On 7/22/2014 5:03 AM, Carl Moreschi wrote:
The LP-100A has two connections to the sensor. One is for current and
one is for voltage. Power is simply the product of voltage times
current. So for any load impedance, power will always be calculated
correctly.
Most less expensive meters only measure voltage. They calculate power
as Voltage squared divided by 50. They always assume the load is a
pure 50 ohms. But with a 1.5 swr, you could have a 75 ohm load or a
33 ohm load or a complex load with both real and imaginary
components. If you have a 33 ohm load, your power will read high. If
you have a 75 ohm load, your power will read low. The point is unless
the swr is exactly 50 ohms, your power measurement will be wrong by a
lot.
There's a third component of the power computation -- the PHASE angle
between the voltage and current, and the LP100A measures that too.
On 7/22/2014 4:37 AM, K8JHR wrote:
Gee... Sorta condescending and demeaning. And not particularly helpful.
I often forget ham radio is only for experts with careers in electrical
engineering - the rest of us hobbyists are merely tolerated, perhaps
only because we buy stuff and keep prices down.
You've got an Extra Class ticket. This is NOT advanced EE stuff. The
equation for the computation of power is covered in the earliest chapter
on the fundamentals of electrical circuits in the ARRL Handbook. If you
bothered to study that (as opposed to cramming for the exam questions)
you would know that.
OK, I read the N8LP web page and, I am sorry because I do not see where
it addresses Carl's point about some meters being less accurate if the
load varies from 50 ohms, or how the LP-100 is immune from that problem.
Perhaps someone can explain this on the E-brand reflector... As far as I
can see, both units might be affected similarly as they will be used
exactly in the same way. I suppose I missed it, but I tried.
Like I said in my earlier post, N8LP's home page has a rough description
of the LP100A. To see detail and the specs, you must click on the LP100A
link.
And while I am at it, this whole "contest" thing seems a bit overblown.
How many contests have been lost because the the operator could not
determine amplifier power with sufficient accuracy?
With an SWR of 1.5:1, the error in the power calculation would be +/-
1.8 dB. Several years ago, N6ZFO did some statistical analysis showing
that 1dB translates into a 2.6% increase in our score in the ARRL
Sweepstakes. So that 1.8 dB error becomes a 4.5% better (or worse)
score. In the recent WRTC competition, the difference between 1st and
2nd place was 3.7%, between 2nd and 3rd was 4.3%, and between 3rd and
4th was 1.3%.
When you're loud at the other end, a couple of dB doesn't matter, but
when you're close to the other guy's noise level, it's the difference
between a QSO and no QSO, or between getting him on the first call, or
several calls. That translates into score.
73, Jim K9YC
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