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Author Topic: Dummy Loads  (Read 2839 times)
W0DLM
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Posts: 75




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« on: April 05, 2010, 08:00:59 PM »

Okay, so I am a newbie.  I got myself an HF radio and now I'm trying to figure out what I need next.  Power supply, obviously.  SWR meter, I'm pretty sure.  Dummy load?  I think so, but what kind?

The radio has 100 watt max output.  How big of a dummy load do I need?  I see some that can handle 100 watts continuously.  They're a lot more expensive than the ones that handle 100 watts for only a minute or so.

So, when you use a dummy load, how often are you transmitting into it continuously, and for how long?  If I get one that can handle 100 watts for 30 seconds or so, is that going to be adequate?

Thanks!

W0DLM
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VA3RTX
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« Reply #1 on: April 05, 2010, 08:24:34 PM »

For the money, about $150 new or $100 used, I would suggest a MFJ-949E. This is a combination Antenna Tuner, SWR/Power Meter, 300 Watt Dummy Load and Antenna Switch. I've been using one for over a year now and find it very useful. You can read many reviews on this unit here at eHam...

        73's VA3RTX
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N3OX
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« Reply #2 on: April 05, 2010, 08:29:44 PM »

If your radio is a modern solid state one and not a tube hybrid or something, 100W for 1minute or so is fine.  You will only need it for things like making sure a problem isn't inside your radio or what I use mine for: adjusting digital modes or transmit audio... If you need to learn to tune up a tube rig or you build or fix stuff, a higher power load is useful.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K0ZN
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Posts: 1525




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« Reply #3 on: April 05, 2010, 09:53:26 PM »

Hi,

Truthfully, a dummy load is not something you will use a lot, but they are very valuable at times. i.e. if you think you have an antenna or transmission line problem, you can switch to the dummy load and make sure your rig is operating OK and the problem is NOT your rig. NICE to know! You can also use it to check and "play" with transmitter functions and settings with out radiating needless interference on the band.
It is very poor practice to do a lot of on the air "testing and tuning".

Frankly, I would recommend just biting the bullet and buy the MFJ oil filled dummy load. If you are just running 100 watts you can buy it dry without the oil and save some money... about $50 I think, and use non detergent motor oil instead of the more expensive transformer oil. (DON'T run it dry!!) The MFJ will have a good long run time at 100 W. It would still be useful if you ever add and amp to your rig too. I have a "400 W" dry dummy load and it gets hot FAST at 100 W...not real useful....so I bought the MFJ oil one.

If you buy the MFJ Dummy, BE SURE to carefully look over the connection at the coax connector to the resistor. The one I got was broken and took some repair. If someone had tried to use the one I got, they would have transmitted into an Open circuit~! ( Not Good!!) Unfortunately, MOST MFJ products, while useful, do require some serious going over to make sure every thing is tight and connections are good. This is just a cold reality; the end user pretty much has to do the final Q.C. check! ... you can confirm this on most product reviews, unfortunately.

You can also build one, but my guess is that you will spend as much as the MFJ one by the time you are done.

Bottomline: get one...you will be glad you did.

73,  K0ZN
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VK1OD
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Posts: 1697




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« Reply #4 on: April 05, 2010, 10:58:00 PM »

Okay, so I am a newbie.  I got myself an HF radio and now I'm trying to figure out what I need next.  Power supply, obviously.  SWR meter, I'm pretty sure.  Dummy load?  I think so, but what kind?

The radio has 100 watt max output.  How big of a dummy load do I need?  I see some that can handle 100 watts continuously.  They're a lot more expensive than the ones that handle 100 watts for only a minute or so.

So, when you use a dummy load, how often are you transmitting into it continuously, and for how long?  If I get one that can handle 100 watts for 30 seconds or so, is that going to be adequate?

Dummy loads fall into two broad classes, those of instrument quality (ie you can depend on them having a very low VSWR over their specified frequency range) and those that are roughly 50 ohms and provide a substitute for a low VSWR antenna. I have both kinds, and I use all of them frequently.

Give consideration about your future needs. Will you use VHF / UHF?

You can get ATUs with an integrated load (eg the '949E that one poster suggested), but there is advantage in having a separate load with better performance.

