- [Feld-Hell printer built by me]
- [Homebuilt Hellschreibers from other people]
- [Hell/CW audio filters]
Last page update: 31 October 2017
MY HOME-BUILT HELLSCHREIBER PRINTER
I built a mechanical Hellschreiber (receive/print only) in 1983. Most of the structure was made from double-sided printed-circuit board, precision-cut with a CNC machine during a summer job at the Dutch public telephone company (PTT) in 1982. It measures 15x8x12 cm (WxHxD, ≈6x3x5 inch). The printer spindle and the paper transport rolls were machined while I had a student job at Philips Research Labs in Eindhoven in 1983.
My home-built Hellschreiber printer
I used a small motor with integrated down-gearing paper transport, and a servo motor for the printer spindle.
Top-view of my printer
My home-built Hell printer in 3D
(you will need red/green glasses to get the 3D/stereoscopic effect)
The circuitry that I used for the tone detector, solenoid driver, and motor speed control (paper transport and spindle) is rather simple. See the schematics below. The transistor types really date the design! A solid-state implementation of the driver circuit typically needs a diode (installed with reversed polarity) across the magnet solenoid, to avoid damage to the transistor due to the inductive voltage spike upon de-energizing the solenoid. Alternate detector designs are described in ref. 2A and 2B.
Left-to-right: solenoid-driver circuit, paper transport motor control, spindle motor control
(source: ref. 1A, 1B, 2C)
Another simple detector / solenoid driver circuit
(source: ref. 1D)
If the relay is a bit slow to respond when energized, you may try to speed it up with one of the simple circuits described in ref. 2D, 2E. They basically briefly apply almost twice the supply voltage to the relay coil when energizing. The capacitor in the circuit will have to be adapted to the desired speed and relay (decay time).
My ink-roller is made of several round felt pads that I stacked and glued together, and drilled a hole through the center. The pads are simply felt bumper-feet for furniture (the ones with adhesive backing are the easiest), available in several forms and sizes at your local Do It Yourself store. An other option is a felt polishing wheel (or tip) from a Dremel® type rotary tool. I used Pelikan-brand endorsing ink. It is thick and comes in a tube. The label says that it is intended for felt pads. Just what I need. You just may want to use gloves when handling this type of ink! And don't use water-based ink; not only will it cause the spindle to rust, the capillary properties of the paper will cause the ink to be dispersed and make the print fuzzy.
Felt disks for making an ink roller, and printing ink
Paper transport speed calibration is easy: all you need is a stopwatch and a ruler or tape measure. Simply adjust the motor rpm to obtain about 47 cm per minute. Unlike the spindle in a Feld-Hell machine, my spindle is a single-thread (1-start) worm. It needs to turn at 7 columns per character x 2½ characters per sec x 60 sec/min = 1050 rpm. The Feld-Hell machine has a double-thread (2-start) spindle: it has two intertwined windings. Note that the number of threads is not related to the number of turns of each thread! Both the Feld-Hell and my homebuilt spindle have two turns. The Feld-Hell spindle only needs to turn at ½ x 1050 = 525 rpm, to get the same effect as a single-thread spindle at 1050 rpm.
Adjusting the spindle speed to 1050 rpm can be done several ways: 1) first get the electro-magnet to work properly (which you have to do anyway), then feed the printer with "known-good" Hell-audio and adjust the rpm until the print becomes legible and the text lines are horizontal; or 2) use a tachometer.
I used method nr. 2 by building a very simple optical sensor (4 cheap components), attaching a strobe disk to the spindle shaft with a dab of glue, and measuring the sensor output signal with an oscilloscope. You can make a crude strobe card with a black felt marker, but such markers are really not black at all, and the ink does not have good reflectivity. Best is to print one on regular paper with a laser printer. Here is a file with strobe disks that have 1, 2, and 4 black segments and ≈5 cm (2") diameter.
Small strobe disk attached to the spindle shaft
The sensor simply consists of an infrared LED and a phototransistor. No specific types (you can also use an IR diode + transistor in a single package, e.g., an SY-CR102). I shielded them with shrink tube (black on the 5 mm LED, blue on the 3 mm phototransistor in the photo below). As you can see in the photo, I bent the leads such that the diode and transistor are not parallel, but are trained at a point just in front of them (about 2-3 cm, 1"). The largest swing in the sensor output signal (5 Vpp) was observed at a distance of 0.8-1 cm (≈3/8") from the strobe disk. The scope image below indicates that the spindle is turning at 1500 rpm, and needs adjustment. A strobe period of 10 msec (100 Hz) = 100 cycles per second, and 4 black segments on the strobe card means 100/4=25 rps which is 25x60=1500 rpm. The scope is set to AC-coupling.
