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Amateur Radio Info & Exams - Using a Bird 43 to determine SWR

This article covers how to determine SWR using a Bird 43 Directional Wattmeter, or related products.

About SWR

SWR, also called VSWR, represents a mismatch in an antenna system, expressed as a ratio between the desired and actual impedance.

In most cases an Amateur transceiver needs to see as close to a 50 Ω as possible. 1:1 is a perfect match. 2:1 means an antenna impedance of either 25 or 100 ohms. Non-resonant antennas will appear to have inductive reactance if too long, and capacitive reactance if too short. The other situation is that an antenna is resonant, but has a different impedance, such as a delta loop with a 102 ohm impedance. The number and angle of radials below a well tuned VHF or UHF vertical will also alter the impedance.

Common methods to determine the SWR include SWR meters, antenna analysers, nanoVNAs, and professional Vector Network Analysers, the latter costing A$5k to A$100k, and directional wattmeters, discussed below.

SWR meters

SWR meters are built around the directional coupler, a primary feed-line in parallel with a sense line. A network consisting of a diode and resistors is used to interface this second line to a meter movement. Where two sense lines are used, they are set up with the diodes at opposite ends, and they are selected by the FWD and REV switch, or they feed the two movements of a cross-needle meter. PCB stripline is frequently used in these; coax passing through a coil on a toroid is also an option.

Each SWR meter is specified for a range of frequencies, say 3 to 150 MHz, or 100 to 500 MHz; and a range of powers. They often include a power range switch. The vast majority are designed for 50 Ω systems, as are the Bird products.

Finally, in the case of meters with a FWD / REV switch, there is a Calibrate knob. Typical use is to select FWD, transmit on AM, FM, or CW, adjust the knob so that the needles sits on the CAL mark, and de-key. Select REV and transmit again, reading the SWR off the scale. Some include a forward power function.

Directional Wattmeters

The more professional instrument is the directional wattmeter, with the most famous being the Bird 43 Thruline Directional Wattmeter.

"Thruline" is used to differentiate from terminating wattmeters, which include a resistive load. That said, Bird does sell loads or termininators.

These consist of a grey box 100 mm wide, with rounded corners, consistent with its 1950s design. The RF connectors are on each side, linked by the feedline in a cast metal body. Either can be used for input and output. The 30 μA meter movement is on the upper front is round. Below it, in line with the connectors, is the port for the element. This allows us to measure the power going from the transmitter to the antenna; and to compare this with the reflected power. It has a black strap over the top, to act as a handle.

Left: This unit has a N connectors on both sides. The yellow warning symbol indicates that at higher powers if you were to insert your finger into the hole shown for the element, and touch the centre conductor, you could receive a shock, fatal at very high powers. The three rows of numbers end in 25 (implied), 50, and 100. These can also be read as levels such as 250 mW, 2.5, 250, 2500; 500 mW, 5, 500; 100 mW, 1, 10, or 1000 watts, depending on the element. Variants can read to 10,000 watts (10 kW). Below: With a 50 watt slug (element) inserted in this angled view. The hole above the connector can store a spare element.

You purchase the body which includes the meter movement, then add elements or "slugs" which plug in to measure specific power levels and frequency ranges of interest. A standard 43 is priced at US$753.

These slugs have a large arrow on them, and can be plugged in in either direction. They are marked with their power level in watts, and a letter indicating the frequency range. To measure several power levels several elements may be needed. The 43 uses elements which are 7/8″, or 22.225 mm in diameter.

An example: Say we had a 400 watt repeater: Assume we measure exactly 400 watts is going out. If we measure 5 watts coming back, that is quite a good figure, meaning an SWR of 1.25:1. The lower the return power, the better, and the lower the SWR.

To make the readings discussed above, for a 2 metre repeater, the 500C and 50C or 25C elements would needed. The forward reading is taken with the larger slug, with the arrow pointing from transmitter to antenna (load). Next, reverse this slug to check for excessive reflected power (excessive SWR), before fitting the low power slug, being extra careful to ensure the arrow points from load to transmitter. Both low power elements and the meter movement (US$378 replacement cost) can be damaged by incorrect insertion, and using a 10C in the above case would not be NOT recommended, as a poor a connection could cause a spike in SWR, and thus high reflected power, resulting in failure of these parts.

