RF Dummy Loads – Observations and experiments

Hi there! I have been busy and haven’t got the chance to post anything for quite a bit. Well, I’d like to share my observations that I made when I was experimenting with RF dummy loads. Loads and impedance matching itself is very interesting, but quite a broad field on their own, so more on that in future articles.

Well, a dummy load is meant to allow transmitters to perform on-air tests, simulating an antenna, without radiating much power out. A dummy load is essentially, made out of resistors. As you know, a perfectly matched dummy load would have its impedance match the transmitter’s. An ideal transmitter for amateur radio would have an output impedance of 50 ohms, with no imaginary component (50 + j0).

I have been asked questions like: “Hey! Can I use wire-wound resistors for use as an RF dummy load?”. So, to answer that, I’ll be showing you my observations with various RF dummy loads. Let us start with a commercial RF dummy load. Pictured here is one that I use for quick tests.









And here’s the corresponding measurement data:

I’m sorry for the lower resolution on this image. For most of the amateur frequencies, say up to 500 MHz, the VSWR is lower than 1.5:1. That’s quite okay! Now let us look at another load. This time, it was constructed by Mr Jeff, out of axial resistors. Here’s it is:







And of course, the measurement data.

















As you can see, the VSWR is at 3.76:1 on the UHF band. The performance is not as great as the commercial dummy load.

At the start of the year, I asked myself a question: “Why can’t we use wire-wound resistors for a RF dummy load?”. After all, wire wound resistors have high power ratings, and a few could easily dissipate 100 watts from a transmitter. Being curious, I decided to build one myself. Here’s my completed load:











And here is the measurement data:

















Totally not good! The vector network analyzer doesn’t lie! The load performs very terribly! Not a good idea to begin with. Why is that so?

The name itself tells us why! Wire-wound! You see, they are made by coiling a resistive wire! It behaves like an inductive load! A little computer model shows similar (kind of) results:










Just a tiny, tiny amount of inductance is enough to make the VSWR very high, especially on the VHF and UHF band. Just so you know, this is a simulated sweep from 1 – 500 MHz. Granted, this may not be a very accurate model but it does show that reactances do matter in the realm of RF electronics.

Anyway, hope you enjoyed reading my post!

Joyce (9V1AN)

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