Dummy Loads and SWR
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Just a thought on dummy loads, 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. Last year a Dayton I picked up a dummy load, which is a 50 ohm it is 10 or 20 W resistor on a massive heat sink, and easily handles 100 W, designed for use on VHF, it is flat across the bands, at least from 1.8 to 30 MHz. By the way I also have several hb dummy loads for QRP which I know are also 50 ohms. Steve Ray K4JPN ex K1VKW EM82ep Warner Robins GA Elecraft K2 1422 & KPA-100 Heath Fan HW-101, HW-8 http://www.thewinstonator.com/k4jpn.htm _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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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: [hidden email] 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 |
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Ron,
here you remind me of one of my favorite demonstrations; hopefully some of the rest will enjoy my reminiscing. >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. > > Years ago, I used to teach a class on high powered amplifiers, and the importance of neutralization of the tank circuit was a topic frequently covered. I would take out an old knight kit T-60 (Yeah, I agree, certainly not QRO, but effective for the demonstration) and hook it up to a 60 watt incandescent bulb as a dummy load. Then, with no crystal or VFO, I would key it down... the bulb would glow as the parasitic oscillation came up, while we tuned the rig's plate and load controls... Then the challenge would be to neutralize it such that this could not happen... And back then, just a brief warning to turn the exciter off, and ground the plate supply before working on it sufficed; we were a less litigious society in those days I guess.. 73 de W5SV, Dave _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by Steve & Anne Ray
In a message dated 08/06/05 17:50:09 GMT Daylight Time, [hidden email] writes: Just a thought on dummy loads, 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. Reply:--------------------------------------------------- There was a similar query in Radcom quite a few years back, I think in the Technical Topics column about the value of resistors changing in oil filled RF dummy loads. The answer to the problem came back from no less a person than Reg Varney, G5RV who was I understand involved in the chemistry profession at one time. Certain types of oils apparently degrade the carbon material that the resistors are constructed from and over a time alter the resistance value. Vegetable oils were particularly bad in this respect it turned out. As these vegetable oils are available cheaply in all grocery stores, this would be a popular choice for the cash strapped ham, to their eventual downfall. There are oils available that do not exhibit or minimise this effect, though probably are much more expensive to purchase and more difficult to source. Some transformer oils such as PCBs have to be avoided as they are reputed to be carcinogenic. Bob, G3VVT _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by Steve & Anne Ray
I mistakenly gave the wrong christian name for G5RV in reply. Was pointed
out correctly by Peter, G3PDL that it was Louis Varney. Reg Varney was a more infamous UK TV personality of the past. Senility creeps up on you and you don't know it is happening until too late! Bob, G3VVT _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by G3VVT
After my post yesterday about the accuracy of the Heath "Cantenna" dummy
load being found to be "off value", I started wondering about the original specifications and did a little search. Heath spec'd the Cantenna as having a VSWR of "<1.5:1" across the HF range. Now, if the reactance is negligible over that range (the Cantenna is rated for frequencies as high as 900 MHz so the reactance should be very low below 30 MHz), then a perfectly "in spec" Cantenna might show as little as 35 ohms or as much as 75 ohms and still meet "factory specifications". That's something to consider whenever working with older gear. Modern manufacturing techniques have spoiled most of us with cheap and highly accurate devices and parts that would have been simply unavailable or incredibly expensive only a few years ago. A dummy load with an SWR >1.5:1 is a fine dummy load for most purposes, but when doing things like critical power measurements, etc., it's good to realize that the actual value may not be as close to the desired value as one hopes. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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