Dummy Load

Gary T. Lane · Member Joined: 06 Sep 2008 Posts: 155

Hi Guys, I'm ready to test the Xmtr I just built. I think you suggested using a 60 watt light bulb coming out of the Antenna connection for a load in stead of an actual antenna. Now, do I use this dummy load next to the receiver to see if I'm actually putting out a signal? Will it hurt anything holding the key down a second or two without the load?
Thanks to all, Gary

Johnnysan · Posted: Nov Tue 03, 2009 2:51 pm

Lamps don't make good dummy loads. What transmitter do you have?

Norm Leal · Posted: Nov Tue 03, 2009 3:01 pm

Hi Gary

The lamp will glow depending on output power from your transmitter. Don't operate a transmitter without a dummy load, even for a second. This can damage components. If the circuit is solid state transistors may burn out.
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Norm

Curt Reed · Posted: Nov Tue 03, 2009 3:16 pm

If this is for the transmitter you have been talking about, then a bulb would be just fine. Just be aware that they can cause a chirpy CW signal, as the resistance of the bulb changes as it heats up, causing a different loading on the transmitter. Also it is very important to keep the leads from the transmitter to the bulb very short, not more than a few inches if possible. Using the bulb with several feet of wire on it will result in your signal radiating all over town. I once worked a ham several miles away on 80 meter CW and I was using a light bulb for an antenna.
Curt
_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!

Kevin Kuehn · Posted: Nov Tue 03, 2009 3:47 pm

How does the transmitter see the bulb impedance wise? I assume you can't really peak an antenna pi network into it? Of course I'm thinking BC band in my case.

Kevin

Gary T. Lane · Member Joined: 06 Sep 2008 Posts: 155

Hi Fellows, the schematic is on page 2 of HART-65 on this Homebrew page. The engineer who designed the Xmtr says, The Hart-65 was carefully constructed to minimize television interference. A high-C pi-network circuit discriminates against harmonics that want to excape up the antenna feed line.
Also C-1 an (reducing capacitor) can be adjusted to assure a chirp-free starting of the Xtal everytime the key is operated.
This has been a very exciting project for me, I have never built a transmitter before. A lot of you guys encouraged me to build this even if I never put it on the air..Just like getting an old radio to come back to life..I just love the thrill of making something work.
Again thanks to all for your patience and good tips!
Gary

Arnie-AE · Member Joined: 18 Aug 2009 Posts: 110 Location: Berlin, MA

Gary,

You can make a real dummy load pretty easily. Along the lines of an old heathkit cantenna.

If you can get about 5 watts worth of 50 ohm carbon resistors, or even (2) 100 ohm 2W resistors you put in parallel, then submerge them in mineral oil. Heathkit called it a cantenna because they put it into a 1 gallon paint can. with the littler version, you can put it into a 1/2 pint can and fill it with mineral oil. The oil is non-conductive and disipates the heat in the resistors so you can run a bunch more power into them than their rated wattage.

Put your connector on the lid and wire the resistors to the connector and you'll have a nice dummy load good for probably 50 watts.

arnie

Curt Reed · Posted: Nov Tue 03, 2009 8:35 pm

The capacitor you mention as nothing at all to do with changing load on a transmitter. Think of a changing load as the filament of the lamp heats up like a car with a standard transmission. If you let out the clutch, it puts a load on the engine, and the engine will slow down a bit. That capacitor only controls the feedback in the oscillator circuit and has no bearing on the output loading.
Curt
_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!

Jack Shirley · Member Joined: 14 Mar 2008 Posts: 2459 Location: SE USA

The lamp basically tells you that you are throwing out RF which is what you want to know at this stage. The rig probably won't load/tune correctly into the bulb since like Curt sez its hardly a fixed load nor is it a 50-ohm load. Pi-networks are generally designed around a 50-ohm load. The 'glow' could be full of harmonics or other weirdness along with the desired signal so to really nail it down you need a real dummy load.

Rich, W3HWJ · Posted: Nov Wed 04, 2009 2:59 am

Using a lamp bulb as a dummy load was pretty standard in the old days... when most ham transmitters were Class C amplifiers. Heath used to recommend it with the DX-40 that I had as a Novice, but when I got my HW-100 SSB rig, I also built a Can-Tenna.

Gary... maybe this will motivate you to get a ham license???

Rich