RF Wattmeter 101

[W7TFO]

A good and concise treatise on Wattmeters:

http://www.thebdr.net/articles/ops/xmtr/MeasuringRF.pdf

73 dg

[k4kyv]

The diode detector type of power meter, typical of the hammy hambone wattmeters, will not accurately measure average power for anything except a steady carrier (unmodulated carrier, FM, RTTY or slow speed CW), because power is a square-law function of voltage or current whereas the diode detector responds more or less linearly, assuming the signal is great enough to push the diode into its linear region of conduction.

The simplest thermal device is the old fashioned thermocouple RF ammeter, but as the article mentions, it suffers from a limited range of measurement.

Note that most "power meters" are really voltage or current measuring devices, with scales calibrated in watts, assuming some arbitrary resistive (non-reactive) load.  In most cases, this arbitrary load is 50 or 72 ohms.

The word "average" makes this confusing to a lot of people.  Average, or mean, power is an indicator of real power, the actual heat-generating capability of alternating current. It is calculated by taking the product of rms voltage and rms current.  Rms voltage or current is the equivalent DC voltage or current that would have the same heating capability when working into a pure resistance, as the a.c. voltage or current being measured.

Average voltage or current is not the same thing as rms.  The average value of a sine wave voltage or current, or of any a.c. waveform passing through a transformer or coupling capacitor is, by the laws of physics, zero.  The average voltage applied to a plate modulated transmitter is the same as the unmodulated DC plate voltage, since the positive and negative modulation peaks average out to zero.  That's why a Bird 43 does not kick up when audio is applied to a full carrier AM transmitter.

What can make this confusing is that average power is not average voltage X average current, but rms voltage X rms current.  There is no such thing as "rms power".

An diode type of meter does not accurately read average power output with a SSB signal.  The reading is much lower than the real average power output, because the diode rectifier is not a square law device.  The wattmeters discussed in the article are true square law indicating devices.

http://www.measuretronix.com/products/bird-apm-16

[W3SLK]

Correct me if I'm wrong here but isn't a Bird 43 a diode detecting device?

[k4kyv]

Correct me if I'm wrong here but isn't a Bird 43 a diode detecting device?

Yep, it is.

Bird's model 43 and most other wattmeters available today were designed to measure power of constant amplitude, sinusoidal waveforms. Modern wireless communication systems can use a variety of digital techniques to combine many voice data channels into a complex, composite RF signal. Measurement of such signals with a conventional wattmeter may yield unacceptable errors.

http://birdtechnologies.thomasnet.com/item/elements-7-8-line-section-elements/categories-wattmeters-and-line-sections-wattmeters/apm-16?&plpver=10&origin=keyword&filter=&by=prod

[KX5JT]

I wonder how well those "peak reading" addon boards work?  Anyone ever add one on?

[w1vtp]

The following was taken from this url:

http://cp.literature.agilent.com/litweb/pdf/5965-6630E.pdf

"Watt
The International System of Units (SI) has established the watt (W) as the unit of power; one watt is one joule per second. Interestingly, electrical quantities do not even enter into this definition of power. In fact, other electrical units are derived from the watt. A volt is one watt per ampere. <snip>…
Three methods of sensing power
There are three popular devices for sensing and measuring average power at RF and microwave frequencies. Each of the methods uses a different kind of device to convert the RF power to a measurable DC or low frequency signal. The devices are the thermistor, the thermocouple, and the diode detector."

When I'm really interested in measuring power, I use a calibrated power sensor with an accurate attenuator.  The rest of the methods are approximation techniques at best.  When I retire I may invest in one of these power measuring systems.  I already have the accurate 30 dB attenuator.  Meanwhile, I have no allusions about the approximate accuracy of my Bird 43 which assumes a design impedance

Al

PS:

http://www.bird-technologies.com/techapps/app_notes/RF-Power_MeasurementTechniques.pdf

PPS:  This is all good reading -- the thread that is

[ke7trp]

Don does not like the bird.  We get that.

The Bird 43 peak board works quite well.  It works on a 2.828 RMS to peak scale. The big Drawback is that it takes two 9 volt batterys and if you leave the damn thing on, Your going to be buying more 9 volts. Its a nice tool to have when tuning and adjusting the transmitter. It agrees with the O scope dead on for modulation to carrier ratio.

There are a couple of lower priced peak boards on the market.  Both work ok and do not have the battery problem. The newer design Draws less power. 

The only Drawback to these types, Is that they are VERY voltage picky.  If you have dead batterys, Your readings will be lower. So its important to use Fresh batteries OR a REGULATED power supply to power it.  A wall wart wont work. The house or shop line voltage changes to much for steady readings.

