# [TowerTalk] TX RELAYS

David J. Sourdis hk1kxa at hotmail.com
Fri Jul 18 16:59:45 EDT 2008

```Jim and Roger, thank you very much for reply, I knew it was not so simple.

73

David
HK1KXA
EC5KXA

> Date: Fri, 18 Jul 2008 06:46:42 -0700
> To: hk1kxa at hotmail.com; towertalk at contesting.com
> Subject: Re: [TowerTalk] TX RELAYS
>
> David J. Sourdis wrote:
> > Hello All,
> >
> > I need advice on relays. What are the minimum parameters should I look for?
> >
> > http://www.findernet.com/en/products/profiles.php?serie=46&lang=en
> >
> > Would the relay in this link work to withstand 1500W or say 2000W to have a safety margin?
> >
> > I have calculated the following, please correct me if wrong.
> >
> > I= Square root ( Power/R) = SQRT(2000W/50 ohm) = 6,3 A. Based on this, 10A rating would stand up to 5KW = 50 ohm x 10A^2.
> >
> > Voltage would be V= I x R = 6,3A x 50 ohm = 315 Volts.
> >
> > I know it is not the same the ratings for 50 - 60 Hz as it is for 28 MHz...
>
>
>
> Bear in mind also that you've calculated for the proverbial (and
> non-existent) "well matched system".
>
> The typical rule of thumb is to require design to 10dB over or test to
> 6dB over the expected maximum power.  (because that's the worst case
> with a worst case mismatch)
>
> That is, if you calculate 315V, you'd want a relay tested to 630V, or
> designed to about 1kV breakdown.  Ditto for current.
>
> So, the 10A relay isn't going to hack it.
>
> Also, keep in mind that for voltage it's "peak voltage" that counts, not
> RMS.
>
> If your system has any sort of resonant components in it, you need even
> more design margin, depending on the Q of the resonance. A Q of 10
> implies that 10 times as much energy is stored in the system as is
> passing through, that is, sqrt(10) (about 3) times the voltage and current.
>
> (why those contacts on inductor band switches might get burned, eh?)
>
> (In microwave high power breakdown testing, we use a technique called a
> resonant ring to get high average powers when we don't have a source
> that can just crank out as much as needed.

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