Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: AB5NI on November 28, 2016, 04:36:29 AM
Wonderful use of the Dosy meter, Buddy! I don't know if this is true, but I hear that that particular meter (the circuitry) is not too accurate.
I have the perfect dummy-load to go along with it, if you or anyone else is interested. Here's what you need:
An old, glass baby-food jar with a metal lid
An SO-239 or BNC connector
Some mineral oil
2 100-ohm, 3-watt resistors in parallel
A short run of 50-ohm coax
FIrst, take the lid off the baby-food jar, and then punch out (or drill out) the center of the lid so it can accept your desired connector. Mount the connector to the lid, with the female portion of the connector on the top of the lid. This is where you'll screw in your coax. Next, solder the coax to the center/positive connection on the connector, attach the ground of the coax to the lid/connector, and then solder the other end of the coax to both sides to two paralleled, 100-ohm resistors, making sure that the parallel resistors wind up centered inside the baby-food jar. Fill up the jar with mineral oil, and then insert the coax and resistors into the mineral oil and screw down the lid.
When done, what you'll have is a baby Heathkit "cantenna" that should be able to handle around 10-15 Watts or so, but don't push the thing too hard or you'll wind up letting out the magic smoke :D.
73,
Randy AB5NI
Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: AB5NI on November 28, 2016, 05:07:16 AM
BTW, it is also important to note here that this wattmeter is a piece of test gear and meant to be used with a dummload, folks. Please don't try to use this as a station wattmeter while transmitting on air. Your end results will not be kind, especially if you are trying to run a rig in the milliwatt range. For those who wonder why this is so, it is a direct-sampling wattmeter, tied directly to the center conductor. It is robbing a large portion of the RF to drive the meter circuit; moreover, it is changing the input and output impedance, and it will no longer be 50 ohms. You could probably get away with it by using an antenna coupler on the output, but the direct sampling and the tuner will rob too much power for QRPp operation. I guess you could get away with using it with a few watts of power, but I'm not sure I'd try using it that way either.
73,
Randy AB5NI
Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: The Radio Shop on November 28, 2016, 11:09:21 AM
Hi Randy, Yep those meters are not the best. Seldom seen two that would read the same. Thought the best thing to do was give it a transplant :) Thanks for the dummy load example. I made a small salt water dummy load before.
Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: AB5NI on November 28, 2016, 12:33:46 PM
Yo, Buddy!
Great use of the "DOSY-DOH!" meter movement, my friend! I'll be on the lookout for one for sure. Should be a wonderful addition to my test gear. BTW, if you keep this up, I'll wind up having projects that go well beyond the winter months :D.
73,
Randy AB5NI
Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: The Radio Shop on November 28, 2016, 03:06:30 PM
Thanks Randy. I like sharing stuff when I can. Projects are always fun. Plus got to do something all winter lol.
Title: Re: #81 Shop Talk Homebrew Test Gear PT 2
Post by: RJ on November 28, 2016, 06:55:47 PM
Great stuff guys!
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