HP 415 SWR Meters

Brooke Clarke, N6GCE 2003 - 2022

HP 415A, IM-97/USM-37, SWR Meter

CD-ROM HP415E Inside & Rear
Inside & Front PCB
Front Panel
Inside bottom & rear
Inside Bottom & Front of 2709A10766
A3 PCB 00415-60023
Model History
415E Controls & Connectors
    SWR Meter
    Slotted Line


 Back in the days of tube type equipment there was an HP 415 SWR (Standing Wave Ratio) meter and there have been a number of versions ending with the latest "E" model.  This instrument is a very narrow band 1 kHz attenuator and amplifier and a precision rectifier.  The relative amplitude of the 1 kHz signal can be read to scale ticks of 0.05 dB and interpolated to 0.01 dB.  There is also a scale for the original purpose of reading SWR.

Note that if Full Scale is set on the "zero" range then when reading down 10 dB on the 60 range you are reading a signal that's down 70 dB.

The 415E is 6" high and 7 3/4" wide.  This does not appear to be a standard Rack Mount instrument size.

Model History


415B Tube 1961, 1967
415BR Tube
Ge? Xistor

Si Xistor
HP Serial Number to Mfg. date

HP Journal

HP Journal Jan-Feb 1953 Vol 4 No. 5-6: More About -hp- Precision Directional Couplers - mentions the 715A Klystron power supply and the 415A standing-wave indicator.
HP Journal March 1955, Vol 6 No. 7: A New Standing Wave Indicator With an Expanded VSWR Scale - The 415B. (the same issue has the 430C power meter).
HP Journal Oct 1955, Vol 7 No. 2: High - Directivity Coaxial Directional Couplers and Reflectometers - mentioned 416 Ratio Meter (see patent 2852743
HP Journal July 1966 Vol 17, Measuring Attenuation, SWR, and Substitution Loss with a Low-Noise, High-Precision SWR Meter, by Bradford G. Wolley
The 415E is pictured.  The 415A dates from 1952.


This example is marked:

Licensed by Western Electric Company, Inc. Under U.S. patents of American Telephone and
Telegraph Co. and Western Electric Company, Inc. and by Radio Corporation of America.
(See Label Inside)
If this set contains a Resistance Tuned Oscillator, it is licensed under U. S. patent No.
2,173,427 and manufactured under U.S. patent No. 2,268,872.
115 Volts       50 - 60 Cycles
Serial  3428
Manufactured in the United States of America
Hewlett-Packard Co.
Palo Alto, California   (R)

Indicator, Standing Wave
Supply: 115V or 220V 1(phase) 60 (cycles)
A unit of indicator, standing wave AN/USM-37
Manufactured for
Navy Department - Bureau of Ships
By Contractor
Hewlett-Packard Company
Palo Alto   California
Contract  NObsr-64702
U.S. Property

For the past 19 years (on and off) I've been trying to find the HP patent for the 415 SWR meter.  Today (2022 Apr 18) I found a photo on an eBay ad for a 415A (it's on order) that showed patent numbers 2268872 which is THE audio oscillator that started HP.  I don't understand why it's on the 415S? And another patent ending ....427.  I could not find any HP patents between 1941 and 1954 ending with 427.  But did find a photo of a USM-37 at NavyRadio.com  where, with a magnifying glass I made out patent 2173427 assigned to General Radio.  The Inventor was H.H. Scott (Wiki).

PS I bought a new H.H. Scott stereo audio amplifier and it had so much noise out that I traded it in for a McIntosh MC2505 (Wiki).  The Scott amp probably would be fine when used with Acoustic Suspension loudspeakers (Wiki) but not with efficient speakers like the Altec Lansing (Wiki) Voice of the Theater speakers.
GR made the 1232A Tuned Amplifier and Null Detector (IET: manual).  This requires tuning to the desired frequency unlike the 415 which has a single pre-tuned frequency.  Also the 415 has a scale calibrated in SWR whereas with the GR 1232A you need to manually calculate the SWR (Wiki) which involves taking two square roots which in the 1950s was not trivial so the HP 415 was a great time saver.

