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Skyrider Diversity - Model DD-1  -  Part 1

History of Diversity Reception,   Development of the DD-1,

Amateur Radio Station XE1G,   The Production DD-1

Restoration of the Skyrider Diversity DD-1 Console Receiver   - Part 2

Schematic Errors and Engineering Upgrades,   DD-1 Performance Details,

Known DD-1 Serial Numbers,   DD-1 Photo Gallery


by: Henry Rogers WA7YBS - Western Historic Radio Museum


 Skyrider Diversity - Model DD-1


History of Diversity Reception - Diversity receiving equipment utilizes two or more receivers connected together at the second detectors and AVC lines. The receiver inputs are from widely separated antennas. The goal of a diversity receiver is to greatly reduce or eliminate fading radio signals. Fading radio signals have always plagued shortwave reception and are encountered when the signals being received originate from a transmitting station located at a distance that requires the radio waves to propagate via skywave through the ionosphere. As signals fade down to the noise level much of the information being transmitted is lost until the signal level increases above the noise again. Sometimes deep fades will last for half a minute or more, causing program interruption or important messages to be lost. Besides fading, other phenomena occurs when radio waves are refracted through the ionosphere, such as selective sideband fading causing distortion on AM signals along with rotation of the radio waves causing polarization changes.

 Photo Above:  H.O. Peterson (left) and H.H. Beverage testing and adjusting the triple diversity receivers at Riverhead, Long Island, New York.  Photo from August, 1933 - Radio News

In the early 1920s, two engineers working for RCA, Harold H. Beverage and H.O. Peterson, began investigating what was actually happening when signals faded. The event that fomented Beverage and Peterson's interest happened when the two were monitoring signal reception from two points simultaneously. They were utilizing a telephone line to monitor Peterson's receiver at his home and were comparing that to the received signals from the RCA station site, located about one-half mile away. By monitoring the same transmitter signal as received from two different locations simultaneously, Beverage and Peterson noted that each station received the signal with different fading characteristics. More tests showed that different fading occurred with as little as three hundred feet of separation of the receiving antennas.

Beverage and Peterson first connected strip-chart recorders to a combination of three antennas and three receivers. The charts showed that not only was the same signal at different amplitudes at each antenna but, at any one instant, the signal at one antenna was also many times out of phase with that same signal on the other antennas. At first the engineers speculated that radio waves were being refracted in the ionosphere at different angles and therefore some waves would be intercepted by an antenna while other wave-angles would miss the antenna. A large multiple antenna should capture more wave angles and reduce fading. Actually, they found the opposite happened. Fading became worse with larger antenna arrays. The two men then theorized that over-the-horizon radio waves could be refracted simultaneously from different heights in the ionosphere, these waves would travel slightly longer or shorter paths and arrive at a single antenna at slightly different times. The delay of the longer path signals would sometimes cause multiple phases of the signal to occur on a single antenna. Sometimes the phase differences would add resulting in stronger signals and sometimes the phases would cancel causing a drop in signal strength. Since the ionosphere was nearly always in a changing state and since radio waves would frequently be simultaneously refracted at several different levels in the ionosphere, fading due to radio wave phase changes on a single antenna was always going to be present.

 The solution was to use separate antennas and separate receivers to provide the isolation necessary so each antenna and receiver would react to radio waves specific to its time and space relationship. As the signals passed through the receivers, the various phases at the RF frequency maintained their out of phase relationship until they were converted to audio signals at the second detector. The process of detection eliminated the RF out of phase nature of the signals since now the signal information was at an audio frequency. Each second detector output would be tied together with a common load, operating an AVC system that interconnected all the receivers, resulting in a single audio signal the amplitude of which was the result of the strongest received signal at any one time. Through experimentation, it was found that at least a full wavelength of antenna separation was necessary to have consistent difference in the phase of the signals.

