GBPPR X-Band Radar Warning Receiver |
Overview
This is a simple project for the amateur SIGINT enthusiast which can be used to detect and analyze various radars operating in the 8 GHz to 12 GHz (X-band) region. This frequency range is commonly used for marine and airplane weather / navigation radar. There are also several Doppler-based motion alarms which operate in this frequency region. Most notably, the ones used to secure large, open areas like those found around missle silos.
By monitoring the pulse repetition, antenna sweep rate, and carrier frequency (using a RF spectrum analyzer) of different radars, you should be able to identify, track, and monitor all sorts of new surveillance targets. For example, the Department of Homeland Security, and other enforcement agencies, often have marine-based "intelligence units" poking around the various water ports in the country. Since each radar has its own slight differences in carrier frequency (as the magnetron's age) and pulse repetition rates, you should be able to identify and track these operations. Just think, maybe YOU could be responsible for preventing a false-flag nuclear attack on an American city by the Mossad!
Detecting those Doppler-based motion alarms will be a little different. Since most of these alarms don't have an identifying modulating signal, you'll need to monitor their operating frequency range for the "silence" they create when compared to the normal background noise. The CW carrier from the alarm sounds just like - nothing. Rotate the receiver's horn antenna and as it sweeps through the alarm's RF carrier, the background noise will go full-quieting, then come back.
This Radar Warning Receiver (RWR) project consists of a 17 dB horn antenna with a standard WR-90 flange connected to a salvaged WR-90 diode detector mount. The diode detector is a standard 1N23B point-contact diode. These diodes and mounts are farily common items to find at ham radio swapfests or on eBay. They are also fairly easy to construct, if you're a semi-decent machinist. It should even be possible to use an older X-band police radar detector as an integrated horn antenna and diode detector. Those old, clunky Uniden models from the 1980s should work fine. A standard 10 GHz Gunnplexer will also work, as long as you don't power the Gunn diode and just use the mixer diode output as the detector.
The radar warning receiver's diode detector converts the incoming radar's RF pulses into a series of sharp-rising voltage spikes. These voltage spikes are further amplified by a chain of both transistor and op-amp amplifiers. Since most radars have a pulse repetition rate in the audio frequency range (20 - 20,000 Hz), is possible to monitor the output of the radar warning receiver using just a pair of headphones or common computer spectrum analyzing software.

Pictures & Construction Notes

Internal case overview. The case is from an old printer switch box.
Two types of radar warning receiver circuits will be shown here.
This version has an internal 9 volt battery and two signal outputs. One output feeds a high-impedance Piezo speaker element, and the other is routed to panel-mounted banana jacks. The input from the diode detector is via a BNC jack.

On the left is a 17 dB horn antenna with a WR-90 flange. This horn antenna was from an old Alpha Industries automatic door opener.
On the right is a 1N23 diode mount which also has a WR-90 flange. This was salvaged from a box of old radar parts I bought for $1. I swapped out the diode detector's original jack for a BNC jack.
1N23 diodes operate up into the X-band, while 1N21 are meant for the S-band (2 to 4 GHz). The letter after the part number (i.e. 1N23B) determines the diode's noise figure. The "higher" the letter, the lower the noise figure.

Circuit overview of one version of the GBPPR Radar Warning Receiver.
This version has a standard common-base, low-impedance transistor amplifier feeding a peak-holding circuit made from a LM324 op-amp.
The idea was to "stretch" the incoming pulses so they could be more easily measured with a frequency counter, but it didn't work out as well as I hoped.
The circuit worked, but analyzing the received signal's characteristics using software proved to be much easier.
A high-impedance Piezo speaker was used to try and monitor the signal, but it is still too weak at that point and required additional external amplification with a LM386 or LM380 amplifier IC.

Circuit overview of another version of the GBPPR Radar Warning Receiver.
This is basically the stock circuit from one-half of the binaural receiver circuit in the March 1999 issue of QST.

Internal overview of the completed GBPPR Radar Warning Receiver.
It turns out there really wasn't enough gain to drive the internal speaker, you'll need to add an external audio amplifier, like the GBPPR 1079 Audio Amplifier from GBPPR 'Zine Issue #39.
The output is fine for monitoring via a computer's sound card though.

Overview of the horn antenna mounted to the top of the radar warning receiver's case.
This horn antenna will be horizontally polarized, which most radar's use.
Due to the broadband nature of the diode detector and this horn antenna, the radar warning receiver's frequency response can be from below 1 GHz to over 30 GHz.
The quality and age of the 1N23 diode usually determines this. Be sure to take this into consideration if you notice any strange, spurious signals.

Completed GBPPR Radar Warning Receiver.
The output from the diode detector is via a short piece of coaxial cable with BNC connectors.
The final signal output can be monitored via a pair of headphones or sent to the "line input" on a computer for software analysis.
The program shown here is Spectrogram from Visualization Software.
During this initial setup the GBPPR Radar Warning Receiver is powered, but is not receiving any RF signals.
The Spectrogram frequency display is from 100 Hz to 4,000 Hz.
The output signal from the radar warning receiver is connected to the line input on a standard laptop computer.
Test setup receiving a signal from a Furuno Model FR-360 Mark II marine radar operating at around 9.41 GHz.
This radar is fully powered and is using the "long range" Pulse Repetition Frequency (PRF) of approximately 840 Hz.
The radar's antenna is not rotating.
Note the fairly strong harmonics of the true PRF signal.
Radar antenna rotation (24 rpm) is now turned on.
The radar warning receiver is stationary.
Note how the signal peaks as the antenna sweeps through the main lobe of the radar warning receiver's horn antenna.
The radar is now using the "short range" PRF of approximately 3,360 Hz.
The radar's antenna is not rotating.
Radar antenna rotation (24 rpm) is now turned on.
Again, note how the signal peaks as the antenna sweeps through the main lobe of the radar warning receiver's horn antenna.

