Back in August last year I built a dummy load out of a bunch of resistors, a paint tin and some oil. I said at the time that it seems to work fine on HF but not so well on VHF. After I used the load last weekend to test my aerial, I thought perhaps I’d put it on the analyser and actually check and record the performance.
Firstly I checked the SWR and the impedance between 1.8MHz and 29.7MHz, the normal amateur radio HF bands.
The SWR is 1.01:1 at 1.8MHz and 1.11:1 at 29.7MHz and the impedance measures 48.6 ohms at the lowest point and 50.3 ohms at the highest. That’s very acceptable and it’s the best I could have hoped for considering the resistors I used.
On to the mid range bands. I tested between 50MHz and 70.5MHz to include 6m and 4m.
A different story now – The SWR is still respectable at 1.27:1 at 50MHz raising to a not very good 1.41:1 at 70.5MHz but you can see that the impedance of the load varies quite a lot between 57.7 ohms and 65.9 ohms. I’d still use the load on these bands but it’s right on the edge of usability at 4m.
I then tested the load on the 2m band from 144MHz to 146MHz.
The SWR is pretty constant at 2.25:1 and the impedance drops from 91.6 ohms at the bottom of the band to 85.5 ohms at the top. I wouldn’t use this dummy load on VHF.
Finally, for a giggle I tested the load over the whole range of my analyser, from 100kHz to 200MHz. Interpret it as you like, I think the curves are pretty 🙂
I’ve still not dared to use my amplifier into the dummy load yet – The resistors in air are rated at 90 watts and although that should be significantly higher when submerged in oil, I’ve not yet tried. I said before that I’m going to build a higher rated load so I’ll leave that for the amplifier.