I have this nice and small (and expensive) helical filter in my drawer for some time. Looking the TOKO filter data-sheet this is very nice filter for the 1152 MHz local oscillator project, but I want to use the same one in my 23cm transverter, for filtering 1296 MHz. The data-sheet states that it is not possible to tune to the desired frequency. Of course, there is another filter covering that particular part of the band but my drawer is not TOKO store :-)
Going back to the data-sheet... It is clear that this is double tuned helix type filter, center frequency 1195 MHz with the coupling done through the window inside the metal shield. The connection diagram (F connections) in the data-sheet does not give a real situation of the helix inside the shield. From the diagram we can see that there is a helix between the hot end and the ground connection and a small part of the helix continuing after the hot end pin connection. Basically, the idea is just to short the helix to bring the resonance higher using cut and try method, but following the diagram there is nothing much to cut on F version.
On the other side I got the hint from Goran AD6IW that he make the mods. on the same filter very easy and simple and this should work, so this was good reason to try and see what is going on inside the "mystery helix".
Do not be afraid, this is going to be very quick and simple modification. You do not need not even a soldering iron, just a small sharp cutter and a small flat screwdriver. Each of the helix is secured inside the housing with the 4 dents, 2 on each side. Using a tip of a sharp flat screwdriver bent vertically all 4 dents to free the brown helix former. Pull out the the helix former, and you will have the same situation as from the following photo:
You probably ask yourself the same question I ask myself too, WTF is worth 20$ inside this helix filter ?! Do not look and search for any other parts inside the metal shield, there is nothing else inside except the tuning screw :-)
First thing that you may notice is that the real situation is not the same as on the diagram. The "helix coil" between the hot end and the ground pin is just a bridge from one pin to another, no any helix turns at all, as showed on the diagram. The helix is then extending from the live end pin around the coil former exactly 5 turns. What we need to do is just short the helix to bring the center resonance around the 1296 MHz. Cut and try method was the simplest one and gave excellent result. The helix is soldered on one end to the connecting pin and the other free end is just bended wire inside the gap of the coil former. To bring the resonance up, simply cut and remove just 1/4 of the last wire turn from the helix using the tip of the sharp cutter. The wire is stiff enough to stay on the coil former, but if you feel that the helix will be unwound secure the helix with the drop of super glue. I did not use this method because I didn't know what effect can cause the glue to the coil resonance and Q factor (if any?).
Use the same procedure for the other helix in the filter. After all, insert the coil formers back to the shield and secure them with the same 4 dents from the beginning of the modification.
So that's it, just a 1/4 turn shorter helix solved the problem and the filter is tuning smoothly to the 1296 MHz with almost identical response curve. I did check the filter directly soldering the SMA connector to the live pin and ground shorting the ground pin and shield. This was much simpler than soldering the filter on the special PCB. The proper way of tuning will be using the network analyzer but this is not so common among the HAM experimenters so simple generator, oscillator on 1296 MHz and LNB diode detector can do the job also.
Finally, check this cheap and dirty approach, the LNB IF MMICs (C1H & C1E) board with the modified TOKO filter. -13dB of input gives out filtered +12dB of signal on 1296 MHz. You can use this board both way, for RX & TX, so simple, the half of the 23cm transverter is already here :-)
Thanks for information, I have a preamp for ATV 23cm with a Neosid 1300 F/N, can't find it anymore, I find Toko 5HW 120050 : 1225 Mhz on EbayReplyDelete
ON7ARQ - Patrick