This filter, along with the reciprocal 144MHz Low-Pass / 432MHz Notch filter, became a requirement for my station because I co-located a 2m yagi within an EME array of four 70cm yagi antennas. See the picture on the right. Antenna simulations and experience have shown that the 2m yagi will cause negligible deterioration of the 70cm EME array performance as long as it is positioned exactly in the center of the four 70cm yagis.

Still, as I had expected, the coupled 144MHz RF energy into the 70cm yagis was higher than what a typical receiver front end could tolerate. I actually measured 10mW at the 70cm antennas when transmitting 100W into my 2m EME array. This is a -40dB coupling between antennas. It is actually better than what I had expected, but it is still probably too much for the 70cm mast-mounted preamp to cope with. Maybe there is a LC high-pass filter at the input of my 70cm preamp, but I did not want to try it!

Hence the need for a filter. A high-pass filter combined with a notch filter appeared to me as a good way to do this. A commercial diplexer might have done the trick, but I did not want to spend, I wanted to build!

THE CIRCUIT

The main design objectives were: At least 30dB of attenuation of the 144MHz signal. Minimal insertion loss (S21) at 432MHz, < 0.15dB Low VSWR (S11 or return loss) at 432MHz, < 1.5:1 Be able to support at least 100W of 144MHz RF, though my immediate use is Rx only. The design activity was a combination of theory, gut feeling and trial-and-error. The resulting filter circuit consists of a high-pass filter stage (C1, L1, C2) followed by a Series-LC notch filter stage (L2,C3). This is shown on the figure to the right. Simulations were performed using Ansoft Serenade SV v8.50. I used a Q (quality factor) of 50 for the inductors and 500 for the capacitors. The resulting simulation plots are shown on the right. With these results on hand, it was time to heat up the soldering iron!

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CONSTRUCTION

For the enclosure, I used a square aluminum box on which I installed two SMA-female bulkhead connectors. This enclosure did not have compartments which could have improved isolation between filter stages, but I decided to give it a try.

You may build a box using PCB material or use an off-the-shelf enclosure. This is not really critical. I would pick a large enough box so that the inductors do not come too close to the walls. The variable capacitors I used are of 350V ceramic type . These should work fine with a 432MHz RF power of at least 100W. The suggested capacitance ranges are written on the circuit schematic. As for the inductors, their values and construction details are also provided on the schematic above. I used enameled copper wire, but bare copper wire will work the same. Of course, never use stranded wire for winding coils... I recommend you mount the two inductors at right angle one from the other to reduce coupling. Update 02/04/2012: One ham with keen eyes spotted a mis-wiring of the circuit in the enclosure shown on the right. Note that, even though the 144 MHz notch filter part made of L2 and C3 is not connected to the right location, it behaves exactly the same way: notch out the 144MHz signal and get out of the way at 432 MHz.

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TUNING AND RESULTS

The tuning of this filter consists of first nulling the notch filter by using a 144MHz signal source (RF generator level below 1mW or 0dBm) and an RF power meter or 144MHz receiver. Adjusting C3 for minimum signal is the objective. L2 winding spacing can also be changed and C3 re-adjusted to make sure that the best null can be achieved. Tuning is quite sharp, so go gentle on the tuning screwdriver. Using an RF power meter and a signal generator could be tricky as the power meter will also pick up the generator's harmonic spurs at 288MHz and 432MHz. A double check with a 144MHz receiver or a spectrum analyzer is probably a good idea. The second step in the tuning process consists of minimizing the VSWR at 432MHz. This is done by tuning C1 and C2 for best VSWR when a 432MHz transmitter is connected to a dummy load or antenna through the filter. Use a low enough power to minimize the risk of transmitter damage. If a good VSWR cannot be achieved, change L1 winding spread and re-tune C1 for best VSWR. Expect little interaction between the two filter stages. But as a final pass, the above two steps should be repeated so that any interaction, even small, between the two filter stages is taken into account. The resulting performance should resemble what I have measured on my filter. This is shown on the three figures on the right hand side. The first figure shows the resulting notch at 144MHz, a depth of greater than 60dB. The second figure highlights the very small insertion loss at 432MHz. I measured less than 0.1dB. The third figure shows the S11 reflection loss of better than -30dB at 432MHz. This is effectively a 1:1 VSWR. With such a filter, I don't feel nervous for my 70cm preamp if I feed 100W or more at 144MHz into my 2m antenna. This filter paired with the complementary VHF Low-Pass / UHF Notch filter allow a very safe and compact antenna configuration.

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