CIrcuit to measure high variable output power power from PA

[spacehen]

Hi all,

I was just wondering what technique is commonly used to measure relatively high output power that varies from say 0 to 300W from a PA stage?

 I know directional couplers are usually used to sample a small fraction of the input power (which then feeds a power sensor) but I see most directional couplers are only rated for 1-10 watts of input power. My idea is that one could instead measure the output of the pre-amplification stage which is around 1-3W. One would then do some measurements of the output power for various levels of preamplifcation drive and then extrapolate from there in software. Is there a better way?

Thanks in advance  .

[jwet]

Most wattmeters use some kind of somewhat crude coupler.  The general type is called a Breune circuit.   In the ham bands, the usual circuit is a coax going though a toroid with a few turns of wire to sample the current.    The Coax is a 1 turn primary, and the toroid is a secondary.  You can then use a voltage sampler using a capacitive divider and sum the two- transmission line Voodoo math makes it all work.  If you then add diodes and a meter, you can make a directional wattmeter.  I googled a figure below to give you an idea of what it looks like.  Look in any ARRL handbook.  They aren't hard to make and work pretty well.  At higher frequencies, VHF, you can make the sampler on a piece of PCB with the main line going though and secondaries on adjacent pcb lines.  ARRL HB covers these too.  Fancy directional coupler like Mini Circuits are for lab measurements, etc.  You can buy a cheap watt meter from MFJ made with either of the techniques above for less than $50 I believe.

[rfdave#gmail.com]

You could buy a directional coupler that’s rated for 300 watts and get an actual measurement

[geggi1]

If its only the power you want to measure its possible to use a rf-tap.
There are different designs of these resistive, inductive and capacative.

[Wallace Gasiewicz]

Lots of cheap power meters for 300 watts. usual ones can easily go to 5 KW. Old Heathkit and many other brands. They are not very accurate but usually that is not necessary,
For HAM Radio there are meters that read just CW and some also read Peak Power. Most newer ones read forward power and reflected power in both CW and Peak. Again not too accurate but works just fine.
The more accuracy you need the higher the price, of course.
Perhaps a local ham club has a classified listing. Do you have any Hamfests in your area? I pick up working power meters for $20.
Some of the old Linear Amps had a power meter built in and it was a RF bridge, or directional coupler, just really two parallel wires with a detector for powering the meter.

[radiolistener]

I was just wondering what technique is commonly used to measure relatively high output power that varies from say 0 to 300W from a PA stage?

You can put PA output into powerful 40-60 dB attenuator and use logarithmic detector to detect power on attenuator output.
0.1 - 300 W range is just 34.7 dB and can be easily measured.
You can also use some SDR with spectrum waterfall to measure output from attenuator.

[Wallace Gasiewicz]

Please tell us if you wish to monitor the power out of your radio to a Load or an Antenna or perhaps another amplifier?

Also any hints on the actual unit being tested, like frequency and output connection and if there is an antenna, feedline description.
Stuff like that.

This is just a very gross estimate of what I believe your costs would be depending on the way you want to measure your output.
In other words, how much do you want to spend?

For an in line wattmeter, count on maybe $20-50, Typical Ham stuff, not remarkably accurate, Or buy one from  MFJ (link below)
For a good 300W plus Load maybe $300 used, if you are just measuring HF and not concerned about being exact, you can pick up a cantenna for a few bucks or buy one from MFJ maybe $50

https://mfjenterprises.com/collections/meters

For a good attenuator at that power maybe $500 or more used. I have not found any good cheap ones at this power level.
For an accurate "terminated" wattmeter" that will measure the signal out of attenuator maybe another $300 to thousands.

You can spend thousands of dollars,or just a few. Depends on what you want to do and how accurate you need to be.
In most HF radio applications you do not need to be very accurate
In development of a new amplifier, you probably need to be more accurate.

[joeqsmith]

My idea is that one could instead measure the output of the pre-amplification stage which is around 1-3W. One would then do some measurements of the output power for various levels of preamplifcation drive and then extrapolate from there in software. Is there a better way?

For a known load, you could certainly characterize the PA and use the drive signal to infer the what the amplifier is doing.  Of course, any change you make will effect it.  The bigger problem is for you to characterize it, you need a way to measure the PA which it seems is what you are asking for help on.  So I would say in your case, your idea has a flaw.   

As Mr Wallace suggests, you would need to provide more detail.   

[spacehen]

Thanks all,

So just to clarify I am able to measure the output of the 300W PA stage (using an rf sampler and spectrum analyzer), it's just that I would like to design a circuit to measure the power dynamically while the PA is operating. I can certainly use a 300W rated directional coupler, it's just that they are usually quite large and there has to be a simpler method that will take less space. Commerical RF samplers are also quite bulky.

@radiolistener
This is a good suggestion but I suspect that you would then need to route all the RF power into the logarithmic detector/attenuator. I would like to measure the power while transmitting.

@Wallace
I would like to monitor the output of a PA that is driving a 50 ohm load (antenna)

The setup is also pretty simple:
RF source (output 1mW) -> driver stage (0.5 W) -> PA stage (output 300W) (measure here) -> antenna

[Kalvin]

Here is a simple, wide bandwidth RF power measurement circuit using a simple RF-tap:

https://www.qsl.net/sz1a/download/build%20an%20rf%20power%20meter.pdf

A simple RF-tap is shown in Figure 3, and the output J3 of this RF-tap is connected to a RF-power meter. For example, you can use the wide bandwidth RF-power meter shown in Figure 1, or any other traditional diode-based power meter. The RF-tap will sample the high-power RF-signal, and attenuate the sampled signal down to a signal level which safe/suitable for the RF-power meter.

The example circuit is for 100W, but it is simple thing to modify the required attenuation for 300W output power (see the text how the R1 is composed of multiple resistors in order to obtain necessary power dissipation capability. For example this 100W RF-tap is using three 0.5W resistors in series).

[geggi1]

If you build the RF tap and want a DC block using it with a spectrumananlyzer you can add a NP0 type capacitor on the centerpin of the BNC.
This will give you protection against any DC form the PA.