Now I understand - Phase Locked Loops

Every now and then I come across great books or videos that explain a concept in such a way that it becomes immediately obvious what is going on. I'm a great believer in learning by demonstration or even better, learning by doing.
I came across another explanatory video recently that I thought was too good to keep to myself. It covers a topic that was a complete mystery to me: Phase Locked Loops. We utilize them in almost every modern transmitter and receiver yet most people I have talked to view them as a black box that, fortunately, does its job well and usually without interruption.
The video below does a good job on opening the black box and showing just what makes phase locked loops ... well, lock.

What is the AM-6155/GRT-21 ?

FAA AM-6154/GRT-21 Amplifier

From the website of N1RWY

The FAA used the AM-6154 and 6155 amplifiers in the early 1980's as ground-to-air AM transmitters. The 6154 was designed to cover 118-136 MHz and the 6155 to cover 225-400 MHz. Both models were set to 50 watts output, and amplified an AM exciter.

Both models were rack-mount, 7" high, 19" wide, and about 24" deep; they weigh about 75 pounds and have an internal AC power supply that can be used on 120/240 VAC. They are built so they have a slip-in RF drawer that contains the RF amplifier itself. The main chassis holds the power supplies and a small blower. No T/R switching is included. Because they are intended to be used with the exciter, the amplifier circuitry expects to get some DC control signals from the exciter which have to be simulated by some modifications.

The reason the amplifiers are so interesting to V/UHFers is because they are capable of outputting over 400 watts on 2, 222 or 432 MHz with only a few hours work and almost no extra parts. This is because they use the 8930 tube (or the Amperex equivalent, the DX-393), which is basically a 4CX250R with a 350-watt anode. 50 watts average power of AM is actually four times that power peak; or 200 watts; and these things were designed to do that all day, every day. That's why they used such a high-power tube for only 50 watts output power.

The RF drawer of either model can be modified for ANY of 2, 222 or 432; but as it comes from the factory, modifications HAVE to be made for any of the three bands. In general, the mods involve redesigning the RF grid circuit to be more efficient and to tune one of the ham bands. The plate circuit in either model can be used on any of the three bands, although use at 400 watts on 432 MHz places a big strain on the stock plate DC choke and plate blocking capacitor, which almost always have to be reworked for serious 432 MHz use. They have apparently proven able to withstand high power on 144 and 222 MHz without modification, although the mods do not hurt.

The power supply chassis is composed of three smaller drop-in chassis, an additional metering PC board, and a 400 Hz 120VAC blower which is powered by a DC-to-AC converter (one of the three drop-in chassis). The high-voltage power supply uses a very compact and lightweight transformer, a dual-section oil-filled filter capacitor, two screen voltage dropping resistors and a string of three zener diodes to regulate the screen voltage. Another drop-in chassis houses the filament transformer which also provides grid bias power and another winding to provide power to the DC-to-AC converter which drives the very small, compact 400 Hz blower (which is able to cool the large tube due to the high speed, 5500 RPM, of the blower).

A front-panel-mounted meter with 12-position switch is also wired to a PC board which contains circuitry that originally monitored the output power and antenna VSWR in addition to the blower current, filament voltage, grid, screen and plate voltage, and plate current. The RF drawer contains a directional coupler and low-pass filter which are almost always removed from the output of the amplifier (and sometimes rewired to the input side) because they don't handle 400+ watts very well.

So, where can you get one?    

Most of the amps are sold by Fair Radio in Lima Ohio. They advertise in QST. Sometimes they can be found at hamfests and there have been reports that a few have been picked up from government surplus sales. Fair Radio asks $235 for the AM-6154 and $285 for the 6155. (Based on early 1997 figures) Fair Radio has by far provided the lion's share of them, probably well over a couple thousand to US hams alone.

They are extremely popular among US V/UHFers and usually command a $300+ price tag, whether modified or not (because the shipping alone totals around $35 for UPS Ground!). Recent estimates by Harry Brown, W3IIT of the Packrats, are that there are OVER several hundred in the Philly area alone; and Ev Tupis, of the Rochester VHF Group, estimates ALMOST that many in northwestern NY state! They're great amps - not 8877's but the price is hard to beat.

[ Thanks to Steve, KO0U/1, and Harry, W3IIT, for the summary and history of these fine amplifiers. ]

Heathkit's first amateur transmitter - Heathkit AT-1

The Heathkit AT-1 represents the commercial embodiment of the simple Master Oscillator Power Amplifier (MOPA) transmitter using a crystal controlled 6AG7 oscillator plus a 6L6 final output tube.

Although it was possible to design and build a simpler transmitter, the goals of output power and stability could become mutually exclusive when trying to operate with only one tube. For a novice class license holder of 1951 the Heathkit AT-1 represented a solid performing rig that would be relatively easy to construct and operate.

