The AM Forum

Ok, here's the question. I am using a 150 watt light bulb as a dummy load for a DX100 I'm bringing back to life.  

The bulb glows just fine on 160 and 80 meters and it also glows on 40 meters but not as bright and part of this may because of the efficiency of the tank circuit on that band.

Now when I go on 20 meters the light is somewhat dim and on ten meters it will not light even though the tank circuit has plenty of RF floating around in it noted  by observation of generous arcs to an insulated screw driver.  Now I know a non inductive resistor bank would be better and I have all that stuff at my camp but it's snowed in till the end of March.

The question is, why won't a light bulb work on these higher frequencies when I know the RF is there? Is there something else that can be done to make a light bulb work better at higher frequencies? Do I need to use an antenna tuner to match the bulb?  

I've been using two 100w light bulbs in parallel for years with my valiant. I have noticed that they are not a good match on some bands, at least the SWR meter climbs with frequency.  I can usually load up to 20 meters though with out a hitch, full brightness, by adjusting the coupling cap & tuning the tank. Higher than that it gets a bit sketchy.   I did make a "Cantenna" using a slavaged MFJ load from a crapped out tunna.  But I like the positive visual power indication from the Bulbs!

Are you also dipping the dipper and peaking the peaker while observing the plate meter and the light bulb? I've used light bulb dummy loads all the way to 2 meters.

Yup, Pete I am, been dipping and a peaking all my life and still don't know the answer ??? ???

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I've had similar results with light bulb loads. At dc or 60 cycles the resistance of the bulb a full brilliance is 96 ohms. As the frequency goes up the inductance of the filament  and the leads causes the impeadance to rise of course causing the current to go down. Older sets with wide range pi-net output circuits can compensate for this to some extent.
                                                                                                                       Jay-

Try a couple of 100W's in parallel. That should lower the impedance.

Won't cost you much to try.

Jim

Do you have a receiver on the frequency that you're doing tuneup? Can you hear it? Does the S meter increase as you dip and load? Sounds like your final tank circuit is just diddling with parasitics on the higher frequencies.

Well Pete, you may have something there. I just tried the light bulb on a Gonset GS-50 and while it did not make the bulb very bright, it did light it. I need to start first by seeing if the VFO is in calibration. I have my R-390A to set up the band edges and go from there.  Thanks for the help guys, I'll be back in a few days on this project and let you know what I find good or bad :) :) :)

A real dummy (not you, Terrry) would use a CFL ;)

73DG

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Terry,

    Try putting a little oatmeal on the bulb.

There was a time (before Bird dummy loads were invented) that I used (24) 100 watt light bulbs in series parallel. I added a variable L/C network to match it 1:1 to 50 ohms.  Yes, it requires different tuning for different bands.

In 1988, I had it outside in a shed and was testing a new 4X1 AM plated modulated rig. After a day of testing, I put on the antenna and started a QSO.  The FAF broke in and axed why I was testing all day on 3885. I had been S9+10 80 miles away on the "dummy load" and causing QRM to his QSO.   Well, needless to say, now I only use a Bird dummy load inside an aluminum sided house for testing.


My point was, try a simple variable  L/C network with the lightbulbs to get a good match.  (L in, C to ground or C in, L to ground or C to ground L in - whatever it takes for a 1:1 match.)

T

I just removed the remote marine tuna from service yesterday.  I was using it with a 20 foot piece of coax out the window to the eaves of the house, then 6 inch ladder line 30 some feet to the 160 inv V.

I removed the 'hot side' of the tuna first, didn't notice ANY difference in the received signals.......  Then I removed the ground side.  The signals lost about half their amplitude after detection (I was using the scientific method of having the volume up loud enough so I could hear the radio outside).  Well, THIS is counfounding.

Turns out, I had been tuned 1:1 BEFORE removing the ladder line.  AFTER removing it from the ground and 'vertical' side of the tuna, I had a 1.8 or so....   Still copying the same signals, far as I could tell.

SO, sometimes our tuners are dummy loads, sometimes our dummy loads are ...  Tuned? 

Incidentally, I key up 2 repeaters with a bird termaline installed at the antenna jack on my 2 meter handheld.  They are located literally blocks away though :0

It seems that their was a military load that also did the same thing?  I bought a load from Fair YEARS ago that had 12 supposedly non-inductive resistors in it, and then a LARGE cap to tune out the inductance of the 'wirewound' non-inductive resistors....  Like, someone fed the military a pile of goods on the audio non-inductive resistors being use(ful) at rf?


--Shane
KD6VXI

You will probably have to re-adjust the L and C according to the power you put into it, since the filament resistance of incandescent lamps varies widely with temperature.

I once got a complaint from 90 miles away when I was testing a 100w rig into a light bulb.

I often wonder if I am testing through my open T-tuner (two aircaps and a johnson roller inductor on a board) to my dummy load if I am radiating.   After working several Retro 75's I know it doesn't take much to be heard sometimes.  The dummy load is a Cantenna clone from Vectronics. 

Has anyone ever used an electric stove coiled burner element as a dummy load?   I noticed (on stove repair day) that the larger ones are very close to 50 ohms resistance...

A person in the broadcast business told he had used electric oven elements as a dummy load.

As told to me, BIG BC stations have used a 55 gallon drum filled with water and salt. Hang two large metal plates into the water and connect to the coax. Adjust the plate spacing for 50 ohms. Might need an L/C match. 

The dummyload will take huge power before the water boils.

T

I don't see why it wouldn't work...I need a dummy load that will handle 10amps @ 50 ohms

Here's mine.

12ea. 600Ω glo-bars in parallel for 50Ω.  Plans are to split it into three sections of 4 resistors each to make 150Ω per section, to be strapped in parallel for 50Ω or wired in series for 450Ω.

A beer cooled dummy load...nice Don!

Back in my early days I was an electronics tech for a company making HT's and pagers and eventually mobile rigs. At one point we were cranking out a bunch of demo pagers to distribute to the local fire departments. My job was to set them up, then take them in a screen room and run them against a cushman or motorola sig gen with the two tone combinations set to tone them off for the FD tones to make sure they worked right.

That was all going well, until I got to the pager for the town we were located in. Fired off the tones, and the pager didn't respond. But the local fire siren went off. Is it noon already? Checked my settings on the pager and tried it again. No response, but hmmm, the fire siren went off again. That's kind of odd. Set up a different pager with the frequency and tone settings and fired off the generator again and this time the pager responded correctly, but so did the fire siren. *Then* the lightbulb went off. Whooops.   :o 

My boss had assured me the 120 db isolation of the screen room would keep all the signals inside. Umm, not quite.

As a novice I made a couple of contacts with my rig on a 100 watt light bulb. That was kinda cool.  ;D

At Harris we had some nice 10KW water cooled dummy loads. Looked like they were built into a washing machine cabinet.

I still use light bulbs and haven't had a complaint in a long time. Knock wood.

Ive worked about 30 miles on 160 with 100W and a 3000W Bird load. The other guy, K1MEM,  was running 100W into a Cantenna. Signals were Q5 and pee weak naturally but there was no noise.