Additional problems started to surface when it became necessary for the diversity receivers to operate on CW. The interaction of the receivers in maintaining a constant detector load output regardless of variations on the separate antenna-receiver inputs required the AVC to be operational. When a BFO was injected into the second detectors the strength of the BFO oscillator would capture the AVC and not allow it to follow the signal strength variations in that receiver. >>>

 >>> The solution was to either send modulated CW from the transmitting station, which wasn't always practical, or the second detector outputs could be used (without a BFO) to operate an external device called a Tone Keyer. The Tone Keyer provided a clean CW output that was devoid of any noise, static or interference since it was "keyed" from the diversity output of the detectors. This then allowed the system AVC to be in operation and standard CW to be received. Beverage and Peterson, while working at Riverhead, NY, installed the first space diversity stations for reliable, long-distance communication. 

By the late twenties, RCA had 41 triple-diversity receivers installed at Riverhead in communications with 26 different countries. The outputs were routed to a main office in New York City where operators would copy the strip tapes, decoding the messages. By using filters and multiple tones, several signals could be routed simultaneously to the main office without interference. The associated antenna farm, with 1000 feet of separation between antennas, required acres of land. The installation was used for commercial sending and receiving world-wide radiograms.

As receiver design progressed, the diversity stations became more sophisticated. By the forties, RCA had introduced its AR-88 receiver which was adapted to the new triple-diversity receiver, the DR-89 (Navy designation RDM.) These seven foot tall racks contained three AR-88 receivers, a Tone Keyer with combined output meter, a Speaker Panel, a Meter Panel with S-meters for all three receivers, an antenna patch panel and a power supply. Even more complex was the military RCP and RBP-1, also built by RCA for the Navy during WWII. The RCP used double conversion receivers with multiple tuners installed in four, seven foot tall racks. The RBP-1 was a dual-triple diversity receiver that was essentially two RCPs side-by-side. Using six double conversion receivers in seven racks the entire setup weighed a mere one and a half tons! The RBP-1s were still in use as late as the 1970s.

The Signal Corps also used diversity in many of their teletype installations. The system used was "frequency diversity" which used one frequency and receiver for "mark" and another frequency and receiver for "space." The theory was that each frequency would fade differently and rarely would both frequencies fade together. Only either a mark or a space was needed to decode the teletype, (if it isn't a mark - it must be a space.) The popular receivers used were the Collins R-388, R-390 and the R-390A.

Another form of diversity was to use a vertical oriented antenna in combination with a horizontal antenna. The theory was that the radio waves many times rotated as they were refracted through the ionosphere and therefore good diversity action could be achieved with both vertical and horizontal polarization. This form was called "polar diversity" and was generally used when a full wavelength of antenna separation wasn't practical. In the early forties, a QST article featured a novel way to experience a diversity action using only a single receiver. The writer used a high speed antenna switch, running at 400Hz, to switch between two antennas. The average of the signal strength of each antenna was more or less what the receiver responded to supposedly giving some relief to fading. Additionally, the 400Hz switching gave a sort of modulation to the signals so CW was easy to copy without a BFO. 

Amateur interest in diversity reception waned as WWII loomed in the near future. The expense, the complexity, the space required for antennas all made the results not worth the effort. With less expensive equipment most hams could get the same results. So what if the 'phone stations needed to repeat some of their transmission. On CW, there was no obvious benefit to diversity and, in the pre-WWII days, most hams were on CW. After WWII, there is hardly any mention of diversity in any ham magazines. The commercial stations and the military had always been and were to remain the primary users of diversity reception. As technology progressed into satellite communications by the late 70s and other more reliable forms of communication came into use, the need for large, expensive diversity receiving installations all but vanished. Most of the equipment has been scraped or sold into surplus.


Development of the Skyrider Diversity

Amateurs became familiar with the principals of diversity reception by reading various books and magazine articles throughout the twenties and thirties. QST published an article by Chester W. Rice in the August 1927 issue about shortwave uses that mentioned diversity theory and reception. For the avid 'phone operator, having the equipment to eliminate fading signals and the associated distortion, must have seemed like a virtual necessity. To the CW op though, diversity seemed to not be worth the effort, after all, CW copy was not that seriously affected by fading. However, James Lamb, Technical Editor for QST, and engineer James McLaughlin decided that diversity was worth a try and began their research into some kind of practical amateur receiver that would incorporate diversity. With the financial help of Dr. James M.B. Hard XE1G, the first amateur dual diversity receiver was built in the Spring of 1936.

