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Author Topic: "RF Terminator" as dummy load?  (Read 5896 times)
AI8IA
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Posts: 108




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« on: April 28, 2016, 03:53:59 PM »

OK at the risk of being flamed beyond recognition, I'm going to ask this.

I "inherited" a box of several RF adapters from my old employer (where I spent 17 years as an RF field engineer) and I found a couple little Amphenol "1 Watt 50ohm RF Terminators." Can I use this as a makeshift dummy load if I keep the power way down? I don't need a dummy load all that often but the one I have is a big MFJ one. I wouldn't mind leaving this on my "B" antenna on the K2 if I wanna switch over, drop the power all the way down, and practice CW or what-not.

73

http://www.mouser.com/ProductDetail/Amphenol-RF/46650-51RFX/?qs=ukCXrI%2FP3JRTsXHF491B7Q%3D%3D

 
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KE6EE
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« Reply #1 on: April 28, 2016, 05:32:21 PM »

I'm far from an RF engineer but I'd consider the following design parameters:

A. Likelihood of operator error.
B. Accuracy of resistive load.
C. Dissipation capacity, see A.
D. Longevity of device beyond a few seconds, see A and C.

Thus a simple QRPp dummy load design: 2 x 100 ohm 5 watt non-inductive resistors in parallel across a coax connector; resistors exposed to air for heat dissipation. Would no doubt handle a few continuous watts well. Cost: a few bucks.

High-end, sophisticated QRP dummy load design: 20 x 1000 5 watt non-inductive resistors mounted in metal box for some heat sinking and air circulation. Averaging of resistor values gives accurate load. Could handle 100 watts. Cost, with a new connector, some sort of circuit board, a ventilated box, all new components, maybe $40.

An RF terminator is suitable for use with test gear. Won't last long, if at all, as dummy load. Worth only a couple of bucks, however.

« Last Edit: April 28, 2016, 05:34:23 PM by KE6EE » Logged
WW7KE
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Posts: 952




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« Reply #2 on: April 28, 2016, 09:53:30 PM »

OK at the risk of being flamed beyond recognition, I'm going to ask this.

I "inherited" a box of several RF adapters from my old employer (where I spent 17 years as an RF field engineer) and I found a couple little Amphenol "1 Watt 50ohm RF Terminators." Can I use this as a makeshift dummy load if I keep the power way down? I don't need a dummy load all that often but the one I have is a big MFJ one. I wouldn't mind leaving this on my "B" antenna on the K2 if I wanna switch over, drop the power all the way down, and practice CW or what-not.

73

http://www.mouser.com/ProductDetail/Amphenol-RF/46650-51RFX/?qs=ukCXrI%2FP3JRTsXHF491B7Q%3D%3D

I wouldn't put more than 1/2 watt (+27 dBm) continuously into one of those.  They're designed to terminate unused ports on test equipment, not transmitters.  You can build a higher-power dummy load for cheap, using ten 2 watt 510 ohm carbon resistors in parallel.
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WA7PRC
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« Reply #3 on: April 28, 2016, 10:24:58 PM »

A dummy load can be made from many smaller resistors but, the problem with that is, reactance from the interconnections quickly begins to mount. Then, it is less and less purely resistive, and frequency becomes limited. For best high frequency response, you want to limit the number of parts, and the length of interconnections.

At one point, I realized my then employer had been scrapping some good RF power resistors (50Ω/400W) and other parts that would make a good high power dummy load. I used four in series-parallel on two 140 2in heatsinks, cooled by a 100 cfm tubeaxial (aka Muffin™) fan. I tracked temperature using another scrapped part, a LM35DT "Precision Centigrade Temperature Sensor" IC. My test with a 660W @ 40.68 MHz source indicated it should work fine at 1500W. It easily handles my SB-220 amplifier continuously. See http://www.tinyurl.com/wa7prc-dummy for more detail.

So, where can you find resitors for your DL? Henry Radio (http://stores.ebay.com/Henry-Radio) and others sell them on eBay for reasonable prices.  Right now, Henry Radio has 50Ω/100W resistors for $12.95 shipped. They want only $15.95 for a 50Ω/250W resistor. Want to build a 1500W DL? They have two 100Ω/800W resistors for $125.

Bryan WA7PRC
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AI8IA
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« Reply #4 on: April 29, 2016, 05:37:35 AM »

Thanks for the replies so far!

Would a pair of these do ok for a 50ohm 20W DL?

http://www.mouser.com/ProductDetail/Welwyn-Components-TT-Electronics/WHS10-100RJA02/?qs=sGAEpiMZZMtlubZbdhIBIKWTpbnbhCAfHBxUM544rm8%3d

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N8YX
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« Reply #5 on: April 29, 2016, 05:55:20 AM »

Daiwa has a screw-on 30w dummy load and other manufacturers (Maldol, etc.) offer a similar product; some rathed 150-200w.

I've seen these on fleamarket tables for $5-10.

Those port terminators are handy for use on distribution manifolds (i.e., Mini Circuits 1-to-n power splitters) but I wouldn't use them for anything other than RX signal levels.

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KD5PKS
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« Reply #6 on: May 01, 2016, 01:59:23 PM »

You can pair the terminator with a properly rated attenuator and use that as a dummy load.
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W5SRT
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« Reply #7 on: May 02, 2016, 07:14:08 PM »

Sure, it will work fine.  1 watt only. Wink

73

- Dan
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WA7PRC
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« Reply #8 on: May 02, 2016, 10:37:05 PM »

It's a wirewound resistor... much above audio frequencies, it will appear more and more reactive (NOT what you want).

Some people recommend TO-series plastic tab non-inductive resistors (made by several manufacturers). They ALL specify the power dissipation at 25°C (77°F). They derate above that temperature. The RF power resistors I mentioned above are specified full rating at 100°C (212°F). If you want to build a DL, THAT's what you want.

This resistor will sink 100W at up to 100°C, and costs only $12.95 shipped (link). Add a small heatsink and fan, and you're done. If you keep the connections short, it'll work at VERY high frequencies.

vy 73,
Bryan WA7PRC
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W2OR
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« Reply #9 on: July 06, 2019, 06:52:44 PM »

Small dummy load for 100 watt rig:

Use four 450-Watt small RF Termination resistors, each 50-Ohms, each 450-watts, each about $12.00, each about 1.2 inches in length, Mouser.com Part 620-G450N50W4 manufactured by Anaren, a U.S. company.  Mount the four resistors on separate heat sinks, use thermal compound, don't over tighten mounting screws, solder carefully, wire them appropriately, add a fan, and don't burn them out.  Be careful in your construction techniques and use, because 100-watts of heat gets quite hot.

BTW: Anaren makes similar RF resistors at different ohm resistance ratings for use with alternative wiring schemes.  // 73 de Mark W2OR.
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