Audio Readout for the MFJ-209 Antenna Analyzer

Audio Readout for the MFJ-209 Antenna Analyzer

An easy to make adapter that adds audible audio output to a popular antenna analyzer.

Tom Fowle, W A 6 I V G, and Bill Gerrey, W A 6 N P C

Note: Minor modifications have been made to this article by the Courage Handiham System to improve readability by screenreading software and for conversion to Daisy format. Comments and questions about changes may be directed to Patrick Tice, wa0tda@arrl.net.

This article describes additions to the MFJ-209 antenna analyzer to provide audible readout of both S W R and frequency. These readouts make it easy for a blind ham to adjust antennas or antenna tuners. Sighted hams may also find this system convenient, allowing eyes to be kept on the work rather than on the test equipment. The only actual modifications to the 209 are holes in the case ends to accept the machine screws and a grid of holes under the speaker in the back of the case.

Background and General Description

Although various devices such as dip meters, impedance bridges and the like have been used for making antenna adjustments for years, the introduction of analyzers has made this work simpler. The ability to determine S W R while trimming or adjusting almost any antenna at its base, rather than from the shack, greatly speeds up the adjustment process and probably also improves safety, because of the reduced need for trips from the shack to the antenna. A variety of means for audibly reading S W R meters has been known for years, but we wanted the convenience of an entirely self contained and portable analyzer to go with todays increasingly portable stations.

There are two quantities which need to be read from the analyzer: S W R and frequency. Particularly in this example, the S W R is a dynamic value: always changing while adjustments are being made. Making the S W R talk is feasible, but inappropriate, since speech is entirely too slow for a dynamic readout. Our popular dynamic audible readout was chosen for S W R readings; this translates the meter reading to a varying audible tone. The analyzers frequency is read out in Morse code by an outboard frequency counter.

In operation, the analyzers TUNE knob is adjusted for the lowest pitch emitted by the V C O (voltage-controlled oscillator connected to the analog S W R meter). This audible tone is chopped (interrupted so as to pulsate) when the S W R exceeds a predetermined threshold. We set our threshold at an S W R of 2:1. As the frequency is swept over a band of interest, the tone dips and rises with the S W R. When S W R is below 2:1, the tone is continuous. When the measurement is above 2:1, the tone is broken or chopped at a rate of about 2 or 3 Hz. With this system, it is very easy to determine the 2:1 band edges and the center resonant frequency of an antenna. Or, a desired frequency can be selected, whereupon the antenna can be adjusted for lowest S W R by listening to the pitch of the V C O. Frequency measurement is accomplished by connecting the MFJ-209s low-level RF output to the MCount, a nifty little Morse output frequency counter from Jackson Harbor Press.1 The completed project is shown in the lead photo.

Our Plan of Attack

Why, you will ask, did we choose to modify the older MFJ-209 analyzer when so many newer digital readout models are available? The answer lies in the sad fact that the introduction of digital systems often makes it more difficult to gain access to desired signals to produce non-visual readouts. Although it is sometimes possible to connect directly to digital displays and re-interpret the information, usually this requires difficult wiring to existing densely packed boards and/or modification of production microcontroller code. The V C O circuit we used is popular with us as a general purpose meter reader. In many other applications, we include a panel mounted pot with tactile dial which adjusts the point at which the pulsating of the tone begins. This makes a complete meter reader that allows the user to take readings at any point in the range under measurement.

In the case of the 209, it is most often only necessary to know the 2:1 S W R points and the center frequency. Therefore the calibration adjustment was left on an internal pot which is trimmed during final installation. Particularly in the case of this analyzer, only a few simple connections to the existing unit are required. Using a more expensive digital readout device would have no advantages to the blind technician and the lower purchase cost of the base 209 may help offset the costs of making these adaptations.

Making it Happen

The original prototype of this adaptation was actually made on an MFJ-249 with a digital counter. This was done simply because a 249 was in hand. The two units are nearly physically identical, so that if desired, this modification can probably be made to any of MFJs line of analyzers. However, the existence of the low level RF output jack on the 209 makes connecting the counter a snap.

Our two units were hand wired point-to-point on vector board. Although we have worked with printed boards, this is not easy for blind builders. We know of no practical techniques by which blind engineers can design PC boards. Therefore we are proud to keep the hand wired tradition alive. As with retrofitting any commercial device, the most difficult part of this modification is physically installing the V C O board into the 209.

