Steve K4JPM wrote: "...a number of years ago I noticed my Heath Cantenna was giving me a slightly higher SWR reading instead of 1 to 1. When I measured it with a VTVM I found it was 40 ohms, not the original 50 ohms when I built it 25 or so years ago. I posted this on the Heath users Email list and received around 5 or 6 replies all with the same condition of 40 ohms. The interesting point is that my dummy load has never seen more then 100 W; as I have never have and probably never will own a KW. Several of the replies I received from the Heath users group also had never run a KW into their Heath cantenna's."
-------------------------------------------------------- When dummy loads such as the Heath "Cantenna" first turned up in Hamshacks, they were a luxury item intended to replace the standard dummy load of the day: an incandescent light bulb. The light bulb had the added advantage of showing when the rig was tuned for maximum output by how brightly it glowed. The Cantenna and similar loads were first promoted as "TVI-proof" loads that did not radiate. Still, relatively few Hams used them. The appearance "No tune" rigs and increasing popularity QRO amps demanded something better that the simple bulb (although a few Hams persisted with gigantic incandescent bulbs hooked to their QRO amps). How accurate does such a dummy load have to be? A tolerance of 20% or even 30% was perfectly fine. After all, the no-tune rigs were specified to handle an SWR of up to 1.5:1 at least. That would mean a value of anywhere from 25 to 100 ohms was a usable dummy load. Even if a Cantenna measured close to 50 ohms when new, it's not likely Heath cared to spend the money on a resistive element guaranteed to stay close to that value over time. I have also read many reports of Cantennas showing 40 ohms today. Since most resistive elements increase in value over time, I have to wonder if they weren't always 40 ohms. After all, the ohmmeters we have today are far more accurate than what most of us had in those days too. The Cantenna appeared in the day when Hams were measuring and logging their power by the d-c input power to the final amplifier: the collector or plate voltage multiplied by the collector or plate current. Few hams had any way of measuring their output power beyond comparing the brilliance of an incandescent bulb to its normal value. It was common to verity that a 100-watt rig was producing full output by noting that a 60 or 75 watt bulb glowed to "full brilliance" when used as a dummy load, indicating an efficiency of 60% to 75% from a "100 watt" (d-c input power) rig. Of course, that sort of output power measurement went away when no-tune ham rigs no longer tolerated a light-bulb dummy load. And so, in time, wattmeters designed for a 50 ohm load started to appear for Ham use but those wattmeters are anything but exact. After all, a 2 or 3 dB error is virtually undetectable on the air under normal conditions. Most meters were intended more as relative power meters that would show a change indicating something amiss in the rig or antenna. If a meter monitoring a rig putting out 100 watts showed anything from 80 to 120 watts, it was a very good meter. To this day, an error of 20% of full scale is typical of many Ham (and come commercial) wattmeters. Note that is "full scale", so if you're looking at 10 watts output on a meter that reads 30 watts full scale, that means that the actual output power is anywhere from as little as 4 watts to perhaps as much as 16 watts (20% of 30 = 6 watts). Us Hams often like to get on a quest of perfection, even if it makes no sense technically or in terms of on-air performance. That has been encouraged by those who operate the QRP contests who want to know in their hearts that they are running all the power they can, but not one milliwatt over 5 watts output. But that's a recent phenomenon. When the Cantenna came out, all most operators cared about was whether the rig was "happy" with the load. After all, when those Cantennas appeared, official QRP ARCI power for contests and the like was 50 watts d-c input (30 watts or so output - roughly)! Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
