Watts Up: A Simple Amplifier For The Field

I often pack a 2 watt QRP CW rig to the field with a dipole antenna. I seldom have trouble making contacts. Mountain tops are great for QRP work, but much of my hiking is in deep gulches - including the Grand Canyon. Sometimes I've wished for more power when camped at the bottom of a remote,
narrow desert canyon.

The Ramsey QRP Amplifier

Several years ago I purchased a 40 meter Ramsey Electronics QRP amplifier kit on the Internet at http://www.ramseyelectronics.com. These kits have been available for many years, advertised as companions for the Ramsey QRP monoband transmitter series.

Ramsey promises as much as 20 watts output when the amp is driven with one or two watts. At the time of purchase I planned to install an amp in my 4WD truck for use when operating mobile. I immediately built the kit, but set it aside and soon forgot about it. Recently I dug it out, built an enclosure and did some testing with the thought that I might backpack it into Coyote Gulch and use it with my SW40.

The Kit

Ramsey QRP amplifier kits are available for 20, 30, or 40 meters. The price was recently reduced to $39.95 which I think is an incredible bargain. They come with a circuit board and all required electronic parts, including RCA type jacks for the antenna and a push-button on-off switch. This is appealing, since I find it difficult to gather parts for projects.

The kit instructions were very easy to follow, and my amplifier went together in little over an hour. A voltmeter and an ammeter are needed for calibration, otherwise no special tools are required.

Modifications

My amplifier worked immediately, but the output was a disappointing 7 watts for 1.8 watts input using a 12-volt gel cell as power supply. I checked the SWR between the transmitter and amplifier and found it was in excess of 3:1, a problem that several others have mentioned. Removal of two turns from the 6-turn secondary winding of transformer T2 caused the SWR to drop to near unity, and the output to increase to about 18 watts. Setting aside the gel cell, I attached a husky 14.25 volt power supply and found that the amplifier would put out about 25 watts with less than 2 watts of drive.

In my case, the modification of T2 was essential, but no additional changes were necessary. None-the less, I customized the amp by scrapping the RCA-type antenna jacks and using BNC connectors. I also tossed the push-button in favor of a toggle switch and added a low-current 2 ma LED from Radio Shack in series with a 5.6K resistor as an off-on indicator. Mounted in a home-made plastic box, my amp weighs 4 ounces.

The circuit board is very roomy, inviting other modifications and experimentation. An Internet search will reveal many other changes suggested by operators who have been using the amplifier as bones for a PSK31 amplifier. This is an extraordinarily demanding application, and I was surprised to find that they could make it work. It appears that adding a DPDT switch and an output filter for 20 meters could easily make the amplifier a dual-band affair, but I have not tried it.

Operation

The amplifier includes a carrier-controlled relay that functions well, but is noisy in my plastic box. The output transistors did not heat significantly after a 20 minute CW contact at 25 WPM. The signal appeared clean on a spectroscope, and caused no interference to a nearby television set. On-the-air testing provided T9 reports even when the battery voltage dropped to 11.3 volts.

Is it worth it?

I think the price tag for this kit is more than reasonable, and building the amplifier was fun and relatively free of frustration. I have no doubt that my amp will occasionally find use when I run my SW 40 mobile from the truck, or when I am in a stationary camp with a big battery.

But what about using the Ramsey amplifier on the trail? My experiments convinced me again why circuit designers the like of Dave Benson, K1SWL, and Wayne Burdick, N6KR, choose 2 watts output for their trail friendly radios. My SW40 puts out just shy of 2 watts. It draws 16 ma on receive and about 300 ma on transmit. My SST 40 is about the same. My Sierra, which has been modified considerably for 4 watts output on 40 meters and has a digital display, uses about twice as much current. An eight-pack of AA alkaline cells fits in my pocket and runs any of these rigs for the duration of a typical four-day backpacking event with casual operating in the evenings.

The Ramsey amplifier draws over 4.2 amps for 18 watts output . I can switch it off while I am listening, and perhaps turn it off once a QSO has been established, but the battery requirement is still very troublesome for a backpacker. An eight or ten-pack of alkaline D cells appears to be the best option for a trip to the back country, since the amp sucks my 2 AH gel cell to critically low voltage in only a few minutes.
I won't be able to use my little antenna tuners with this amplifier. Even a relatively bulky ZM-2 will not tolerate 18 watts.

The anticipated gain from a ten-fold increase in output power is about one-and-one-half to two "S" units at the receiver. My experiments on the air with the amplifier bear this out. Is the additional weight in batteries worth the trouble? I'm going to think it over very carefully before I take my amplifier to the field.
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Dr. Bob Armstrong, N7XJ, a contributing editor to The ARS Sojourner, is an expert outdoorsman and QRPer who is in medical practice in Manti, UT.