Anytone AT5555N odd power meter behaviour

Having plugged a watt meter into the back of the rig, the on board power meter appears to be reasonably accurate.   However, the weird thing is, if I turn the amp on, it reads high.  The aim is to put 8 watts into the amp.    When I set the wattage at 8, confirmed with watt meter inline before the amp, you turn the amp on and the onboard watt meter shows 20.   I'm not overly bothered about this.  I am confident that I am not over driving the amp as I trust my external watt meter.   I have made a tippex mark so that I know where to set the power with the amp on.   There is no way the radio is chucking 20 watts into the amp, the knob is barely turned from it's lowest setting.   Even the external power meter reads a couple of watts high when the amp is powered on.   Not overly worried but I wonder if anyone knows what is going on here?

The metering functions within your CB are a marketing afterthought or sales pitch usually made as cheaply as possible and hence won't be an accurate instrument. An external meter will usually provide a better reading but this depends on the manufacturer and what measurement circuitry they utilised. The easy way of knowing is price. (A more expensive meter is usually more accurate, the additional components required and calibration adds to the cost.)

Regardless, all RF power meters are simple RF voltage measuring devices. A voltage reading isn't much use to us and to calculate the power in watts we also need to know a resistance. The 'resistance' in this case is the resistive impedance Z and as we know the typical value for coax and the output of our radio is a nominal 50 ohms. Therefore the meter is calibrated to 50 ohms.

Taking Ohm's Law and utilising our resistive impedance Z of 50 ohms as R,

V = SQRT ( P * R ) or the square root of Power in Watts multiplied by the Resistance in ohms, therefore :

V = SQRT ( 8W * 50 ohms ) = SQRT ( 400 ) = 20 Volts

This for your example of 8 Watts.

So when your meter is reading 8 Watts it is actually reading 20 Volts, (in this case RMS Volts).
This will only be accurate if the impedance is a true 50 ohm, a decent dummy load being the only way of accurately measuring your actual power. If your impedance varies from that nominal 50 ohms we get different results.

So taking Ohm's Law again but for the case of P or power and utilising your 8 Watt example with what we've already calculated :

P = V^2 / R or the Voltage squared divided by the Resistance, therefore :

P = 20^2 / 50 = 400 / 50 = 8 Watts.

This confirms our first calculations. 

If the resistance changes we will get different results, let's say a nominal 15 ohms either way :

P = 20^2 / 35 = 400 / 35 = 11.43 Watts for a 35 ohm load

P = 20^2 / 65 = 400 / 65 = 6.15 Watts for a 65 ohm load

So if our impedance is lower at say 35 ohms the output wattage will be higher and conversely a higher impedance will give us a lower wattage.

Now, is the actual Wattage higher or lower due to actuality, or errors in our meter measurement?

That's where the real fun of uncertainty comes to play......and we haven't even touched base on the reactive components of L and C, (Inductance and Capacitance), that make up our impedance of Z let alone RF current. (The only way of gaining a more insightful answer.)


All gets a little complicated doesn't it?


What you've seen is that when the linear is 'off' your meter is giving a reading based on what comes after the linear as it is bypassed, so I assume your antenna. When the linear is 'on' it's circuitry is now inline and you are reading a higher value. From the mathematical examples above we can assume that the input impedance is lower on the linears than it is on your antenna.

That, is all we know for certain.

Measurement is one thing, knowing what we're measuring is quite another.



Anyway, all of the above was a good bit of fun for my grey thinking matter even if it only proved my brain is still working. If anyone learned anything from my musings to this question then I'm pleased, as that was my only intention. 

Besides, despite knowing the mathematics to such things I much prefer working by 'rule of thumb' and 'er' on the side of caution. It's far less problematical and usually gives you less of a headache.


All the best,
Victor

Hi Victor, thanks for this detailed explanation.   Although I must confess that I still find Ohm's law a bit puzzling, but I understand the basic premise of what you are saying.   At least I know I can safely disregard the inbuilt watt meter and rely on my Tippex mark.   I used the amp for rather a long time last night and it was fine.   This old Zetagi B507 I am running quickly lets you know with aromas when you are over driving it!

No worries, it was fun and you've obviously got the gist of it.

It can all get complicated pretty fast and lead you up the garden path, which is why common sense can be far more important. You know you don't want to blow your linears and definitely don't want to cook them so took precautions to prevent it. Far more important at times than thinking you know the maths. 


All the best,
Victor