RF Relay Contact Rating

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Relay contact rating and current rating at dc or low frequency ac have almost nothing to do with RF performance.

Published or advertised contact ratings are usually for hot switching into specified load types at low frequencies (or dc) and at fixed voltages. At radio frequencies, things are quite different. At radio frequencies, effects unimportant at dc or 60 Hz come into play.

Skin Effect

Skin effect pushes current to the outer edges or outer surfaces of conductors. While skin effect doesn't have a large effect on the small area actually connecting in relay contacts, it has a large effect on contact bar and wire heating in relays. Current handling of the very flexible woven braid used in relay movable contact wiring is greatly affected by frequency. With the loose, soft, fine-wire weave often used in very flexible relay leads, the safe current carrying capacity of wires can be 20% or less of the dc or low frequency AC rating.

 Voltage Rating

Nearly any time a component is rated for power handling, PEAK voltage must be considered (not RMS voltage). This is because voltage breakdown RMS is used only for heating problems or power calculations, not arcing.

Relay operating voltage with a 1:1 SWR into 50 ohms would be 1.414 times the sqrt of (P*R). This means the operating voltage would be 388 volts times a safety factor. If we wanted to handle a 2:1 SWR operating voltage would be 550 volts. With SWR, we must multiply the square root of the SWR times the normal matched peak voltage. The relay must have the result as an additional safety factor. 

For example:

Power = 1500 watts into 50-ohm system.

Operating voltage = 1.414 times the sqrt (1500*50) = 388 volts peak

388 volts peak into a matched load. With 2:1 SWR maximum voltage = sqrt 2 * 388 = 550 volts

Current

Current causes heating, so RMS and time-averaged values of current are required.  I = sqrt (P/R). RF current = sqrt (1500/50) = 5.5 amperes

We should also use a time, or short-term averaged current, because the failure is often caused by the accumulation of heat over time. The time interval over which we have to integrate or average current  depends on the thermal lag or thermal inertia of the contact path and wiring.

Again we need the same SWR correction and a safety factor. With current we use the same SWR correction method as with voltage.

Hot Switching

Contacts can instantly be destroyed, even in a very large relay at low power, if contacts are opened or closed while RF is present. Opening is particularly damaging because a small opening arc will ionize the air surrounding the contacts and create an arc-sustaining plasma. A second effect is standing waves. When a contact is open, the transmission line feeding the contact acts like a transformer. Under the right conditions, because of transmission lines and standing wave effects, voltage at the source can be stepped up dozens of times to extremely high voltages.

Contacts also can be ruined if high power RF is applied and an external voltage surge triggers a very tiny arc. A distant lightning hit several miles away can induce enough voltage into an antenna to cause a relay to arc. The arc ionizes air between contacts, and the resulting plasma lets high power RF follow a new path. The effect is very much like striking an arc with a welding rod. Once started, a peak RF voltage as low as 100 volts can sustain an arc 1/4 inch or more in length. Transmitter RF will sustain the arc until something fails.

Published Contact Ratings

There isn't a good way to use dc or low frequency AC ratings of a relay to determine RF ratings. Relays almost always must be tested and inspected in a working system because:

1.) Closed contact wiring and contact support bar current capacity is reduced. RF causes more heating, especially in any twisted or braided wires carrying current. 

2.) Hot-switched RF rating is much smaller than the hot-switched rating at low frequencies. Hot-switched failures in RF circuits can occur at surprisingly low operating current and voltage levels.

3.) Closed contact current carrying is usually much more than the hot-switch rating, and is not often published. Manufacturer's ratings are normally for switching a live circuit (hot switching) at low frequencies. Closed contact capacity is generally many times the published switched rating.

The RF contact rating and general switching performance is very much different than the dc or low frequency ac hot switch rating manufacturers publish.

Bottom line is we have to pick a likely choice and test the relay to see how it actually behaves.