[TowerTalk] Re: [Amps] Relays for RF switching

[Geoff]

I believe the key to handling QRO is in the construction of the relay.

Contacts are often just staked to the arms, this makes for poor RF
contact. A bit of silver solder works well, probably regular solder is
OK also.

The arms are often made of stainless; I have no idea what alloys are
used but these will heat at RF, increasing as the frequency goes up.

The flexible leads to the solder terminals are often not copper and are
spot welded to the stainless arm. I replace with very flexible RG-141
braid.

Soldering to stainless requires an acid flux. You can use plumbers
tinning fluid, dilute hydrochloric acid or diluted muriatic acid which
already is 40% diluted hydrochloric. Very, very little acid is needed,
just a tiny drop off a toothpick works fine.

I have used bargain 10-12A rated relays up thru 2M and 1500W for many
trouble free years once I isolated the problem areas.

Geoff




----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Vic Rosenthal" <vic at rakefet.com>; "Pete Smith"
<n4zr at contesting.com>
Cc: <towertalk at contesting.com>
Sent: Saturday, April 17, 2004 8:48 AM
Subject: Re: [TowerTalk] Re: [Amps] Relays for RF switching


> > >$140+ to Ameritron.  I used some relays that I had purchased from
All
> > >Electronics for $1.50 each; they are "power relays" with contacts
rated
> > >15a at 120 VAC.  After all, 1500 watts into a 50 ohm line is only
about
> > >280 volts and 5.3 amps.
> > >
> > >Hey, if it worked I'd be in the miser's hall of fame.  However,
what
> > >happened is that the relays got quite hot after about 15 seconds at
the
> > >1500 watt level.
>
> I've seen large open frame 30 amp relays overheat at 1500 watts. The
low
> frequency AC or DC switching rating of a relay has almost nothing to
do with
> how suitable it is for RF applications. I have some small fifty cent
relays
> that will handle the equivalent of 20kW CCS into 50 ohms at 50MHz, and
some
> fifty dollar two pound relays that overheat at 1kW.
>
> The same is true for rotary switch ratings.
>
> 73 Tom
>
>
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