The XE1G Receiver - The First Amateur Dual Diversity Receiver

James Lamb and James McLaughlin had been experimenting with diversity reception as early as 1931. At that time, the two engineers used two shortwave converters connected to two identical TRF receivers used as IF amplifiers. Both TRF receivers had been modified to have amplified AVC. The performance was satisfactory enough for each to continue their research and designs. For the next few years, each pursued their own careers but decided to have an informal meeting in the spring of 1935. The purpose of the meeting was to discuss each others thoughts on what technical advances might be used to create a practical amateur diversity receiver. At about the same time, well-known amateur operator Dr. James M. B. Hard,  XE1G from Cuernavaca, Morelos, Mexico, let it be known he was ready to finance the construction of a dual-diversity receiver of Lamb's and McLaughlin's design.

Both Lamb and McLaughlin thought that the tuning of commercial diversity receivers was too cumbersome. In commercial installations, where usually only a few frequencies were used, tuning each of the receivers to a specific frequency was a task only performed once or twice a day. In an amateur station, the ability to tune though each amateur band easily and switch from band to band quickly was an obvious necessity. The XE1G receiver uses a single Local Oscillator between the two receiver front-ends to provide "single dial" tuning. Other circuits incorporated in the XE1G receiver were chain-drive for the band switch, separate pre-selector tuning (not ganged to the main tuning,) three power supplies, separate audio amplifier using a modified Collins 7C audio amplifier, tuning coverage from 1.7MC to 30MC in four tuning ranges, amplified AVC and a separate Jensen speaker with an add-on tweeter unit. The completed receiver was delivered to Dr. Hard in the Spring of 1936.

Photo left shows the XE1G receiver as it appeared in the May 1936 issue of QST along with an article describing the receiver in detail. The rack contains the dual receivers, pre-selectors and multiple power supplies. Shown on top of the speaker is the modified Collins 7C audio amplifier. The speaker cabinet contains a Jensen speaker with an add-on tweeter unit.

photo above: Dr. James M. B. Hard, XE1G, in his station. The May 1937 issue of "RADIO" magazine published in Los Angeles, California ran this photo of Dr. Hard's incredible station located in Cuernavaca, Morelos, Mexico.

XE1G - Dr. James M. B. Hard


Above is Dr. Hard's 1938 QSL card. Note that the call is now XE1GE. Also, listed as No. 1 Receiver is the Dual Diversity Receiver by James J. Lamb and James L.A. McLaughlin. Most of the equipment listed on Dr. Hard's QSL can be seen in the photo to the left.
XE1G (later, XE1GE,) was an incredible station as can be seen from the photo above-left. The Dual Diversity Receiver is dominate in the left foreground of the photo. To the right of the Diversity Receiver is the Jensen speaker cabinet and, in front of it, the Collins 7C audio amplifier. By the time this photo was taken the Dual Diversity Receiver had been repainted black. Other equipment included an RME-69 receiver with Peak Preselector, a Hammarlund Super-Pro receiver (against the wall, far right,) an oscilloscope and frequency meter are on the right side of the operating position table and to the right of the clock is a variac. A 1KW Collins 202X transmitter is against the wall left and a Collins 30DBX 100W transmitter against the wall right. The antenna feed lines drop through glass plates in the ceiling. Between the two transmitters are large meters that monitored the voltage and current coming into the shack. Dr. Hard had a private 3000vac line running one half mile from the power company main line to a pole transformer on his property. The HV line was stepped down to 240vac and then run underground some 800 feet to his hacienda. All lines in the house were run in iron conduit that was embedded in the concrete floors. The shack earth-grounds consisted of 2" copper ribbon buried under the foundation of the building. The XE1G antenna was supported by three 160 foot masts. Using a system of weights and pulleys any of the radiating systems could be raised or lowered or even tilted at angles as much as 45 degrees. XE1G was a famous amateur station in the mid to late 1930s and Dr. Hard worked the world from his QTH in Cuernavaca, Morelos, Mexico.

