POWER METERS FOR THE VHF’er

By Dave Hallidy K2DH

I don’t think a day goes by when some VHF ham isn’t wondering how much RF his transmitter is putting out. In order to determine this, probably the most important piece of test equipment we can have in our shacks is a power meter- or better yet, several different ones, each designed for a specific purpose. In this discussion, I’ll talk about some of the popular power meters, the usefulness of each in the ham shack, availability, and the up and downsides of each. I’ll also make my recommendations as to what you might consider having in your own shack in terms of RF power measuring equipment.

There are many different types of RF power meters out there. Here are my notes on some of them.

BIRD Model 43 Thruline Wattmeter

  • The first on my list, and probably the one we’re most familiar with (and I bet most everyone has at least one in their shack) is the Bird model 43 Thruline Wattmeter and its associated “slugs”.
  • The Bird does a good job of accommodating many needs by offering plug-in detector units (“slugs”) for many different frequency and power ranges from 450kHz to 2.7GHz and from 100mW to 10kW. Just plug the right slug into the line section and read the power! Well, it’s just about that simple.
  • The upside of these meters is their simplicity of use- insert the line section into the transmission line, rotate the slug so it’s pointing toward the load end of the line and give it some RF. It will read the Forward power in Watts. Rotate the slug 180o, put the same amount of RF into the line and it will read the Reflected power in Watts. From this you can easily determine the VSWR.
  • At low frequencies (HF through VHF) this is fine and the accuracy is pretty good- Bird claims +/-5% of the full-scale reading as the accuracy of the model 43. You can leave the line section in place all the time- it uses very little of the available RF in the line, so you can get a continuous readout of your transmitter’s power output.
  • The downside is at UHF and above. The line section can (does) add a slight impedance “bump” in the transmission line, which can make certain amplifiers behave abnormally, and it can (does) add some extra loss to the line. Also, depending on where the line section is placed along the line, you can get completely bogus readings (high OR low) because the line section is actually a significant part of a wavelength at UHF and above.
  • Also another downside, I suppose, is that it can’t be used to measure really low power levels, such as you would find at the LO of a transverter, or the output of a MMIC stage that drives a TWT or other high gain amplifier.
  • The Bird 43 is not cheap- the basic meter, line section, and case with no slug is in the range of $400 new. New slugs are over $100 each. Surplus, we can usually find the meter/line sections for $150 or so, and used slugs for around $50 with no guarantee that they’ll be anything close to accurate- dropping a slug is a sure way to “uncalibrate” it!

HP-430, 431, 432 Microwave Power Meter Family

  • The models in this series of power meters are actually called “Microwave Power Meters” by their manufacturer, Hewlett-Packard (now Agilent). But don’t let the name fool you- they can be used not only at microwaves, but at HF, VHF, and UHF as well. HP (Agilent) has come out with a whole series of power meters in the 4XX family over the years, but the later ones, notably the 435, 436, 437 and beyond are generally priced out of the range of the amateur, so I’ll concentrate on the ones that are the most affordable.
  • The “trick” to these meters is that they were designed to measure very low levels of RF, from –20dBm (0.01mW) to +10dBm (10mW). They all use some sort of a device that detects the presence of heat energy to change the balance in a bridge circuit, the imbalance causing a meter deflection on a meter face calibrated for milliwatts of RF power. All of these meters use a remote sensing device or “head” on a cable, and the head is connected to the source of RF. The head is a 50ohm load and the thermal device in the head is coupled closely to the load. As the load changes temperature with the presence of RF, the thermal device changes its resistance. As you might imagine, this is a very sensitive device (being capable of measuring the heat generated by <10uW), so it’s very important that the head be well-cared for (more on this later).
  • The HP43X series power meters are very accurate- in fact, the HP432, which originally came out in the 1960’s, is still in production today, costing nearly $4000. The reason it’s still made is that the meter reading can be verified by substitution of a precision DC voltage source, thus providing far more precise calibration than any other type of thermistor power meter on the market.
  • The oldest model was the HP430- it contained vacuum tubes and first came out in the ‘50s. It was replaced in the early ‘60s with the HP431, which was solid-state. The 432 replaced the 431 in the late ‘60s/early ‘70s with more design improvements and a more compact case. The 431 and 432 use a different style sensor from the 430. The 430 simply had a resistance that heated up and unbalanced the bridge in the meter. The 431 and 432 actually have two thermistors in the head- the one for actual measurement of RF and another, connected to another bridge circuit, to detect changes in the ambient temperature of the head, providing greater stability on the highest sensitivity scales. You could not hold on to the head of a 430 while trying to measure very low levels of RF- the heat from your hand would peg the meter! This problem was partially solved in the 431, and completely solved in the 432 (which also contained an auto-zero circuit).
  • I think the greatest upside to these meters is their ability to measure very low levels of RF. If you want to measure the output of the local oscillator in your transverter or the drive level to your final amp, you can- very accurately.
  • Some might say that the next feature is a downside, but I don’t think so. If you want to measure higher power than 10 milliwatts, you must use one or more attenuators such that whatever the expected power level is, by the time it goes through the attenuators, it doesn’t exceed 10mW (+10dBm- the max the head can withstand). Otherwise, damage to the sensor may result. For example, if you’re trying to measure 10W (+40dBm), you would need 30dB of additional attenuation (and rated for 10W) ahead of the sensor/meter (which is capable of reading +10dBm) and then adding 30dB to whatever reading you get on the power meter.
  • You can also use these meters in line continuously to measure your transmitter’s performance and feedline VSWR by using a directional coupler in the line and connecting the power meter to the appropriate port of the coupler. Here are a couple of pictures of typical directional couplers, both coaxial and waveguide:
  • RF passes through the coupler with very little attenuation and a portion of it is tapped off on the coupled port (the port at right angles to the straight through portion in these images). If, for example, the coupled port is –20dB from the straight through (and the coupler should be marked with this “coupling factor”, usually over it’s entire operating range), then to read the 10W we used in the previous example would require only another 10dB of attenuation in addition to the –20dB of the coupler (+10dBm for the meter, 10dB for the additional attenuator, 20dB for the coupler=40dBm full-scale power on the meter). Some couplers have two coupled ports, for reading both forward and reflected power simultaneously. But if it only has one port, you can still read the reflected power, simply by turning the coupler around in the feedline.
  • Here are some tips for picking out a microwave power meter in this family: Try to stay away from the HP430. It is about 50 years old, uses vacuum tubes, and except for the highest power scales, is pretty drifty with changes in ambient (or sensor) temperature, as I mentioned earlier. If the price is really good though, it might still be worth considering as a power monitor in your transmit feedline with a directional coupler on the highest power range. The 431 is better than the 430, but still tends to be a bit unstable on the highest sensitivity scales. It and the 432 use the same sensors and cables. The HP432 is the best of the lot- compact, much more state of the art design and stability, and compact. The HP430, with a sensor and cable (it just uses a chunk of RG-58) should cost no more than $20-25. An HP431 (just the meter- no cable or thermistor) will run about the same. An HP432 without cable or sensor will typically be less than $50. For the HP431 and 432, the most common thermistor (RF sensing head) is the model 478, which is coaxial (type N) and covers from 10MHz to 10GHz. These can typically be found for around $50 with no guarantee of condition. Be suspicious of a really cheap one- it’s probably been hurt, or worse. The cables that go between the head and meter are very hard to find- you’ll be lucky to find one for less than $50. All that said, sometimes you’ll find a complete unit- meter, cable, and thermistor for less than $100. A good quick check of a 430, 431, or 432 (with cable and sensor) is to plug it in and see if the electrical zero works. If the bridge circuit zeroes (the meter can be adjusted electrically to zero from controls on the front panel), then you can buy it and be sure it will work, at least to amateur accuracy needs. If it won’t zero, then either the meter or (more likely) the sensor is damaged. By the way- there are many different sensors available, covering the range all the way (in waveguide) up to 100GHz.