My analog optical tachometer
A couple of years ago, I bought an inexpensive digital tachometer (model DT-2236A) on eBay, for maintenance of the motor-generators of my Feld-Hell machines. It can be used optically (laser + a marking or strobe disk on the rotating object), or by contact (surface speed or shaft speed, with various attachments).
My DT-2236A digital tachometer
HELLSCHREIBERS BUILT BY OTHERS
Below are photos and descriptions of Hellschreiber printers (and even electro-mechanical senders) built by other radio amateurs, with varying degrees of sophistication. See ref. 1A-1J.
- [Mart, PA0MJS; 3 printers]
- [Eckhard, DG9JO]
- [Mart, PA0MPR, & Ans, PA0ARB; 2 printers]
- [Kees, PA3AFD]
- [Bastiaan, PA3FFZ]
- [Dale, VK1DSH/VK2DSH]
- [Jan, ON4AS/EA3DPB]
- [Wim, PA0WV]
- [Arie, PE1AQB]
- [Luca, IZ6HPJ]
- [Gerhard, VE6AQO]
- [Michal, SQ3TCE]
- [Rommert, PE1DCF]
Mart, PA0MJS (SK), built a Feld-Hell printer in 1979:
Feld-Hell printer built by Mart (PA0MJS) in 1979 − front view
Feld-Hell printer built by Mart (PA0MJS) in 1979 − top view
The circuitry of the spindle-motor speed regulator is interesting: it comprises a Wheatstone-bridge, in which one branch is formed by the motor winding, and another branch by a resistor with the same value as the motor's DC resistance.
Mart built another one in 1980 (ref. 1A) and a third one in 1982 (ref. 1B). The last one used a bicycle dynamo for a DC-motor.
Feld-Hell printers by Mart (PA0MJS)
(source: ref. 1A and 1B)
Eckhard Bremer, DG9JO/DL4EBV, was inspired to build his printer, seeing Mart, PA0MJS, and his machine at a ham fest in 1980. Eckhard used the motor (with a flywheel) and speed regulator from a cassette tape player.
Front & back of the Feld-Hell printer built by Eckhard, DG9JO
(source: ref. 1C)
Martinus (Mart) Rooth (PA0MRB, SK), and XYL Ans (PA0ARB, SK), built a complete and modular Feld-Hell "clone". The keyboard sender is based on a modified typewriter keyboard! See ref. 1E. However, it does not have a spinning character drum with slip-contacts. Instead, there is a single, stationary character-disk. It has 7x14=98 contacts (one for each pixel of the Hell font) and a rotating arm with a two brush contacts. The conductive metal patches of the character drum are replaced by a large diode matrix (±800 diodes). For each character on the keyboard, there is one set of diodes for the associated black pixels. The printer helix is made of 2 turns of a heavy-gauge steel spring (5 mm ID), stretched to 10 mm (for 10 mm wide paper). The amplifier is based on an ECF80 triode/pentode. The construction project took several years.
Complete Feld-Hell "clone" built by Mart (PA0MPR) and Ans (PA0ARB)
Rear of the Feld-Hell "clone" built by Mart (PA0MPR) and Ans (PA0ARB)
(the wire "spaghetti" on the right is part of the diode matrix)
Bottom view of the Feld-Hell "clone" built by Mart (PA0MPR) and Ans (PA0ARB)
(the wire "spaghetti" is part of the diode matrix)
Top view of the Feld-Hell "clone" built by Mart (PA0MPR) and Ans (PA0ARB)
(note the character-disk with rotary contact on the left; source: ref. 1E)
Top view of the Feld-Hell "clone" built by Mart (PA0MPR) and Ans (PA0ARB)
(the 750 rpm motor in this photo was taken from a Hell "Kleinfax" KF-108)
Close-up of the character-disk with rotary contact
Mart also made a simple printer:
Feld-Hell printer built by Mart (PA0MPR) − top view
Feld-Hell printer built by Mart (PA0MPR) − front view
Feld-Hell printer built by Mart (PA0MPR) − close up
The adjustment for the motor speed is quite interesting: it is a continuously variable transmission (CVT). The motor drives the spindle via a string. The motor shaft is conical. The string passes between two washers that can be moved by turning a knob on the front panel. This moves the string on the conical shaft: the larger diameter part of the shaft results in a higher spindle speed (at constant motor speed). Likewise, if the string is moved to the smaller diameter part of the conical shaft, then the spindle speed is decreased. The motor is mounted on a spring-loaded swivel, to maintain tension on the pulley.
