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F.M. on two meters

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A simple rig which provides deviation to suit all receivers.

Narrow-band f.m., as used on 10 and 6, is not generally useful on 2 as yet, because of the predominance of broad receivers. Here is a simple set-up which will provide substantially distortionless deviation up to the amount required by most superregens, yet it is also capable of narrow-band work with a degree of stability that makes it readable on the sharp superhets. With careful adjustment of the deviation, and a little cooperation on the part of the receiving operator, the signal can be made quite satisfactory on any of the wide variety of receivers now in use on 144 Mc.

Some time ago an article in QST entitled " Simplified F.M."(1) described a method of obtaining narrow-band f.m. by lightly amplitude-modulating a self-excited oscillator which was followed by a Class C multiplier to multiply the frequency deviation and to wipe out the amplitude modulation. The article concluded that the system provided good results when used with an f.m. receiver, but that reception of the signal on most superregenerative receivers was not satisfactory.

Further investigation, along with a review of the results obtained by other amateurs using narrow-band f.m. on the old 2½ and the present 2 meter band, led to the conclusion that while superregenerative receivers were unsuitable for the reception of narrow-band f.m. they might do a reasonable job on f.m. signals of much wider deviation. The actual amount of deviation required would depend on the characteristics of the individual receiver but in general would increase with increases in channel frequency as the selectivity of the receiver was decreased. In simpler language, a 2 meter superregenerative receiver would require less frequency swing than a 1 meter set because in general the 2 meter set would have a sharper selectivity characteristic.

In order to test the validity of these conclusions an f.m. transmitter capable of a frequency deviation of approximately ±45 kc. was put in operation on the 2-meter band. After some preliminary tests to determine the proper audio gain-control adjustment to provide the necessary frequency deviation, and with the aid of patient and careful tuning of numerous superregenerative receivers at stations worked, this transmitter gave a good account of itself. In the period from August 29, 1946, to November 9, 1946, a total of 241 contacts were made, with 97 different stations.

A large. number of these contacts were with local stations within a ten-mile radius, but no trouble has been encountered in working regularly up to thirty miles regardless of band conditions. Completely satisfactory communication with stations as far as fifty to sixty miles away was quite commonplace, and the best DX worked included W3HWN at Mechanicsburg, PA, 150 miles away, and W3GKP at Silver Spring, MD, at a distance of 180 miles. The receivers used at the various stations contacted included every possible type: sharp and broad a.m. superhets, sharp and broad f.m. superhets, and superregens. In all cases the operators using f.m. receivers reported excellent quality when the deviation was adjusted to suit the receiver. Both sharp and broad a.m. superhets also reported very good quality in most cases after the deviation was adjusted to match the receiver selectivity. In nearly every case the operators of rush-box receivers reported satisfactory reception with wide deviation and in those cases where reception was actually unsatisfactory, experience in tuning for best reception of the f.m. signal nearly always resulted in a subsequent satisfactory contact.

The schematic of the transmitter used for these tests is shown in Fig. 1. It consists of a triodeconnected 7C5 oscillator on 16 Mc., a pair of 6AG7s tripling to 48 Mc. exciting an 832 tripling to 144 Mc., which in turn drives the neutralized 829-B operating straight through at 144 Mc. The grid of each 6AG7 is operated with a current of about 200 microamperes resulting in 50 volts of grid-leak bias. The drive to the 832 results in a total grid current of about 2.5 mA or a bias of 125 volts. With the 832 drawing 50 mA of plate current the 829-B grid current is 10 mA.

Fig 1
Fig. 1. Schematic diagram of the 144 Mc. f.m. transmitter.

