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Author Topic: Dummy Load Question from a Dummy  (Read 1328 times)
IRABREN
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« on: January 07, 2008, 09:43:00 AM »

Yes I am still that newbie - but learning !
If I want to test/ setup the radio ( Yaesu Ft-950) , using s Sigma 40XKR ( vertical dipole ) off air -  - What is the best dummy load ( for this dummy )  for me to use ? ( I think I would prefer a smaller - air cooled one( ? ) - not too costly )
I don't care for oil, mineral oil(except for constipation @ - or paint cans ! )
Thanks and 73
Ira
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WA3SKN
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« Reply #1 on: January 07, 2008, 10:12:33 AM »

Oil dummy loads can dissipate a lot of heat!  They are also heavier and can be messy.
Air dummy loads are cleaner and dissipate less heat, and can cost more.
There is a surplus market for dummy loads you should investigate, but make sure it is the proper wattage and frequency rating... as well as the expected connector!
You CAN make your own!  Check the back of the MFJ catalog... they will sell you the parts!  I am not sure of the savings, though!
You WILL want one.  When the questions arise, you will want a known good load for testing purposes!
73s

-Mike.
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IRABREN
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« Reply #2 on: January 07, 2008, 10:18:36 AM »

Thanks - but I am still unclear what to get ?
"but make sure it is the proper wattage and frequency rating"
Does this mean - I am testing with the load at a particular frewquency and wattage ? - Wouln't 1 dummy load suffice - or do I need more than one ?
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W3JJH
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« Reply #3 on: January 07, 2008, 10:21:55 AM »

First of all, the dummy load will replace the antenna as the load on the transmitter.  You can use it in the shack to test the radio and out at the antenna end of the transmission line to test the radio and the line.

Second, the HF dummy load I usually recommend is the Heathkit Cantenna or its modern replacement the MFJ-250.  Both of these use a one-gallon paint can to shield the dummy load resistor and mineral oil in the can to conduct heat away from the load.  If you insist on a dry load, the MFJ-260 (or something similar) will do for a 100 W radio as long as you stay within the time-power derating curves.

If you suspect that you have a problem with your antenna, you should get your hands on an antenna analyzer.  The MFJ-259 is the probably the most commonly used by hams.
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KB9CRY
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« Reply #4 on: January 07, 2008, 11:01:17 AM »

I am testing with the load at a particular frewquency and wattage ?


What they are trying to say is that there are dummy loads rated for HF only, for VHF only, and for UHF only.  On top of that they come in different wattaged ratings.  Some are 300 watts continuous carrier for maximum of 1 minute or 1500 watts max of 5 minutes.

Personally I use a Cantenna, the oil filled paint can.  It can handle any wattage (on HF only) that I throw at it.  If you think you'll never have an amp, I'd still reconsider.

MFJ makes some nice air cooled units.  
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WA3SKN
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« Reply #5 on: January 07, 2008, 11:03:37 AM »

The larger air cooled units are more expensive, that is why I mentioned the surplus market.  But the surplus market has some dummy loads designed for mirowaves, and higher... with connectors you do not need.
For HF, you will want PL259/So239 connections.  make sure any surplus load has them.
The large air-cooled units are expensive.  If you are planning kilowatt plus operation in the future, do consider the oil cooled units.
73s

-Mike.
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W3LK
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« Reply #6 on: January 07, 2008, 11:15:40 AM »

How are you coming on getting a license?

Good Luck with it.

73,

Lon - W3LK
Naugatuck, Connecticut
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K5LXP
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« Reply #7 on: January 07, 2008, 11:25:01 AM »

You probably already have the parts on hand to build one like this yourself.

<http://www.qsl.net/k5lxp/projects/SaltLoad/SaltLoad.html>

Mark K5LXP
Albuquerque, NM
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IRABREN
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« Reply #8 on: January 07, 2008, 11:27:01 AM »

 I am scheduled for the Tech Exam Jan 30 and using the W5YI book for study - an excellent book ! I have ordered the antenna - but it will take 12 weeks. Haven't ordered anything else yet.
Thanks to all for the info!
73(s)
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KM5Z
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« Reply #9 on: January 07, 2008, 11:47:07 AM »

The rig you specify (Yaesu Ft-950) has a maximum output of 100 watts, so a simple, little MFJ dummy load like this one:

http://www.mfjenterprises.com/Product.php?productid=MFJ-260C

Would be suitable. It handles 300 watts for about $40 or so.

I have an older model of this exact dummy load I picked up for about $20 used.

Mike Yancey
KM5Z
Dallas, Texas
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VK1OD
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« Reply #10 on: January 07, 2008, 12:04:48 PM »

A newbie needs to understand that dummy load specifications are often deceptive.

>...It handles 300 watts for about $40 or so.

It is actually rated at 300W for a maximum of 30 seconds.

If you use it withing that limitation, it may be fine, but be aware that you could damage the dummy load with a 100W transmitter, this load's continuous rating is just 25W.

Owen
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AC5E
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« Reply #11 on: January 07, 2008, 12:11:13 PM »

Well, Ira, since you have a little time - check the ARRL website (arrl.org) and check the index for hamfests in your area. You can often pick up a good used dry dummy load rated at 300 watts or better for a few bucks.

Twenty bought the Drake I use for everything except high power, and I paid less than $100 for the 200 Watt 50 ohm resistor and five gallons of transformer oil I use for high power. Of course, I had to do a little metal work on the big load.

But the Drake does well at least up to 2 Meters, Ten Tec, and Palstar also have dry dummy loads you can acquire at a reasonable price.

Good luck on the test - and as soon as you get your Tech go ahead and study for the General. Because once the Tech is behind you the General is not that much of a hill to climb.

Best Regards  Pete Allen  AC5E
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KI4NX
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« Reply #12 on: January 07, 2008, 12:57:15 PM »

Hi, Ira:

A while back, I bought four 100W, 50 ohm resistors, wired them in series/parallel to maintain the 50 ohm load and sunk them into a three gallon oil can. The thing will run just about anything I can throw at it, power-wise and not even get warm to the touch.  It's a great project for a weekend.

I also have several small air cooled dummy loads ranging from 10W to 100W.  However, most of the time I still use the old three gallon can.  And nothing beats building it yourself.

GL and 73
Ken
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AA4PB
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« Reply #13 on: January 07, 2008, 01:20:39 PM »

Dummy loads generally have a MAXIMUM frequency rating and a MAXIMUM power rating for a given amount of time. If, for example, your load is rated to 200MHz then you can use it on any frequency up to 200MHz without the SWR going too high. If its rated to 30MHz then you can only use it up to 30MHz. If its rated for 300W at 10 minutes then you can pump 300W into it for 10 min without overheating it. You can use any lower power for up to 10 minutes and probably longer, depending on how low the power is.
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IRABREN
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Posts: 273




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« Reply #14 on: January 07, 2008, 01:38:15 PM »

Thanks again for all the great responses. I've got the idea !
"as soon as you get your Tech go ahead and study for the General. Because once the Tech is behind you the General is not that much of a hill to climb"
           That is exactly my plan ! - Thanks
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