Antenna Noise Bridge Detector

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Tune your antenna, then measure its radiation pattern with this pocket-sized instrument

J, Frank Brumbaugh KB4ZGC Box 30 - c/o Defend in i Salinas PR 00751

Using an antenna noise bridge is one of the best and simplest means of measuring antenna characteristics. It is invaluable for tuning an antenna, especially a beam, to resonance at a desired frequency. Normal!) ihe station receiver is used with the noise bridge as a detector to indicate die resonant frequency of the antenna, and the rvsis-lancc and reactance (if any tat the fcod point.

The feed point of most antennas, and practically alt beams, is high in the air or at the top of a tower It is extremely unhandy to carry the station receiver/transceiver up a ladder or tower, dragging a long extension cord. All this climbing increases the possibility of falling, which can be detrimental to yourself and whatever you are carrying. Of course, you could make your measurements on the ground, at the end of a multiple of half-wavelengths of transmission line, running up ihe tower to make an adjust* meni, then back down and into the shack to adjust the noise bridge and receiver To make the job easier, i designed this simple-to-build instrument for you.

This detector, used in conjunction with your antenna noise bridge, substitutes for the station receiver. It even fits in your pocket so you can use both hands io climb to the feed point. Measurements at the feed point of your antenna are quick, easy and accurate. Even better, you probably already have the necessary pans in the junk box, so it can be constructed in an hour or two. Even if ail parts must be purchased new or surplus, your total expense shouldn't exceed five dollars, not including the enclosure.

How it operates

The circuit is illustrated in Fig, 1, The noise output from your antenna noise bridge is applied through a coax jumper cable to J K an SO-239. This noise, which usually will peak siightly below

1,0 volts, is broadband white noise and is fed through C1, a I GO pF capacitor, to a pair of small signal diodes connectcd as a rectifier/voltage doublet

The rectified fX^ voltage, filtered by C3, a 0.1 |ii~ disc capacitor, is then applied to the base of a small signal NPN transistor

Noise Bridge
Fig. L Noise bridge detector schema!it diagram. 76 73 Amateur Radio Today ■ May 1996

Qt, which serves as a meter amplifier. Meter Ml is a small surplus 2(H) |j A meter. It monitors collector current through Ql,

The emitter of Ql is connected to the wiper of GAIN potentiometer R2+ which controls the emitter voltage between zero and about +1.5 VDC. On/Off sw itch S2 is mounted on the GAIN control which, in series with R3. forms a voltage divider across batterv BTK a volt batterv which

powers this instrument. The GAIN control is wired so the wiper travels inom the end of R3 to ground as the knob is rotated clockwise. This sets the emitter bias and the point ¿it w hich Q1 will go into conduction rectified noise voltage is applied toQI base.

The current drain from the battery is approximately 8 ^tA with no input, increasing to slightly over 200 \xA with the meter at full scale. With such low current drain an alkaline battery should last for years, even if you forget to turn the instrument off!

Frequency coverage

! his instrument covers the range from below 40 meters to above 10 meters in two bands: 40 and 30 meters; and 20-1715-12-10 meters. Eighty and 160 meters were not included for two reasons. First, there are very few 80 and 160m beams. Second, eliminating these frequencies made band switching considerably simpler, and reduccd the cost and complex-it*

Bandswitch Si, an SPOT toggle or slide switch, selects the frequency range. The tuning capacitor C2 is a small 150 pF air variable, although one of the small variables with Ihin plastic sheets between the plates, such as used in many small portable radios, may be used instead. C2. in conjunction with inductances LI or 1.2. allows peaking the frequency response of the instrument in the ham band in which your antenna is to be adjusted. Thus, it readily and handily substitutes for the station receiver, which otherwise would have to be used.

Construction

The detector should be constructed in an aluminum box, or an enclosure made of printed circuit board stock, because it must be well shielded to function accurately, Other than compact construction on a small piece of perf board, of a general-purpose printed circuit board such as are available at Radio Shack™, and short, direct leads carrying RE parts placement is not important. Only those connections between CI, C2, LI, L2, Rl, J1 and SI carry RF. The other leads carry only DC, so their routing is unimportant.