For repeaters on 70 cm, D or E suffix elements are needed. C elements are US$271 each; D are $259. E were US$274 with some covering up to 800 MHz, or 800 MHz to 1 GHz, so C and E-400 might be a better combination, depending on what other bands are also used, if you can get them used. With both A and B covering 6 metres, the choice may be whether 10 metres is also of interest, or the licensed mid-band spectrum used by WICEN is. Hams elsewhere may select elements based on the specifics of European 8 metre (40 MHz) and 5 metre (60 MHz) beacons and repeaters, 4 metre (70 MHz) beacons; or repeaters in the 1.25 m band in the Americas.

There are also high power, low power, and milliwatt elements, plus J series high UHF units, and the P series for the medium wave band, at broadcast powers.

As you can see, the same meter could also be used for testing a community broadcaster's AM or FM antennas, with them supplying just a new slug or two.

Calculations

The ratio of power, expressed as φ (Phi) is the reflected power over forward power, written as a number, or percentage.

A value of φ = 5 / 400 = 0.0125 (1.25%) is derived from the powers above.

SWR is calculated using ρ (Rho) thus: ρ = (1 + √φ) / (1 - √φ)

ρ = (1 + √0.0125) / (1 - √0.0125) = (1 + 0.11180339887) / (1 - 0.11180339887)
  = 1.11180339887 / 0.88819660113 = 1.25175371924, or a ratio of 1.25:1.

Note that the relationship between the 1.25 figures is coincidental only.

An alternative expression is:

We can use this for a second, high SWR calculation: Assume we read 100 watts forward, and 35 reflected (35%), which would only require a single slug:

ρ = (1 + √(35/100)) / (1 - √(35/100)) = 1.591607978309962 / 0.408392021690038
  = 3.89725531787681, meaning an SWR of 3.9:1.

Clearly, φ = 0.35, had you used that method.

Other models: There are several additional versions of the 43: The 43-UHF expands usage up the spectrum. The 43P, which reads peaks in pulse and SSB systems requires two PP3 9 volt batteries, as does the 4314C. There are several other 43xx and 44xx models. While the standard version has two N sockets, the 43-BNC uses BNC sockets. A wide range of “Quick Change” sockets and panel-mount plugs are available, which can be swapped into the meter body with just a screwdriver. They also sell high quality adaptors. Special Pulse elements are available, likely designed for use in things like radar systems, which have large peak power.

A benefit for professional and regulatory users is that the meters and elements can be formally calibrated. It can be used for permanent monitoring of forward and/or reflected power, with the option of removing the line section and placing it at the transmitter, and connecting its DC output to the meter, via RG-58 coax. Additional RPK43-3 line sections can be obtained for US$316, allowing forward and reverse monitoring. Other RF Line sections are available here: 7/8" RF Line Sections

The manual discusses testing cables.

I expect people developing linear amplifiers could use these meters to assess the input impedance seen by the transceiver at various frequencies, likewise check that the corrent power is flowing from the transceiver, and that amplifier output is correct, and at a legal level.

Coaxial Dynamics

With various patents expired, Coaxial Dynamics builds meters in a similar format, but in blue folded metal cases, and with large rectangular meters. One option allows two elements to be fitted, and has a switch used to select between forwards and reverse readings. Some feature a USB output to allow Forwards, reverse, and SWR to be displayed simultaneously. Another has an LCD display. They also have a "Web Wattchman" product. See: coaxial.com

They sell a full range of slugs, including the E series. They are compatible with Bird meters, while Bird cna fit there meters.

Links

Product information for the Bird 43: BirdŽ Model 43 RF Wattmeter - The Industry Standard

The manual includes graphs for determining lower and higher values of SWR (ρ) from the percentage of reflected power (φ): https://birdrf.com/hubfs/discontinued-manuals/920-4330_4331-rf-directional-wattmeter.pdf

You can read the relevant US Patent, 2891221 at: Google Patents: US 2891221

If the USPTO site behaves you can search by patent number at: uspto.gov

Other Bird products: The 4305A is a high power version, in a heavy sheet-metal case. Beyond the 43 series there are similar looking products, such as the the APM-16 average reading unit for things such as CDMA, which requires specific APM slugs; and remote sensor bodies, large diameter line sections for hard-line, using these or similar elements, rack mounted systems (one or two meter movements), and equipment which can be integrated into remote monitoring systems for radio stations. The 4410A is a high dynamic range unit, at US$1272.