We built the V C O circuit, Figure 1, including its speaker, on a rather Z-shaped piece of vector board that mounts behind the analyzers main board. The shape of this board (see Figure 2), is such that the batteries in the MFJ unit slide past cutouts in our board. Cut-outs and board position are shown in Figure 3, the bottom view of our modified instrument. The batteries, two four-packs of double As, are mounted inside the back cover (not shown) of the 209. To support the ends of our added board, we made two custom brackets from 1/2 inch aluminum angle stock, tapped for 4-40 machine screws. These were secured to the end panels of the analyzer. If you have a 4-40 tap and the appropriate size of drill bit, we strongly recommend tapping the mounting holes in the brackets to avoid a hideous time trying to fit nuts under and inside these brackets.

The vector board is 6.5 inches long, just long enough to fit inside the length of the 209s case. With the cases back and sides removed, and viewing with the connectors up, the upper section of the board is 2.125 inches square and its right edge is nearly against the right side of the case. The center section of the board is slightly less than an inch wide. The lower section of the board is also 2.125 inches square and extends to the left of center opposite the upper square section. The upper section carries the op-amp and associated parts while the 556 and its components are strung out down the skinny center. The lower square of the board carries the speaker. Because it is a close fit, position the board between the battery holders. We also suggest making the board and brackets (shown in Figure 4) and then fitting them into the case, making sure that the board passes between the installed batteries before you do the wiring. The upper L bracket is held by one screw that also secures the SO-239 antenna connector; this bracket must be filed a bit to fit around the insulator of the connector. The lower bracket is the width of the boards lower square and carries two #4 machine bolts in each side: two holding it to the case and two holding the board to the bracket. We drilled sound holes in the back panel of the removable piece of the cabinet, positioned adjacent to the loudspeaker. It seems wise to put a business card or other handy insulator between the speakers magnet and the analyzers board to avoid shorts. The frequency counter is kept external to the analyzer, connecting it only through a short RCA cable. This leaves the counter available for other uses and reduces the difficulties in modifying the 209.

Putting it to Work

The counter, built in an Altoids box, can be attached to a side of the 209 with sticky backed hook and loop material, allowing for a single package, but leaving the counter free to go wherever else it is needed. Two other advantages for blind hams of the 209 are:

The RF oscillator that produces the analyzers output is modulated at a few hundred Hz. If you dont have the frequency counter available, it is quite easy to find the 209s signal in an external receiver, even a portable AM only unit.

The small pointer on the TUNE knob allows quick approximate adjustment as the user gets used to band locations on the pointer dial.

We equipped the rear panel of the 209 with a Braille chart showing the ranges of each band. Power for the V C O is taken from the 209s 12 V battery. A single wire is soldered to the hot meter contact where it protrudes through the main board. This provides input to the V C O. In operation, the V C O comes alive as soon as the analyzers power button is pressed.

With no load connected to the antenna jack, a high-pitched tone from the V C O will be heard, pulsating as the S W R value goes to maximum. As soon as a load is hooked up, the tone pitch will reflect the S W R and the presence of the chopped tone will immediately indicate whether the reading is over 2:1. The user simply selects the correct frequency band and sweeps the tuning knob for lowest pitch to find the resonant frequency of the load. The MCount is then used to read the chosen frequencies.

Note that the counter needs to have its pre-scaler switched out below 50 MHz. With a little experience, the blind operator can determine upper and lower 2:1 S W R points and a center resonant frequency within a very few seconds. It is also completely practical to make antenna tuner adjustments and perform all other functions mentioned in the analyzers manual.

Voltage Controlled Oscillator

The V C O circuit is built around the ancient NE556 timer chip. One half of this timer is the V C O whose charging current is supplied through the first half of a CA3240 op-amp. This op-amp and a 2N2222 transistor make up a voltage-controlled current source that drives a current mirror. This provides the necessary current source referenced to the plus supply line. This configuration produces a V C O with a wide frequency range and very smooth operation. The CA3240 was chosen because its inputs operate properly down to the negative supply rail, which is necessary as the analyzers meter is referenced to ground. The second half of the CA3240 op-amp is a comparator that compares the incoming meter signal to a calibrated voltage, which is set to reflect a 2:1 S W R reading. The output of this comparator enables the second half of the NE556.

This second oscillator, running at about 3 Hz, turns the V C O on and off, providing the chopping effect when the S W R is high. A temperature stable reference diode, LM336, keeps the calibrated voltage accurate, regardless of battery state. The output of the V C O, the first half of the NE556, drives the speaker through a 47 Ohm current limiting resistor. The magnetic speaker can be replaced with a non-resonant type of piezo sounder, eliminating the need for the 47 Ohm resistor.

