Note
The “Cantenna” RF Load was the longest running and most successful product Heath ever made. It sold for 30 years and spanned more than three quarters of Heath’s amateur radio life. According to former Heath Vice President Chas Gilmore, more than 200,000 Cantennas were sold. Other products came and went, but the Cantenna remained.
It is difficult not to wax nostalgic over this humble product. The HN-31 was housed in a standard one gallon paint can and sold for $9.95, it remained plain black until 1969 when it was adorned with the familiar Heath logo and a derating curve diagram.
The original Cantenna came topped with a small aluminum minibox fitted with an SO-239 and an RCA DC pickoff jack for relative power measurements.
In 1983, the box and the RCA jack were removed and the SO-239 was mounted through the lid of the can. At the same time, Heath repainted the can in a flashy three-color motif. Later the fancy paint job was replaced with a three-color stick-on label.
The HN-31 uses a surprisingly small 50Ω ±10%, 30-watt resistive element (Heath part number 1-2-10) to provide a non-reactive load with an SWR of 1.5:1 from 1.5 to 300 MHz. An aluminum shield surrounds the resistor to reduce stray reactance and helps maintain the 50Ω impedance well into the UHF region.
The load will handle up to 1000 watts, provided the can is filled with oil to cool the resistor. With transformer oil the Cantenna will dissipate 1000 watts for 10 minutes. With mineral oil the 1000 watt rating drops to less than a minute. At power levels below 400 watts, mineral oil will serve as well as transformer oil.
Warning: Never use motor oil.
Caution: Some transformer oils contain PCBs—a suspected carcinogen. If you’re not sure what’s in your Cantenna, handle it with care. Heath did not supply the oil.
The Cantenna should be filled with oil to about 3/4 of an inch from the top of the can. The oil level should be at least 1/4 of an inch above the top of the resistor and shield tube when the lid is installed on the can. This level permits proper circulation of the oil as it heats up in the vicinity of the resistor.
It would be prudent to keep the Cantenna in a pail or other containment to contain leaks and minimize the possibility of tipping and spills.
The original HN-31 had a 3:1 SWR spike just above 225 MHz and a 2:1 SWR above 400 MHz. Refer to chart. The “A” version moved this spike up to about 350 MHz and lowered the SWR above 400 MHz to less than 1.5:1.
Note: the value of the resistive element can change over the years, but should read within ±10% to be considered acceptable.
No shack should be without this piece of history. It’s hard to go to a swap meet and not see at least one Cantenna.
References:
New Apparatus. QST. Jul 1962, p .29.
Review. 73 Amateur Radio. Jan 1963, p. 62.
Review. QST. May 1984, p. 42.
Use on UHF. 73 Amateur Radio. Jan 1965, p. 34.
Use as an RF Wattmeter. QST. Mar 1966, p. 79.
Finding oil for. QST. Aug 1970, p. 53.
Improved metering. 73 Amateur Radio. Aug 1979, p. 124
Impedance: 50Ω, ±10%
SWR:
less than 1.5:1 up to 300 MHz
less than 2.0 above 400 MHz
Power dissipation: 1000 watts maximum
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