Although any small 140-150 pF tuning capacitor can be used (take a took in your junk box), an APC type is recommended for compactness, This was a common WW II surplus item and can often be found at hamfest flea markets, or from Fair Radio Sales, Inc., Box 1104, Lima OH 45802. Small surplus meters with 100 or 200 \xA movements, originally made for CB and home entertainment equipment, arc also available from Fair Radio Sales and other mail order dealers as well as at hamfest flea markets. The toroid cores (T50-2) are available from Amidon Associates, Box 25867, Santa Ana CA 92799, and from Radio Shack. Other components may be found at numerous mail order parts dealers. However you probably can find just about everything you need in your junk box, from other local hams, or at the next hamfest you attend.

Calibration

There are several ways to calibrate this instrument. A source of low level RF covering the ham bands from 40 through 10 meters, with accurate frequency determination, is required, Your station receiver, or a frequency counter, can be used to check the frequency of a signal generator or a dip oscillator.

To calibrate with a dip oscillator, temporarily wind a two-turn loop around LL Lightly couple the dip meter coil to the link, tuned to a ham band (40 or 30 meters). Be sure SI is in the proper position. Tune C2 for a dip in the dip oscillator meter. Reduce coupling and tune C2

until the dip is just barely discernible. Mark this spot on the dial of C2, Using your station receiver to make certain the dtp meter is on the right frequency will enable you to mark the edges of the wider bands on the dial.

Repeat this procedure with a fink on L2 and S1 set for the high band. The 20, 15 and 10 meter bands are wide enough so you can mark both edges on the diaL

You can also use a signal generator to calibrate this instrument, using your station receiver to maintain frequency accuracy because signal generator dials arc seldom very accurate. Connect the signal generator output, set for low level, to JL With S1 set for the low band, tune C2 on this instrument for a peak on ML Mark this point on the C2 dial. Repeat the process for each ham band.

You can also use your station equipment to calibrate this instrument by feeding your transmitter into a dummy load, or use very low power if you are feeding an antenna. Place a short piece of wire into the center terminal of J1 to act as an antenna. Key the transmitter on CW (key down) and tune C2 for a peak on Mi. Mark this spot on the C2 dial.

Operation

This device can be used to replace the station receiver when adjusting an antenna. It can also be used as a linear field strength meter to measure the field around the antenna and plot its radiation pattern.

Operation as a noise bridge detector

Connect a short length of coaxial cable between J1 and the "receiver" connector on your antenna noise bridge. Connect the "unknown" connector on the bridge to the feed point of your antenna.

Tunc the noise bridge detector to the ham band of interest. Then follow the directions in your antenna noise bridge manual and adjust your antenna, substituting the noise bridge detector wherever "receiver" is mentioned in the manual

Measuring antenna radiation pattern

After you have adjusted your antenna you can use this instrument as a linear field strength meter. Connect a short antenna or a remote dipoie to J L With low power applied from your transmitter to your antenna, enough to register on the meter of this instrument, either rotate the beam to check its radiation pattern, or move the detector and its antenna around the antenna, noting the meter indications at each point. The distance between the station antenna and the detector or its remote antenna should be several wavelengths at the frequency in use for greatest accuracy.

When used to indicate field strength, the lower fifth of the meter scale will be nonlinear because of diode conduction knees. However, the upper 80% of the meter scale provides linear indications, so increases and decreases in the meter indication in this upper scale portion indicate equivalent changes in radiation intensity from your antenna.

Caution!

This instrument is extremely sensitive. It is easy to pin the meter when using it in conjunction with your antenna noise bridge, and also during the calibration procedure. Use the GAIN control as ncccssary to keep the meter needle from wrapping itself around the pin!

Parts List

BTI 9-Volt alkaline battery CI 100 pF mica or poly capacitor C2 140-150 pF variable capari tor

(see text) C3 0.1 |iF disc capacitor C4 0.01 |iF disc capacitor D1,D2 Germanium diode: 1N34, 1N60, IN90, IN270, etc. J1 SO-239 UHF female connector

(or builder's choice) LI 33 turns #26 enarn, wire on T50-2 toroid (40-30 meters) L2 11 turns #26 enam. wire on T50-2 toroid (20 through 10 meters)

Ml 100 or 200 |iA meter {see text)

Q1 NPN small signal transistor; 2N2222, 2N3904, 2N4124, etc.

