Random-Wire Antenna

Posted by Fred C (W6BSD) on Jan 31 2021

Is Resonance required?

This is an extract from the ARRL Antenna book1

Please recognize that an antenna need not to be resonant to be an effective radiator. There is in fact nothing magic about having a resonant antenna, provided of course that you can devise some efficient means to feed the antenna. Etc, etc.

This means that a wire of any length should work as well as a resonant antenna. In the quote above, it says efficiently feeding the antenna is critical. Since the antenna is not resonant on any frequency, the feed line losses due to high SWR will be significant. This is why the Antenna Book advises to connect the antenna directly to the transmitter or a matching network fig: (b) to minimize the feed line losses2.

Random-wire connection (click to enhance)

A random-wire antenna is often used for QRP or SOTA activations because it is easy to deploy. However, such an antenna can have issues with high power. When using a random-wire, the antenna system is composed of random length wire plus the entire station. Chances are that you will have RF hot spots at some frequencies. It can be an issue with high power (RF burns, RF in the shack, RFI).

The second problem is that it cannot be entirely random. At multiples of half wavelengths of the transmit frequency, the impedance is too high for some tuners. The solution is to cut the wire at a length that is not a half-wave multiple of any frequency you want to transmit on.

To help you avoid the length of wires that are the worst, I wrote the following program. This program is inspired by the work of Mike Markowski AB3AP. This version is written in Python and will generate a .png image with wires' length to avoid.

The program is called randwire, and the full source code is located on my GitHub account

Example

The following example will generate a graph showing the length of wires to avoid for a random-wire antenna working on the 40, 30, 20, 15 and 10 meter bands.

√ % randwire --bands 40 30 20 15 10
16:03:04 INFO: Bands: 40, 30, 20, 15, 10
16:03:04 INFO: "wire.png" saved
√ %

The following image shows the length of wires to avoid.

In the graph, blue means DO NOT USE THIS LENGTH.

Length of wire (click to enhence)

The following example is for an antenna working on the 160, 80 and 40 meter bands. This time the length is shown in feet.

Length of wire (click to enhence)

In the shack

The random-wire antenna can be an option for operators with a small lot or HOA. They are also a good option for EMERCOM, field day, or any event requiring to quickly deploy an antenna. Still, the operator should take some precautions when installing such an antenna.

  • Since the antenna needs to be connected directly to the transmitter or the antenna tuner, you will need a remote antenna tuner installed outside.
  • Make sure to connect your tuner to an excellent ground.
  • If you are experiencing RF in the shack, you will have to play with different lengths of counterpoises.
  • Use a good choke on the coax, as well as the wires controlling the tuner to block any common-mode current getting inside your shack.

  1. ARRL Antenna Book 23rd edition page 1-6 

  2. ARRL Antenna Book 23rd edition page 10-26