Hallicrafters Dd1 Communication Receiver Instructions

							DD 1 INSTRUCTION BOOKLET 
CONTENTS PAGE  
1 Discussion of Diversity Action 
2 Antenna Recommendations  
3  
4  
5 Operating Suggestions  
6  
7 
8 
9 Rejecter Theory and Operation  
10 
11 
12 Four Curves on Rejecter Operation  
13 I.F. Selectivity Curve, Sensitivity Curve, S units, M/V input curve. 
14 Image ratios curve  
15 A.V.C. characteristics curve, Audio Fidelity Curve, Overall Fidelity Curve 
16 Functional Schematic of Receive 
  
DISCUSSION OF DIVERSITY ACTION 
The advantages to be gained from diversity reception with the model DD1 can best be 
appreciated by a general understanding of diversity action. 
Before describing this new receiver in detail, it might be well to briefly review the principles 
of diversity reception with its benefits as compared to the best single-receiver methods. 
The principle improvement is, of course, in the reduction of fading. Fading is the result of 
several waves from the same transmitter arriving at the receiver over different paths. The 
signal delivered at the output of the receiver is the resultant of the several waves which arrive  
						

							over paths differing in direction and length. Additionally, these waves upon reaching the 
receiver are of varying amplitude and phase.  
Fortunately for our solution to the problem of fading, a signal does not fade identically in two 
antenna locations at the same instant - even when the two antennas are spaced a relatively 
small distance apart, or when they are near to each other and in different planes of 
polarization. What is happening is that the phase angle between waves arriving over varying 
paths is continuously rotating. When the amplitude is the same in two waves and the signs are 
opposite, a condition of cancellation, or zero signal, takes place. (We have all experienced the 
condition of a signal taking a rapid and vicious dive into the noise level).  
Diversity reception takes advantage of this vulnerable spot in fadings armor. 
The basic idea is to pick up the signal waves on two different antennas and then combine the 
signals in a common receiver circuit. While it would be convenient to couple these various 
antennas to our receiver the result would not be the correct answer, With signals of various 
phase differences arriving over separate antennas it would still be possible to have complete 
cancellation of the input to the receiver if the signals on each of the antennas were of equal 
amplitude but directly out of phase. Because of this condition it is impossible to obtain 
diversity action through the use of several antennas on one receiver. 
The only place where combining the signals should take place is in the output of the final 
detector where we have the rectified envelope of the signal to work with, The final detector 
outputs of the two receiver sections are tied together across the common load of the 6J5 audio 
amplifier. Combining the signals after rectification results in audio output which will be the 
average of the two signals. By virtue of the common A.V.C, the receiver with the greater 
signal input takes control of the gain of both receivers and supplies practically the total 
output. The gain of the other receiver, at that instant of time, is so reduced that the noise it 
would otherwise contribute is made negligible. The resultant signal-to-noise ratio is that of the 
particular receiver in control at that instant and results in a considerably higher average 
signal-to-noise ratio than can be obtained with the single receiver method of reception.  
In addition, selective fading, or that phenomena which occurs when the carrier fades and the 
side bands remain at their original level with attendant distortion, is  
						