The Novice remained the primary entry license until the Morse code requirement was eliminated for Technician licenses in 1990. On HF it permitted code transmissions only, with a maximum power of 75 watts, (input to the transmitter's final amplifier stage) on limited segments of the 80, 40 and 15 meter bands.

For $29.50 and the loan of a few tools you could get some use out of that new novice license

The earlier MOPA circuit from the ARRL handbook of 1941 below shows a layout remarkably similar to the circuit of the AT-1 although it is designed for plug in coils rather than the band-switching arrangement of the later Heathkit transmitter.

MOPA transmitter using a 6L6 and an 807 as the power amplifier (ARRL Handbook 1941)

For a little added complexity MOPA transmitters generally offered better stability of frequency and keying waveform than single tube crystal controlled or self exited rigs. The straight forward design of the AT-1 should have looked familiar to novice class hams after studying the ARRL handbook or other radio publications.

Heathkit AT-1 Circuit diagram showing band-switching arrangement and link coupled output

Once the novice had upgraded his license the AT-1 could be expanded by the addition of the Heathkit VF-1 variable frequency oscillator to allow transmission on any frequency within the allowed band.

The Heathkit VF-1 Variable Frequency Oscillator

The VF-1 covered 160-80-40-20-15-11-10 meters and used an OA2 voltage regulator tube to provide a stable voltage for the oscillator. Ceramic coil forms, solid construction and high quality components were used to help increase stability.

It was recommended that to correctly couple an antenna to the AT-1 you would use an antenna coupler such as the Heathkit AC-1 which also included a low pass filter.

The Heathkit AC-1 Antenna Coupler. Designed to attach to a single wire by the insulated post on the front panel.

Heathkit AC-1 Antenna Coupler circuit diagram

Although Heathkit did not produce a AM modulator for the AC-1 there is provision for modulator connection on the rear panel. The earlier ARRL manuals have several suitable circuits for modulators that would work with the AC-1. Most functioned by driving a modulation transformer with the output from an audio power amplifier. The secondary of the modulation transformer would be carrying the DC plate supply for the power amplifier tube plus or minus the instantaneous voltage of the audio waveform. By changing the plate voltage to the final amplifier tube the radio frequency output would be controlled by the amplified audio frequency resulting in amplitude modulation.

South Texas Balloon Launch Team launches balloon aimed at China

My daughter and I made the short trip to the No Label Brewing Company in Katy TX to watch the South Texas Balloon Launch Team launch a helium balloon aimed at Nanjing China. Thanks to Tom AE5QB for letting us know about this event!

To track the balloon in real time go to : http://aprs.fi/?call=a%2FKT5TK-11&_s=mb

From the press release of the South Texas Balloon Launch Team:

The South Texas Balloon Launch Team is pleased to announce the upcoming launch of its twenty-eighth, helium-filled, unmanned balloon in twenty one years. The purpose of this flight is to establish a world record for distance by floating a balloon from Katy, Texas to Nanjing, China.

The balloon will be released at approximately 3 P.M. CST on Saturday, February 11, 2012. The site of the launch is at the western end of the No Label Brewery complex at 5373 First St., Katy Texas, near the old rice grain silos.

The public is invited to this free event, with a special invitation to science students and teachers. Free helium-filled balloons will be available to the first 100 students. Sorry, no pets allowed in the balloon area.

The balloon payload package weighs only about five ounces (150 grams) and contains a high altitude GPS tracking system and a VHF amateur radio transmitter. To conserve weight and battery life, no camera equipment will be on board. The maximum altitude is expected to be above 100,000 feet, with horizontal speeds between 100 and 150 MPH. The balloon size will increase from about five feet to about 39 feet at maximum elevation. Recovery of the payload package is not expected.

Individuals may follow the balloon's progress on the Internet by logging onto APRS, filling in the "track callsign" field with "kt5tk-11", and change the "show last" to 24 hours.

The South Texas Balloon Launch Team is composed of about twenty active amateur radio "Ham" operators from a variety of occupations who donate their time and expertise.

We appreciate the continued support by No Label Brewing Company for our amateur radio projects.

Now I understand - Standing Wave Ratio (SWR)

There are great books out there that explain radio principals in a plain and straightforward manner. Many are vintage military manuals whose primary aim was to give a functional understanding of the theory involved without getting caught up in the interesting but ultimately unnecessary details.

Modern material on the other hand can sometimes miss the mark of providing a functional understanding in favor of trying to completely address the high level theory and leaving practical matters to be addressed later ... or perhaps never.

The AT&T training video certainly hits the mark when it comes to explaining and showing the properties of radio waves. It shows how they are affected by termination and changes in impedance, what resonance looks like, how terminated and non-terminated lines reflect waves and much more.

Material like this is well worth the time of anyone who has an interest in radio or electronics.

More video are available from the AT&T archives here : http://techchannel.att.com/showpage.cfm?ATT-Archives