Ampliphase
Ampliphase: amplitude modulation via summing phase-modulated carriers.
Ampliphase is a brand name for an amplitude modulation system that works by summing phase-modulated carriers. RCA originally marketed this family of modulation and amplifier technology for AM broadcast transmitters, but it was not developed by them. Instead, McClatchy Broadcasting acquired the technology through a patent purchase in the mid-1930s. The design was first proposed by H. Chireix in 1935, who called it "Outphasing."
The system splits a carrier signal into two identical signals, then shifts them 135 degrees apart to establish a baseline power output with zero modulation. Each signal is phase-modulated by the audio signal—one positively, the other negatively. After amplification to the desired power, the two signals are combined in the transmitter's final output filter stage. When the signals are closer in phase, the output amplitude increases; when they are more out of phase, it decreases. A complication is the need for a "drive regulator," which is simple to implement at 10 kW or lower power levels but more complex at higher levels. Drive regulation is most effective when instantaneous power output nears zero.
McClatchy Broadcasting—a former owner of AM, FM, and TV stations and a California newspaper publisher, not to be confused with the present-day McClatchey Broadcasting LLC—installed the first Ampliphase transmitter at KFBK in Sacramento, California, running 50,000 watts full-time. Later installations included KOH in Reno, Nevada (5,000 watts days, 1,000 watts nights). Other McClatchy AM stations, such as KBEE in Modesto and KMJ in Fresno, both in California, used conventional transmitters. Unlike most commercial AM broadcast transmitters, Ampliphase units do not require expensive or large modulation transformers or reactors, saving on upfront capital costs. The downside is higher maintenance requirements. The design trades lower capital cost for higher operating expense and achieved a modest improvement in overall efficiency before other amplification and modulation schemes replaced it.
RCA turned Outphasing transmitters into mass-produced products. Their first models operated at the 50,000-watt level, with lower-power versions like 10 kW and 5 kW following later. Only one known transmitter of this type remains in use: KFBK in California keeps an RCA BTA-50H—the "last gasp" of the Ampliphase concept—as an auxiliary transmitter.
- Original proposer
- H. Chireix
- Original name
- Outphasing
- First developer
- McClatchy Broadcasting
- First marketer
- RCA
- First transmitter power
- 50,000 watt level
- Later power levels
- 10 kW and 5 kW
- Last model
- RCA BTA-50H
Lore & Background
The Ampliphase system was not developed by RCA, but by McClatchy Broadcasting (a former group owner of AM, FM and TV stations, also a California publisher of newspapers, not to be confused with the present-day McClatchey Broadcasting LLC). The first Ampliphase transmitter was KFBK in Sacramento, CA (50,000 watts full-time). Later known installations were KOH in Reno, NV (5,000 watts days/1,000 watts nights). Other McClatchy AM stations like KBEE, Modesto, and KMJ, Fresno, both of CA, employed conventional transmitters.
Unlike most other commercial designs of AM broadcast transmitters, Ampliphase units do not require expensive or large modulation transformers nor modulation reactors, thereby saving on the initial up-front cost. The down-side is the Ampliphase units require more maintenance. The Ampliphase concept trades a lower 'Capital' cost for a higher 'Expense' cost. Ampliphase achieved a modest improvement in transmitter overall efficiency before other amplification and modulation schemes could replace it.
Only one known transmitter of this type is still in use. KFBK in California maintains an RCA BTA-50H (the 'last gasp' of the Ampliphase concept) as an auxiliary transmitter. Radio Caroline has a working RCA BTA-50H on display aboard its radio ship Ross Revenge, however this transmitter has fallen out of use and is unlikely to be put back on the air since the Ross Revenge currently broadcasts via relay to a more efficient land-based transmitter.
Reader's Guide
The Ampliphase system is notable as an early alternative to conventional AM modulation that avoided large modulation transformers and reactors, reducing initial capital cost. However, this came at the expense of higher ongoing maintenance. The design achieved a modest improvement in overall efficiency before being superseded by other schemes. In the post-WWII era very few transmitters of this type were made, except for special purpose orders that kept the design nominally commercially viable into the 1970s. With the oil crises of the 1970s and 1980s, Pulse Duration and Pulse Step Modulation schemes for AM transmitters took hold at power levels at or beyond 50 kW. Currently, the only manufacturer of AM transmitters that uses phase modulation to achieve AM is SRK Electronics, but their design is implemented entirely in the digital domain, avoiding the maintenance issues of the analogue Ampliphase transmitters. KFBK's main transmitter is now a solid-state Harris unit, the prototype for which was later named the DX-50. KOH has long since scrapped its home-built 'outphasing' transmitter for conventional units.
Did You Know?
- The Ampliphase design was originally proposed by H. Chireix in 1935 and termed 'Outphasing' by him.
- Only one known transmitter of this type is still in use: an RCA BTA-50H at KFBK in California as an auxiliary transmitter.
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