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] 2 Next   Go Down
  Print  
Author Topic: Dummy load questions  (Read 5106 times)
KJ6ZOL
Member

Posts: 340




Ignore
« on: September 29, 2012, 10:35:10 PM »

As everybody knows, I'm new. I want to know, what exactly is the use of a dummy load, when would you use it? I gather that it's for adjusting the transceiver without accidentally transmitting spurious or illegal signals.
Logged
N8CMQ
Member

Posts: 353




Ignore
« Reply #1 on: September 29, 2012, 11:33:39 PM »

Basically, it is a big resistor at the same impedance as the transmitter.
Hopefully, it is the same or larger wattage, or you will have to keep transmissions short.
It is used to tune a transmitter before you hook it to an antenna.
That way you don't interfere with other hams.
Logged
G3RZP
Member

Posts: 4366




Ignore
« Reply #2 on: September 30, 2012, 04:35:35 AM »

Also, if you get strange effects caused by RF feedback - reports of distortion, keyer going mad, tx shutting down and so-on, it's the first thing to use in trouble shooting. A good wattmeter/SWR meter to go with it is useful too.
Logged
KG6BRG
Member

Posts: 116




Ignore
« Reply #3 on: September 30, 2012, 06:35:09 AM »

As everybody knows, I'm new. I want to know, what exactly is the use of a dummy load, when would you use it? I gather that it's for adjusting the transceiver without accidentally transmitting spurious or illegal signals.


It's a known 50 ohm load for the transceiver to "see" during testing, trouble shooting, as well as tuneups on boat-anchor rigs.  An oil filled dummy load is a useful piece of test equipment and is a easy first project for a new ham.  A few resistors and a empty quart size paint can, and a bit of mineral oil will give you a 100 watt dummy load for about $25.00 in parts.  cheers.
Logged
K8AXW
Member

Posts: 3651




Ignore
« Reply #4 on: September 30, 2012, 07:07:27 AM »

All of the above!  However, the main thing to know is a dummy load is a KNOWN load of a KNOWN impedance for testing your transmitter.  This is the main use of a dummy load.  The only other alternative is an antenna which is seldom a KNOWN impedance or even if it's working properly.

Every ham should get one at the same time they buy their transmitter.
Logged
KJ6ZOL
Member

Posts: 340




Ignore
« Reply #5 on: September 30, 2012, 03:53:44 PM »

As everybody knows, I'm new. I want to know, what exactly is the use of a dummy load, when would you use it? I gather that it's for adjusting the transceiver without accidentally transmitting spurious or illegal signals.


It's a known 50 ohm load for the transceiver to "see" during testing, trouble shooting, as well as tuneups on boat-anchor rigs.  An oil filled dummy load is a useful piece of test equipment and is a easy first project for a new ham.  A few resistors and a empty quart size paint can, and a bit of mineral oil will give you a 100 watt dummy load for about $25.00 in parts.  cheers.

I found the N4SPP dummy load plan via google. If anybody is familiar with this plan (google "diy dummy load") can you tell me what I should look for in a heatsink? There are all different kinds of large heatsinks on Ebay. How do I know which one I should buy?
Logged
PA0BLAH
Member

Posts: 0




Ignore
« Reply #6 on: September 30, 2012, 04:03:52 PM »

Don't spent money for a dummy load. When you have a 2 kW amplifier, just take a 2 kW electic heater, or a water cooker, or a bunch of lightbulbs in parallel, able to consume that power. Put them after your antenna tuner, tune to SWR 1, and you have your dummy of 50 ohm at the input of the tuner.

Remember that lightbulbs have an impedance that is dependent of the amplitude of the dissipated power.
« Last Edit: September 30, 2012, 04:09:50 PM by PA0BLAH » Logged
G3RZP
Member

Posts: 4366




Ignore
« Reply #7 on: October 01, 2012, 01:14:22 AM »

Any load fed via a tuner can be useful. But there are problems where a proper non-reactive wideband non-radiating load is required to fix them - and strings of lightbulbs etc will radiate.
Logged
KG6BRG
Member

Posts: 116




Ignore
« Reply #8 on: October 01, 2012, 07:00:59 AM »

KJ6ZOL



I found the N4SPP dummy load plan via google. If anybody is familiar with this plan (google "diy dummy load") can you tell me what I should look for in a heatsink? There are all different kinds of large heatsinks on Ebay. How do I know which one I should buy?

[/quote]

check this homebrew D/L out.  cheers.

http://k4eaa.com/dummy.html

Logged
G3RZP
Member

Posts: 4366




Ignore
« Reply #9 on: October 01, 2012, 09:15:28 AM »

The heatsink apporach is if you use resistors that can be mounted on a heat sink, and there are transistor size (TO220) resistors out there. Figure out how many watts you need to dissipate and the maximum temperature that the resistor is allowed. (I'd go for 90 degrees)Then the  amount of heatsink is one that gives a 70 degree temp rise for the amount of power you want to dissipate.

For example, you want to dissipate 100 watts, then you need a thermal resistance from resistor element to ambient of 10/100 watts or 0.7 W/deg C.

Because there's a number of resistors, the figure will be somewhat modified, and again, you won't necessarily need a 100% duty cycle.

By the time you've finished, you would probably be as well off by going to MFJ for one....unless you like building.
Logged
K8AXW
Member

Posts: 3651




Ignore
« Reply #10 on: October 01, 2012, 09:22:37 AM »

BRG:  You don't buy a heatsink for this dummy load.  These resistors are suspended in a can of oil. The new Caddock and Bourns TO-220 non-inductive resistors would be mounted on a large heatsink.  I've just started to use these interesting devices.

