Re: Topband: TX relays

[Bill Wichers <billw@waveform.net>]

That is called "sealing current". It's been used in the telecom industry for 
ages. The spec is <20mA though, and you can get by with only a few mA. It helps 
to prevent various corrosive forces from damaging contacts in relays and the 
like over time.

The problem is that every time you switch you will get a very small arc at the 
contact point and you'll pick some of that up in the receiver. Regardless of 
the value of the "suitable choke" the arc will have a wide spectral content 
that will extend up into the lower HF bands.

It might be possible to keep a sealing current on the system when it's not in 
use, and then go to a regular no-current (dry) system while operating. I'm not 
sure if that would accomplish the same function as having the current always 
present though.

   -Bill

> One solution to the receiving problem when using high current relays may be
> to have dc current (eg 100mA) pass through the contact in RX.
> This would be particularly easy to implement when the antennas are zero
> ohms at DC because the dc current source can then be placed in the shack
> (with a suitable choke to isolate the RF from the supply and capacitor to
> isolate the DC from the rig).
> 
> - Paul
> 
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