ARRL announces second homebrew challenge

The first ARRL Homebrew Challenge, announced in the August 2006 issue of QST, offered a prize for the best 5W, 40 meter CW and voice transceiver our readers could build. The only requirement was that the rig had to be built for less than $50 of new, readily available parts.

In 2009, ARRL ups the ante in the Second Homebrew Challenge by upping the power limit to 50 W. All entries are due no later than February 28, 2010.

"Low power (QRP) operation is fun, but often a little more power is even more fun," said ARRL Technical Editor Joel Hallas, W1ZR.

"For our new challenge, we are asking readers to design, or adapt a published design (with appropriate credit), and build a 50 W amplifier to follow a 5 W/40 meter transceiver. This could be used behind one of the radios from the original challenge, or from other QRP radios at similar power level."

There will be two cash prizes for this challenge. In addition to the prizes, the winners will receive the usual QST page rate for published articles.

The amplifier must operate on 40 meters. It must operate as a linear amplifier capable of accepting a QRP input signal and putting out a minimum of 50 W PEP into a 50 ohm load. It must meet all FCC requirements for spurious emissions. The amplifier must include control circuitry to switch from transmit to receive through a single contact closure and provide a path from antenna to receiver during receive mode.

Hallas suggested taking a look at the two part series by Rick Campbell, KK7B, "Designing and Building Transistor Linear Power Amplifiers," in the February and March 2009 issues of QST.

Hallas listed some basic requirements for the Homebrew Challenge:
* It must meet all requirements while operating from a 13.8 V dc power supply.
* It must put out 50 W PEP on 40 meters with a 5 W PEP input.
* It must be a linear amplifier with third order and higher
intermodulation products must be below -28 dBc (28 dB below the carrier) as measured using the procedures described in the ARRL Lab product review Test Procedures Manual.
* It must be capable of key down operation at 40 W CW output for 5 minutes without more than a 5 percent reduction in output level.
* Single contact transmit/receive switching is required.
* Parts must be readily available either from identified national retailers or by mail order. No "flea market specials" allowed.
* The total cost of all mechanical and electrical parts required for
duplication of the amplifier cannot exceed $125.
* All entries must be received at ARRL Headquarters no later than February 28, 2010.
* The amplifier that meets all the basic requirements at the lowest cost will receive a prize of $200.
* An additional prize of $300 will be awarded for an amplifier that meets all the basic requirements and, in the sole judgment of the reviewers, provides the most useful mix of the following additional attributes within the $125 cost limitation:
* * Full output with an input lower than 5 W, perhaps as low as 2 W, to allow operation with other popular radios.
* * Use of aluminum oxide or other types of power transistors rather than beryllium oxide.
* * Operation on additional bands besides 40 meters.
* * Operation in support of full break-in (QSK) mode.

Hallas noted that some RF power transistors, and even some conduction cooled vacuum tubes, are built on a beryllium oxide substrate or heat conduction base. "Beryllium oxide is inert and safe if properly handled," he said. "If crushed, drilled or filed, however, the resulting dust can be harmful if breathed. Many new power transistors use an aluminum oxide substrate that avoids this problem. It is recommended that aluminum oxide be used if possible. If you do choose to use beryllium based transistors for your amplifier, please treat them with the respect that they deserve, and dispose of them as hazardous material outside of normal household waste disposal channels if they fail or are no longer needed."

Each entrant must submit the fully operational amplifier, as well as a documentation package. This package should include a draft QST article with a discussion of the design considerations and tradeoffs and a description of the construction techniques. The article will also include test and alignment steps, operating instructions, along with a
readable schematic with a list of all parts used, their source and price.

Those who do not have the capability to measure the desired and undesired output products accurately may submit amplifiers early for an unofficial pre-test in the ARRL Lab. Hallas advised entrants allow two weeks (from ARRL's receipt of the package) to be notified of the results.

The station will first be evaluated in comparison to the basic
requirements by the ARRL Laboratory. Entries determined by the Lab to be acceptable on the basis of FCC spectral purity and output requirements will be further evaluated by the QST Technical Editorial Staff.

While the basic rules and evaluation criteria were provided in the original announcement, Hallas said that there are always questions on interpretation. "We have thus again provided a Web resource to provide the questions and answers (FAQs) to all interested participants. Answers provided should be considered a part of the rules and are binding on all entrants."

Source: The American Radio Relay League