Feld-Hell printer built by Mart (PA0MPR) − speed adjustment mechanism
Kees Grauwelman (PA3AFD, SK) built a Hell-printer ca. 1984 (ref. 1K):
Feld-Hell printer built by Kees (PA3AFD) − front view
Feld-Hell printer built by Kees (PA3AFD) − front view, zoomed-in
Feld-Hell printer built by Kees (PA3AFD) − top view
Feld-Hell printer built by Kees (PA3AFD) − rear view
This printer uses standard 9.5 mm (3/8 inch) wide paper tape, like the start-stop Siemens-Hellschreibers. The spindle is actually a steel threaded insert, from the do-it-yourself store. It has a coarse (large-pitch) screw thread on the outside and a fine thread on the inside. It was cut to size, such that the thread makes exactly two turns.
Feld-Hell printer built by Kees (PA3AFD) − threaded insert used as spindle
Louis Crijns, PE1DXH/PA3DSA 1982 machine used separate motors for paper transportation and the spindle. The spindle was electrically heated, and "printed" on heat-sensitive paper tape.
Feld-Hell printer built by Louis, PE1DXH/PA3DSA
(source: ref. 1J)
Also ca. 1982, Germain Geerinck (ON4IB, SK), built a magnificently machined compact Feld-Hell printer. It uses a strong 1200 rpm asynchronous motor that is kept running at the required 1050 rpm with a centrifugal regulator.
The beautiful Feld-Hell printer built by OM Geerinck, ON4IB
(source: ref. 1J)
Ca. 1997, Bastiaan, PA3FFZ, built the Hell-printer shown below. Ref 4. The spindle is etched from a brass rod. The two motors are tape transport motors from a VCR (high torque and easy to control speed, unlike VCR video-head motors).
Front & back of the Feld-Hell printer built by Bastiaan, PA3FFZ
(source: ref. 1D)
Dale, VK1DSH/VK2DSH, built his Feld-Hell printer/sender in 1999. Ref. 1F. It uses an ASCII keyboard from a TTY terminal, a Single Board Computer (SBC) with a 65C02 processor, a spindle motor (with optical encoder) from a DEC line printer, paper transport stepper-motor from a 5¼" floppy disk drive, electromagnet from a telephone relay, 16-character LCD, electronics for motor speed control, tone generation, and tone detection/magnet driver. It prints onto standard 11/16" telex paper tape.
The Hellschreiber sender/printer of Dale, VK1DSH/VK2DSH
(source: ref. 1F)
Close-up of the printer mechanism of the machine built by Dale, VK1DSH/VK2DSH
(source: ref. 1F)
Jan ON4ASZ (ex EA3DPB) has built several Hell-printers over the years. In his first machine, he used a 220 Vac squirrel-cage motor for paper tape transportation, and a 24 Vdc motor for the printer helix. Coarse speed control is done with a rheostat/potentiometer. The printer's electro-magnet came from an electro-mechanical pulse counter (mfr: Hengstler). It prints on standard 11/16 inch wide Telex punch tape.
The first Hell-printer of Jan, ON4ASZ/EA3DPB
Print-outs made with Jan's first machine
Jan completed his second machine in August of 2010.
Close-up of the printer mechanism of the second Hell-printer of Jan, ON4ASZ/EA3DPB
The inside of Jan's second machine.
Jan finished construction of his third machine in February of 2012.
Front-view of the third Hell-printer of Jan, ON4ASZ/EA3DPB
Close-up of the printer mechanism of Jan's third machine
In 2008, Wim, PA0WV, made a fully electronic Feld-Hell "printer", ref. 1G. It has a 256x128 pixel graphic display LCD (an old Optrex DMF610 with a 12.7x7 cm viewing area) and an AT90S8515 processor with an 8 MHZ clock.
