So WHAT IS the manufacturers part number of an adequate relay?
Carl
KM1H
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Tim Shoppa" <tshoppa@gmail.com>; "Mike Waters" <mikewate@gmail.com>
Cc: "topband" <topband@contesting.com>
Sent: Thursday, October 09, 2014 9:46 AM
Subject: Re: Topband: TX relays
Because there are many things that go into relay selection that do not
show on a data sheet, I always dissect and test relays.
I have found 30 amp power relays that overheat at 5 amps at 28 MHz, and
relays that have high contact voltage ratings that make the pole inside
the coil hot with full RF. They wind up with 20-30 pF capacitance from
armature to coil.
Another issue is resistance and reliability at near zero contact voltage
when receiving. This is probably the single biggest relay issue in our
applications. A small bifurcated contact relay is better for receive
reliability, and a high current hot switch design is by far the worse for
receive reliability.
One particularly troublesome high power area for current are the relay
internal leads, and the contact support bar materials. The things that
make the wires and contact bars last a long time in repeated cycles create
very high radio frequency resistances. This is why some large 30 amp power
relays will discolor contacts or melt insulation at several amps on higher
frequencies.
The same thing applies to contacts. Contact materials and platings that
optimize hot switching create RF resistance and low level signal
connection issues. A gold flash on a soft contact, for example, is
excellent for receive but will instantly deteriorate if hot switched at
more than a few hundred milliamperes or with an inductive load. It might
handle 20 amps of closed contact RF current, but only be rated for a few
amps of hot switching current. In contrast, a silver cadmium oxide contact
can take tons of hot switch voltage and current, but is lousy for relay
receive pass through.
Contact support bars, and the wires used in some relays, can also be very
problematic. This is because the materials and any weave in wires is
designed for flexibility. Alloys and construction that improves mechanical
cycle life greatly reduces RF performance.
> Mike, I would be concerned about using these small relays for
non-resonant
antenna switching where the impedance at the switch point may be wildly
away from 50 ohms.
This is the relay I use to switch tuning networks at non-50-ohm points:
http://www.deltrol-controls.com/products/relays/power-relays/900
Deltrol is the brand you get if you order from McMaster-Carr but all the
big relay manufacturers sell these open frame relays.
I also bend the relay contacts for wider spacing as recommended by N6RK
in
QST (page 66, May 2009 QST Hints and Kinks, "Increasing Relay Voltage
Handling"). Open contact gap of 0.5 inches is readily achievable.
Tim N3QE
On Thu, Oct 9, 2014 at 6:19 AM, Mike Waters <mikewate@gmail.com> wrote:
This relay looks exactly what I was looking for to remote-switch my 160m
inverted-L to other bands, because it will withstand a lot of voltage.
From
the PDF:
High insulation
Insulation distance (between coil and contacts): 10mm min.
Dielectric strength: 5KV
Surge strength: 10KV
73, Mike
www.w0btu.com
On Thu, Oct 9, 2014 at 2:23 AM, Thomas PA1M <t.b.tinge@gmail.com> wrote:
> Fujitsu FTR-K1CK012W
>
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