Antenna Pre-amp - Part 1 - First Attempt

On the to-do list since we moved: get my amateur radio station set back up. We have a larger yard and a two-story house and I've been scheming of how to make best use of the space without running afoul of the wife, dogs, and neighbors. In the vein of taking baby steps first, I'm going to try putting in a shortwave listening antenna but rather than going up high like you might think antennas should, I'm going to be placing this one six feet or so off the ground along one of our fences. From what I've been reading, this style of antenna (something like a Beverage but not exactly) is very good and not picking up noise. It is also not very good at picking up radio signals but better at not picking up noise.

OK, let me try that again. The antenna reduces the amount of noise it picks up more than the amount of radio signals I might want to hear. This reduction in the signal-to-noise ratio is great but it has the side-effect that you need some way of boosting the signal back up to a useable level. To provide this boost I'm building a small amplifier called a "pre-amp" that will go in between my radio and the antenna.

The design I'm using  is one I've found with extensive documentation. The designer, Larry (call sign W7IUV),  has put a lot of time into building, testing, and documenting his work; this is very helpful for people like me who have not spent any time in building radio-frequency circuits before. In fact, I'm going to be copying an entire portion of his radio set-up by also building a few switchable attenuators like he has; I have no idea how effective the antenna and pre-amp will be.

After ordering the required parts and finding some cheap scrap PCB on eBay to use as the circuit board, I went to work making the cuts in the double-sided PCB to form the nodes in the circuit. The designer used a Dremel to make the cuts in his PCB but lacking such a tool, I used a utility knife. As you can see, mine didn't turn out super neat but it was good enough. (I took the picture after starting to populate a few of the capacitors.)

As you can see in the photo, the parts span the gaps in the copper, connecting the various nodes. The back side of the board is ground and I drilled a few vias to connect a few top-side ground pads to that larger ground plane. The RF-gurus say having a big ground plane helps reduce noise in the circuit; sounds good to me.

Here's what things looked like after I fully assembled the board:

(The components sticking out the sides are just resistors used in testing and not part of the pre-amp proper.)

Construction and testing of the circuit revealed a few problems:

• The largest by-pass capacitor, a 4.7uF tantalum, shorted-out on me twice. I don't know why this is happening. The input voltage is ~13V and the capacitor is rated at 50V. I don't know if I got a bad lot or if I'm doing something wrong. It may be related to the other problem I'm having...
• The amp draws ~110mA, documentation says it should be somewhere more along the lines of 75mA. The testing process suggests making a comparison in the amount of current drawn when the input and output are terminated (as shown in my picture) and unterminated. If the current goes up when it is unterminated, the designer says this is an indication that the tran-sistor is oscillating at a very high frequency. I don't see a difference terminated vs. unterminated; its just high all the time.

Maybe the two are related, maybe they aren't. Maybe my pre-amp is oscillating, maybe it isn't. The good news is that I found somebody (WD8DSB) who, based on the pictures the designer provided of his pre-amp, made up a diagram of showing how the designer laid out his board. I've got the components and now that I've built it once, the second time shouldn't take too long. I plan on building this second version following his plans and comparing the current draw to the first. Stay tuned....