Topband: Antenna relays

Tim Shoppa tshoppa at
Mon May 18 09:24:29 EDT 2015

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

Tim N3QE

On Mon, May 18, 2015 at 8:49 AM, Tom W8JI <w8ji at> 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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