Above: The GB3VHF Beacon on the 2 metre band, featuring a 4527, Dual Meter-Dual Element RF Wattmeter with Sampler Port at the centre. The other modules have been painted to match the meter panel. Full sized image

The meter unit, including panel, costs US$1401. It includes a 100C and 5C, for 100 and 5 watts, at 100 to 250 MHz, ideal for its 144.430 MHz frequency. You can also buy components to build your own metering system.

Read more about the GB3VHF and GB3UHF beacon system. The unit has been placed into a taller rack, incorporating the 70 cm beacon. It includes an additional 4527, with slugs suitable for its 432.430 MHz frequency.

The "Wattcher" series (the two Ts are a play on Watt) has rectangular meters, and allows interfacing to transmitter site monitoring systems. There are a wide range of other remote sensing systems for both onsite and offsite monitoring.

The new 4480A is a $4k digital display unit which does not need slugs. The the 4391A PEP-Dual Element RF Power Analyst requires elements. There are also several wideband, wide dynamic range analogue display products.

Conclusion: While likely not a tool for the typical ham, these would be useful for those with a professional use, for organisations with multiple repeaters, beacons, or similar systems, and perhaps for those doing equipment development. For clubs commissioning repeaters there are limited rental options.

The company: Originally set up in 1942 in Cleveland, Ohio as Bird Engineering, the company is now Bird Technologies, of Solon, Ohio. This is 327 km NE of Xenia, the new Hamvention venue. Perhaps you can find one at an outdoor seller, if visiting this event.

Sources

Martin RF Supply sell new new and used Bird meters and elements; and Coaxial Dynamics equipment at: Martin RF Supply in Arlington, Massachusetts.

RF parts sells a wide range of RF electronic components, and measuring equipment, but no longer export used equipment. Their Coacial Dynamics elements aer here: https://www.rfparts.com/old_site/element.html#coax. These are ofen under US$100, so represent a significant savings over Bird.

Test & Measurement Australia, located in Mount Riverview (lower Blue Mountains / upper Western Sydney), NSW sells Coaxial Dynamics gear: www.tandm.com.au

Secondary markets: These may turn up at flea markets, in estate sales, on ham specific sales pages, and eBay. eBay often displays prices in A$, so remember this when comparing to those above. You may have to buy meters and slugs separately, as if included, they will reflect the seller's application. You do need to ensure that elements are suitable for your meter. Note that a few listed as “Bird 43” on eBay are FA-1584C, a product built for the Federal Aviation Authority for testing VHF air navigation systems such as VORs, which were auctioned by the GSA (General Services Administration).

An alternative ways to discuss reflected power from an antenna is termed Return Loss, expressed in dB. As you saw above, we had two powers, the forward power, also termed incident power Pi; and reflected power Pr.

The greater this "loss", the better. This means whatever the sign, the larger the absolute value of the numbers.

While a variation on the standard dB formula below can be used, getting to 5 watts from 400 is a quarter, a tenth, then a half, or 6, 10, and 3 dB, totalling 19 dB. It would often be incorrectly signed as -19 dB. 4 watts would be one 100th, or a loss of 20 dB, so 19 is at least close to correct. The formula gives 19.0308998699 dB. Generally that would be considered a good result after an installation, or a check after maintenance or a transmitter swap. It can be wise to keep a log of works at a repeater site, including these values, to look for deterioration in antenna the system.

In the other case above, 10 × log(100/35) = 4.5593195565, which generally would require adjustment or remedial work.

Reversing the position of the two powers simply changes the sign. You can read more at Wikipedia: Return Loss.

When entering a log with a base 10, or log₁₀ into many computer systems the base 10 is assumed, with the natural log, or log_e coded using ln.

Note that when testing the losses in whatever feedline is between the meter and the antenna contributes twice to the return loss, or masks poor SWR. This however can be useful, as an HF dipole on 15 metres of RG-58 may well not have an excessive SWR on 2 metres, handy if your HT goes flat mid-QSO, and you want to transmit to the repeater, and don't have a suitable outdoor (VHF) antenna.

Bird also sells antenna analysers, and VNAs which can provide characteristics such as resistive and reactive components of its impedance, such as 23+30j Ω, or 107-18j Ω. They also sell spectrum analysers.

Written by Julian Sortland, VK2YJS & AG6LE, July 2026.

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