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Radio Frequency (RF) Applications

Because of their good insulation qualities and low contact resistance, vacuum relays are quite commonly used in RF applications such as transmitters, antenna couplers, semi-con-ductor processing equipment, and power supplies. However, operating any relay at RF im-poses current and voltage limitations. The "skin effect" of RF current on the surface of the conductor creates the current limitation. As frequency is increased, the effective cross-section of the conductor path is decreased, resulting in heating of the conductor surface. This heating limits the maximum current the conductor can carry due to the inherent tem-perature limitations of the ceramic-to-metal or glass-to-metal seals.

When the relay is used as an insulator, there is RF voltage across the open contacts, or between contacts and ground. For all practical purposes, this insulator has a high voltage capacitance of from 1 to 2 picofarads. The leakage current flowing through this insulator/capacitor results in the heating of the lossy elements of the insulator which, again due to the temperature limitations of the seals, limits the maximum RF voltage that can be handled. In addition, contact gaps simply break down at lower voltages with RF than DC.

Figure 20
Rockwell International-Collins Avionics and Communications Division makes extensive use of Kilovac relays in the CU-2314(V) / ARC-190 Antenna Coupler Family.

These effects make it necessary to derate the current and voltage specifications for operating at RF. Ceramic relays can handle higher RF power than glass relays due to their reduced susceptibility to failure from localized heating; however, both types are commonly used. Many of the Kilovac relays recommended for RF applications have pure copper contacts for better heat conduction and increased ratings. Because of this, relays in RF applications should normally not be switched under load as a catastrophic failure caused by melting of the relay contacts may occur. If you have an application which requires high frequency power switching, call Kilovac’s Application Hot Line at 1-800-253-4560.

Typical RF Derating For High Voltage Vacuum Relays

Figure 21
Specific relay deratings are shown on the product pages.

Relays Recommended for RF Applications
Part Number Rated Voltage (dc) Current Carry @ 32 MHz Contact Form
S03BJA335 1.5 kV 3 amps A
S06CBA335 2.0 kV 6 amps A
HC-1 3.5 kV 7 amps C
S06FNA218 5.0 kV 8 amps A
K41 series 5.0 kV 12 amps A,B,C,P,R
K40P 5.0 kV 8 amps P
K45C 5.0 kV 6 amps C
S06HBA318 7.0 kV 8 amps A
S06JNB218 8.0 kV 8 amps B
K47 series 8.0 kV 3 amps A,B
K44P 8.0 kV 20 amps P
H-18 8.0 kV 2 amps 2C
K43 series 10.0 kV 10 amps A,B,C,P,R
H-14 12.0 kV 8 amps 2C
H-16 12.0 kV 4 amps 2C
KC-2, KC-11 15.0 kV 10 amps C
H-26 15.0 kV 4 amps 4C
H-19 20.0 kV 6 amps 2C
H-17 25.0 kV 8 amps C
KC-20<, KC-30 25.0 kV 30, 15 amps X,Y
H-23, H-24 30.0 kV 7 amps A,B