[Elecraft] K3 Watt-meter question!

[David Yarnes]

Hi All,

Over the years I've seen Bird Wattmeters for sale at dozens 
of hamfests, as well as over the internet.  Typically these 
are very "pricey", and require all sorts of accessories.  I 
have no doubt that this is, more or less, "the standard" 
when it comes to wattmeters.  However, my question is (and I 
ask this seriously), just how often am I apt to really need 
something that is so precise?  I have an Elecraft W1, and 
several other wattmeters around the shack, including an OHR 
W-1.  I think the two I specifically mention are relatively 
accurate, although perhaps not quite up to "Bird" standards. 
All wattmeters need to be calibrated, and need to be 
re-calibrated if you change ranges significantly.  To be 
honest with you, the only time I have any real concern about 
being 99.9% accurate is if I'm crowding the 5 watt limit in 
a contest.  My solution is to just set it for 4.5 watts, and 
my conscience is clear!  Otherwise, whether it's 5 watts vs. 
5.2 watts, or 100 watts vs. 101 watts, doesn't seem to make 
much difference.  Should I care????

Dave W7AQK


----- Original Message ----- 
From: "Ron D'Eau Claire" <ron at cobi.biz>
To: <elecraft at mailman.qth.net>
Sent: Friday, May 29, 2009 9:10 AM
Subject: Re: [Elecraft] K3 Watt-meter question!


> Jim, you said "supposedly accurate". A Bird wattmeter is 
> accurate only if
> it's been properly calibrated and handled carefully - 
> especially the
> "slugs". Bumps and bangs can easily ruin the calibration 
> (That's why all
> Birds used in critical applications carry calibration 
> stickers showing when
> it was last checked - normally no less often than 
> annually - and is
> carefully handled and stored in approved containers).
>
> Let's assume the Bird is recently calibrated and treated 
> kindly:
>
> 1) Its calibration is then dependent upon using the right 
> slug. You probably
> know better, but a surprising number of Bird owners think 
> the frequency
> range on the slug isn't very important. I believe the 
> proper Bird 43 element
> for 6 meters is the type A slug spec'd for 25-60 MHz.
>
> 2) Are you allowing for the normal error in even a 
> perfectly-calibrated
> Bird? That's +/- 5% of *full scale*. IIRC the Bird 6 meter 
> slugs have full
> scale ranges of 25 or 250 watts. If you're measuring 100 
> watts with the 250
> watt slug, the possible error in a properly-calibrated 
> Bird is +/- 5% of 250
> watts *anywhere* in the scale. So at 100 watts indicated, 
> the actual power
> may be anywhere between 87 and 112 watts.
>
> That's plenty good enough accuracy for nearly all field 
> use, including for
> showing that transmitter power levels comply with FCC regs 
> (although when I
> demonstrate a marine transmitter to FCC engineers, it's 
> *their* Bird that is
> the final word ;-)
>
> Now, the K3 has simply another wattmeter built in, so it's 
> normal for the
> two wattmeters to disagree by the sum of their possible 
> errors.
>
> You asked, "Which one is more accurate?" One of the 
> Elecraft engineers would
> have to answer that, but let's assume the K3's built in 
> wattmeter is as good
> as the Bird. In that case a perfectly calibrated Bird on a 
> perfectly
> calibrated K3 may disagree by as much as 15 watts at 100 
> watts giving you an
> indication of anywhere between 85 and 115 watts on the 
> Bird when the K3
> meter says 100 watts.
>
> And don't forget that assumes everything is in perfect 
> calibration. In the
> real world, it's not unusual to see a greater error.
>
> That's why it's not normally considered important for 
> owners to worry about
> the K3's power calibration. Unless you have some 
> laboratory standards to
> compare to each other and to the K3, it's almost 
> impossible to improve on
> the default factory settings.
>
> Ron AC7AC
>
>
>
> -----Original Message-----
>
> Gentlemen,
>
> How accurate is the built-in watt meter indicator in the 
> K3???
>
> I'm asking because when I've used a supposed;y accurate 
> "Bird 43" while
> measuring the K3's various
> outputs on the various HF bands there seems to be a slight 
> discrepitancy
> between what the K3 is
> showing and what's actually there according to the 
> "BIRD!!!"  I'm
> particularly interested in the
> output readings as concerns "6 metres!" Which is MORE 
> ACCURATE???
>
> Regards,
>
> Jim/nn6ee
> S/N 2406
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: 
> http://www.qsl.net/donate.html
>