Topband: TX relays

[Tom W8JI]

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 at 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 at gmail.com> wrote:
>>
>> >  Fujitsu FTR-K1CK012W
>> >
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