The First Prototype DD-1

The Prototype DD-1 built by James McLaughlin and Karl Miles in 1937

Photo QST, Dec. 1937

Shortly after the QST article about the XE1G receiver was published, Hallicrafters became interested in building a Dual Diversity receiver for the amateur market. Karl W. Miles, Hallicrafters' chief engineer, was given the task of developing the working prototype. Working directly with James McLaughlin, Karl Miles was to build a dual diversity receiver, based on the XE1G receiver, that was within the capabilities of the company to manufacture.

The first prototype was given ample space in the December, 1937 issue of QST.  The simplified schematic shows that the prototype now has the preselector stage ganged to the second RF and Mixer stages. The Diversity Meters are now in a small cabinet that can be placed near the receiver. The Infinite-Rejection system is now incorporated into the IF amplifier. Separate Power Supply and Amplifier are now in much smaller cases which can be placed on each side of the receiver but they are not shown in the schematic. Obvious in the photo of the prototype are the many Hallicrafters parts utilized in the construction.

By the time McLaughlin and Miles started work on their new prototype, the XE1G receiver had logged in 5000 hours of reliable operation. Dr. Hard was still enthusiastic about his receiver and stated that many times, with poor conditions, only it could be relied upon for intelligible reception. He mainly operated on 20 meters with two antennas spaced at 50 feet, not the ideal spacing but sufficient to provide usable diversity action. With improvements that had been developed since mid-1936, mainly in mechanical design, IF amplifier improvements and the Infinite-Rejection system, Miles' and McLaughlin's new version would cover .54 to 36 MC in five bands, have a simpler band switch system and be housed in a smaller cabinet. The article concluded with a rather vague statement about how the audio modulation for CW is not really working satisfactory and there were still "bugs" to be worked out. >>>

>>> This earliest prototype is very different from the later prototype and the production DD-1. Some of the more obvious changes for the Hallicrafters production units included changing the rotary bandswitch to the massive push-button band switch assembly built by Mallory-Yaxley. The top button is the power switch while the remaining six buttons select the bands. Frequency coverage was also changed from the early prototype with the new range being .54MC to 45MC in six bands. With this new higher frequency, the old 6K7 RF amplifiers were changed to the new Raytheon VHF amplifier tube, the 1851. The LO tube was changed from a 6J5 to a 6K6, affording higher output in the VHF region. IF Gain controls were on the front panels of the prototype but were eliminated for the production version. Also, the production DD-1 uses two IF amplifiers while the early prototype had three. The meter arrangement is opposite of the early prototype on the production model. The panels slope on the production version and a transparent scale is placed over each tuning dial rather than the articulated pointer.

The Production Skyrider Diversity DD-1

The new Diversity Receiver's advertising had been carefully nurtured and the publicity was going to be more important to Hallicrafters' reputation than any profits derived from sales of the receiver. Following their tradition of naming receivers, the DD-1 was dubbed the "Skyrider Diversity" and the June 1938 issue of QST features no less than 10 pages of ads devoted to the new receiver. All of the major component suppliers, such as Mallory-Yaxley, Crowe, Aerovox, Jensen, Stancor, Aladdin and Raytheon weighed-in with their own ads plugging themselves along with the DD-1. It was the largest ad campaign for a new receiver up to that time and this was just QST! Advertising for the DD-1 was everywhere.

The advertising campaign for the DD-1 didn't last very long - only about six months. Bill Halligan and probably everyone involved with the DD-1 project knew that the Skyrider Diversity was not going to be a major seller. With initial prices between $300.00 and $500.00, depending on options, the diversity receiver was just not in most enthusiasts' budget. The base price for the receiver was to be $300.00, with the Power Supply adding another $40.00 and the Audio Amplifier adding another $50.00. The Diversity Action Meters sold for an extra $20.00 and the 12" Jensen table speaker sold for $12.00. The total for the receiver with the standard accessories was $422.00. If the purchaser wanted the ultimate, he could spend a full $500.00 for the Skyrider Diversity Console DD-1. That price included the Diversity Action Meters installed in a matching wooden top. The huge machine-age style speaker cabinet has shelves in the rear (right side) to house the Power Supply and the Audio Amplifier. The 15" Jensen Ortho-dynamic speaker is mounted in a Bass Reflex enclosure that takes up most of the center and left side of the cabinet (as viewed from the front.) A small table is provided as an arm rest for fine tuning the DD-1 or it could be used as a small writing table for logs or whatever. The speaker has a 5000 ohm to 8 ohm matching transformer mounted on it. The fabulous cabinet design was the work of Auther Haggstrom, who had also designed cabinets for Jensen, E.H.Scott and McMurdo Silver. When one looks for a price comparison to get a feel for what could be purchased for $500 in 1938, look at the Chevrolet coupe, selling for $550 in 1938 - essentially, the Skyrider Diversity Console DD-1 was the price of a new car! >>>