BOONTON Model 4200 Microwatt Power Meter

  • While not nearly as well-known as the HP series of power meters, the Boonton 4200 is probably the “Cadillac” of affordable microwave power meters.
  • Like the HP’s, there are different sensors available to cover different frequency ranges, up to 100GHz.
  • Unlike the HP’s, there are also different sensors to handle different power ranges (up to 5 or 10W direct).
  • Also unlike the HP’s, the 4200 can read power all the way down to –70dBm (50dB more dynamic range than the HP432).
  • The 4200 has a digital display, which for continuous power level monitoring is good, but for tuning and peaking a circuit is not (I like the fact that you can see an analog meter respond, even if it doesn’t stay up long enough to read the peak- the digitals take time to react).
  • The 4200 has a built in calibrator (HP did this in later models, after the 432).
  • The 4200 is at the top of the affordability range, in my opinion. Typically, on eBay, they run in the $400-500 range, with cable and sensor, and if sold by a reputable test equipment house, as much as 5-10 times that price! Nice meter for the workbench, but maybe a bit of overkill.

DEMI All-Band Power Meter (ABPM)

  • Last, but certainly not least, I’d like to mention the ABPM or “All Band Power Meter”. This unit may be the “sleeper” of all time. Our friends at Down East Microwave, working with Paul Wade W1GHZ, have come up with a complete kit (case included) for $75 that will allow you to measure at least relative power all the way up to 24GHz. Paul needed a way to be sure that when he was portable or roving, his transmitter was making power. He came up with this design, which is really sort of a glorified field strength meter.
  • It uses a couple of state of the art power detector IC’s- the AD8307 and the LTC5508 to provide coverage from 10kHz to 10GHz. The outputs of the chips can drive a DMM, or for quick checks, it is set up to illuminate the bars on an LED bar graph display (part of the kit).
  • Because of the quality and repeatability of the response of the IC’s, with careful calibration this unit can be used (with appropriate external attenuation) to measure absolute power in dBm.
  • It uses a 9v battery for power and can be connected to a small sense antenna (as in the picture) for use in the field for a quick check of why the other guy isn’t hearing you.
  • It has an absolute power range from –70dBm to +5dBm on the low frequency end and from –20dBm to +13dBm on the high frequency end. And while it’s only spec’ed to work to 10GHz, it’s been confirmed to work at 24!
  • I’m so impressed with this unit that I’m going to buy one, and I actually think it might make sense as the next RVHFG club project.

So, to sum up- my recommendations for RF power measurement in the ham shack are as follows:

1)A Bird model 43 with a few of the slugs you need to measure VHF RF is probably a really good idea. Yes, it will cost considerably more than a “Japanese special” do-all wattmeter, but you get what you pay for.

2)If you’re thinking about a microwave power meter, to do work on projects or to use as a power monitor/VSWR meter, I’d spend the money on an HP432. There are always plenty on eBay. Remember- you’ll need attenuators rated for the power you want to measure, and/or directional couplers. Yes, the money adds up- but again, you get what you pay for.

3)I think we all should have one of the new DEMI ABPM’s- I think at $75 and a little bit of construction it’s a steal, and will serve rovers/portable stations really well, and the folks at home will know that their transmitters are working, too.

There you have my take on RF Power Meters. I welcome any comments or questions about this subject, and if the consensus of the club is to make the ABPM a club project, I’d be happy to head that up.