Fig 1
HP 415A, IM-97/USM-37, SWR Meter
Fig 2
HP 415A, IM-97/USM-37, SWR Meter
Fig 3
HP 415A, IM-97/USM-37, SWR Meter
Fig 4 Note aluminum can marked 1 kc.
In theory it could be unplugged and replaced with a circuit for a different frequency.
HP 415A, IM-97/USM-37, SWR Meter
Fig 5 Terminal Board construction
HP 415A, IM-97/USM-37, SWR Meter

415E Controls & Connectors

Front Panel

The analog meter movement is a mirrored type where you look for the reflection of your eye's pupil bisected by the needle and thus eliminate parallax in the measurement.  There is also a mechanical meter zero pot.  The correct way to use this is to turn the pot in one direction so that the needle moves up scale and and then comes back down scale and you stop turning when the needle is over "2" on the expanded scale.  If you over shoot, do not turn the screw in the opposite direction, but rather, continue turning and approach the zero position from up scale.

The POWER switch has positions for OFF, ON, (and for instruments with the optional internal battery supply: Battery: ON, TEST and CHARGE).  

The INPUT switch has positions for XTAL IMPED: HIGH, LOW or BIASED (+1.3 VDC on the center contact with 12.6 mA of current capability) and BOLOMETER: 4.5 mA or 8.7 mA.  To match a wide variety of input sources.  The HIGH position is also used for signal sources that are not from a detector or power sensor.

The INPUT is a BNC(f) connector with the shell 50 Ohms above ground.

The FREQ adjust pot allows the center frequency of the 415 to be set to match the frequency of the source generator.

The BANDWIDTH control adjusts the width of the pass band and therefore also controls the signal/noise ratio for faint input signals.  If the 415 is used with a sweeper this control should be set fully CCW to increase the BW to allow for signal variations as the sweep progresses.  Also the amplifier output should be used to drive a scope or chart recorder instead of the recorder output since the latter has a low pass filter that limits it's bandwidth.

The RANGE-DB control has an outer knob with positions for 0, 10, 20, 30, 40, 50, and 60 dB a very large dynamic range, and an inner EXPAND knob with positions of: NORM, 0, 2, 4, 6 and 8 dB.  By using the expanded (red) scale positions you can read amplitude to 0.01 dB over the full 60 dB range.  On the 0-NORM range you can bring the needle to full scale with an input signal between 0.120 VAC and __?__.

Comparing the 34401 DVM with the 415E shows that the noise bandwidth of  the 34401 limits its use:
FS 10 range
1.4 V
FS 20 range
140 mv
FS 30 range
14 mv
FS 40 range
1.4 mv
FS 50 range
0.01 mv*
FS 60 range
can not get signal this low

* 34401 is reading noise and 415E has a Full Scale reading.

The Gain & Vernier controls will bring the needle to Full Scale with an input of more then 0.12 Volts up to more than 5 Volts.

Square Law

Note that the 415 reads 10*LOG(voltage ratio), this is called "Square Law" and when the 415 is fed from a microwave detector that has a low level signal at it's input the 415 will read the same as a power meter because the detector has a "square law" response.  If you were trying to measure the dB difference in two voltages you would normally use dB = 20 * LOG(voltage ratio), but that's not what the 415 was designed to do.

Rear Panel

On the rear panel there is a BNC(f) RECORDER output that has a 0 to 1 VDC signal to drive a chart recorder and a pair of banana jacks on the standard 3/4" centers that has the amplified 1 kHz signal.  There is a covered hole that can be used to change the input connector from the front panel to the rear panel.


SWR Meter

I used the 415E for making microwave Smith Chart plots in conjunction with a slotted line and for making attenuation and/or gain measurements using a crystal detector.  The 415E is very good for both of these measurements and probably is the highest dynamic range scalar measuring instrument around.   It would also work fine for making optical measurements with an optical to DC converter.

A key feature is the narrow (slightly adjustable) bandwidth. Note slotted screw pot at lower right of front panel.  One article says the Q is 25 to 35 centered at 1 kHz, so in the range of 40 to 28 Hz.

Side Bar: Lock-in Amplifiers (Wiki, SRS AN3) & Autocorrelators (Wiki)

I worked with an engineer from the UK many decades ago.  One of the jobs he worked on involved measuring the impulse response of a steam powered electrical generation plant.  Since feeding the turbine an impulse of steam would destroy pretty much everything some other method was needed.  He used an Autocorrelator.  This is a method even more advanced than the Lock-in Amplifier.  It involved a "T" connection on the steam line where a piston that could be used to make a very small change in the line pressure and a very long digital signal.

