On 7-11-2011 11:37, Scott wrote:
On 7-11-2011 08:08, Stuart Longland VK4MSL wrote:
On Jul 11, 12:19 pm, Kickin' Ass and Takin' Names
wrote:
On Mon, 11 Jul 2011 10:29:08 +1000, wrote:
Wouldn't a simple power divider do the job ?
some sort of resistor arrangement acting as a pad

Or maybe a few feet of lossy coax.

It would, and I aim to use one of these. The plan is a small module
with two SMA connectors (or similar) plugs in and provides the
attenuation. A resistive pad… the amount of coax needed here would
not be practical unless it was *very* lossy. A few feet would be
quite inconvenient, and again, would offer fixed-value attenuation
only, unless I missed something.

However, I can design an attenuator to give me 50mW from 500mW
(-10dB), all will be fine, but the moment 5W gets pumped in, for
whatever reason, up goes the amp in smoke, as it will be 500mW, not
50mW that gets through.

I'd just like something cheap that can let out the smoke rather than
the amplifier module. Smoke isn't a bad thing in itself, as it'll
tell me I've done the wrong thing, and if it's a cheap part/module
that can be field-replaceable, that is even better. An amplifier
module does not classify as a cheap field-replaceable part however.

Maybe a simple fuse in-line like they do on signal generators to protect
them from people transmitting into them. You can calculate the current
for your low power setting by using Ohm's Law and then add say 50% to
that value to find the fuse rating. You could test it by hooking up a
dummy load in place of the amp and set the radio to low power. The fuse
shouldn't blow. Set the radio to the most likely power if you forget to
go to lower power and try it. The fuse should blow quickly. They make
fuses that look like leaded resistors, so you could probably break the
PC trace on the RF input line inside the amp and solder the fuse across
the break. You'd have to open the amp to replace, but having it in the
trace will keep lead length down.

N0EDV



Calculating for 50 mW at 50 Ohms, yields about 32 mA. DigiKey (in the
USA) sells 50 mA fuses in surface mount packages, which would be ideal
for putting across a cut in a circuit trace inside the amp. A 50 mA
fuse should blow at about 125 mW.

http://search.digikey.com/scripts/Dk...=478-6482-1-ND

N0EDV