MODIFIED CAL.AMP. MMDS FOR HAM S-BAND:
Who said microwaves were difficult?... With some few modifications one can make an MMDS antenna used for pay-TV distribution in some countries into an S-Band receiving antenna at 2400MHz. Suitable for UO-11 S beacon reception and in the very near future for Phase3D downlink!!

I made these tests back in may-1998 with California Amplifier model 130194 which is shown in the photograph. It seems that this model has been discontinued and replaced by others. Searching CalAmp pages I found the closest current model (electrically and mechanically) to be model 131534, others may perform as well as or even better than mine though. Be carefull with the "radar immune" models as they most probably will block our 2400MHz band portion!
With this modified antenna I receive UO-11 S-band beacon at 2401.5MHz quite well. I am confident to say that its reception is better than some commercial Ham systems using parabolic reflectors. Listen to a .WAV file recorded from an SSB Electronics downconverter and another from my little antenna. Due to the unmodulated nature of UO-11 beacon a constant CW tone should be heard, but given the wild Doppler effect at these frequencies (and particularly with LEO sats.) the frequency correction software was forced to make huge steps wich are clearly heard in this recording.
In my opinion the key to success here was the excellent quality of the California Amplifier downconverter and having used the highest gain/lowest noise model available. Important also was using one that does not include the PCS/WCS/Radar filter wich would have wiped-out faint signals at 2400MHz.

MOD DETAILS: There are two modifications to be done: change the PLL synthesizer crystal and bypass the output IF filter. The PLL-controlled Local Oscillator comes at 2278MHz which would convert 2400 into 122MHz, what is outside the reception coverage of many rigs. So we need to lower the L.O. enough to make the 2400 be converted inside our rig's range. In this case the reference crystal resonating frequency must be 256 times lower than the wanted LO frequency. Ideally an HC-49 crystal with a resonating frequency of 8.8125MHz should make a signal at 2400MHz be converted downto 144MHz. But in my case I used an HC-6 (much BIGGER!) crystal at 8.772 MHz, so you see how flexible this thing is... With my un-ideal crystal I am getting UO-11's beacon at 2401.5 converted to 155.76MHz, so my LO is near 2246MHz what makes the crystal reference oscillator near 8.7724MHz. This seems acceptable having used an un-specified surplus crystal from Nebraska Surplus with 1/2" lenghtened leads!. I had to use long leads because the HC-6 case did not fit in the same place as the original HC-49 and had to place it on the top side of the PCB!. The original 8.8984MHz crystal is placed on the bottom side of the PCB (opposite to all other components). To reach the bottom of the printed circuit I had to unscrew all the screws that fixed the PCB and unsolder the antenna terminal (shown with a circle in the picture below).

BYPASSING OUTPUT IF FILTER: The Output IF filter must be bypassed because it is designed to let out signals between 222 and 408MHz, and we are quite away from there. The filter itself is formed by some inductors and condensers located between an SMD component marked 'RF 6X' and the output F connector. We have to place a bypass (piece of wire) between this SMD component and the output connector. Do not reach directly the output connector as there are 24Vdc there!, finish the jump at the cold-side of the dc-blocking condenser just before the output. The recomended path to follow is indicated with a line on the picture, the component marked with a cross must be unsoldered.

GOOD LUCK!

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