R1,R3 10k 5% 1/4 Watt resistor R2 1000 Ohm linear taper poten tiometer, with switch S2 SI SPOT toggle or slide switch (Bandswiteh)

Crrr's corner

Analog-to-Digital (A/D) Converters

Joseph J. Carr K41PV P.O. Box 1099 Falls Church VA 22041

Amateur radio astronomy has been a long-time favorite topic in this column. I've discussed the topic of frequencies from near-DC to daylight (literally?), tot there always seems to he more to say. One constant question, however, is how can one collect the (essentially) analog data that results and store it in a computer? The answer is to use an analog-to-digital converter {ADC or A/D),

How the DC signal is derived

The receiver used for radio astronomy observations, whether VLFt HF, or VHP, Will produce a vailing signal level according to the strength of the received signal. It is that signal level variation that you warn to record. This job requires thai the signal be converted into a DC voltage; or. more correctly, a unidirectional varying signal that looks a lot like unstable DC,

This is done by using a rectifier and integrator circuit. Like most tilings, wo can do this plain or fancy, depending on how you want it and what you want to do. Fig. 1 shows I he plain solution that I teamed from Bob Sickles and Jeff Lichtman. Jl uses a plug fPl) 10 the speaker or headphones jack of the receiver Once the receiver is tuned to the frequency being observed, the plug is inserted so that this circuit comes into play. The rectifiers consist of a half-wa\e voltage doubler made up of CI, DI and D2: C2 is also pan of the voltage douhler, but it serv es a dual role as the integrator. Capacitor CI can be \ uF tantalum, 01 some other form If the capacitor is a polarized variety, then the larity as shown should be observed. The diodes are plain vanilla 1N60 germanium diodes. These diodes arc also sold by some dealers under the designations ECG-109 and NTH-109 (these are intended Ibr the radio and television repair industry).

Hie integratorcocsisLsof C2 and R I. The time constant of the integration can be varied by changing the values of C2. I've used values from 10 pF to 470 pF with only a small observable change in the way it operates. The output signal is a DC voltage or current that goes to a meter, an oscilloscope, strip-chart recorder, or the input of an ADC.

The fancy approach is shown in 1. This circuit is a tneau value amplifier, and is made from circuits seen in most good operational amplifier books, including my newly released (March 1996) title from Buticrworth-Heinemann (313 Washington Street. Newton, MA 02158-1626:1-617-92^2500). The book was released in England, but is available in the USA through the address shown. What is a mean value amplifier? Well, itTs really just a rccti fierfaitegrator writ large to justify using the op amps.

Amplifier A1 is a precise rectifier (half-wave), while A2 is a Miller integrator. The op amps can be CA-3140s (which use 741 pinouts), or a single CA-324G (two 3140s in a single DIP with ihe pinouts of the popular LM-E458 device). The values of the resistors are not critical, but the ratios (R and 2R> should be followed. 5k ohm and 10k ohm, or 10k ohm and 20k ohm are likely candidates for value. Hie diodes are germanium f N60s, although I've used silicon 1N4148 with no observable problems. The integrator time constant is R limes the value of the capacitor. Stan with I j.tF and work up ~Ul you seta good response. The compensation resis tor between the t + i input of A! and ground should have a \alue of K, or that input can be grounded if some small DC offset can be

tolerated Ory it first, however, because integrators like A 2 don't like of f set voltages at their inputs).

A/D converters

The job of the ADC is to lake an analog input voltage and render :t into a binary word Lhat represents it Cheapo com, eners that di\ ide up the analog input voltage range into 256 states are available in 8-hii lengths A 12-bit converter costs a little more, but can divide up the analog voltage into 4,096 different values. In the 12-bit case, on an ADC that allows analog input voltages from 0 to 5 volts. 0 volts might be represented by binary 000000000000, while +5 volts would be represented by binary 11111111111L Actually, because of the 1-LSB and zero problem, the maximum input voltage would be only (4.095/4,096) x 5 volts, or 0+9998 x 5 volts = 4.999 volls.