The XYLCD project of Wim, PAØWV
(the LCD measures approximately 12½x7 cm (5x2¾"), including the bezel)
The circuitry of Wim's Hell-LCD
(source: ref. 1G)
Arie van Ooijen, PE1AQB, has built has built a very nice modular Hellschreiber printer system. He demonstrated the system at the 33rd annual Hell Meeting (October 2010). It includes his latest addition: two interchangeable printer heads for the start-stop Hell-80 mode - a "world's first"! One printer head uses a continuously spinning spindle, the other uses a 7-pin needle printer-head and ribbon cassette from a PC-printer. The paper transport module uses a stepper motor. The electronics are implemented on several circuit-cards in a rack.
Left-to-right: paper tape transportation module, needle printer-head module, paper tape roll
The Hell-80 character set, printed by Arie's needle printer-head.
Arie's paper-tape transportation module (left) and Hell-80 spindle-printer module
Close-up of the start-stop spindle-printer module
Making a start-stop Hell printer (or more precisely: a printer that is compatible with the start-stop Hell-system) is tricky. The original Hell start-stop equipment (Hell-72, Hell-80) uses a continuously turning motor and a complicated clutch mechanism - not suitable for home-building! Arie has kept the continuously turning motor, but does not use a clutch. His ingenious solution detects the start position of the spinning spindle, with a pin that protrudes radially from the hub of the spindle shaft (see photo above). The pin passes through an optical detector. The detector's output signal signal is sampled, and shifted through a FIFO-register. The depth of the register is equivalent to a single column of the Hell-characters. Upon detection of the start-pulse of a character (5 pixels in the first column of the Hell-80 font; effectively at least ≈ 3 msec of the "mark" tone), the FIFO buffering is used to delay the received pixels until the pixel stream is properly aligned with the position of the spindle. See ref. 1H.
The latest creation of Arie is another Hell-80 start-stop printer. The paper transport motor and spindle motor with optical detector are as described above. The helix shaft, printer magnet, and chassis parts are from a salvaged Hell-72 machine. The original printer spindle was damaged; the damaged spindle thread was removed and replaced with a 2-turn steel spring (i.e., with intent to use it as a Feld-Hell printer). The photos below show operation with 9.5 mm wide paper tape, whereas the modified 2-turn spindle was dimensioned for 1/2 inch wide paper tape.
Another start-stop Hell-80 printer head
Luca Bronzi, IZ6HPJ, has built a Feld-Hell mode printer mid-2010. The helix is a steel spring. The ink roller is a wooden piece that he turned on a lathe, and covered with felt. The motors are from an old printer and copier. The speed of the spindle motor is controlled with pulse-width modulation (duty cycle), based on a 555 timer IC. The paper transport speed is adjusted via the motor voltage. The pinch roller is also wooden, with two rubber O-rings. He uses rolls of cash-register paper tape, cut back to 15 mm width, and Rexel-brand ink. He also wound the 400 mH solenoid of the electromagnet
Front of the Hell-printer built by Luca, IZ6HPJ
Rear-view of the Hell-printer built by Luca, IZ6HPJ
Side-view of the Hell-printer built by Luca, IZ6HPJ
Luca's printer in action
Gerhard Oberforcher, VE6AQO/DL9BBR, has been working on a beautiful compact printer. The photos below show the status of his drive-unit module in 2012.
The drive motor drive module of the Hell-printer built by Gerhard, VE6AQO
The motor is a Buehler Tacho that came out of an old Tandon Floppy Disk Drive. The associated speed controller circuitry is re-used, to maintain accurate and stable speed. The drive-shaft for the paper tape transportation is driven by down-gearing the spindle-shaft with a 40-1 worm gear drive. The shafts are held in place accurately by slightly pre-loading the ball bearings.
Hell-printer built by Gerhard, VE6AQO
On the back of the spindle shaft, there is a combined flywheel and optical encoder disk sensor. The sensor has two outputs. One output provides the pixel clock for sending pixels. The other output provides an index pulse twice for each full shaft-rotation. The pixel clock has a period of nominal 8.16 msec ( = 2x standard Hell-pixel duration). The index pulse has a period of 57 msec. The slotted strobe-wheel has 14 pixel-cutouts ( = 1 pixel column) and two index-cutouts. A slightly longer opto-interrupter detects the index pulses. The index pulse is active a little before the pixel clock. This allows the interrupt handler of the PIC processor to detect the presence of the index pulse. This linkage will synchronize the sending of character pixels the same way it is done in a Feld-Hell machine with its character drum that is rotated continuously by the central motor.