Note that the 1 Hz Resolution Bandwidth of the HP 4395A spectrum analyzer goes a long way to seeing very weak signals.

Also see Microwave Test Equipment\Hot-Cold Noise Source.

I'm considering using it to measure the output of magnetic field sensors, where it's narrow noise bandwidth may greatly extend the sensitivity of the sensor.

When a 5.04 k Ohm resistor is connected to the input and the range set for Xtal Impedance High and both the gain and vernier knobs are turned fully clockwise the meter reads 6.5 dB below full scale.  The manual addresses the theoretical KTBR noise (Wiki) of a 5 k Ohm resistor and how the 4 dB noise figure of the 415E will show up in the measurement.  This shows how sensitive the meter is.

Typical uses for the 415E are: SWR measurements using a slotted line, Power measurements using a Bolometer (Wiki), Relative power measurements using a crystal detector.  In all cases the source must have a 1 kHz square wave modulation.

Slotted Line

When a short is installed on the unknown port and theslotted line (Wiki) carriage is moved back and forth there should be a couple of nulls a half wave apart.  This means as the test frequency gets lower the slotted line needs to get longer.

Record the location of a null.  Install the Device Under Test (DUT) and find the nearest null.  The movement of the null and it's direction (either toward the DUR or away from the DUT) corresponds to be rotation on a Smith Chart (Wiki).  The SWR will determine the radial distance on the Smith Chart.  This is relative to the transmission line impedance of the slotted line, typically 50 Ohms.

The slotted line in patent 2673963 below.


2173427 Electric oscillator, Scott Hermon Hosmer,
                  General Radio, 1939-09-19
2173427 Electric oscillator, Scott Hermon Hosmer, General Radio, 1939-09-19, - (on back of 415A) - Filters in feedback networks.
2268872 Variable frequency oscillation generator,
                  William R Hewlett, HP Inc, 1942-01-06
2268872 Variable frequency oscillation generator, William R Hewlett, HP Inc, 1942-01-06 - THE HP patent with light bulb stabilization. (on back of 415A)

Makes use of positive feedback for determining the frequency and negative feedback to reduce distortion using a light bulb.  This clearly is making use of the GR patent 2173427.

See: A real gem: HP's audio oscillator patent turns 60 -  Model 200A (Wiki)

2673963 Ultrahigh-frequency measuring apparatus,
                  David Packard, HP, 1954-03-30
2673963 Ultrahigh-frequency measuring apparatus, David Packard, HP, 1954-03-30, - 809 (USM-37D) Universal Carriage

This carriage can be equipped with waveguide or coax lines to cover different frequency ranges.

500 - 4000 MHz
500 - 4000 MHz
3 - 12 GHz
* 805A is a slab line two parallel grounds with a rod center conductor.
2852743 Electronic ratio meter, Nicholas L
                  Pappas, HP Inc, App: 1954-07-23
2852743 Electronic ratio meter, Nicholas L Pappas, HP Inc, App: 1954-07-23, - by measuring relative to a reference signal variations in the signal generator amplitude are automatically cancelled.  This probably was the HP 416 Ratio Meter

---------------- Other old HP patents ------

3294988 Transducers,  Packard David, HP, 1966-12-27, - piezo pressure or temperature - can be used for medical probe



TM 9-4931-321-34P Direct Support and General Support Maintenance Repair Parts and Special Tools Lists Standing Wave Ratio Meter, Hewlett-Packard model Y10-415E (NSN 6625-00-815-6273)

TB 9-4931-321-35 Calibration Procedure for Standing Wave Ratio meter Hewlett-Packard model 415E and PRD model 277D

00415-90009 SWR Meter 415E covers serial prefix 0990A- and with Appendix II Manual changes covers 719-, 545- and 530-.  The Yellow Manual Changes sheets dated 3 February 1986 have changes for: 1143A, 1709A, 1750A, 2031A, 2433A, 2507A, 2547A and 2509A.
But 2709A is too new for these changes.


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Page created 28 March 2003.