brainwagon

Like many people, I live in a development with a fairly restrictive HOA. This means that I have to be fairly careful to use largely invisible or stealth antenna setups. Because of a lack of suitable trees on my property, this means that I’ve had fairly compromised setups: I’ve gotten the most use from a simple 40m dipole which probably only averages about six meters in height.

But at least for receive there might be an interesting alternative. PA0RDT has developed a simple active antenna which has intrigued me.

Here is DL1DBC’s excellent description.

This article points out something which may not be entirely obvious until one thinks about it: antennas designed for transmission and reception have different goals. A transmitting antenna is designed for efficiency: to send as much of the input power out as radio energy as possible. In this context, short antennas have a very low radiation resistance, so other losses tend to dominate, and you lose a lot of power as heat. But receive antennas aren’t concerned with efficiency: you really just want to preserve the signal to noise ratio of the incoming signal. As long as the noise generated by the preamplifier is low, it doesn’t actually matter how efficient the antenna is. A short antenna picks up less of the desired signal, but also picks up less of the surrounding noise. So, these antennas can work reasonably well, especially on frequencies where full sized antennas are impractical such as VLF.

PA0RDT has created a simple preamp circuit, including a way to power the preamp through coax so you can easily mount this antenna remotely, say 50 feet from your house, and avoid the electrical noise that dominates many active antenna setups. A very neat solution, and inexpensive.

Some more links: Scott built one, using the more ubiquitous MPF102 and 2N3904 transistors, instead of the recommended but harder to find J310 and 2N5109 transistors.

Peter Marx got an official one that was built by it’s creator, Roelef Bakker:

You can read about Peter’s experience here.

A neat little project. Roelef’s articles (esp. the second one) linked from DL1DBC’s give lots of details. Worth looking at, particularly if you are interested in VLF/HF/shortwave listening.