Flywheel and stroboscopic sensor (left) and beveled gear (right) of Gerhard 's printer drive-module
The ¼ inch tape drive-shaft is driven via a 1:1 bevel gear. The ink felt roller is made of felt, from salvaged inkjet printers. The entire sending of characters will be done with a PIC, programmed to clock out characters pixels with the timing of the drive train.
Michał Zielaskowski (SQ3TCE) started building his own printer during the spring of 2016. What's special about it? With few exceptions, all parts - including the spindle - are made with a 3D-printer! I don't have any photos yet, but here is a sample of early printing tests:
First print-out of Michał's printer - still showing some motor speed control issues
(©2016, Michał Zielaskowski, SQ3TCE)
Michał uses the tone-pulse detector circuit design from Max Perner (DM2AUO).
Rommert (PE1DCF) built a Feld-Hell equipment set in 2017. He demonstrated it at the 40th Annual Hellschreiber Meeting that was held in October of 2017.
Front view of the Feld-Hell printer built by Rommert (PE1DCF)
Top view of the Feld-Hell printer built by Rommert (PE1DCF)
Close-up of Rommert's printer spindle and printer magnet
The spindle is a steel threaded insert from the do-it-yourself store, just like Kees (PA3AFD, SK) used in his 1984 printer.
Print-out made with Rommert's printer
(print out of a QSO during the weekly Sunday European Feld-Hell net on 80m, recorded on 22-Oct-2017)
Rommert's Feld-Hell keyboard sender with a diode-matrix memory for the 7x14 bitmap font
The electronics box with power supply, tone filter, and driver amplifier for the printer magnet
HELL/CW AUDIO FILTERS
When I started getting involved with "Hell" (late 1970s), I only had an old WW2 receiver (BC348/BC224). It crystal filter was InOp (now I think that the IF frequency was probably misaligned). So in 1983, I decided to build a very sharp AF filter for my printer (-3 dB bandwidth is 150 Hz, -60 dB is 680 Hz!). The filter output impedance is that of land-line POTS: 600 ohm. This matches the audio input impedance of Feld-Hell machines, as they were intended for use over telephone land-lines. The filter design is from Klaas, PA0KLS (ref. 3A, 3B). It uses five standard 88 mH toroids, surplus from telephone companies; 88 mH loading coils (in various forms, sizes, materials) have been used extensively on long analog telephone lines worldwide since the early 1900s. The 88 mH telephone toroids have become rather hard to find. There is a very good alternative: the Toko 10RB series. I bought two sets of five at a very reasonable price at JAB Electronic Components in the UK, for use in a filter per ref. 2J. Note that that you can find inductors of the same value for considerably less money elsewhere. However, you will find that they do not work in these filter applications: their DC resistance is much too high - at least five times that of the Toko inductors!
I also have the parts lying in my drawer for a very nice active AF filter, with very few parts. It uses a switched-capacitor filter IC - much better than OpAmps! See ref. 2K. References 3C-3H discuss some other passive designs for narrow filters.