There are basically two kinds of dummy loads.  One is the "dry" type which uses cluster of non-inductive resistors or one large non-inductive resistor mounted in a cage to minimize RF radiation.  Some of these "dry" dummy loads are cooled by air circulating through the cage and some have a fan to force cooling air for the resistor(s).

The second kind is the "wet" type dummy load which consists of a bank of non-inductive resistors or a single large resistor suspended in a can of transformer oil or mineral oil.  The oil acts as a coolant.

While the homebrew dummy load you note will work great, it must be understood that it's rated for less than 200W and for a short period of time.  In most cases this will be fine.  If you have to do extended testing of your transmitter then you'll find yourself waiting until the dummy load cools.  Dummy loads can't be "pushed" too far beyond their ratings because as the resistor(s) heat the resistance changes.  

If you have a linear or intend to get a linear sometime in the future, you might want to consider biting the bullet and buy an MFJ Cantenna type wet dummy load which will handle 2KW.  

I built a dry dummy load for QRP which works fine but I also have a Heath Cantenna at my operating position for my 100W transceiver and 2 amps. I use a laboratory dial type thermometer to monitor the temperature of the oil.



« Last Edit: October 01, 2012, 09:30:49 AM by K8AXW » Logged
KG6BRG
Member

Posts: 116




Ignore
« Reply #11 on: October 01, 2012, 10:16:12 AM »

BRG:  You don't buy a heatsink for this dummy load.  These resistors are suspended in a can of oil. The new Caddock and Bourns TO-220 non-inductive resistors would be mounted on a large heatsink.  I've just started to use these interesting devices.

There are basically two kinds of dummy loads.  One is the "dry" type which uses cluster of non-inductive resistors or one large non-inductive resistor mounted in a cage to minimize RF radiation.  Some of these "dry" dummy loads are cooled by air circulating through the cage and some have a fan to force cooling air for the resistor(s)

The second kind is the "wet" type dummy load which consists of a bank of non-inductive resistors or a single large resistor suspended in a can of transformer oil or mineral oil.  The oil acts as a coolant.

While the homebrew dummy load you note will work great, it must be understood that it's rated for less than 200W and for a short period of time.  In most cases this will be fine.  If you have to do extended testing of your transmitter then you'll find yourself waiting until the dummy load cools.  Dummy loads can't be "pushed" too far beyond their ratings because as the resistor(s) heat the resistance changes.  

If you have a linear or intend to get a linear sometime in the future, you might want to consider biting the bullet and buy an MFJ Cantenna type wet dummy load which will handle 2KW.  

I built a dry dummy load for QRP which works fine but I also have a Heath Cantenna at my operating position for my 100W transceiver and 2 amps. I use a laboratory dial type thermometer to monitor the temperature of the oil.





You misunderstood my reply, I am only offering the original poster another option for a dummy load, I have an oil filled D/L, I built years ago and it has served me well.  I use it on all my repairs of hy-brid rigs and it works great.   Here it is again. Take a look ZOL.

http://k4eaa.com/dummy.html

to be clear, this is another type of D/L that uses oil to dissipate heat created by RF.  cheers.
Logged
KJ6ZOL
Member

Posts: 340




Ignore
« Reply #12 on: October 01, 2012, 11:38:00 AM »

You misunderstood my reply, I am only offering the original poster another option for a dummy load, I have an oil filled D/L, I built years ago and it has served me well.  I use it on all my repairs of hy-brid rigs and it works great.   Here it is again. Take a look ZOL.

http://k4eaa.com/dummy.html

to be clear, this is another type of D/L that uses oil to dissipate heat created by RF.  cheers.


I saw that. It looks quite difficult to build. The N4SPP load looks easier to build.

Edit: http://www.nonstopsystems.com/radio/frank_radio_antenna_dummy.htm 

This is the one I'm talking about. He doesn't specify the type of heatsink required. I noticed that he used a heatsink with many fins, as opposed to few fins.
« Last Edit: October 01, 2012, 11:44:07 AM by KJ6ZOL » Logged
NO2A
Member

Posts: 754




Ignore
« Reply #13 on: October 01, 2012, 11:51:03 AM »

The built in dummy load in the MFJ 989C will handle 100 watts just fine,however their claim of 100 watts for ten minutes or 1kw for 10 seconds is very misleading! I`ve used mine at 100 watts to test power out levels on each band. That resistor gets very hot. Loading a kw into it would melt it and your amp. Shame on you MFJ for making a claim you know is a lie.
Logged
KJ6ZOL
Member

Posts: 340




Ignore
« Reply #14 on: October 01, 2012, 12:41:15 PM »

You misunderstood my reply, I am only offering the original poster another option for a dummy load, I have an oil filled D/L, I built years ago and it has served me well.  I use it on all my repairs of hy-brid rigs and it works great.   Here it is again. Take a look ZOL.

http://k4eaa.com/dummy.html

to be clear, this is another type of D/L that uses oil to dissipate heat created by RF.  cheers.


I saw that. It looks quite difficult to build. The N4SPP load looks easier to build.

Edit: http://www.nonstopsystems.com/radio/frank_radio_antenna_dummy.htm 

This is the one I'm talking about. He doesn't specify the type of heatsink required. I noticed that he used a heatsink with many fins, as opposed to few fins.


I believe the term is "high density" heatsink, as opposed to "low density". Would any high density heatsink off Ebay work for this load?
Logged
Pages: [1] 2 Next   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!