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Reviews For: Heathkit HN-31 Cantenna

Category: SWR & Wattmeters & Dummy Loads

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Review Summary For : Heathkit HN-31 Cantenna
Reviews: 3MSRP: 19.95
Heathkit's classic oil-filled dummy load produced from 1961 through to 1983 when it was discontinued in favour of the HN-31A. One of these can handle continuous transmission at 200 watts, and up to ten minutes at one kilowatt. It sold over 200,000 units in its first ten years and was described in "Heathkit - A Guide to Amateur Radio Products" as the most successful product Heath ever made. Though it and its successor the HN-31A have been out of production since 1993 (when Heathkit stopped operations), it is still wifely available used. The MFJ-250 is a virtual duplicate of the Heath design.
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# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
KK8ZZ Rating: 2018-11-21
Death in a Paint Can? Time Owned: more than 12 months.
These work great, and there's a zillion of them around, BUT.... RUN, DO NOT WALK, from these.....the early Heathkit recommendation was to fill them with "transformer oil" (1960s-1970s).... "Transformer oil was high in PCB, a very active cancer agent". It is now banned nationwide but an older one you might find at a hamfest might still be filled with the deadly stuff. My dear brother, a lineman for the city we live in, died of non-Hodgkins lymphoma - attributed to prolonged contact with transformer oil...ummmm like sitting under your radio desk in your office space or bedroom? DO NOT BUY THESE UNLESS YOU ARE CERTAIN -- ABSOLUTELY CERTAIN -- that they are filled with Mineral oil. OR .... just go out and buy a nice little MJF air cooled dummy load that's really not likely to kill you, or leave a mess on your floor. de KK8ZZ
G4AON Rating: 2016-11-29
Stood the test of time Time Owned: more than 12 months.
I'm surprised at how few reviews there are for what was the top selling Heathkit. I bought mine as a kit in the 1970s and use it regularly. Other than an oil change a few years ago I've not done anything to it.

They do not have the best cooling arrangement, getting very hot around the top half of the can when running powers between 500 and 1000 Watts for more than a short time, but it's more than adequate for what it is and mine is still the same as it was 40 years ago, which says a lot for the original design.
VA1DER Rating: 2016-11-14
A lucky purchase Time Owned: 3 to 6 months.
I bought mine on a trip when the purchase of a dry one fell through. This was the only other one available, and this was the only other one available. If I knew then what I know now, I wouldn't have even considered the dry one.

This elegant piece of kit started being produced back in 1961. Heathkit stopped making it in 1993 only because they ceased operations. It was then taken up, essentially unchanged, by both Vectronics and MFJ (now Vectronics' offering has been discontinued in favour of them carrying MFJ's).

An oil-filled paint can may seem chintzy to some, but it's actually a very elegant design. An internal sleeve around the large non-inductive resistor creates a convective flow that brings cool oil from the bottom of the can up. The resistor is held by clips made of silver plated brass so as to offer good conductivity without being magnetic. A very elegant design. Determining whether it is safe to continue transmitting is a simple matter just feeling how far down the hot area is. If just the top couple inches is hot, then you're good to keep going. When it gets close to the bottom, time to let it cool down. This convective flow makes it important, though, that the unit is never operated when the oil is low. The oil should be within .75" from the top of the can. It must never be below the top of the sleeve tube around the resistor or else the sleeve just becomes an insulator keeping the hot oil in place.

Other amateurs are daunted by the idea of trying to find proper transformer oil to fill it with. That was my issue. While real transformer oil is considered best, it's perfectly acceptable to fill it with mineral oil. I use medical grade light mineral oil, which I find is ideal for good convective flow. The derating curve suggests less transmit time is possible at high power when using mineral oil, but that has not been my experience. I have operated almost continuously for ten minutes at close to 900 watts. But then today's medical/food grade mineral oil is far more refined now than it was in 1961 when the derating curve was made. I first thought it would be difficult to source enough, but I found four litre jugs of medical grade mineral oil at a local farm animal supply store (Feeds & Needs in Nova Scotia) with little trouble. Veterinary supply outfits, or even veterinarians themselves that cater to larger animals also carry the jugs.

Make sure when you buy one used that you ensure the resistor is still ok. The fellow I bought mine from operated it 3/4 full of oil, which, as mentioned above, inhibited the convective flow and overheated the resistor. The resistance when I got it home read close to 80 ohms. While that was frustrating, I was able to find a replacement resistor with little trouble. Original Carborundum resistors are available on eBay, and new manufacture ones are available from MFJ and from Kanthal Globar.

I highly recommend a Cantenna. At the kilowatt level a lot of other dummy loads measure transmit time in the seconds. Either that or they cost a (not so) small fortune.