following drawing of an interdigital filter :
(click to enlarge)

Looks nice, but where do I get the parts to build this thing?
After a quick look in our garage I decided to redesign the thing a little bit and use some other materials to build it :

The outside of the filter will be build from epoxy board with 2 layers of copper. For the poles inside the filter, I used an old tentpole ;-)

Let the fun begin!

The tentpole cut into 7 neat little poles, each 47.5mm in length.

Now take the epoxy board and cut the top, bottom, front and back sides out of it. Make the front and back 10mm wider than the image says, so they will overlapp 5mm on each side with the top and bottom of the filter.
I used a pair of scissors to cut it, that gives more straight lines than using a saw.

The right image shows how the parts will fit together later on.

Now draw a line in the center of the front and back panel and place some marks for the poles and the tune-screws. Drill the holes for the tune-screws and the BNC-plugs.

Now I first soldered the BNC input and output plugs in place, since they are very hard to reach when the poles are fastened.

With the BNC plugs at the right place, it's time to solder the first 4 poles. I sanded the poles before soldering, which made the soldering a real piece of cake. Take your time to position the poles in straight angles, since we want the tune-screws to fit into them from the other side :)

Solder the front and bottom panels together. This is just to get the distance to the back-panel right, so just solder them together at 4 points or so. We'll solder it closed tight in the end. This also allows you to re-adjust the angle of the panels, which would be impossible if we soldered the entire thing together at once.

Solder a 5mm wire to the BNC plugs for the connection with the poles. Now take the back-panel and solder the last 3 poles onto it. Then solder it, again at 4 points, to the other panels. If all fits well, you should be able to solder the 5mm wire from the BNC plugs to the poles.
I should have drilled holes for the tune-screws, but as you can see in the images, I blatently forgot them. It would have made more sense if I had done it while drilling the holes for the BNC-plugs. No big deal, just drill the holes carefully.

Now take a nut, solder it over a tune-screw hole and put a screw into it. Repeat this action for all the holes you've drilled. The result should look something like this :

We're almost done here. Put the top panel and the end panels into place now and solder them together.

Now it's time to try the thing out. The outmost poles are meant to suppress 950MHz and all the others must be tuned to be resonant at 23cm. To start with, I just turned the outmost screws a couple of centimeters into the poles, and all other screws halfway between the panel and the poles.
Since I don't have a spectrum analyser I had to tune the filter in another way. I took my 1250MHz ATV transmitter (output power +/- 10mW) and connected it to the input. The output was connected to my ATV receiver. With the signal-strength meter I could easilly tune all screws for maximum signal. When the signal reaches the receiver's maximum, just add an attenuator somewhere between the transmitter and the receiver.

But the outmost screws were not yet in their 'best' position. So I connected the receiver to my Yagi antenna, tuned to 950MHz and rotated the antenna untill I found a fairly strong GSM signal. Now place the filter between the antenna and the receiver, and simply tune the outmost screws for minimum signal.

I had a full-scale signal level without the filter, and zero with the filter in between, so I couldn't tune it better with my equipment. Someday I'll hook the thing up to a spectrum analyser and put some real-life numbers here, but for now, I can only say : losses are VERY small, and supression of 950MHz is very good.
I certainly don't regret it took me an entire afternoon and evening to build the thing, it is worth every minute it took. ;-)

Update 08-06-2002 :
I had the chance to connect the filter to a spectrum analyser.
Some numbers :
145 MHz-36 dB
430 MHz-23 dB
950 MHz-45 dB
980 MHz-50 dB
1250 MHz-0.9 dB
1280 MHz-1.1 dB

The -3 dB bandwidth of the filter is about 95 MHz. The poles I used are a bit wider than the poles used in the filter on the drawing, so I should have made the poles a couple of milimeters shorter. This would have improved the pass-band suppression.

Some images of the filter as it is right now :

(note I still have to solder the edges to get it waterproof)

- Created by Daan Vreeken - PA4DAN -

Email me with questions/comments : Daan <Daan @ pa4dan . nl>