In this discussion, a number of guys have suggested using PCB power relays
for switching 1500W of RF. For example, the Tyco RTB14012F
(http://catalog.tycoelectronics.com/TE/bin/TE.Connect?C=1
<http://catalog.tycoelectronics.com/TE/bin/TE.Connect?C=1&M=BYPN&PN=RTB14012
F> &M=BYPN&PN=RTB14012F) is available in both DPDT and SPDT contacts, the
latter handling 16Amps. It's very cheap (about $2) and reliable and would
appear perfect for such an application.
However, I've shied away from such relays in previous RF applications,
because of their minimum load ratings. Most people aren't aware that relays
designed for DC switching have a minimum load rating, For example, the
aforementioned Tyco relay specs its minimum load at 12V/100mA. At a lower
load, switching reliability isn't guaranteed. Most of us have had the
experience of losing receiver sensitivity, especially in cold weather, but
having it restored after clicking the PTT switch a couple of times. This is
a common problem with Transco switches (not relays), which I've stopped
using outdoors for that reason. It can also be a problem with power relays.
Relays that switch DC loads are particularly prone to this problem.
I wrestled with this issue in a receiver multicoupler I built two years ago.
It used 11 PCB relays, all intended for switching low-level RF. I finally
settled on the Tyco V23105 relay, which has a minimum load rating of
10uA/10mV and haven't had any problems, even with RF voltages in the uV
range. Relays like the V23105 are called "signal relays" and are
specifically designed to switch low-level voltages and currents. Obviously,
as the name implies, they aren't suitable for RF power applications.
Unfortunately, there's a tradeoff between minimum and maximum load ratings,
and one can't find a relay that has both low minimum and high maximum load
ratings.
I'm dealing with this issue again on a current project, a station control
unit that will allow a user to switch easily between multiple transceivers
and amplifiers from one operating position. It will transfer all the CAT
serial lines, packet, RTTY, microphone, key, etc., between stations, a total
of 25 different signal lines. Here, I'm using the V23105 for low-level
switching, but still searching for a good high level RF relay. For my own
station I'll use vacuum relays for the high level switching, but want to
find a more practical and inexpensive solution for other builders who may
want to duplicate the device.
73,
Jim W8ZR
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