							greatly minimized. The reason for this is clear when we understand that the receiver in control 
at any instant is the one which is delivering the greatest signal to the amplifier.  
The Hallicrafters Dual Diversity receiver, model DD1 differs from the usual commercial 
installation in that instead of using separately tuned receivers with individual high frequency 
oscillators, a common oscillator is used which feeds the first detectors of each of the two 
receivers. This eliminates the expensive precautions taken in the multi-receiver system to 
keep the high-frequency hetrodyne oscillator of one receiver from feeding into the other 
antenna or input circuits. Additionally, with separate oscillators it is practically impossible to 
obtain perfect synchronism for any length of time. 
In addition to reducing fading, diversity reception also reduces heterodyne interference in the 
reception of telephone signals. This may be considered as a species of phase selectivity 
peculiar to the diversity system, irrespective of its frequency selectivity characteristics. When 
the audio frequency outputs representing the envelopes of two modulated signals are 
combined in the audio Load circuit, they will add arithmetically. However, beat frequency 
heterodyne products from an interfering carrier will add up vectorally. Hence, with the dual 
diversity system the interference to signal ratio will, at worst, be the same as with a single 
receiver when the heterodyne outputs of the two detectors are in-phase aiding and of equal 
amplitudes. Under all conditions, the beat-note products will have a tendency to cancel each 
other. The combined beat-note output may be reduced to zero when the separate beat-note 
products are of equal amplitude and opposite phase, Since the relative phase of the interfering 
heterodyne outputs of the two detectors depends on the radio-frequency phase relationship at 
the inputs of the receiver, and since this relationship is likely to shift continually, the average 
heterodyne interference is considerably reduced.  
The Hallicrafters Model DD1 Diversity system of reception does four important things: 
1. Elimination or great reduction of fading in high frequency reception,  
2. An average increase in signal strength better than that obtainable with any single receiver, 
3. Greatly improved signal to noise ratio. 
4. Reduction of heterodyne beat-note interference to a marked degree. 
The Model DD1 overcomes so many handicaps attendant with normal methods of high 
frequency reception as to represent a receiving system without a parallel.  
ANTENNA RECOMMENDATIONS  
As previously mentioned, diversity action depends on the fact that a radio signal will not fade 
in two antenna locations at the same instant. Antenna systems used commercially for diversity 
reception are spaced by thousands of feet. While such an arrangement affords the ultimate in 
antenna design, practically such an installation represents an impossible installation problem 
for the normal user of the DD1 receiving system. Additionally, practice has shown that 
satisfactory diversity action is afforded by having the two antennas separated only a quarter to 
a half-  
						

							wave length apart. Suffice to say, regardless of the type of antenna system you should choose 
to use, diversity action will be better when the two antennas are separated by as great a 
distance as possible.  
At this point it might be well to bear in mind that no one aerial, regardless of type, will work 
at peak efficiency throughout the tuning range of the DD1 receiver. The most practical 
antenna system for general coverage reception will be that consisting of two inverted L 
antennas approximately 100 feet long overall, Fig. 1A. Running these two wires at right 
angles to one another will give very satisfactory pick-up in a location where the noise level is 
fairly low.  
Considering the fact that the antenna input impedance of the model DD1 and the average 
terminating impedance of the single wire type of antenna, at high frequencies, is 
approximately 400 ohms, a good match Is effected. 
NOTE - When using this type of antenna the jumper should remain connected between A2 
and G, or ground. 
Where reception on the short wave broadcast channels is most desired, two of the 
commercially available all wave doublets erected at right angles to one another and spaced 
as far apart as possible will provide excellent diversity reception on these frequencies. 
However, it might be well to point out at this time that in no case should the twisted pair 
transmission Line as supplied with this all wave type of antenna be connected directly to the 
diversity. Only through the use of the matching transformer supplied with these kits can a 
satisfactory match between the 72 ohm impedance of the twisted pair and the 400 ohm input 
of the receiver be attained. A serious mismatch with a resultant loss of sensitivity and poor 
reception will occur if a direct connection is made. If you should wish to give preference to 
any one frequency your logical choice would be an aerial cut one half wave length long and 
coupled to the diversity by means of the P matching stub and the 440 ohm open wire line. A 
440 ohm line is conveniently constructed using #10 wire separated by 2 glazed porcelain 
spacers. The length of the half wave flat top and dimension for the Y matching stub may be 
determined by referring to graph A and Fig. 1 B respectively. 
In view of the numerous types of antennas that can be constructed these suggestions cover 
only the most practical and easily erected systems. All three types have demonstrated their 
worth over a period of years. It is to your advantage to erect as efficient an antenna system as 
possible in order to enjoy the capabilities of the DD1 receiving system to the fullest extent,  
NOTE 1 The antenna lead-ins should be kept as far apart as possible when entering the 
receiving position and should not be allowed to parallel one another unless separated by at 
least 10 feet. 
NOTE 2 Unlike the average receiver, the DD1 requires a good ground connection. If a low 
resistance external ground is an impossibility a cold water pipe is advised. unless a good 
ground is used objectionable interference may result.  
						

