Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Use of light bulb for dummy load  (Read 396 times)
SCUBA
Member

Posts: 74




Ignore
« on: March 18, 2007, 08:46:38 PM »

Hi,
How good is the use of a light bulb for a dummy load?

73/Jack
Logged
N4LI
Member

Posts: 397




Ignore
« Reply #1 on: March 18, 2007, 09:07:38 PM »

Very poor.  

The problems are many.  First, resistance varies widely with the heating of the filament.  That's bad news, particularly on modern solid-state transmitters.  Further, they tend to radiate, rendering the "dummy" part of the load for naught.  

It seems that I remember reading about this stuff in one of the license manuals (I can't remember which test; I took 'em very closely together).  But, that was a few years back.

Dummy loads are cheap.  Just go pick one up.

Peter, N4LI
Logged
WD8PTB
Member

Posts: 670




Ignore
« Reply #2 on: March 19, 2007, 05:22:18 AM »

It worked somewhat with older radios because the older radios would not cut back the power and would put out some power with a high swr. Don't expect a 1:1 swr.  73 Don WD8PTB
Logged
K8AC
Member

Posts: 1477




Ignore
« Reply #3 on: March 19, 2007, 06:21:56 AM »

If you have no alternative and if your rig doesn't fold back power at the resulting SWR (it will be higher than 50 ohms), then a 100 watt lightbulb will do the job.  Unfortunately, the term "dummy load" is a misnomer as it will also radiate fairly well.  Back in my Novice days (1959) it was quite common to use a bulb as a dummy load.  One day I was testing on 40M CW running maybe 25 watts to a 100 watt bulb and was answered by another Novice about 300 miles away.  Quite a surprise!  
Logged
AA4PB
Member

Posts: 12980




Ignore
« Reply #4 on: March 19, 2007, 07:15:55 AM »

It depends on what you are testing. If you want to check out one of the automatic tuners like an SGC, a 100W lightbulb is ideal. If you are trying to connect it directly to a 50 ohm transmitter then all of the previous responses apply.

Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #5 on: March 19, 2007, 08:27:25 AM »

"it depends on what you are testing. If you want to check out one of the automatic tuners like an SGC, a 100W lightbulb is ideal."

I like to use a second antenna tuner with a 50 ohm dummy load on the "input" to test other matching networks.

Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KI4NX
Member

Posts: 121




Ignore
« Reply #6 on: March 19, 2007, 09:53:49 AM »

Then there was one ham who hooked up a 100W bulb on top of a 30 foot pole and worked the world!  Kinda strange for an antenna but it did radiate.

GL
Ken
KZ4Y
Logged
AD4U
Member

Posts: 2177




Ignore
« Reply #7 on: March 19, 2007, 12:33:05 PM »

A 250 watt light bulb will have around 52 ohms resistance when lit by 120 VAC.  I used light bulbs from 100 watts to 250 watts as a "dummy load" for the first 20 years of my ham career until somebody told me it would not work.  Back then most every rig had tube finals and either a pi-network or some type of tunable output network that would tune into the light bulb very well.

Of course the light bulb was no substitute for a real dummy load and it would radiate a signal.
Logged
W7ETA
Member

Posts: 2527




Ignore
« Reply #8 on: March 19, 2007, 01:27:54 PM »

Not a bright idea.
Logged
SCUBA
Member

Posts: 74




Ignore
« Reply #9 on: March 19, 2007, 03:49:52 PM »

Thanks for enlightening me.
Logged
W5ONV
Member

Posts: 0




Ignore
« Reply #10 on: March 19, 2007, 04:50:49 PM »

 I remember in the late 1960's when I built my Heathkit DX-60 transmitter they gave instructions on how to build a simple light bulb dummy load. They recommended to use that to do the test and tune up tests after building the kit. It worked great and really lit up that 100 watt bulb. 73, Jim
Logged
W7ETA
Member

Posts: 2527




Ignore
« Reply #11 on: March 19, 2007, 08:49:35 PM »

Guess I have a dim view of this when 50 ohm dummy loads are readily avaliable.

Same for using a 4 foot ground rod instead of wire radials for verticals. Might work better if we attach a light bulb to the ground rods?

Neither excites my electrons enough to get them into outer shells.  So, of course, they ain't gonna cough up any photons.

Bob
Logged
WA9SVD
Member

Posts: 2198




Ignore
« Reply #12 on: March 20, 2007, 02:16:23 AM »

Not a bright idea with modern solid-state rigs.
    In daze gone by, tube output transmitters had Pi-network (or Pi-L)output circuits, and could match a wide range of impedances.  And a light bulb has a wide range of impedances at RF; mostly resistance, which varies widely from cold to approximate full brilliance.  The old output circuits could match the bulb's resistance, even if it needed adjustment as the power increased; eventually, the resistance (and impedance) would stabilize.  THAT won't happen with fixed impedance outputs in modern S-S technology.
Logged
AA4PB
Member

Posts: 12980




Ignore
« Reply #13 on: March 20, 2007, 05:48:11 AM »

For QRP we used to use a #47 pilot lamp for a dummy load.
Logged
AD4U
Member

Posts: 2177




Ignore
« Reply #14 on: March 20, 2007, 08:46:18 AM »

Glad I could "enlighten you".  Maybe that is why my Mom called me sonny (sunny).
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!