Measuring Peak Power

 

A peak-reading meter is one which measures the peak value of a waveform, rather than its mean value or RMS value (like an average).

There are power meters sold that have switches labeled PEP or Peak, but MOST do not provide accurate PEP or Peak readings.   A true peak reading circuit can read the peak value and hold that value long enough to drive the meter even when the input signal is dropping. Most meters use a capacitor as a simple “hold” on the charge. But the peaks still only charge the capacitor to an average value, a value higher than when switch is in RMS /Average mode but not a true peak either.

All of the meters sold by Paradynamics, Dosy, Astatic and many others that say Peak or PEP on them will only read an enhanced “average” power. Generally they display about 60-75% of actual true peak power.  If you are set up for low dead key and high peak power these meters will be even further off and read only a fraction of your true peaks, more like 30-40% of true peak power. Even with this said, these meters are still of decent quality, relatively inexpensive and for most people its a good choice for personal use.

A Bird 43P meter (with an peak reading circuit installed) can hold the value and drive the meter to show true peak readings as verified on an oscilloscope. The peak RF voltage is input to an op amp circuit that will hold that charge for as long as it needs to.  The Bird meter is very expensive and not practical for most users. These meters are usually used by technicians and by users who operate at increased power levels (above 2KW) that can only be measured by these type of meters. 

The oscilloscope is the most accurate PEP measuring tool, as it will display the crest (peaks) of the modulation envelope independently of the rise-time or audio-frequency response. An o-scope is also very expensive and not practical for most users. These are usually used by technicians. Do to the quantity of surplus USED units out there, there could be more in regular use by serious users. The problem is they are large, complicated, have many controls and tend to scare of the average user.

Do you have to own a Bird 43P meter or an O-scope?
NO, a regular Dosy meter is perfectly fine for a consumer to use to monitor his equipment.  You do have to realize the limitations to average commercially available meters and know that your true peak power is more than you are seeing.

For instance…

• A tuned up Galaxy DX-959 (Mosfet version) can be tuned for a peak power output of 28-30 watts on the Bird 43P.  The same radio as measured on a Dosy meter would show 18-20 W. You must understand that the 20W you see is actually only 70% of  the true value. 20 / .70 = 28.57 watts. 

• Lets say you are trying to set up your linear amplifier for good drive and clean audio. To do this you know (and remember from old newsletters) that you want a 4:1 ratio power. So you want your peak power to be 4 times your dead key. First you read your peak output using a Dosy meter and you might read 300W.  So by knowing the limitations of the meter you decide that your true peak power is really about 400W and set your dead key to about 100W.

• Say your technician sets up your radio and amp and tells you it is dead keying 250 watts and peaking 1000 watts (perfect). When you get home and check out your meter you see 240W with peaks to 650-750W.  Did he do it right? Did you set everything up right? Is something wrong? What should you do? …..NOTHING, because you know the limitations to your meter to read true peak power. So you smile and say yep, that’s just about right. 

I only touched on a few basics, as you can tell by now I try to tone the tech-talk down a bit so I don’t lose any readers.  There are lot’s more things to know about reading power.  Like, did you know the best reading are done on a dummy load? Your antenna’s reactance can cause different readings than at the shop.