							OPERATING SUGGESTION 
(General)  
The receiver has reached you in a sturdily constructed container. The packing case was 
designed to give maximum protection to the instrument while in transit. If the case bears 
visible evidence of having been damaged in shipment it is suggested that you get in touch 
with the carrier at once.  
The power amplifier and power supply are packed separately. The cables which will connect 
these units to the receiver are packed in the same container. The A. C. cord to connect the 
receiver, and indirectly the audio amplifier and power supply, to the A. C, line is packed with 
the receiver. 
If the Diversity meters have been purchased that unit will be in its own container complete 
with the connecting cable.  
Check all the units to be sure that nothing is missing. 
If you plan on having the audio amplifier and power supply at each end of the receiver, the 
power supply should be put at the right and the audio amplifier at the left when looking at the 
receiver from the front. When you are locating the Hallicrafters PM 12C speaker and case it Is 
inadvisable to place it on top of the receiver cabinet proper. The speaker wires should be 
connected to the 5000 ohm terminals on the rear of the power amplifier. The other two 
terminals on the power amplifier are of 500 ohm impedance.  
It is assumed, before going into the suggested operating procedure, that you have installed 
two antennas and have brought their lead-ins, or transmission lines, into the room at a point at 
which you intend to operate the receiver. If conveniently possible it is recommended that the 
amount of lead-in from the window or lead-in insulators to the receiver be kept reasonably 
short.  
Connect the audio amplifier and powers supply to the receiver. It is impossible to connect 
them incorrectly because of the different types of sockets on each unit. The cable for the 
power pack has an 8 prong plug and the one for the amplifier a 5 prong plug. Both an 8 and a 
5 prong socket will be found on the back of the receiver into which each of these plugs will 
fit. 
The seven prong socket takes the connecting cable for the Diversity Action meters, should 
they have been purchased with the receiver. The Diversity Action meters can be placed in any 
position, so that they can be conveniently viewed by the operator. 
The A.C. cord packed with the receiver plugs into the recessed male receptacle in the back or 
the chassis. When this cord is connected to the power mains you have also automatically 
connected the audio amplifier and power supply, The top button in the vertical row of band 
switch buttons in the center of the receiver turns all three units On and Off, It will be 
noticed that the receiver is off when this button is pushed in. In order to turn the receiver on 
all that is necessary to do is to push the button covering the particular range of frequencies to 
which you wish to listen. When anyone of the 6 band switch buttons are operated the AC 
off button will automatically release and connect power to the receiver, audio amplifier, and 
power supply.  
						

							FREQUENCY COVERAGE  
The Hallicrafters Model DD1 Receiving System tunes from 545 KC to 46 mc. This frequency 
range is covered by 6 bands. They are as follows: 
1 - 540 KC to 1500 KC 
2 - 1.5 MC to 4.2 MC  
3 - 4 MC to 10 MC  
4 - 10 MC to 20 MC  
5 - 20 MC to 36 MC  
6 - 34 MC to 46 MC 
When switching from one band to another, pressing the button that is identified by the 
frequency marking on its front will switch all seven coils for that range, There are a total of 42 
coils in the coil and switch section. For protection and isolation a metal cover is firmly held in 
place over the mechanism by screws Which are threaded Into the coil partitions. 
TUBE FUNCTIONS 
The tube functions will be treated for only one receiver section inasmuch as the R, F. 
amplifiers, 1st detectors, I. F, amplifiers and 2nd detectors are similar in both receiver A 
and B.  
The R. F. amplifier consists of two stages in which the high-gain #1851 type tubes are used, 
This tube represents the latest advancement in tube design and provides substantially 
increased gain on the higher frequencies.  
The first detector-mixer is a 6L7. The injector grid of this tube is extremely well shielded 
which adapts it for the first detector - mixer function.  
The high-frequency oscillator is a 6K6 tube, The output of this tube is coupled to the injector 
grids of the 6L7 first detector tubes in both receiver sections. In commercial practice, where 
separate receivers are used for diversity reception, the most objectionable fault is the leakage 
experienced with the high frequency oscillator of one receiver feeding into the input circuit of 
the other. Expensive shielding precautions found necessary with separate oscillators are 
obviated by the use of the 6K6 oscillator which is common to both first detector circuits in the 
DD1 receiver. The choice of the tube is dictated by the fact that the parallel injector grids of 
the two 6L7 tubes represent a capacitive load which, if applied to full output of the oscillator, 
seriously limits the tuning range. As a result the 6K6 tube, in providing greater signal output, 
allows the load represented by the two first detector tubes to be tied into the oscillator at the 
cathode tap. Coupling the high frequency oscillator output at that point enables the tuning 
range to be extended, Additionally, a higher degree of oscillator frequency stability is 
afforded. In conjunction with its performance in filling the above requirements the greater 
conversion factor at higher frequencies recommends the 6K6 as a tube ideally suited for its 
purpose in the DD1 receiver.  
The first stage of 455 K.C. I.F. amplification uses a 6K7 tube. A 6L7 follows in cascade as the 
second stage. The 6L7 was chosen as the second I.F. amplifying tube  
						