You have already discovered the inflated ratings, kW loads that have a long term continuous rating of way less than 10% of their rating. Whilst a short term rated load can be quite adequate for a lot of things, it becomes an issue if you are chasing a problem and require longer run time.

I suggest you look at the MFJ-264N which is rated at 1.5kW at 1 to 650MHz. The ratings are competitively extravagant, its continuous power handling is probably more like 70W, and the VSWR at 650MHz is a bit shabby. This is not an instrument quality load, but it performs close to it up to the 2m band. The N on the part number is for N type connector, buy the load with an N type and put an adapter on if you must for UHF series connectors. As you progress, you may learn the value of the N type connector and appreciate committing to it on the load which you will probably use for a long time. The MFJ-264N is not very expensive, and good value for a HF to low UHF load.

I use a MFJ-264N for experiments / measurements etc on my TS2000 / AL811H combination. My TS2000 has an added facility to send a rapid pulsed 1kHz tone at 10% duty cycle, so it can run on the load continuously and allow tuning / measurement of the AL811H under conditions similar to SSB voice (low duty cycle, low power supply sag, maximum PEP).

A load like this is something you can always pull out to checkout a transceiver that you think is not working properly, to adjust drive levels etc for new modes, check power levels simply, checkout VSWR meters, etc.

I expect a string of responses assuring you that you don't need one, fostering the new age ham shack which is a transceiver and power supply, possibly an ATU and a random length wire as an all band radiating dummy load. But, if you are interested in the technical aspect of the hobby (a dying group from all accounts), a medium quality dummy load such as I recommended will be a good addition to the shack and of long term benefit.

Have a search here for the instances when a newbie comes up with a problem and the responses are "what does it do on a dummy load". It is a most useful first test in localising many transmitter problems.

Owen
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N3OX
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« Reply #5 on: April 06, 2010, 06:11:00 AM »

If you have a piece of heat sink material or even a pretty thick aluminum sheet, you can make a fairly inexpensive dummy load by buying a thick film heatsinkable power resistor.

This doesn't work out inexpensively if you don't already have something high thermal conductivity to use for a heatsink, but this is one of my dummy loads:

http://n3ox.net/files/dummy_load.jpg

That uses a Caddock MP9100-50 resistor, which is something like $10 to $15 from Mouser.

It's got very low SWR on HF and usably low even on UHF because I kept the leads short and the resistor is designed for high frequency use.    I used a BNC connector mounted through the heatsink.  This whole thing is about the size of my hand; it's not even close to being a 100W continuous dissipation load.  But it will take 100W for 30s or a minute.  So if you've got a reasonable heatsink or can pick one up for a few bucks at a hamfest, this could be an option for a basic low-duty-cycle dummy load.

73
Dan

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

Monkey/silicon cyborg, beeping at rocks since 1995.
TANAKASAN
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Posts: 933




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« Reply #6 on: April 06, 2010, 06:28:54 AM »

All agreed on the post from N3OX but in my case I used two 100 ohm resistors in parallel across the BNC. Not only does this spread the heat across two devices but of one goes open circuit you only get 2:1 SWR.

Tanakasan
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AA4HA
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« Reply #7 on: April 06, 2010, 06:47:04 AM »

So, when you use a dummy load, how often are you transmitting into it continuously, and for how long?  If I get one that can handle 100 watts for 30 seconds or so, is that going to be adequate?

If you get into making alignments of the internal stages of the transmitter you might need a dummy load that can sustain a higher transmit cycle. Not many folks get that far into their gear where they are tuning up stages. If you were into boat-anchors (old tube based radios) this would be a much more common experience.

Generally it only takes a few seconds to check your power output from the transmitter, through a wattmeter and into a dummy load. If you are checking the quality of your transmitted signal (with a receiver nearby just monitoring the transmitter by leakage alone) this may be less than a minute.

Sometimes if you are testing digital modes with a nearby receiver and you want to make sure that your transmitter is sending a clean signal and a separate receiver (attached to a TNC or computer running a program) then this may be a minute or so.

You are right on to be asking about duty cycle ratings and devices rated for continuous duty.