C110 µF 25 volt electrolytic.
C23 µF 400 volt paper.
C335 pF per section variable (should be mechanically and electrically stable).
C4100 pF 400 volt silver mica.
C5,C8,C9,C11,C14,C150.0022 µF 400 volt mica.
C6,C725 pF 400 volt silver mica.
C1035 pF per section variable
C12,C13100 pF 400 volt mica.
C16,C19,C2125 pF per section variable.
C17,C18,C200.001 µF 400 volt mica.
C220.001 µF 1000 volt mica.
C23,C14470 µF 400 volt mica.
R1820 Ω, 1 watt.
R2500 kΩ volume control.
R35 kΩ, 5 watts.
R415 kΩ, 1 watt.
R5,R6220 kΩ, 1 watt.
R7,R815 Ω, 10 watts.
R91 kΩ, 10 watts.
R10,R11100 kΩ, 1 watt.
R1225 kΩ, 20 watts.
R13200 kΩ, 5 watts.
R146 kΩ, 5 watts.
R1510 kΩ, 20 watts.
R1640 kΩ, 10 watts.
L115 turns c.t., No. 14 enameled wire close-wound on 1-inch diam. ceramic form with turns cemented.
L28 turns c.t., No. 14 enameled wire, 1 inch diam., turns spaced to occupy 1¼ inches, self-supporting.
L32 turns c.t., No. 10 bare copper, 1½ inch diam., turns spaced to occupy 1¾ inches, self-supporting.
L4,L5Single-turn loop connected with 300 ohm Twin-Lead.
L6Hairpin of No. 12 wire, 1 inch spacing, 1½ inches long. (Effective length of inductance is about 2-7/8 inches because of addition of condenser plates.)
L7Hairpin of No: 10 wire spaced 1¼ inches, 3½ inches long. (Effective length of inductance is about 5 inches because of addition of condenser plates.)
L83-inch antenna-coupling hairpin.
L9Primary of 6N7 Class B input transformer.
L10Primary of plate-to-voice-coil output transformer.
B1Microphone battery.
CNHeavy wire near tube envelope opposite fiat side of plate.
MA10-5 d.c. milliammeter.
MA20-150 d.c. milliammeter.
MA20-15 d.c. milliammeter.
MA40-300 d.c. milliammeter.
RFC1,RFC22.5 mH choke.
RFC3No. 28 silk-covered enameled wire wound to a length of 1 inch on a 3/8 inch form.
RFC4,RFC5Ohmite Z-O choke.
S1Tune-operate switch.
S2High/low-power switch.

In this transmitter, as in the previous model, f.m. is accomplished by simple Heising amplitude modulation of the self-controlled oscillator. The a.m. is wiped out by the succeeding Class C stages while the frequency deviation is multiplied nine times by the two tripler stages. The amount of frequency deviation is adjusted (up to the limit of the 6J5 modulator) by the audio gain control, R2.

The amount of reasonably linear deviation available from such a modulator is closely related to the L/C ratio of the oscillator. Curves of plate voltage vs. percentage of frequency shift for several values of oscillator-tank L/C ratio are shown in Fig. 2. These curves were run with the same coil and different condenser settings and show that an increase in this ratio results in a greater available percentage of swing. Curve C of Fig. 2 representing the conditions used in these tests is reproduced in Fig. 3 to show actual frequency vs. plate voltage.

Fig 2
Fig. 2. Curves showing the percentage frequency shift of the 7C5 oscillator, using different L/C ratios.

Fig 3
Fig. 3. Curve C of Fig. 2, reproduced to show actual frequency variation with plate-voltage changes.

Although this curve is not quite as linear as was hoped for it does show a reasonably straight characteristic of about ±5 kc. centered around 145 volts. This deviation when multiplied by nine results in a final deviation of ±45 kc. Actually the plate voltage was set at 150 volts since this value was convenient to regulate with a VR tube. From a practical standpoint some distortion from this source is not objectionable on voice reproduction. Since the audio equipment is so simple an1 operating at such a low level its speech reproduction is excellent; hence the plate voltage/frequency characteristic is the only place throughout the transmitter from which distortion can originate.

There is a second reasonably straight portion of the plate voltage/frequency characteristic centered around 70 volts which provides a linear deviation when multiplied by nine of nearly ±60 kc. Operation over this portion of the curve with a plate voltage of 70 was also tried and found successful. However, the grid drive to the following stage was, of course, reduced and the very small amount of audio required for proper modulation made the gain control difficult to adjust so that in general the higher plate voltage gave better performance. However, with some additional adjustments this lower-voltage operating range seems to offer the possibility of better performance than the high-voltage range.

With the oscillator-coil dimensions shown, the oscillator resonates around 16 Mc. with a total capacity of only about 20 pF. Since most of the required capacity is available in the tubes and stray capacity, the tuning condenser is set at near the minimum value. Although the L/C ratio of the oscillator tank is unusually high, the use of a large oscillator tube running light, along with careful physical construction, results in good stability. The drift of the oscillator frequency is low enough to be entirely unobjectionable. The oscillator must, however, be built solidly throughout to prevent frequency change because of mechanical vibration (when the table is hit or a door slams, etc.). No claim is made that this oscillator arrangement is the optimum for this type of operation. No other oscillator circuits have been tested, and no circuit parameters have been varied except the oscillator-tank L/C ratio.

