50 Ohm Resistor Networks for Dummies

It's called a dummy load. No seriously, it is.

What's it for? Well, it gives the radio a place to send its power without radiating RF.

Why? It provides a nice consistent load for tuning the final amplifier stage on my radio before I transmit. It can also be used for testing the radio after a repair. Also, some radios transmit while receiving a program download from a computer (though they shouldn't.) This gives you a way to prevent such transmissions from interfering with others.

Why not just disconnect the antenna? Well, the radio forms one half of a resonant circuit. The antenna forms the other half. The most efficient circuit is one where both halves have the same impedance. For amateur radios, that ideal impedance is fifty ohms.

If you have an impedance mismatch, not all of the energy makes it from the transmitter to the antenna. The law of conservation of energy says that the energy that doesn't make it to the antenna has to go somewhere, and in this case, it gets reflected back to the radio. When it gets back to the radio, it gets turned into the heat in the final amplifier stage. Enough heat, and the radio dies.

As far as impedance mismatches go, a disconnected antenna is about as bad as you can get. That's why you should never run a radio without an antenna attached. When you fire up the transmitter, all the energy goes to the antenna connection, and finding nothing there, is reflected right back to the final amplifier. Since all the output from the radio gets instantly turned into heat, failure happens pretty quickly. The 50 ohm dummy load gives the power a place to go. It still gets turned into heat, but the heat is in the dummy load instead of the radio. And the dummy load is built to shed heat.

My dummy load is a network of 1k ohm resistors in parallel. The net resistance of this network is 50 ohms. The resistors are each able to dissipate 3 watts of power, so together the network can handle 60W.

However, my VHF radio is capable of putting out 75W, and my HF radio can pump out 100W. So, the resistor network sits in a 1L paint can filled with mineral oil. This will allow the resistors to take more heat before failing.

This build is based on a design by K4EAA. He also sells the pack of resistors needed to build it. (Not just any resistors will work, since some are built of a coil of wire. Such wire-wound resistors are OK for DC, but as soon as you pass RF through them, they act like inductors instead of resistors, and their impedance changes. These are metal-film resistors which have the same impedance at any frequency.)

Now, if only I could remember to switch back to the antenna after tuning the finals...