>>> The initial prices might seem high but after the first advertising campaign was over the prices were increased,...substantially. Certainly the second production run of about 20 to 30 receivers cost the prospective buyer dearly. The prices listed in the 1940 ARRL Handbook ad for the DD-1 are $450 for the receiver, $135 for the amp and power supply and another $135 for the 15" speaker in the Haggstrom cabinet. Total cost for the DD-1 console was $720! The price of the DD-1 table version with diversity meters and speaker was $627!  

Of course, Hallicrafters realized they weren't going to sell very many DD-1 receivers. The price, the size and the usability was just not what the average ham could justify. Hallicrafters probably estimated that somewhere around 100 to 150* receivers would fill all of the orders besides all of the publicity donations that might be needed. The major benefit to Hallicrafters was to their reputation. They now had proven that they were capable of successfully engineering and building a complex, dual-diversity receiver.

*For the past several years, I have been collecting serial numbers of known DD-1 receivers in an effort to estimate production accurately. It now (2014) seems likely that a production quantity of 200 units is far too high. This estimate was originally based on Halligan's 1980 statement regarding DD-1 production. It appears from the reported serial numbers that an initial run was made that was somewhat less than 100 receivers and used serial numbers in the H-80500 to H-80600 range. A few months later a very small production run was made, probably to use up the remaining DD-1 parts inventory and this run accounted for a few receivers up to maybe 20 or less receivers. Only three DD-1 receivers have been reported with serial numbers in this later range, H-85617, H-85629 and H-85648. The current information leads me to believe that only about 120 DD-1 receivers were produced and that total quantity came in two production runs of approximately 100 units and later approximately 20 units. See "Estimated DD-1 Production" section below for more details.

The DD-1 Advertising Images

This is an enlargement of the artwork that was used for most of the DD-1 advertising by Hallicrafters. It is artwork - not a photograph. Most advertising at the time used detailed drawings for their images and though the Hallicrafters DD-1 artwork is detailed, it is not a photograph. Whether it was based on an actual prototype or if it was a "mock-up" is not known but the receiver is different from the standard production DD-1. Obvious is the lack of the Heterodyne Oscillator implying that the BFO was a last-minute addition. Also, individual RF Gain controls on the far right panel with a "Master RF Gain" control (large knob) differs from the production DD-1. The tuning and BS dial logging scales are different and if there is any scaling on the tuning dial it is too light to see in the finished artwork. The four smaller panels are plain with no decorative border. Additionally, the toggle switches are mounted lower than center of the panels while the production DD-1 has these switches mounted center of the panel. The location of the larger knob on each lower panel is different than the production units. The standard production DD-1 used 21 tubes in the receiver, four tubes in the Power Amplifier and one tube in the Power Supply, bringing the total to 26 tubes.







photo above: Bill Halligan and some of the engineers with the DD-1. This is an actual photograph showing a production DD-1. Although it is shown with the console speaker, the wooden top with the Diversity Action meters is not installed.

 Both photos to the right also show production DD-1s. Though all of the images are small it can be seen that the toggle switches are centered in the smaller panels, the panels have a decorative border and the knob arrangement is standard production. Also, the wooden top is installed on the W6LYY DD-1.

More on photos right - The Skyrider Diversity Console DD-1 is shown in the shack of W6LYY, Barney Boyd of San Diego, California. Boyd's shack was to be the state-side link for communications to the Archbold New Guinea Expedition. The table version DD-1 that was delivered to New Guinea for the Archbold Expedition. This set-up featured a special, 15" Jensen speaker mounted in the familiar Hallicrafters speaker cabinet.



Photos - QST, Jan.1939



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