							because of it having an injector grid into which can be fed the output of the 6J5 Heterotone 
oscillator. The Heterotone oscillator modulates the signal flowing in the I.F. amplifier. The 
electron stream in the 6L7 is already modulated by the signal in the normal operation of the 
tube. When the voltage at the heterotone frequency is applied to the injector grid, the electron 
stream is further modulated at this frequency. The product of these two modulating voltages 
appears in the plate circuit of the 6L7 with the result that the signal delivered to the 6H6 diode 
tube has a modulation envelope at the heterodyne frequency. After rectification the CAW 
carrier disappears and the modulation frequency appears as an audible note in the output of 
the 6H6 tube. 
The output of the 6H6 second detector is then coupled to the grid of the 6J5 audio frequency 
amplifier tube which is mounted in the receiver chassis. The output of this tube is transformer 
coupled through a 500 ohm line to the audio amplifier which is in its separate cabinet 
complete with power supply.  
  
AUDIO AMPLIFIER 
The audio amplifier consists of one 6J5 driving two 2A3s in push-pull, A 5Z3 rectifier 
supplies voltages for these three tubes. Having a separate power supply for the audio section 
is advisable, A separate input filter is provided so that the hum level of the amplifier is 50 DB 
below full output. The output transformer on the audio amplifier has both 500 and 5000 ohm 
output terminals. By referring to the fidelity curve you will see the excellent frequency 
response to be expected from this amplifier.  
POWER SUPPLY  
The power supply unit for the DD1 receiver delivers 250 volts at 110 milliamperes. A 5Z3 
tube is used as the rectifier. 
A single section filter of conservatively rated components adequately filters the direct current 
supplied to the tubes in the receiver. The unit is similar in appearance to the audio amplifier. 
Channel type of construction with flame-welded joints is employed which contribute to the 
mechanical rigidly of the unit.  
OPERATION OF CONTROLS  
As mentioned previously, the frequency to which you wish to listen is selected by pushing the 
band switch button covering that range. When that range is in the circuit the coverage dial, 
which is the large calibrated dial to the left and controlled by the knob below it and to its left, 
is turned until the frequency appears under the edge of the transparent plate mounted in front 
of the dial. The calibration of the main dial will be accurate only when the band-spread dial, 
which is the right hand large dial, is set at minimum capacity or O. This dial is rotated by 
the handwheel below and to its right, When using the band-spread control the coverage dial 
should be set to read higher in frequency than that on which the desired station or group of 
stations transmits. Adding capacity by turning the bands spread control from its minimum 
capacity position will allow you to resonate on 
   
						