Following V C O board completion and installation in the 209, attach a 100 ? resistor to the antenna connector of the analyzer. At power up, you should get a tone from the speaker. Turn the calibration pot until you find the point below which the tone is smooth and uninterrupted, and above which the tone begins to pulsate. With the 100 Ohm resistor at the antenna connector, the S W R will be 2:1 (100/50 Ohms. In other words, a 2:1 S W R indication will now be just on the edge of the start/stop of the chopping. If you want the chopping of the tone to occur at a different S W R, you can calibrate the instrument by selecting a different test resistor. If a non-inductive resistor is connected to the antenna SO-239 connector, the test S W R will be the value of that resistor divided by the ideal 50 Ohms.

One note of warning about this, and probably most similar antenna analyzers: If you live in a very high RF environment, as does W A 6 N P C, you may find that, on some bands, you will never get a good low S W R reading even with an antenna you know to be matched. This is due to the ambient RF being wrongly interpreted by the instrument as reflected power.

Circuit Description in Text Form

The negative line of the adapter circuit goes to the ground plane of the MFJ analyzer (this is also the negative side of the visual meter in the MFJ device). The Vcc line of the adapter circuit goes to the on position of the MFJ off/on switch. Hereafter, ground will refer to the negative line of the adapter circuit. The Vcc line is bypassed by 100 µF (negative of this electrolytic at ground). Pin 7 of a 556 dual timer is grounded. Pins 4, 10, and 14 are tied together and go through a 10 Ohm .5 W resistor to Vcc. Pin 14 is bypassed to ground by the parallel combination of 0.1 µF and 100 µF (negative of this electrolytic at ground). This pi network isolates any noise generated by the 556 from the Vcc line. The hot terminal on the 209s meter goes through a 2.5 mH RF choke, then through a 220 kilo ohm resistor to pins 3 and 5 of the CA3240 dual op-amp.

Each end of the choke is bypassed to ground by 0.047 µF capacitors. The junction of the resistor with pins 3 and 5 of the op-amp is also bypassed to ground by 0.1 µF. Pin 4 of the CA3240 is grounded while pin 8 goes to Vcc. The comparator that controls the chop oscillator is the second half of the 3240. Pin 7 of the 3240 goes through a 22 kilo ohm to pin 12 of the 556. Pin 6 of the 3240 goes to the arm of a 10 kilo ohm calibration pot. The bottom of this pot is grounded. An LM336 2.5 V voltage standard has its anode grounded and its cathode goes through a 3.3 kilo ohm resistor to Vcc.

This cathode (the 2.5 V output) goes through a 100 kilo ohm resistor to the top of the 10 kilo ohm calibration pot. The third adjust connection to the LM336 is left unused as it would serve no purpose here. Pin 1 of the CA3240, the output of the first op-amp, goes to the base of a 2N2222, whose emitter goes to pin 2 and through 3.3 kilo ohm to ground. This forms the current source, which reflects the input voltage from the meter. The collector of this 2N2222 goes to the current mirror consisting of two 2N2907 PNP transistors. The bases of both of these transistors go to the collector of the 2N2222. The emitters of these two 2907s are tied together and go to pin 9 of the NE556, the output of the slow running chop oscillator. The collector of one of these two 2N2907s goes through 22 kilo ohm resistor to pins 2 and 6 of the NE556 to provide charge current for the V C O.

The second 2N2907 has its collector tied to the two bases forming the current mirror. On the 556, pins 2 and 6 are tied together and go through 0.0047 µF to ground as well as to the previously mentioned charge resistor from the current mirror. Pin 2 also goes through 10 kilo ohm to pin 1. Pin 5 of the 556 goes through 47 ?, then through the loudspeaker to pin 14. Returning the speaker directly to pin 14 keeps the speakers load from disturbing the Vcc line. On the second half of the 556, which is the chop oscillator, pins 8 and 12 are tied together and go through 2.2 µF to ground (negative of this electrolytic at ground). Pin 12 goes through 10 kilo ohm to pin 13.

The counter kit is available from: home.att.net/~jacksonharbor/mcount.htm

Acknowledgments

The list of folks we need to thank for help leading to this work is endless but must include: Howard Moscowitz, KB3ZX, for thinking up the modulated V C O years ago. Our dear colleague Albert Alden, real analog engineer for the schematic, endless patience and more. Smith-Kettlewell Institute and our boss and friend Dr John Brabyn, for putting up with us! Toms wife Susan Fowle, NY6D, for the original schematic drawing, meter reader board design and everything that matters!

