K2 power meter/wattmeter question
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K2 power meter/wattmeter question
 
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Hi Fellow Elecrafters,
How accurate have you found the internal power meter/wattmeter to be on the K2??? I ask this because there is a large difference between what I set the power for, on the K2, and what it displays as compared to the external meter (contained in a Ten Tec tuner, scale 0 to 200 watts, first division being between 0 and 10 watts). To get a half scale reading between 0 and 10 watts (5 watts) on the Ten Tec I have to set the K2's power level to over 8 watts. If I set the K2's power level to 5 watts I get an indication on the Ten Tec meter that falls below a half scale reading between 0 and 10 watts. In other words, appears to be less than 5 watts. I tend to want to trust the Ten Tec meter a bit more, since it has always given me a good indication while running powers up to 200 w and in accordance of what the power out was in a particular rig. Anyway, any comments would be appreciated. And, (maybe) should I just purchase a good QRP wattmeter kit or invest in a Bird/Byrd. Thanks In Advance, Mike KT2E K2 4777 _______________________________________________ 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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Mike, I wouldn't put much stock in any wattmeter, including Bird, at 10% of full scale. Larry N8LP Mike Monger wrote: >Hi Fellow Elecrafters, > >How accurate have you found the internal power meter/wattmeter to be on the K2??? > >I ask this because there is a large difference between what I set the power for, on the K2, and what it displays as compared to the external meter (contained in a Ten Tec tuner, scale 0 to 200 watts, first division being between 0 and 10 watts). To get a half scale reading between 0 and 10 watts (5 watts) on the Ten Tec I have to set the K2's power level to over 8 watts. If I set the K2's power level to 5 watts I get an indication on the Ten Tec meter that falls below a half scale reading between 0 and 10 watts. In other words, appears to be less than 5 watts. > >I tend to want to trust the Ten Tec meter a bit more, since it has always given me a good indication while running powers up to 200 w and in accordance of what the power out was in a particular rig. > >Anyway, any comments would be appreciated. And, (maybe) should I just purchase a good QRP wattmeter kit or invest in a Bird/Byrd. > >Thanks In Advance, > >Mike >KT2E >K2 4777 > >_______________________________________________ >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 > > > > 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 |
RE: K2 power meter/wattmeter question
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In reply to this post by Mike Monger
Mike, KT2E asked:
How accurate have you found the internal power meter/wattmeter to be on the K2??? Reply: --------------------- Mine is well within the 1/2 watt at 5 watts as stated in the specifications. Measuring RF power at QRP levels is really not difficult. A low forward-voltage drop diode from the 'hot' side of the RF line to a capacitor that goes to ground will give you an accurate peak RF voltage across the cap. Measure that with a high-impedance meter like most DMMs today. RMS value is .707 times that reading. To get the power, multiply the RMS voltage times itself (square it) and divide the result by the resistance: 50 ohms. Actually, that's exactly the circuit the K2 uses to measure power. It gets a little tricker as you go up in power only because most diodes can't handle the voltages involved. One the places where many ops go "wrong" evaluating power meters is in the load. You *must* use a good 50 ohm load. An antenna tuner adjusted for an indicated SWR of 1:1 is not good enough. Not even an Elecraft ATU gives you a reliable load accurate enough for this. The mismatch presented by such a load can really throw off the power indications. That's because the SWR meters simply don't need a high order of accuracy. There's no useful difference between the efficiency of an antenna system or the rig when presented with an SWR of 1.2:1 or 1.3:1 compared to a true 1.0:1 match. Most SWR bridges are very vague below about 1.5:1. Yet these errors can really upset many power meters, including that used in the K2. 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 |
RE: K2 power meter/wattmeter question
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In reply to this post by Mike Monger
Mike,
The K2 internal RF metering is quite accurate - BUT it depend on having a precise 50 ohm non-reactive load. The wattmeters built into the KAT2, KPA100 and KAT100 remove the dependency on a 50 ohm resistive load, but need to be properly calibrated first. Many (IMHO most) external wattmeters are less acurate than the internal K2 metering, particularly at low power levels. A typical specification for them is 20% of the full scale reading, and on your 10 watt scale, that is a 2 watt error. In addition, you may not be setting the tuner for a true 50 ohm resistive load to the K2. If you have an antenna analyzer, you can do an experiment - tune the antenna as normal and then disconnect the coax from the K2 and measure the impedance. I will venture to guess that it will not be 50 ohms resistive unless you were quite lucky. Even in bypass mode with a dummy load on the far side of the tuner, you will likely see something other than a 50 ohm reactive load at the input side. While you have the antenna analyzer out, check your connecting cables too, they can become lossy and produce strange results, and deserve to be checked every now and then. 73, Don W3FPR > -----Original Message----- > > How accurate have you found the internal power meter/wattmeter to > be on the K2??? > > I ask this because there is a large difference between what I set > the power for, on the K2, and what it displays as compared to > the external meter (contained in a Ten Tec tuner, scale 0 to 200 > watts, first division being between 0 and 10 watts). To get a > half scale reading between 0 and 10 watts (5 watts) on the Ten > Tec I have to set the K2's power level to over 8 watts. > _______________________________________________ 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 Ron D'Eau Claire-2