One approach to the ADC problem is to build one yourself The Maxim people seem to h,ne a lot of really interesting circuits, one of which is their MAX-187 ADC. h comes in an 8-pirt mini-DIP package, and offers 12-bit conversions and a serial output. The circuit ol Fig. 3 uses a M AX-187 and is connected to the computer via the paral Eel printer interface. The parallel printer interface is normally regarded as an output because it sends data to the printer, but there arc five handshaking and signaling lines on the interface connector that cither go back to the computer or are bidirectional. Wire the connector as shown, and then plug it into the printer output at the back of the computer. Make sure that the gender of the councilor lhat you u^e is opposite that of the parallel printer output (otherwise, it's still a DB-25 connector*.

You will have to write a short BASIC program to input the data. 1 have a candidate program, but it belongs to someone else. If you send me an SASE or reach me \ ia L-rnajl. I ll tell you where to get the BASIC listing that runs this ADC,

The MAX-187 can be bought in single quantities from dealers such as Digi-Key (701 Brooks Avenue South, P.O. Box 677, Thief River Falls MN 56701-0677; 1-800*3444539). By the way, if you're at all interested in electronic construction projects you need the Digi-Key

Photo A. Pico Technology, Ltd ADC-16 serial port ADC.

catalog. It is chock full of neat ports: there's not a lot in (he RH realm, but ii offers nearly e\ cry thing else (¡md they do have at least some RI: p;utsY Fhey are a source of the NF.-f ii )2 chip used for a lot of direci-conversion radio receivers, for example.

Another solution is to buy a ready-made ADC from a commercial source. Several models are available that plug into cither Ihe parallel printer pirl or the RS-232 serial asynchronous communic; it ion ]> Jrt on tl )e hack of the computer. Photo A show s one that 1 bought from J*ico Tochtwlogy Ltd (Broadway House, 149-151 Si, Neots Road. Hanlwick. Cambridge CB3 7QL England]. They lake Visa cards, so you can < irder and not worry about sending a check (or as they say, "cheque") denominated 111 pounds ceding.

The version shown in Photo A. is the Pico ADC-16 model <although I understand a newer version is now out). It has eight software-selectable analog lor

"analogue"*) inputs, resolution that is programmable between S and 16 bits, and a 12.5 volt input range. Pico Technology, Ltd, also offers two software programs for MS-DOS and Windows computers lhat allow y ou to run their \ arious ADC products. PicoScope makes your computer work like .1 storage

Photo B. Radio Shack computerinterface digital multimeter.
Fig. 2. Fum y rectifier integrator (or "mean value amplifier *).

oscilloscope when the ADC is connected, while PicoLog makes the computer into a nifty data logger I use bothsoftware packages

The final method. shown here in Photo B,. is the Radio Shack1 * digital multimeter thiii interfaces to IBM-Ft compatible computers thnxigh ihe RS-232 serial port. The advantage of this device is fiui it ajso functions as a combination frequency counter (although not to a high ttequency i md digital multimeter, when not being used for ADC. Also, because of the ditYerent ranges and functk>ns, a wider range of parameters can he input to the computet Ihe Radio Shack instrument also comes with iLs own software for MS-DOS or Windows machines.

The Radio Shack computer-interface digital multimeter comes in a couple of different models/Hie one I bought is not easily found anymore; however, the manager of my local Radio Shack told me that he stocks a couple of varieties thai replaced it Cheek with a Radio Shack store to see whether or not Lliis is a good idea for you. I paid around $130 for mine, but don't know what the current price is.

Tlie use of tlie computer allow5 yot l i( • make a lot more observations. as w ell as do some neat data anal v si s thai would be harder bs hand. Strip-chart reo tidings look like "real science" but [ire a pain in the neek to read and use properly. I prefer the computer approach, if only for that reason.

Note: if you write a BASIC or Visual* BASIC program to read data from am sort of ADC and wani 10 time and date stamp ti*e data using the computer's internal dock/calendar, then make sure that vou store the iata in a tile in comma-delimited format This means putting a comma "CHRSiOr character between "printfT statements for the ADC data, time and date. If you do it right, then you can displa> the data through an Excel, Lotus 1-2-3, or other spreadsheet program. Also, the graphing Junctions of those programs can be used to plot your daui if the ADC software lacks its own version.

Connections

1 can he reached at P.O. Box 1099. Falls ChuirfiVA22041, or via Internet E-mail [email protected] i welcome your input.

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  • averardo
    How do you use a antenna noise bridge?
    2 years ago

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