Lee Szuba About

Mini Whip Antenna

I currently have two different antennas for testing. A 20 m band dipole at home in Toronto, and a small, active PA0RDT mini-whip that travels to wherever I am currently living.

The mini-whip is an active antenna, originally designed for long wave and medium wave reception, but that works well all the way up through the shortwave bands. Mine is built in a piece of PVC pipe. One of the transistors original transistors is a TO39 metal can device (2N5109) that doesn’t really exist anymore. It was replaced by a SOT223 package device with similar, if not better performance.

Mini-whip in its plastic pipe

Detail

The antenna is powered by +12V injected through the coax by a small power supply module (a few filter capacitors and an inductor). It is built in a small box constructed from copperclad scraps, with two BNC connectors and a 12V power input.

Power supply

Advantages and Disadvantages

The biggest disadvantage to using an active antenna is the inability to transmit with it.

The mini-whip isn’t resonant on any shortwave frequency, which gives it an incredibly wide usable bandwidth (0-30 MHz). This means only one antenna will receive almost any signal the current version of SDR is capable of processing and still provide a meaningful output. Unfortunately, the mini-whip can easily overload the receiver’s input, causing intermodulation distortion. The bandpass filter described earlier will take care of most of this problem, but it has a small but significant insertion loss, and only covers one band. Alternatively an broadband attenuator will work at the expense of sensitivity.

The non-directionality of the antenna can also be useful or a potential drawback. It is excellent when you do not know which direction the signal is coming from, but it also means it picks up all signals equally. In Toronto, a dipole can be oriented with one of the nulls at the ends of the wire facing downtown. This greatly reduces the amount of AM/FM radio interference. Using the mini-whip, all the broadcast interference gets through.

The biggest advantage of the mini-whip is it is tiny and very easy to set up. Living away from home for school, in a small apartment, it is almost impossible to set up an real antenna. The mini-whip can be mounted on a balcony railing with receive performance very similar to a dipole.