The oil-filled "Cantenna" devices are fine. If you buy an old one from a hamfest just be aware that the really old ones were filled with PCB oil. All the oil is for is to increase the surface area of the load resistors or elements and to conduct heat away. I would not suggest automotive lubricating oil and would suggest one of the following;

1. Transformer oil, this may be hard for you to find.
2. Automatic transmission fluid. This is intended to carry heat away and has a much higher flash point than motor oil.

You could go with an air-cooled device with either fins or vents. To slightly increase the wattage rating of this type of dummy load, use a surplus computer fan to blow air across the radiator or through the dummy load to pull heat away.

I will not speak about particular models or the pro's and con's. There are plenty of folks who can give that advice.

Tisha Hayes, AA4HA
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AD4U
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Posts: 2150




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« Reply #8 on: April 06, 2010, 08:14:28 AM »

IMO the best place to find a dummy load is at a hamfest.  Four weeks ago I attended a local hamfest and bought a brand new BIRD 150 watt continuous duty, dry (no oil), dummy load for $25.  

The ham selling them had several, and they were not selling.  Go figure.

Dick  AD4U
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W0DLM
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Posts: 75




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« Reply #9 on: April 06, 2010, 09:40:15 AM »

"buy the load with an N type and put an adapter on if you must for UHF series connectors. As you progress, you may learn the value of the N type connector..."

Okay, so tell me the value of N-type connectors.  My radio is an Alinco DX70.  It has SO-239 connectors for both HF and 6m.
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VK1OD
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Posts: 1697




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« Reply #10 on: April 06, 2010, 10:59:32 AM »

...
Okay, so tell me the value of N-type connectors.  My radio is an Alinco DX70.  It has SO-239 connectors for both HF and 6m.

The UHF series connectors depend on the tightness of the coupling ring for a good / reliable shield connection. It is very easy to assemble them so that they work loose. You can overcome this by making sure that the V points are properly engaged in the bottom of the notches, wiggling them so that don't falsely catch on the side of the notch as you tighten the ring, then tighten the ring quite tight with a pair of multi-grips.

UHF series is not alone in this tightness requirement, others include mini-UHF, SMA and F connectors, but it is the UHF series and mini-UHF that have the V and notch that frustrates proper tightening.

In a measurement situation, you want reliable connections, and N type is better for that.

When I want to use my MFJ-264N on a radio like yours, I have a fly lead with N type on one end, and UHF plug on the other. That way, I only have one connector that needs extra care that it is properly connected.

Owen
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TANAKASAN
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« Reply #11 on: April 06, 2010, 12:05:24 PM »

"Okay, so tell me the value of N-type connectors. My radio is an Alinco DX70. It has SO-239 connectors for both HF and 6m."

1) An N-type has better performance than a 'UHF' connector above about 100 MHz and provides a constant impedance.

2) The connectors are watertight. This means that an oil-filled dummy load with N-type connectors won't leak.

3) They provide a connection which is physically secure.

Tanakasan
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W4VR
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« Reply #12 on: April 06, 2010, 01:00:26 PM »

Back in the old days I used a 100 watt light bulb.  Not sure how that would work with today's solid state radios.  Eventually I bought a real dummy load and used it about once every 10 years.
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VK1OD
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Posts: 1697




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« Reply #13 on: April 06, 2010, 02:34:31 PM »

...
2) The connectors are watertight. This means that an oil-filled dummy load with N-type connectors won't leak.

All N types are sealed between body outsides using a gasket that is clearly visible inside the male connector.

If you want to seal the inner as you suggest, you must buy a special connector that has an internal seal. You can get them (and BNC) with a hermetic seal (ie a glass seal) which is very effective, but O ring seals are adequate for most purposes. I made an experimental dummy load using R12 refrigerant (a long time ago) using a hermetically sealed BNC connector, and the liquid is still in the load.

Owen
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N7WE
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Posts: 13




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« Reply #14 on: April 07, 2010, 02:22:42 AM »

Great item to try your hand at "homebrew!"  And one of those items that it is "Better to have one and not need it than need it and not have one."  

Check out this site  http://www.k4eaa.com/dummy.html  Buy his resistor kit LK-R1 ($12.50), make a quick trip to your local Ace Hardware, and have at it.  Mine works great at 50 ohms +/- <0.1 from 1.8 to 50 MHz and with the mineral oil fill, I've never had it get hot at 100W.

Rick - N7WE
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