A Communicator Reprise... October 2011
"I made this here dummy load and can’t figure out why I’m getting a 25:1 SWR!!” “Got such great deal on this 100 watt 51 ohm resistor off of eBay and though it would make a great dummy load for the HF rig,” he beamed as handed me the homebrew project. Did a nice job putting it into an aluminum project box, with both a UHF [SO-239] and a BNC connector...
|[Note: 3” discs would have been better in retrospect]|
The discs I cut from a piece of thin PCB, the resistors from eBay after I verified with the seller that they were not wire wound.
After drilling the 22 holes in the discs and soldering all 20 resistors into place, one disc becomes the braid or shield side, the ground of the SO-239 and the piece of bare copper wire #10 or #12 bare copper goes from the centre pin on the connector to the disc on the bottom [of the photo].
|The resistors were 5 watts, so 20 times 5 = 100 watts. |
It gets hot enough in 30 seconds that you would not
be able to hold onto it for long.
What you have when finished is 1000 divided by 20 or 50 ohms. But these are 5% resistors, and I didn’t ask for 20 hand measured resistors, so of course they were not all 1000 ohms or higher. All it takes is one to be 5% low and it brings the whole array down below 50 ohms. This one here measured 49.6 ohms.
Close enough for its purpose. Yes, this does present a small degree of capacitance and inductance – the discs do that, but it’s not enough to cause a problem.
The SWR didn’t change when put in the can with 1 litre of pure mineral oil from the drug store. You could have used it as a dry load, but the oil makes it usable for minutes, rather than seconds and the paint can does reduce the RF radiated by roughly 60db.
As we can see from the sweep below, from 1.8 to 30 Mhz. the Impedance [green line] is pretty darn close to the 50 ohm reference line all across the entire HF band. The SWR 1.02:1 at 160 metres and about 1.26:1 at 30 MHz. OK, not a perfect dummy load, but it costs less than $20, you can make it all at home and it does the job of giving your transceiver a non-radiating load for testing. The fancier commercial ones have LCR circuits incorporated in the array to compensate for the inductance and capacitance of the device – in other words, they doctor the load to get that perfect 1:1 across the band.
You can see above, from 50 to 55MHZ, the SWR is about 1.3:1 across the six meter band,
so it could be used if you needed to. But by the time you get to the 2 meter band, it’s above 1.6:1 and not a good load. Thus commercial products tend to get more expensive at higher frequencies because you have to make them differently with higher tolerances, higher or more precise engineering and a few more parts to null out any inductive or capacitive reactance.
Next, the sweep below, from 1 MHz. to 180 MHz. is actually a good representation of most HF Cantennas, MFJ and other ‘non-VHF’ dummy loads. The green line represents the impedance of the load. From 1 to 21 Mhz. it’s pretty good at staying near 50 ohms and with an SWR of less than 1.1:1. But as you can see by the time it climbs out of the 30 Mhz section, the impedance drops and the SWR climbs, so by the time it reached 180Mhz., the limit of my MiniVNA, it’s almost 2:1 with an impedance closer to 30 than 50 ohms.
This is mainly due to the construction of the dummy load, the 5% resistors soldered between two copper discs act somewhat like a capacitor, with the inductive reactance of lead lengths and 1¼ inches of #12 wire from the centre pin of the SO-239 and the lower disc, going through the centre of the discs, which must have some inductive quality. So not a perfectly symmetrical or resonant device, but as you have seen perfectly “HAM” in nature and quite adequate for the few times you need to test at full power without annoying others on the air.