							the wanted frequency. The band-spread control will be of great help in tuning in short-wave 
stations by virtue of the fact that they can be more accurately resonated due to the capacity 
change with this control being but a small fraction of the change experienced should the 
coverage control be used and moved through the same number of degrees. Directly below 
each of the large dials will be seen an aperture through which is visible an indirectly lighted 
vernier dial, Using the markings on this 100 division dial in conjunction with those appearing 
on the larger dials will allow the receiver to be accurately re-set to a known frequency once it 
has been tuned in and logged. It takes 25 revolutions of the hand wheels to complete the 333 
degree are of the large dials.  
In order to become familiar with the operation of the receiver it is recommended that No. 1 
band covering the standard broadcast frequencies be used at first, The operation and functions 
of the other controls as follows:  
To the right of the hand wheel which controls the coverage dial is the head-phone jack. 
Inasmuch as no direct current flows in the head-phone circuit crystal type head-phones can be 
used. For most satisfactory operation high impedance type headphones are recommended, On 
the same plane as the fone jack and to the left of the band-spread hand-wheel is the ANL or 
automatic noise limiter on and off switch. This switch controls the 6H6 noise limiter tube 
which operates in the input circuit of the audio frequency amplifier. The limiter in no way 
affects the signal being received but when surges of static, ignition or other types of 
interference of a pulsating nature are picked up, the limiter cuts off the peaks of interference 
to the level of the signal. because of the interference peaks being of so short a period of 
duration, the signal will predominate. The ANL will be of benefit in minimizing noises so 
objectionable to high-frequency reception.  
The left hand control on the upper panel to the left of the main tuning control is the stand-by 
switch. This switch removes plate voltage from the tubes in the receiver when placed either in 
the up, or down position. In the down position the switch locks in place and must be moved 
back to the neutral or on position manually. The up position of the switch is for 
momentary stand-by. The switch does not lock in this position, returning to center as soon as 
your hand is removed. 
The meter on this panel is the average carrier intensity meter which is calibrated in S units, 
this meter indicates the output of both receiver sections. It will be interesting to note that the 
variation in signal strength, as viewed on this meter, will be minute in comparison with the 
wide variation in carrier intensity shown on the Diversity Action meters. 
The first step in adjusting this Meter is to remove the antenna from each receiver. Turn the 
Master gain control on full. Leave the A & B balance gain control in the center or upright 
position. Raising the lid of the receiver will disclose a knurled shaft protruding through the 
R.F, chassis near the 6J7 meter amplifying tube. With the above conditions taken care of turn 
the knurled shaft until the average carrier intensity meter reads 0.  
To the right of this meter is the Heterotone - Heterodyne Switch. In the center or neutral 
position neither oscillator is operating. In the up position of this switch the Heterotone 
oscillator is connected. In tube functions attention was drawn to the coupling; of the 
Heterotone audio frequency modulator to the 6L7 2nd I. F.  
						

							stage. It is recommended that the heterotone be used for the copying of code signals when it is 
desired to use the receiver in the dual position. The Heterotone gives either 500 or 1000 cycle 
output. The Hi-Low switch in back of the panel near the Heterotone oscillator tube will 
allow the choice of either of those two modulating frequencies. 
In the down position of this switch, the Heterodyne oscillator is connected in the circuit. The 
pitch of the beat note can be varied by the knob marked Pitch Control mounted on the lower 
left panel. The Heterodyne oscillator can be used with the receiver operating in the DUAL 
position but little is to be gained. This is explainable by the reason that when the Heterodyne, 
or B.F.O., oscillator is operating the receiver has no A.V.C. In view of that, no diversity 
action is possible. As a result when the BFO is used it is recommended that either receiver 
A or B be used.  
The controls on the lower left hand panel looking from left to right are: The Pitch control, 
Tone control and Infinite Adjacent-channel Rejection control. The Pitch control, when the 
Heterodyne switch is in the Heterodyne position, will enable you to vary the frequency, or 
pitch, of the beat note.  
The operation of the Tone control Is easily recognized by referring to the Fidelity curves 
appearing later in this manual.  
The upper right hand panel incorporates the I. F. gain switch, Balance meter and Dual switch. 
The I.F. amplifier gain switch in neutral, or center position, gives normal I. F. amplifier gain 
to the 6H6 second detector. In the +10 DB position the switch shorts out a portion of the 
cathode resistor in both the 6K7, 6L7 tubes.  
In the -10 DB position the cathode bias resistor in the I. F. stages is increased in value. In 
many instances it is advisable to operate the receiver with the I.F. gain switch in the -10 DB 
position, This suggestion is explained by the fact that, due to the A.,V.C. action of the 
receiver, reducing the I. F. gain automatically increases the R. F. gain, As we all know the 
greater the R. F. gain before the first detector the more favorable will be the signal to noise 
ratio. 
The circuit balance meter registers the balance of gain that exists between Receivers A and 
B, This meter and its associated controls will be treated later in greater detail. 
The Dual switch to the right of the circuit balance meter is in the plate circuit of both Receiver 
A and B. When in the center position plate voltage is applied to each receiver and dual-
diversity reception is possible. then moved to receiver A position, plate voltage is removed 
from receiver B so that it does not contribute any signal to the audio load after its second 
detector. In the B position receiver A is out of the circuit. 
The first two controls on the lower right hand panel, the Master, and A and B balance RF gain 
controls, are adjusted in conjunction with the Dual switch. Let us examine what the A and B 
balance R. F. gain controls accomplish.