[Boatanchors] Should dummy load oil ever need to be changed?
Drew P.
drewrailleur807 at yahoo.com
Sat Aug 25 11:00:50 EDT 2012
Brian wrote:
(snipped)
"The Heath Cantenna uses a carbon resistor (good) in a parallel sided tube
(very bad). What this means is that as a dummy load, the main dummy is the
user. In a word, the Heath Cantenna is, in this format, only good for DC.
The air-surrounded MFJ jobbie is just as useless. For a proper dummy load
look for a Bird Termaline.
If you have a friend with a very good lathe, get a negative exponential cone turned up to house the carbon resistor - then it will have performance flat beyond at least 1 GHz. I have designed such a cone, used one surrounding that self same carbon resistor and achieved the performance I specified at least up to 500 W."
How good does the dummy load have to be?
Yes, the performance of the Cantenna deteriorates as frequency increases. It would be most useful at lower frequecies - HF region. In its use, one might declare and not go above a "cutoff frequency" at which the VSWR has risen to an objectionable level, say, 2:1.
On the same topic, Rob wrote:
> Now, having written all that, let me also add that for AM the Cantenna
> is inadequate in my opinion, except for brief test transmissions.
> That sucker will get hot if you dump a carrier into it for more than a
> minute or two, especially over 300 watts. Use it, but keep looking
> for something better.
I like the idea of a big salt water dummy load - easy to build and cheap. A Google search turned up this link and many other references.
http://www.deserthound.com/amateur-radio/rfdl01.html
(Salt water dummy load text is at the bottom of the referenced page.)
Drew
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