cheap 250 watt dummy load

[nikdfish]

Just for grins, put together a dummy load from a couple of amazon items, an aluminum heatsink & 50 ohm resister (RFP 250N50) & parts laying around. Parts cost, delivered, was < $18

Construction mainly consisted of designing/printing the 3D printed bracket for the SO-239 & drilling and tapping some M3 holes in the heat sink. I might have been able to avoid the plastic bracket but it was the quickest/easiest approach considering the space available ... It works well enough in the HF range (SWR 1.01 - 1.16 for 1Mhz - 30Mhz) & acceptable in VHF ( < 1.5 at or below 160Mhz ) for quick testing purposes.

The bracket was printed in ASA so it is pretty temperature stable (good to about 95C).

 

[Klondike Mike]

Nice work. I am suprized your SWR @ 160Mhz is as high as it is. Component limitation and SO-239 connector at fault here. I have a project I'm working on utilizing a 800w resistor and I will be using a N connector. I'm hoping to get a low SWR out to 500Mhz. Other consideration of concern with your open design would be radiated RF but overall a worthwhile endeavour vs purchasing a more costly, finished product.

 

[brandon7861]

Cool! I did something similar a while back.

You can probably improve that design by swapping the plastic part for a piece of angle aluminum.. If that is not possible, maybe running a shorter ground wire between the SO-239 screw and the resistor mounting screw and route it tight along the center pin so it more closely resembles a transmission line (so some of the capacitance can cancel some of the inductance).

As it stands, you have at least an inch more wire in the ground path than you need, and an inch of wire at 24nH per inch is 25ohm at 160MHz. If you want it to work at higher frequencies, stray reactances must be considered.

 

[nikdfish]

You guys are absolutely right about the ground line. I made a quick change:

& the difference was quite noticable.

The AA-170 only plots 100 points to the screen, but the general trend is there, pretty much <= 1.2 from 1mhz to 160mhz

I think I have a partial roll of aluminum flashing in the basement, should be pretty simple to fab a shield starting on the back side of the SO-239 & pop-rivet it in place.

 

[brandon7861]

I have an idea. Maybe scrap that angle mount completely. Drill a hole in the middle of that heat sync for the back of the SO-239 connector to pass through and then mount the resistor right next to it so the center pin and the resistor tab touch.

This configuration would have significant benefits over the current setup. The ground would be as close to ideal as you could get. There is plenty room in that flat area to make a little metal shield to go over the resistor and connection, so it would be very easy to properly shield. The heat is centered in the heat sync and makes better use of the surface area. And with the SO-239 through the middle, when sideways to connect a coax, the fins are oriented in a way that favors air convection.

Edit : or maybe drill it closer to one side so you could stand it up on end without the coax tipping it over. Could print some feet to keep it vertical if too top heavy.