radio8z

EDIT TO ADD: Apr. 17 2011. See my post later in this thread about the efficiency breakthrough.
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EDIT TO ADD: Apr. 17 2011. See my post later in this thread about the efficiency breakthrough.
END EDIT

In another thread (http://www.part15.us/node/2714) I reported on my efforts to increase the efficiency of the Ramsey AM-25 transmitter. Subsequently, since I am reluctant to use my Ramsey or SSTRAN outdoors, I was inspired to construct a transmitter based on my study which could be mounted in a watertight enclosure for outdoor use and I would like to share the project here. This is not meant to be a builder’s guide but rather to show (and show off) what I did in case it may inspire others to develop a similar or improved project.

The first picture attached below shows the assembled transmitter board and the second picure illustrates how it mounts in a standard 4″x4″ marine watertight electrical enclosure.

The transmitter is divided into three sections: audio modulator/bias, oscillator, and RF final amplifier/filter. The audio section is very similar to the Ramsey AM-25 design which provides adjustable audio gain and DC bias to the final. The oscillator is crystal based and uses a digital IC to produce the drive for the final. The final amplifier is a BJT based circuit with a 5 pole low pass filter. Provision has been made for an RF isolation/impedance matching transformer which is yet to be installed on the PC board. This transformer will provide isolation between the circuit ground and antenna system ground to prevent ground loops.

The circuit was built on a PC board using a modified “dead bug” technique. All components are surface mounted to the board which allows a continuous ground plane on the back side. This method also makes circuit changes very simple for further experimentation. The PC board was made using the “toner transfer” method and was etched in ferric chloride. The board will be coated with clear lacquer prior to use to prevent oxidation of the copper traces. I hand tinned some of the traces and then decided this was not worth the effort.

The transmitter checks out with an efficiency of 65% and modulation up to 95% without noticeable distortion. The frequency is stable to +/-2 Hz from cold start to warm but I have yet to check this at the expected ambient temperature extremes so this is still a work in progress. My intent is to mount this at the base of a yet to be built base coil loaded outdoor antenna so details about cable connections have yet to be determined. My plan is to feed a differential audio line level signal, DC power, and ground via a multi conductor cable. I intend to report here when the installation and testing are completed, probably in the next few months.

My well stocked junk box meant I had to buy only the two toroid cores for the filter and the enclosure. It is satisfying to stir a pile of parts with a soldering iron and end up with something which works and then share the experience with others who understand it.

Neil