RF Relay Contact Rating
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.
Contact Materials
Contrary to some opinions, a larger relay is almost always not better. Larger
relay contacts often have less pressure per square inch of mating area, and are
often materials designed for hot switching. The ideal contact material would
have a gold flash with the smallest contact patch area the steady-state make
current allows. At all costs, avoid silver or cad plated contacts, or
exceptionally hard contacts. The ideal receive side relay would be designed with
bifurcated (split) contacts.
Contact reliability in cold switched, near zero contact current and near zero
contact voltage applications, and this is a cold switch application,
rapidly decreases as the contact is made larger than the minimum size possible.
Reliability also decreases significantly if hard contact materials are used, or
if the contact is cadmium or silver plated.
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.