Posted by
Ron D'Eau Claire-2 on
Jun 08, 2005; 6:40pm
URL: http://elecraft.85.s1.nabble.com/Dummy-Loads-and-SWR-tp379143p379147.html
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
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