The F-4 Phantom II, a legendary aircraft of the Cold War era, relied heavily on its sophisticated radar systems for air-to-air and air-to-ground operations. As a former MOS 6657, airborne missile fire control technician on F-4J and F-4S Phantom IIs in the Marine Corps, I gained extensive hands-on experience with these intricate systems. Let’s delve into the nuances of the APG-59 and AWG-10 radars, exploring their functionalities, modes, and the intricacies of their operation.
The top image showcases the LRU-1 radar antenna, a critical component of the F-4’s radar suite. Notably absent are the IFF (Identification, Friend or Foe) antennas in this particular configuration. Positioned on the port side, or the right side from the viewer’s perspective, is a rectangular, funnel-shaped structure – the CW illuminator feedhorn. This component was essential for guiding AIM-7 Sparrow missiles. The system employed a main KPA (Klystron Power Amplifier) at 1525 watts and a CW illuminator KPA around 900 watts. The seemingly lower power of the illuminator KPA was due to the main KPA’s duty cycle limitations. Operating at less than 50% duty cycle for transmission and reception (TX/RX) significantly reduced its effective power compared to the continuous wave (CW) KPA. Signal from the CW KPA was also routed via coaxial cable to the rear missile stations, enabling Sparrow missiles to achieve target lock. Early versions of Sparrow missiles utilized mechanical tuners, which were notoriously unreliable. One memorable incident involved an ordnance technician resorting to a large hammer to “unstick” a tuner, a method that understandably sent more cautious personnel scattering.
The APG-59/AWG-10 radar systems featured three primary operational modes, each optimized for different scenarios and ranges:
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Short Pulse Mode (10 NM Range): This mode utilized a 0.65 microsecond (uSEC) pulse. Triggering the transmitter released a pulse of the same duration (0.65 uSEC). This was the standard mode for short-range operations, effective up to 10 nautical miles.
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Chirp Mode (Extended Range): For enhanced range, the system employed “chirp” mode. A 0.65 uSEC pulse was processed through a “delay line,” essentially an inductor grounded at one end. This induced the inductor to resonate, prolonging the transmitter pulse duration to approximately 65 uSEC. Upon signal return, the same delay line compressed the pulse back to around 0.8 uSEC before reaching the receiver (LRU-2A8). While this process resulted in a slight reduction in resolution, the significantly increased pulse duration dramatically improved detection range due to the stronger return signal.
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Pulsed Doppler (PD) Mode (Velocity-Based Targeting): The most powerful mode was Pulsed Doppler. In this configuration, the transmitter fired for roughly 40 uS, followed by a 40 uS reception period. The PRF (Pulse Repetition Frequency) was dynamically adjusted to prevent “target eclipsing,” a phenomenon occurring when the transmitter is active while the return signal arrives, causing signal loss.
Returning to the LRU-1 antenna image, a rectangular panel situated about a third of the way down the antenna is visible. This is the Beam Spoiler, activated in PPI/MAP (Plan Position Indicator) mode for ground mapping. Extending approximately 1 inch, it deliberately distorted the radiation pattern to facilitate ground scanning. On the radar scope in PPI/MAP mode, the sweep pattern differed from combat modes. Instead of vertical sweeps, the scan was horizontal, traversing 120 degrees (+-60º). The bottom of the scan was fixed, while the top, representing the furthest range, displayed a fan-shaped or pie-section appearance. In combat situations, the radar sweep transitioned to a vertical orientation, covering the entire display screen for air target acquisition.
The second image highlights the front of the antenna, now equipped with eight black T-shaped IFF antennas. The white tips on one end of the “T” denote antenna polarity, crucial for correct signal transmission and reception. Incorrect polarity configuration could disrupt the IFF system’s functionality.
The feedhorn, projecting prominently from the antenna’s center, plays a pivotal role in directing and receiving radar signals. Its forward end is sealed with thin fiberglass covers, epoxied to maintain pressure within the waveguide system. Pressurization with dry air at 14 lbs/in² was necessary to prevent RF energy arcing (shorting) within the waveguide. The feedhorn directed transmitted energy towards the dish and, conversely, received return signals. Upon target lock initiation by the RIO (Radar Intercept Officer), the feedhorn support began to rotate at 66 RPM, a process known as “nutating the feedhorn.” This rotation created slight positional shifts in the feedhorn, causing the radar to effectively “paint a donut” around the target. The radar system then analyzed signal return variations around this “donut” to automatically reorient the antenna for optimal signal strength.
Antenna control was achieved through servos and resolvers, powered by the aircraft’s hydraulic system. While the F-4’s primary hydraulic system operated at 3,000 PSI, the antenna system utilized a regulated pressure of 1,200 PSI.
Among the radar modes, Pulsed Doppler (PD) presented the steepest learning curve for operators and was often more complex to troubleshoot. Unlike other modes displaying targets in terms of range, PD mode indicated targets based on closing velocity (Vc). Targets positioned higher on the scope were approaching rapidly, while those lower were receding. The velocity range spanned from approximately 1,600 knots closing to 500 knots opening. To ascertain target range in PD mode, a lock-on was necessary, upon which a range gate would appear as a blip on the display.
The introduction of the AWG-10A radar marked a significant advancement. Key improvements involved replacing LRUs 15, 16, and 17 (analog computers) located in the turtleback (panel 19 behind the RIO). The updated system incorporated digital LRUs 15 and 16, and eliminated LRU-17 entirely. The analog system’s missile envelope representation, a truncated cone, was deemed inadequate. The AWG-10A offered a more accurate and effective missile envelope, often described as mushroom-shaped, better reflecting the missile’s lethal zone.
In the context of the second image, the “6” equipment rack is shown in the down position. LRU-6A2 is situated at the bottom, with LRU-6A1 above it. The 6A1 primarily managed antenna control, while the 6A2 was dedicated to the CW illuminator functions.
Adjacent and forward of the “6” rack is the “5” equipment rack, and behind the X-frame lies the “4” equipment rack. The “4” rack houses LRUs 4A1 (top), 4A2 (middle), and 4A3 (bottom). Notably, LRU-4A2 contained 290 crystal ovens, each operating at a distinct frequency. These ovens resonated with signal returns in PD mode, providing the velocity closing (Vc) range, spanning roughly -500 knots to 1500 knots.
Correction: It was the 4A1A7 board, not the 4A3A7, that was crucial for frequency selection. The 4A3A7 board, however, had its own vulnerabilities. A blown block of Zener diodes on this board could manifest as “picket fencing” on the radar display – vertical streaks caused by PRF fluctuations every 60 milliseconds.
Later F-4 versions incorporated VTAS (Visual Target Acquisition System). This system utilized a specialized helmet equipped with four IR transmitters, complemented by IR receivers in the pilot’s cockpit. The pilot’s helmet integrated a reticle. By aligning the reticle with a visual target and pressing the acquisition switch, the radar would initiate a sweep in short pulse mode (10-mile range) to acquire the designated target. While memory fades over time, VTAS significantly enhanced target acquisition capabilities within visual range engagements.
Conclusion
The radar systems of the F-4 Phantom II were marvels of engineering for their time, demanding specialized knowledge for maintenance and operation. From understanding the nuances of Pulse Doppler mode to appreciating the advancements of the AWG-10A, technicians and pilots alike relied on these systems for mission success. The insights shared here offer a glimpse into the complexities and capabilities of these Cold War era avionics, highlighting the critical role of radar technology in air combat.
