Much has been posted recently about the Heath Cantenna and the use of
mineral oil to help increase its short-term power rating. While doing a
search on a slightly similar subject, I ran across an interesting fact
about the resistors used in the Cantenna which conveniently explained a
problem I had seen with my unit.
The original non-inductive resistors used in Heath's Cantenna were made by
Carborundum Corporation's Electric Products Division (which was purchased
by Kanthal Globar Elektrowarme GmbH in 1993). Heath, as a company, was
known for reasonably good engineering, but it was never known for using
quality components. After all, the company started its electronic kit
business using surplus components. My Cantenna (HN-31) was a good example.
I purchased my Cantenna kit in the late 1960's and used transformer oil
provided by the local electric cooperative to fill the paint can. It was
much later that I learned that the oil was contaminated with
polychlorinated biphenyls. When new, the resistance was close to 50 ohms,
but over several years the resistance gradually rose to approximately 100
ohms. When checked with my VSWR meter, the dummy load showed a consistent
2:1 VSWR. I thought I had "cooked" the resistor by using high power for an
extended period, never thinking that the problem was inherent with the
resistor itself.
It turns out that Heath had supplied a type A or AS resistor rather than
the type SP which should have been used. I discovered this accidently in a
search of QST articles. In the January 1989 Technical Correspondence
column, there is a quote from the Carborundum catalog discussing the use of
a heat transfer fluid to increase the power handling capability of these
non-inductive resistors:
"Unless the resistors are protected by a coating that the fluid cannot
permeate, such as an epoxy, these fluids cause the resistance of the
Type AS
resistors to increase. With some silicones, this increase is as little
as
10%; with some mineral oils this can be as much as 100%. Generally,
the
resistance will rise as the fluid permeates the resistor body, and it
will
finally stabilize..."
The Type SP resistors are already coated at the factory, and these should
have been the ones Heath used. Note, however, that the epoxy coating
reduces the heat transfer from the silicon carbide resistor to the oil to
some extent. It also reduces the maximum voltage rating by as much as 50%.
In an earlier post, I had suggested the use of Bourns power RF flanged chip
termination resistors stocked by Digi-Key (a 250 watt unit selling for
$27.50). I have since learned that they also stock 800 watt units selling
for $78 at the time of this writing. These are good to 1 GHz. They do
contain Beryllium Oxide so be very careful when installing these. A data
sheet from Bourns may be found at
http://www.bourns.com/data/global/PDFs/CHF190104CBF.pdf. These devices
require careful attention to heat sinking, even if they are approximately
1" x 2" in size.
73, Dr. Barry L. Ornitz WA4VZQ
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