My implementation of Klaas' filter design
(additional circuitry on the right is an LM386 AF amplifier (bottom) and a TL082 input buffer amp)
Band-pass characteristics of my filter
(spec curve in red, measured curve of the bandpass filter in my Feld-Hell is shown as reference (curve shifted from 900Hz to 1000 Hz)
- Ref. 1: Descriptions of home-built Hellschreiber printers
- Ref. 1A: "PAØMJS maakte zelf een mechanische hellontvanger", Dick Rollema (PA0SE), Electron, nr. 10, October 1980, pp. 557-559
- Ref. 1B: "Schrijftoestel voor ontvangst van hell- of morsesignalen", Martien, PAØMJS, Electron, 5/1982
- Ref. 1C: "Selbstbau eines Feld-Hellschreibers", Eckhard, DG9JO, RTTY, No. 2, 1981
- Ref. 1D: "Mode HELL, zelfbouw (deel 1-2)", Bastiaan Edelman (PA3FFZ), CQ-PA, 10/1997, pp. 348-349 and 11/1997, pp. 385-387; some more details are described here by PA3FFZ; personal communication, June 2008, in Dutch
- Ref. 1E: "Hellschrijver voor zelfbouw (deel 1-5)", (description of home-built Feld-Hell of Martinus Rooth (PA0MPR) and A. Rooth-Beems (PA0ARB)), Bastiaan Edelman (PA3FFZ), CQ-PA, 2/1994, pp. 32-34, 3/1994, pp. 63-64, 4/1994, pp. 97-98. 5/1994 , pp. 121-122, 12/1994, pp. 322-325
- Ref. 1F: "The Feld-Hellschreiber. A home-built, direct printing telegraph system", Dale Hughes (VK2DSH), Amateur Radio Magazine (Wireless Institute of Australia), Vol. 68, No. 11, November 2000, pp. 4-10
- Ref. 1G: "De XYLCD - Deel 1 & 2", 128x256 LCD with micro-controller that can be programmed to be a hardware Feld-Hell receiver, see photo below), by Wim, PAØWV, CQ-PA, 2008, nr. 11 & 12
- Ref. 1H: "Zelfbouw HELLschrijver met nonstop Helix" & "Gereedschap voor HELL80 in startstop bedrijf", Arie van Oijen (PE1AQB), 2011, 10 pp.
- Ref. 1J: "Hellschrijven wint terrein", Dick Rollema (PA0SE), Electron, nr. 2, February 1983, pp. 73-76
- Ref. 1K: "Schrijven over hellschrijven", C. Grauwelman (PA3AFD), in "Electron", 2/1984, pp. 77-80
- Ref. 2: Circuits/schematics for signal detection, printer magnet drivers, motor speed control:
- Ref. 2A: "Radiomodem voor Hell, Morse en RTTY", Koos Fockens (PA0KDF), Electron, 10-12/1986 [in Dutch].
- Ref. 2B: "CW/Hell-skrift demodulator", Jan-Martin Nøding (LA8AK), Amatørradio, 1987, nr. 41, February, Nr. 41, pp. 41-43, Nr. 42, March, pp. 66-69
- Ref. 2C: "Accurate motor speed control", H.S. Malvar, in "Wireless World", August 1980
- Ref. 2D: "Relay speed-up", Frits Geerlings (PA0FRI)
- Ref. 2E: "Increasing Amplifier relay Speed", Bob Wolbert (K6XX)
- Ref. 3: Audio filters for Hell and CW
- Ref. 3A: "Laagfrequent filter voor Hell en CW", in "Reflecties door PA0SE", Electron, 6/1981
- Ref. 3B: "CW-Filter mit 200 Hz Bandbreite", E. Kantz, Funkamateur, Nr. 8, August 1981, pp. 402-403
- Ref. 3C: "Die Frequenztrennung", Funktechnische Monatshefte (FTM), Heft 5, May 1942, pp. 66-68
- Ref. 3D: "Laagfrequentfilter van ON4ASZ voor telegrafie en hellschrijven", in "Reflecties door PA0SE", Electron, January 1992, pp. 5-6
- Ref. 3E: "Audiofilter - realisiert nur mit L und C" [ON4ASZ/EA3DPB filter], Frank Sichla (DL7VFS), CQ-DL, 3/1999, p. 223
- Ref. 3F: "CW-Signalregenerierung ganz einfach" [ON4ASZ/EA3DPB filter], CQ/DL, 9/1999, p. 735
- Ref. 3G: "Audiofilter - einfach, aber gut" [ON4ASZ/EA3DPB filter], CQ-DL, 12/1999, p. 984
- Ref. 3H: "Passive (LC) narrow band AF filters" [with ON4ASZ filter], Pat Walker (G3VA), in “Technical Topics”, Radio Communications (RadCom), March 1998, pp. 62-63
- Ref. 3J: "Een passief laagfrequentfilter voor de ontvangst van telegrafie", Roelof Bakker (PA0RDT), Electron, January 1997, pp. 15-20 (incl. PA0KLS CW/Hell filter). Here is a small Excel spreadsheet that I made for calculating the filter component values.
- Ref. 3K: "NEScaf: A Switched Capacitive Audio Filter", [few parts, adjustable center frequency, adjustable bandwidth, steep; source: New England QRP Club. Also see N5ESE's version of the NEQC NESCAF kit. [pdf]
External links last checked: October 2015
©2004-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.