One of the kits includes a neat feature.. It allows you to place a switch as a multiplier for a single slug.  A 50 can then be used as a 100, or 250 watt. ect..

 

Clark

[k4kyv]

Don does not like the bird.  We get that.  

You are making an unwarranted assumption. I have nothing against the Bird.  The 43 is probably the best of the diode-rectifier type "watt"meters (and its price reflects that).

What bothers me is the Hammy Hambones who think there is something infallible or divine about the Bird 43... kinda like the ones who kneel down and pray to the SWR god three times a day.

The 43 is fine for measuring the power of an FM, CW or RTTY signal, or the carrier power of an AM signal, but it is not designed to measure the power of signals that do not have constant amplitude, sinusoidal waveforms.  This is per company literature:

Bird's model 43 and most other wattmeters available today were designed to measure power of constant amplitude, sinusoidal waveforms. Modern wireless communication systems can use a variety of digital techniques to combine many voice data channels into a complex, composite RF signal. Measurement of such signals with a conventional wattmeter may yield unacceptable errors.

"Complex composite waveforms" includes AM and SSB phone, as well as multiplexed digital/analogue voice and data signals.

Bird's JS add-on allows the 43 to accurately measure peak power, but this tells very little about the real power output, i.e. the average or mean power of signals that may vary widely in peak-to-average ratio.

The Bird APM-16 would be a more appropriate choice for accurate power measurements that could include all modes of emission regardless of waveform. It is important to remember that one of the fundamental requirements for all of these meters is to have a 50Ω coaxial transmission line somewhere along the way between transmitter and antenna radiator.

[ke7trp]

The math from the Birds RMS reading of AM modulated signal, works out exactly with the peak reading of the bird kit. Do the math and find out.

C

[k4kyv]

The math from the Birds RMS reading of AM modulated signal, works out exactly with the peak reading of the bird kit. Do the math and find out.

RMS?  I assume you are talking about current or voltage?  There is no such thing as RMS power.

[K5UJ]

I just use the 43 into 50 ohm dummy to get carrier then do rough calculation for peak.  My 43 is way off what I get by calculating carrier power based on plate current, plate v. and 30% efficiency (which does square with other watt meters I have)--off by 50 watts.  Stock meter on front panel of amp says 250 watts.  Bird says 200.  Way more than 5% off.  I am going to put the bird right at the output of the amp and see if it changes.  Got the 1 KW slug for it out in flea at Dayton last year.  That may have been a boo boo.  If it's off, then I've been running around 350 w. when I thought I was running ~300.   That might explain some arcing 

But anyway, I'm wondering if anyone has had any experience with the Coaxial Dynamics meters?   Wondering how they shape up to Bird.  I've always wondered about them, well, for a few years now at least.

R.

[ke7trp]

I have owned Coax Dynamics. They work fine.  The trouble with these meters is the slugs. unless you have good slugs, the readings can be way off.  I have several of the same rating slugs, Some are low, Some are high.  I took the time to have my 43 calibrated and modified by Dennis #12.  Its certified acurate. When I find a slug is out, I get another one.  Accidents happen.. I keyed the amp up with the 50 watt slug in and got a few words out before realizing my mistake. That slug, was toast. I got another one...

From carrier to modulated AM to peak, My Bird is 100% dead on.   To be honest though. I just use it as an indication that the transmitter is doing what it is supposed to do.  The tuning really occurs when watching the O scope, plate, and Grid meters. Its also a Great indication of reflected power. On transmitters that can be mistuned on a harmonic, A good reflected power meter is important.

Clark

[Jim, W5JO]

But anyway, I'm wondering if anyone has had any experience with the Coaxial Dynamics meters?   Wondering how they shape up to Bird.  I've always wondered about them, well, for a few years now at least.
R.

Over two years back I compared the Coaxial Dynamics to this one:

http://www.telepostinc.com/lp100.html

It has been flawless.  A friend who had a Coaxial Dynamics for the past 2-3 years bought an LP100 because of flaws in the Dynamics.

Shop carefully and read the technical specs.

[K5UJ]

Thanks for the information on the slugs etc. Clark--Looks like I'll be taking my 43 to the Bird booth at Dayton and find out if they can calibrate it.  It's an old one, probably 40 to 50 years old.  Probably have to get a new slug.

R.

[ke7trp]

You can do it yourself if you dont care about the slugs seal.  There are articles out there on how.  But yeah. Its best to have it calibrated proffesionaly.

C