Just to complete the record, the power to the final amplifier of the transmitter was 80 watts. The antenna consisted first of two half-waves in phase and later of 5 extended half-waves in phase stacked vertically. Both were fed with 300-ohm Amphenol Twin-Lead matched with a quarter-wave impedance transformer. The second antenna provided a definite gain over the first, but the difference was not too astounding. The antenna did, however, provide good all-round coverage without the bother of rotation. QRM was in most cases not too bad on a good a.m./f.m. superhet.


F.m. communication was found to be quite satisfactory on 2 meters provided the transmitter is capable of supplying a wide undistorted frequency deviation. A frequency deviation in excess of ±45 kc. should be available to allow reception on all types of receivers to be encountered. The need for wide undistorted swing was brought home most forcibly during initial tests, when the oscillator L/C ratio was reduced. This change caused the undistorted frequency deviation to be materially reduced. As a result operation on the band immediately became unsatisfactory. Sharp a.m. and f.m. receivers requiring low deviation continued to receive an excellent signal, but stations using broad a.m. superhets and rush-boxes which previously reported good quality began to report that the speech was considerably distorted. Reducing the audio level to the value at which distortion was no longer present reduced the audio recovery of the broad receivers to an unsatisfactory level. The number of contacts showed a sharp decline. Restoring the oscillator to the L/C ratio shown in the schematic diagram remedied the trouble and results were again found to be satisfactory.

The need for a large frequency deviation seems to rule out the present use of narrow-band f.m. for general work on the 2-meter band. In order to provide sufficient swing, the limited deviation available from crystal-controlled modulators requires so many multiplications that an unwieldy number of tubes and tuned circuits is involved. The system of modulation used during this investigation seems to offer very good possibility of providing sufficient frequency swing with a small number of multiplications and hence with few tubes and few tuned circuits. This arrangement has been found most useful in making quick frequency changes. The number of tuned circuits could be still further reduced in the transmitter shown in Fig. 1 by using an untuned p.a. grid circuit. Link coupling was used only as a matter of physical convenience. In order to provide better all-round performance of the self-excited oscillator the final design of this transmitter will also include an untuned push-pull isolating stage between the oscillator and the 6AG7s.

During contacts with a large number of stations using superregenerative receivers it became evident that, given an undistorted f.m. signal of sufficient deviation, good reception depended on careful adjustment of the receiver. Since the superregen is essentially an a.m. detector, audio must be recovered from an f.m. signal on the slope of the selectivity curve and the receiver must therefore be tuned to one side (generally the high side) of the received carrier. Results were found to improve tremendously by proper adjustment of the regeneration control in conjunction with this slight detaning from the center of the received signal. Best reception was usually found to be at that point where the receiver was just beyond the threshold of superregeneration, this being the most selective condition for superregenerative reception. In some cases it was necessary to adjust the antenna coupling to the detector to provide the correct set of conditions for good f.m. reception. General directions for tuning in an f.m. signal on a rush-box are as follows: back off the regeneration control until the receiver is just superregenerating and then tune off the center of the carrier on the high-frequency side until the best compromise between audio level and good quality is obtained.

The information presented in this article is in no way meant to infer that wide-band f.m. is either superior to or preferred over narrow-band f.m. for v.h.f. systems in which control of the type of receiver used is possible. It is meant to show what can be done with f.m. on 144 Mc. under present conditions. Actually the use of f:m. with most of the 2-meter receivers in use at present offers only the advantage of transmitter simplicity. The weak-signal reception and noise-suppression advantages with f.m. may only be realized with a true f.m. receiver and a good one at that. The mere addition of a discriminator to an existing a.m. receiver, while it will give better f.m. reception, will not show the f.m. in its true light. To make use of all the advantages of f.m. for communication work a receiver using a conventional discriminator must have at least one and preferably two good limiters, and the over-all gain must be sufficient to allow complete saturation of the limiters on extremely weak signals. With these requirements fulfilled, f.m. reception takes on an entirely new aspect.

The receiver used at this station consisted of a Hallicrafters S-36 modified for 2 meters. This receiver provides for a.m. or f.m. reception. In its original form the set seemed to lose its pep in the f.m. position, and for weak-signal reception was inferior Co a.m. The set was further modified to correct a few obvious deficiencies so that the limiter would operate correctly, and become completely saturated at low signal levels. These changes gave the 2-meter band an altogether new dimension. Complete receiver quieting is now accomplished with a weaker signal on f.m. than on a.m. Signals from modulated oscillators which are too weak for comfortable copying on a.m. often jump right out to you as R5 signals when the set is switched to f.m. The broad and narrow i.f. and a.m./f.m. detection make the S-36 a versatile and quite useful receiver on a band where every possible variety of signals will be encountered.


  1. Geist, "Simplified F.M.," QST, December, 1945.

J.C. Geist. W2STZ.