Radar and associated projects
9.4 GHz RTE
Initial CAD drawing
Prototype radar target, which enhances the radar return from small boats by using an amplifier and pair of antennas. The filter was designed using Wavecon PARFIL on 0.8mm Taconic RF35 laminate. PARFIL turned out to be highly inaccurate and I would not use it again, although perhaps 0.8mm dielectric was a bit thick at X-band.
The printed filters were replaced with Minicircuits 7 GHz High Pass, although band pass filters are required to prevent out of band transmission. The device uses a pair of slotted waveguide arrays to give a horizontally polarised omni response (see antenna page). The whole device had to be made very cheaply in production quantities, without setup or tune, no milled housing and ideally with the antennas soldered to the microstrip feed on the RF board (impossible to connect to test equipment).
Field-testing a target on Hayling Island, near Portsmouth harbour. The measurements
of RCS were made at Qinetiq Funtington.
Unfortunately, the project ended in disaster and a lot of wasted time and money and the customer cancelled the contract. I had spent many thousands of pounds on microwave PCBs and photo-etched antennas, let alone months of time.
The reason it failed was the difficulty in obtaining a sufficient RCS at large heel angles (at least 7.5sq m at 20 degrees or 49.6 dB gain at 9.41 GHz) something which I had not anticipated. The antenna proved the most difficult - I had to get a ripple-free omni response at up to +/- 20 degree elevation (heel) within a 30mm diameter cylindrical radome made from 2mm thick polyethylene pipe.
It was the first time I had used waveguide to make an antenna. Initially a 3 slot device was tried, but when the elevation beamwidth was too narrow, I used a 2 slot. It worked very well as an omni at 0 degree elevation, with about 4 dBi gain, 18 dB return loss over 300 MHz and good horizontal polarisation, but the problem was at large elevation angles (when the mast of the boat is leaning over). A friend of mine simulated the antenna on a 3D package and his results agreed exactly with what I obtained, even down to the shape of the return loss plot, thanks Richard.
Here we see an interim model of the RF pcb, with the Hittite HMC590LP5 power amp on the right and Minicircuits LEE29 amps. The unit was controlled by a 12F675 PIC, which also generated the negative voltage (charge pump) for the PA bias. Hittite make some fantastic microwave ICs and thanks go to Tom Cantle for samples and advice.
RTE version 1 with 2 filters, too much insertion loss/ 600 MHz out
A solution would have been to increase the RF gain to around 60 dB, but these devices run full duplex to eliminate delay time (which causes range error), and it proved impossible to prevent feedback between transmit and receive antennas, especially with the radome. Whilst it was just possible to prevent oscillation in the lab with RAM and no metal objects nearby, the product could have transmitted 4 Watts EIRP out of band in the field, and could jam radar and satellite links.
RTE version 2 with 1 filter, impossible to tune
Prototype SAW interrogator for remote torque measurement, 50 kHz bandwidth
Prototype homodyne radar / altimeter, aerials by European Antennas Ltd
(300 mW transmitter)
Contact details:
JWD Ltd
Telephone/Fax: +44 (0)1403 261251, Mob: 07973 173808
email:
julian @ jwdltd.demon.co.uk