[TenTec] Model 1225 SWR and Wattmeter Kit

[Jim Brown]

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