Saturday 24/06/17

34GHz Radar Transmitter Simulator

The 34GHz transmitter shares many basic features with the other frequency variants. The magnetron is a surplus item of unknown origin, and we were unable to find any original manufacturers data. A number of carefully configured experimental setups were required in order to establish the operating characteristics of this tube. Some assumptions have been made due to the lack of manufacturers data. The unit has been in use for a number of years, and so it can be assumed these assumptions were broadly accurate and the tube is not being operated beyond it's intended ratings.

The Magnetron

The 34GHz magnetron

The magnetron is of the familiar double horse-shoe configuration, with the wavegude 22 flange located on the side. Both ends of the tube have glass seals and so the greatest of care was necessary at every stage. We had only a single tube on hand, and have subsequently been unable to locate any spares.

The cathode is visible through the glass seal on the waveguide port, and it was hence possible to use a non-invasive infrared temperature measurement to establish the correct heater voltage.

Magnetron Testing

Toroidal Current Probe

Magnetron Testing

Without manufacturers data, testing a magnetron is a rather risky business. It would be very easy to damage the tube by exceeding it's ratings.

We utilised a spare partially built power supply unit, and mounted the magnetron in a temporary location outside of the enclosure. A current probe was added to the "cold" end of the pulse transformer, providing vital information regarding the cathode current. Any slope in the cathode current during the pulse can indicate that the cathode current x time rating is being exceeded, and this could cause irreparable damage to the cathode.

Assumptions can be made regarding the maximum heatsink temperature.

We are making use of an automated heater control to regulate the cathode temperature, and this gives vital feedback regarding the amount of self heating caused by secondary electron bombardment. As the average power is increased, the heater drive is seen to back-off autoatically. We can choose an essentially arbitrary but "safe" maximum for this effect.

By taking these factors into consideration, we can arrive at a "conservative" but safe overall rating that gives a warm feeling we are not going to prematurely destroy the tube.

Chassis Layout


Chassis Layout

The physical shape of the 34GHz magnetron did not lend itself to an easy chassis layout. We want to use essentially the same chassis as for the other frequency variants, but this will work only if the magnetron is positioned inside the inner high-voltage compartment in a somewhat inconvenient "flat" orientation. Inconvenient, but workable.

The top image shows an early stage of the high voltage compartment assembly.

For this unit, the pulse transformer is layed on it's side to gain a little more vertical space inside the enclosure.

The IGBT/PFN/saturable inductor are more closely assembled than previously. This is in fact a better arrangement because it reduces stray inductances, but this was not the reason for doing it. There was just a lack of space!

Assembly HV Box

Assembly HV Box

The HV Enclosure

These images show the HV enclosure nearing completion, with all major parts in place and the magnetron fitted and connected to the pulse transformer. We fabricated cathode/heater connection clamps from copper sheet because there was little chance of obtaining any "original" parts for this purpose.

In the top image, the waveguide 22 output flange can be seen where it exits from the enclosure.

The compactness of the IGBT and associated components is also evident.

The lower image also shows the fan and RF gasket. The fan is used to extract heat from the enclosure. Holes in the lid (seen in the images below) direct cool air onto the finned heatsink around the centre of the magnetron.

The fan itself is an a.c powered unit. During early trials with D.C powered fans, problems were encountered due to the proximity of the fan to the magnetic fields around the magnetron, which interfered with the operation of the Hall effect sensors inside the fan. Any potential for this problem is eliminated by using an a.c. fan.

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RF Output, Waveguide 22

This image shows the side of the final unit where the waveguide 22 flange is secured.

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The Completed Unit

These images show the completed unit assembled into it's stainless steel case.

The compactness of the unit is evident, with the D.C power supplies and a.c. controls outside of the HV enclosure.

In the top image, the time switch can been seen. This isolates the main HV supply until the heater has had a 3-minute warm-up. If the power is interrupted for any reason, the 3-minute delay is reset.

In the lower image the two external vent holes are visible. In this unit, they are both fitted to the same side of the box. Air is drawn in at the right, flows into the HV enclosure directly onto the magnetron, and is then expelled via the hole on the left. There are two fans fitted.

The large number of screws used to secure the lid of the HV compartment are there to ensure the compartment does not leak any troublesome RF fields. What we care about here is not the microwave energy produced by the magnetron, but any switching transients and other low frequency noise that could be emitted by the relatively fast, high energy electronics.

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