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My 24 GHz
by Joe Street VE3VXO
UPDATE:
On September 17, Y2K at 17:06 EDT a new Canadian DX record for 24 GHz WBFM was set at Aurora Ontario from grid square FN04FA (VE3VXO) to FN03XA (VE3SMA). A distance of 42 km.
My 10 and 24 GHz hilltop set-up is a combination of VHF and SHF equipment. The VHF portion uses a regular 2m FM mobile rig and a home brew collapsible 5 element quad of my own design. This is used for talk back to coordinate aiming the dishes and tuning. Since the microwave gear can drift several MHz with temperature and the link only happens when both dishes are pointed directly at each other AND tuned to the right frequency, it takes a bit of fiddling to get it set, and the VHF COM's are vital to this process.
Now for the microwave gear. This consists of a tripod and surplus military dish which started out its life as targeting X-band radar in a CF-100 fighter plane. It now resides on top of my tripod and is fed with a switchable feed 10 or 24 GHZ. The feed assy. also contains a low noise preamp for the receiver home brewed from a microwave transistor which delivers about 30 dB of gain and a noise figure better than 1 dB at the IF. The receiver has IF shift capability and is home brewed from a TDA7000 FM IC and has a sensitivity of about 2 microvolts without the preamp. The modulator is optimized for low noise and has a deviation control to allow compatibility with other equipment which may have different receiver bandwidths. There is also tone modulation available for when the path gets tough and CW is the only way to make the QSO.
I have taken pains to match the preamp to the video impedance of the microwave mixer and extra shielding and input filtering to keep out the nasty RF's that live on the hilltops we typically operate from. So all told the system has an overall noise figure of about 12 dB which is good for a wideband FM rig and the dish offers 30 dB gain on 10 and over 40 dB on 24 Ghz so it is looking like a high performance WBFM rig now.
My hope is to break the Canadian DX record on both of these bands this year which is long over due and perhaps 200 km will be possible with 5 mW on 10 Ghz. With 24 Ghz I have a whopping 10 mW but the atmosphere absorbs a frightening amount of that energy depending on the amount of water content in the air and the path length so it is much more challenging, and the antenna beam widths get down around 2 degrees further increasing the difficulty on 24, but my equipment has considerably more gain in the antenna and pre-amplifier although it has less power than equipment which was used previously to establish the records so we shall see.
Vy 73
de Joe VE3VXO