Microwave beams will partially reflect off what they hit, and antennas for focusing beams work just as well in reverse, for detecting and resolving microwave sources. Consequently, the larger HPM cannons with agile steerable beams work simultaneously as targeting multimode radar units. Remember that an aim maneuver with targeting radar allows the beam to "lock on" to a target, for a +3 to hit (although the justification in this case is a bit different. The usual explanation is that the exact range to the target is known. This has no effect for microwave beams, rather the bonus is because the microwave beam is the same beam as the radar being used to lock on - just much more intense. The bonus does not apply to any other sensors, just to cases where an HPM cannon picks up a blip on its radar and the radar operator clicks on the blip and hits the "kill this one" button.).
HPM guns are generally narrowband but tuneable over a fairly broad frequency range. Usually, the waveform is swept or chirped through the frequency scale to best couple to whatever vulnerabilities are present. However, if a specific frequency is known which will cause damage to a device, the HPM gun can be set to that frequency in order to better disable that device. Initially finding the right frequency for a particular device requires analysis- this takes a day, basic equipment, either access to the device or designs and schematics, and an Electronic Operations (Electronic Warfare) or Engineer (Electronics) roll (Gadgeteers can reduce this time to one hour, Quick Gadgeteers to one minute). If the device was specifically hardened against HPM, the roll is a contest against the designer's Engineer (Electronics) skill. Tuning the HPM gun to that frequency will give a -1 penalty to the HT roll against that item and identical items (for example, same make and model), and an additional penalty equal to the margin of success.
As with any focused beam weapon, HPM weapons can defocus their beams to cause damage over a wider range. This gives the usual HT bonus on the reistance rolls for spreading beams described for counter-electronics weapons. They can also increase their ROF if the listed ROF is 5 or more by reducing the power of the individual beams. This gives a bonus to the HT roll equal to the number of columns the ROF shifts on the Speed/Range table (i.e. from ROF 5 to ROF 7 is +1 to HT rolls, to ROF 10 is +2, to ROF 15 is +3, and so on).
Because microwaves propagate differently through the air than bullets, there are differences when using the advanced rules given in GURPS Tactical Shooting.
| TL | Weapon | Damage | Acc | Range | Mass (kg) | ROF | Shots | Min ST | Bulk | Rcl | Cost | LC | Notes |
| 8 | Light HPM Cannon | HT-17 aff | 18+4 | 120/- | 400/3.5p | 5 | 50 | 75M | -10 | 1 | $500 k | 3 | |
| 8 | Medium HPM Cannon | HT-25 aff | 18+4 | 180/- | 1100/70p | 5 | 50 | 120M | -10 | 1 | $1.2 M | 3 |
Descriptions
Light HPM Cannon: A microwave cannon for mounting on light trucks, aircraft, or for point defense on ships and bases. It has a large dish for focusing the microwaves that can be turned and aimed independently of the microwave generator and power supply equipment. Radar range: 25 km. Skill: Gunner (Beams). Aperture 150 cm. Beam energy 0.5 kJ. Energy used per shot 1.2 kJ. Extra power cell 3.5 kg, $1000.
Medium HPM Cannon: A microwave cannon with a phased array antenna dome for indepedently steering the beam. Suitable for mounting on large aircraft, heavy trucks or tanks, ships, or bases for area defense of bases or convoys. Because it uses a phased array for beam steering, it can divide up its ROF between different targets (including those in widely different directions) if it has the software and/or manpower to track and aim independently at each target. The HPM cannon can simultaneously serve as a targeting multimode radar sensor and scanner. Radar range: 60 km. Skill: Gunner (Beams). Aperture 200 cm. Beam energy 10 kJ. Energy used per shot 30 kJ. Extra power cell 70 kg, $20000.