1. http://home.att.net/~jacksonharbor/ham.htm

2. A plain text version of this article and an audio demonstration file are available on the A R R L Web site at: www.arrl.org/files/qst-binaries/fowle0207.zip

Bill Gerrey, W A 6 N P C, was born in Reno, Nevada in 1947 and has been totally blind since a very early age. He was first licensed as W V 6 N P C in 1960 and now holds an Amateur Extra class license. He took his BS in electrical engineering from California Polytechnic University, San Luis Obispo in 1971 and has worked at the R E R C for over 35 years. Other interests include piano restoration and collecting cylinder and other old recordings. He lives in San Francisco with his wife.

Tom Fowle, W A 6 I V G, was born in Berkeley, California in 1946 and has also been blind from an early age. He was first licensed in 1959 as W V 6 I V G and has held an Amateur Extra class license since 1979. Other interests include sailing; he is past commodore of the San Francisco-based Bay Area Association of Disabled Sailors and is a certified skipper to the American Sailing Associations Basic Coastal Cruising standard. Ham interests include casual CW operating. He lives in Hayward, California with his wife Sue, N Y 6 D.

The authors can be reached at The Smith-Kettlewell Rehabilitation Engineering Research Center, 2318 Fillmore St, San Francisco, CA 94115 or bilge@ski.org, fowle@ski.org. Figure 1: Schematic diagram of the V C O. All parts are readily available standard items. Figure 2: View of the component side of the V C O board before installation in the analyzer. Figure 3: Bottom view of MFJ-209 analyzer showing the V C O board installed with its solder side visible. Figure 4: Detail of the angle brackets mounted to the bottom of the case and the board interconnections. Mcount Morse code Output Frequency Counter Kit

The Mcount Morse code output frequency counter kit used here might be considered the second generation of such PIC based kits to be available. It features a prescaler, thus making its range extend to 500 MHz. It also includes a sophisticated menu system whose settings are always saved in nonvolatile memory. Among several other clever things you can do is to define how many of the nine possible digits will actually be sent with each count. When used with the analyzer here, it is of little use to hear frequencies to a resolution of greater than 1 kHz. Setting the MCounts DL (digit low) range at 3 might give a readout of 7.143 rather than a possibly tedious 7.143.039 or the like.

A minor difficulty with the counter is that at frequencies above 50 MHZ, the input needs to be moved from the prescaler to the direct input. This is done by moving jumpers on the board in the standard version. In our version, we added a second RCA input jack, and using the supplied DPDT on-off-on switch, arranged the input of the counter to be switched between the prescaler and the direct input jack. This still requires the connecting RCA cable to be moved between the two inputs depending on the band in use. There are probably other switching schemes which could solve this minor difficulty.

Why not, you may demand, make the counter talk? That would probably triple the cost, triple the size and significantly increase the power consumption of such a counter. A rant which Ive not finished yet: Why do not the designers of commercial equipment, not only ham gear, use sound in logical ways? Why does equipment intended for handheld field and mobile use have toggling functions whose state can only be determined by taking your eyes off the work at hand? A key press that only beeps is merely annoying, it communicates very little about what youve done. Were so close to this, its hard to write about it! The Mcount is now available as a completed unit with UHF/VHF switching, an improved speaker and other nice touches or $45 postpaid anywhere in the U.S. from: Chuck Carpenter, W5USJ, at: www.w5usj.com/mcountbyw5usj.html

The Smith-Kettlewell Rehabilitation Engineering Research Center

The Smith-Kettlewell Eye Research Institute is a 501(c)(3) nonprofit scientific research institute that performs basic and clinical research into many aspects of vision. The institute hosts the Rehabilitation Engineering Research Center on low vision and blindness. The Rehabilitation Engineering Research Center receives its funding through the National Institute for Disability and Rehabilitation Research of the federal Department of Education. The Vocational and Daily Living Lab, in which the authors work as rehabilitation engineers, is tasked by this RERC grant to use off the shelf technology to improve the quality of life for blind, low vision and deaf-blind people.

As such this group has spent nearly 30 years designing and building proto-type devices, both as research tools and for individual people with particular needs. An important note, the RERC does not sell any products of any sort, but is happy to consult with blind, deaf-blind and low vision people on accessibility related needs which are not met by other sources. Much of the authors other work can be found documented in the Smith-Kettlewell Technical File, a technical journal for blind technicians and engineers published here since 1980. The journal is available at: www.ski.org/rerc/sktf/.

This journal includes, aside from design articles for accessible equipment of many types, a seven part series on soldering for blind technicians.