Ron D'Eau Claire wrote:
>Mike, KT2E asked: > >How accurate have you found the internal power meter/wattmeter to be on the >K2??? > >Reply: --------------------- > >Mine is well within the 1/2 watt at 5 watts as stated in the specifications. > > >Measuring RF power at QRP levels is really not difficult. A low >forward-voltage drop diode from the 'hot' side of the RF line to a capacitor >that goes to ground will give you an accurate peak RF voltage across the >cap. Measure that with a high-impedance meter like most DMMs today. RMS >value is .707 times that reading. To get the power, multiply the RMS voltage >times itself (square it) and divide the result by the resistance: 50 ohms. > > Now! Someone pointed out to me several weeks ago that using the Elecraft DL1 (dummy load) with a DDM and a calculator ( the formula on the assembly sheet) works very well and is accurate. de "rc" kc5wa _______________________________________________ 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 Mike Monger
Mike Monger wrote:
> How accurate have you found the internal power meter/wattmeter to be on the > K2??? The wattmeter in a basic K2 (without the KAT2 or KPA100) just measures the RF voltage and computes the power. Therefore, the accuracy is highly dependent on having an accurate 50 ohm resistive load. The KATs and the KPA contain directional wattmeters, which are more accurate if the load varies from exactly 50 ohms. Of course, you calibrate them when you build the rig, and the accuracy depends on what you use to calibrate them! Also, as someone else has pointed out, the accuracy of a wattmeter with a 200 watt scale is quite poor at the low end. Bird rates their meters at 5% of full scale, so if the Tentec is the same then it could be off as much as + or - 10 watts! Because of the nonlinearity of diodes in the low part of their curves, usually such meters are worse at the low end. The OHR WM2 is a good and accurate QRP wattmeter. A calibrated scope or DVM with RF probe and a known accurate 50 ohm load can also be used. -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ 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 |
Re: Re: K2 power meter/wattmeter question
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In reply to this post by Mike Monger
I have the OHR WM2 and a Bird 43 with a 50W 2-30 MHz slug. Both the Bird & the OHR showed about the same power at 10W out of the K2 on all HF bands into a good 50 ohm non-inductive load. Power settings at various power levels from 0.1 to 10W going through the OHR WM2 show pretty good results across the whole range. I haven't measured it on any lab grade power meters that I have at work yet, because frankly, it just doesn't matter.
I notice power settings that appear to be off by as much as 0.5W at 5W set power on the K2 but that is going through an external antenna tuner to the antenna. I think the "error" is more in the slight VSWR mismatch causing measurement problems. The power readings are only accurate into a non-inductive 50 ohm load. If you convert your measured power to the set power ratio to dB it is likely a very small amount. For instance if the apparent measured power is say 6W and the K2 is set for 5W, this power difference is only 0.8 dB. For "ham" requirements this is likely fine, but for "lab" or "professional" measurements this may not be acceptable. An old saying "Don't sweat the little stuff" may apply here. Mark, NK8Q K2 #4786 > >The OHR WM2 is a good and accurate QRP wattmeter. A calibrated scope or DVM >with RF probe and a known accurate 50 ohm load can also be used. > >-- _______________________________________________ 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 |
Re: K2 power meter/wattmeter question
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In reply to this post by N8LP
Guys,
If you read the specifications carefully and do the math, at 10% of scale the reading can be off by a huge percentage even with a fine instrument like a Byrd. Rare to see proper slug for HF qrp for a Byrd. Wayne K9NE ----- Original Message ----- From: "Larry Phipps" <[hidden email]> Cc: <[hidden email]> Sent: Thursday, April 14, 2005 11:52 PM Subject: Re: [Elecraft] K2 power meter/wattmeter question > > Mike, I wouldn't put much stock in any wattmeter, including Bird, at 10% > of full scale. > > Larry N8LP > > > > Mike Monger wrote: > >>Hi Fellow Elecrafters, >> >>How accurate have you found the internal power meter/wattmeter to be on >>the K2??? >>I ask this because there is a large difference between what I set the >>power for, on the K2, and what it displays as compared to the external >>meter (contained _______________________________________________ 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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