The AM Forum

What are some of you using for a dummy load to dump your mauls into? Commercial or homebrew? Your comments and suggestions please.

Well, Dave.  I don't know what constitutes a "maul"  ::), but I use a modern day Heathkit Cantenna.

Hi Dave,
           I have a large Bird type military slurpuss oil filled radiator type. It is rated 500w continuous without fan cooling and 1 kw continuous with fan cooling. Nothing ever seems to bother it. It takes the 4x1 transmitter and laughs.
                                                                           The Slab Bacon

I have a couple of Cantennas filled with mineral oil. Work fine for tuning up.

The six non=inductive resistors that make up the dummy load for the Gates BC1G have a length of feedline on them and pull duty on all the TX's here....Steve

Used to be the dummy-est of all dummy loads, 3890 on a certain schedule.

...h/brew...500watts...paint can,and erl...

12 ea. 600-ohm Glo-bar resistors in parallel.  Not sure of the wattage, but each resistor is about 1" in  diameter and 18" long.  It will take 1 kw of rf as long as I feel like generating it.  No cooling fan or oil, just convection cooling by mounting the resistors vertically.

I like that, do the glo-bars glow? Did you build with the 'end clips'? I suppose you rescued the resistors from a broadcast source.

an ol' Cantenna here... seems to be fine for tuneups of the homebrew KW rig!

Boid Termaline.

umm,  a 200watt lightbulb...

doubles as a power meter... and sometimes when I forget a modulation meter too...

Those Bird dummy loads with the fins can sometimes be found cheap around the flea mkts. Last year I bought a 1KW one for $40. The second one free - someone left as junque. It needed cleaning and paint, but was good to 100mhz flat at 1KW. The connector was some strange bird too and needed an SO-239 JSed on.

I keep one in the shack on the ant switch.

On eBay they are expensive, like $200 sometimes, but not when under the flea table sitting in the dirt.

Funny thing is that sometimes a dummy load cannot be used for the job - ya gotta throw a carrier on the air. How often is a coax antenna exactly 50 j0?  ;D  Also, when testing for RF in the audio or other rf problems, a carrier on the air is necessary. But do it way down the band, just like when Yaz runs way out in the field away from the house to take a dump.

BTW, how's my new P3 2.4 ghz computer looking?  ::)  It was about time.

T

 A standard Australian 1.25 Kw  250V electric stove element is about 50 ohms. Works fine on 160m and the rating is , well 1.25 KW. 

I like this one too as it could keep the coffee warm while testing! Unfortunately not available in the US.

Thanks for all replies. I need a dummy load capable of a kw (or two) for obvious testing puposes but agree with you, Tom, nothing like connecting to the final load checking for rf, etc.

I have been looking for non-inductive resistors in the 100 to 250 watt class to build a dummy load....still looking. That's unless one of you has one to shake loose.

I have three loads that I use:
1) A Bird 500 watt oil filled for SSB around 1200 watts PEP, but keeping it under 500 watts average (as I don't want to break it, and call to Bird recommended not overloading the thang). It has the rubber bladder inside the end for thermal expansion, I don't know how long these things last before they leak. It is broadband, well into the hundreds of MHz.

2) A homemade load using three 150 ohm 18 inch 'globar' style resistors. These were made by "US RESISTOR" and are not very stable. I can run a kW from BC rig into it, but over 5 minutes the resistance drops and it eventually trips off the VSWR protection. I have a forced air fan on it in an aluminum chassis. With that the surface temp on the globars is about 110 deg C after running key down for a while. It works up to 80 meters fine.

3) A Ohmspun load, came from an old 5 kW Collins TX installation, I believe. It was gotten from KBIM Roswell for $50, and needed some TLC, as a number of the elements were burnt. Since they were wired series/parallel to get the rating, it was failrly simple to reduce the number of elements and reconfigure the thing. The frame is cast aluminum about 2 feet long, and each card is an asbestos fabric with resistive wire woven in. The cards plug in, which was one of the weaknesses, as they had burnt fingers on the edges. I was able to make about a 3 kW load from it, and I added a series mica cap to tune out the reactance at 1230 KHz, which is my BC TX frequency right now. With this load I can run 125% AM peaks, tones, whatevever, with no problems in the load.

# 1 and 3 are my good loads, #2 is NG for 1 kW but fine for lower power like 375 Watt carrier + AM.

Recommendation: Look for some globar (carborundum) resistors and parallel them in a box, put a fan on it, and crank it up. I may be able to help you with a handful of resistors if you do the construction.

Tom,
You look like you're strappin' from this end OM.  What did you end up getting, brand and features? And if you don't mind me asking what did it run you?
Third of Three

The glo-bars do not glow, although I have heard of it in high power transmitters that use them in parasitic suppressors.  Mine came from an estate, new in box, never used by original owner, apparently WW2 surplus.  They are designed for use with end clips, although these were not included, but I had ample supply on hand.  I mounted them vertically, in a circle so that the whole thing is like a cylinder.

The cold resistance is exactly 50Ω, but when hot, the net resistance changes a couple of ohms.  However, the SWR meter shows negligible change in reading, so I don't worry about it. Maybe a fan under the assembly would reduce the instability.

I've previously used a 130 foot linearly disposed, air-cooled dummy load made with #14 insulated stranded elements. It would take any amount of power and never got hot.

Bob,

I bought a new Dell P3 2.4 ghz machine wid monitor, 256M ram and a CD driver for about $360? I think.  In fact I liked it so much I ordered another one last night. Gonna get rid of the 233mhz machines I've nursed and run these two as semi-redundant in case on crap outs.  Nice having one on the web for order entry and the other for trading analysis.

The guys were making fun of me for the last year for running those old klunkers that cud barely keep up in the A/V room... ;D  Now I BE BACK.

T

Hey Tom,
That sounds like a pretty good Dell deal.  I just retired a 233MMX myself though I may put it out in the workshop for those times when I want to get on the net and I don't have to go into the house with crapped up feet.  I upgraded the 233 to a 533MHz machine for the shack. It was a freebie from a friend.  It works OK in the A/V room (audio only) though a faster machine might be much better for those times when there are a lot of people running video.

Bob