I agree generally with these comments, except that my experience with
CdAg contacts on receiving hasn't been that bad, provided that the
application exercises the relays occasionally. Thus, using them as,
e.g., an outdoor antenna switch where the antenna isn't regularly
switched, and where there are large temperature fluctuations, can
definitely lead to receive problems. At one point I thought about
using a small dc bias current through relay contacts, but found it
wasn't necessary.
I agree that 60Hz tests of current capacity don't automatically
translate into RF performance. The relays I mentioned earlier I tested
at 30MHz with 2500W (maximum I can generate in my station) into 50
ohms. I've never noticed any dielectric heating of the housing of any
small modern power relays, but some brands use hookup wire connected
to the contacts. With 50ohm loads, I've never had any problems with
arcing or voltage breakdown. Generally, large contacts, with short
wide straps to the relay pins, and the coil oriented to minimize
coupling, give the best results. Obviously mismatched or high-Z loads
are another story entirely. Some relay brands seem to work well up
through 6m, but all become unusable at 2m and higher.
73,
Jim w8zr
Sent from my iPhone
> On May 18, 2015, at 9:24 AM, Tim Shoppa <tshoppa@gmail.com> wrote:
>
> With silver-cadmium relay contacts I use on my ladder line switching, I
> find I have to occasionally transmit a dit to get receive to work again,
> especially if I haven't been on the radio in hours or days. My mental model
> is that I have to "blow through" the surface contamination. Would
> definitely never use the silver-cadmium contacts in a receive-only path.
> (Even though I have been known to make some Q's while transmitting on my
> receive antennas, it was purely by accident!)
>
> I picked up a tray of some dinky 24V-coil microwave relays that KM1H and
> W3LPL recommended from Electronics Goldmine and they work great for
> receive-only applications. (And have survived the occasional accidental
> transmit!).
>
> Tim N3QE
>
>> On Mon, May 18, 2015 at 8:49 AM, Tom W8JI <w8ji@w8ji.com> wrote:
>>
>> DC or low frequency AC is an almost meaningless test, although field life
>> is a good data point.
>>
>> The only valid bench tests for a relay in these applications are RF
>> heating with high current, and voltage breakdown. Some relays even run
>> current or voltage through the coil pole center, making a "capacitor" to
>> the control pins. The relay has to be opened and inspected, or measured for
>> stray capacitances.
>>
>> Almost always the heating is in the contact bar or wire to the contact
>> bar. Heating there is aggravated by RF current and skin effect, so it must
>> be tested at radio frequencies.
>>
>> Contact materials should be gold overlay and ideally have a small contact
>> area, otherwise they develop receiving issues more frequently. The larger
>> and more robust the hot switching, the less reliable the receiving becomes.
>>
>> I find relays that can reliably carry 20-30 amps or more at 60Hz that heat
>> and fail at 4-5 amps at 10 MHz.
>>
>> 73 Tom
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