A Guide to Random Long Wire Antennas for Ham Radio

M. Lubbe ZS1ML

Introduction

A random long wire antenna, as the name suggests, is a simple and cost-effective antenna solution for ham radio enthusiasts. It is essentially a long piece of wire, typically longer than a quarter wavelength, that is strung up in the air and connected to the radio. The length of the wire is not resonant, hence the term “random.” This type of antenna is popular due to its simplicity, affordability, and versatility. It can be used for both receiving and transmitting on various frequencies.

Understanding the Random Long Wire Antenna

The random long wire antenna is a type of end-fed antenna, meaning the feed point is at one end of the antenna wire. The other end is either left open or grounded. The length of the wire is not tuned to a specific frequency, which allows it to operate over a wide range of frequencies.

The performance of a random long wire antenna can be unpredictable due to its non-resonant nature. It can have high impedance and unpredictable radiation patterns, which can lead to high SWR (Standing Wave Ratio) values. However, these issues can be mitigated by using an antenna tuner, which matches the impedance of the antenna to the radio, reducing SWR and improving performance.

Building a Random Long Wire Antenna

Materials Needed

  1. Wire: The wire should be strong, weather-resistant, and capable of handling the power output of your radio. A common choice is 14-gauge stranded copper wire. The length can be anywhere from 10 to 100 meters (30 to 328 feet), but lengths not resonant to the bands you want to operate on are preferred to minimize standing waves.
  2. Insulator: Attach an insulator to the end of the wire. This will be used to secure the wire to your support structure.
  3. Feedline: This is the cable that connects the antenna to your radio. Coaxial cable is a common choice.
  4. Antenna tuner: This is a device that matches the impedance of the antenna to your radio, reducing SWR and improving performance.
  5. Support structure: This could be a tree, a mast, or any tall structure that can support the weight of the wire and keep it off the ground.
  6. 9:1 UnUn: You will need to buy or build a 9:1 UnUn if you do not have a tuner that specifically supports a long wire. This will probably apply to most users. This transformer is designed to match a high impedance (such as that of a long wire antenna) to a lower impedance (such as the 50 ohms of your transceiver).

Steps to Build

  1. Measure and Cut the Wire: Decide on the length of your antenna. Remember, it should be a non-resonant length. A popular choice is 41 meters (134 feet), which is not resonant on any ham bands.
  2. Attach Insulator: Attach an insulator to the end of the wire. This will be used to secure the wire to your support structure.
  3. Connect the Feedline: One end of the feedline is connected to the wire, and the other end is connected to your radio. The connection to the wire should be at the end where the insulator is attached. This can be done using a simple connector or by soldering.
  4. Install the Antenna: Secure one end of the wire to your support structure using rope or cable. The wire should be as high and as straight as possible. The other end can be secured to a lower point, creating an inverted-L or sloper configuration, or it can be strung to another high point for a flat-top configuration.
  5. Connect to the Radio: The other end of the feedline is connected to the antenna tuner, which is then connected to your radio.
  6. Tune the Antenna: Using the antenna tuner, adjust for the lowest SWR on the frequencies you wish to operate on.

Deploying the Antenna

The deployment of a random long wire antenna will depend on the available space and support structures. Here are some common configurations:

  1. Inverted-L: This configuration is useful when vertical space is limited. One end of the wire is attached to a high point, and the other end is secured at a lower point. This creates a vertical and horizontal section of the wire.
  2. Sloper: Similar to the Inverted-L, but the wire slopes down from a high point to a lower point in a straight line. This configuration is easy to set up and requires only one high support.
  3. Flat-Top: If two high supports are available, the wire can be strung between them, creating a flat-top configuration. This usually provides the best performance but requires more space and higher supports.
  4. Vertical: If only a small horizontal space is available but there is access to a very tall support, the wire can be deployed vertically. This configuration can provide good DX (long-distance) performance.

Remember, the wire should be as high and as straight as possible, and clear from any obstructions. The feedline should be routed away from the wire and into your radio station, avoiding any sharp bends.

Antenna Tuning and the Use of a 9:1 Transformer

The impedance of a random long wire antenna can vary greatly depending on its length and the frequency of operation. It can range from a few ohms to several thousand ohms. Most ham radio transceivers, however, are designed to work with an impedance of 50 ohms. This mismatch between the antenna impedance and the transceiver impedance can result in poor performance and potential damage to the transceiver.

An antenna tuner is often used to match the impedance of the antenna to the transceiver. However, not all antenna tuners are designed to handle the wide range of impedances that a random long wire antenna can present. Many antenna tuners are designed to work with antennas that have an impedance closeto 50 ohms, such as dipole or vertical antennas.

If your antenna tuner does not have a specific port and built-in transformer for a long wire antenna, you will need to use an external transformer, often referred to as a 9:1 unun (unbalanced to unbalanced transformer). This transformer is designed to match a high impedance (such as that of a long wire antenna) to a lower impedance (such as the 50 ohms of your transceiver).

The 9:1 ratio of the transformer means that it will transform the impedance by a factor of 9. For example, if the impedance of the antenna is 450 ohms, the transformer will reduce this to 50 ohms, which is suitable for your transceiver.

To use a 9:1 unun, you would connect the long wire antenna to one side of the transformer and the other side to your antenna tuner with coax. The tuner can then be used to fine-tune the impedance match and reduce the SWR to an acceptable level.

Ideal Wire Lengths

While the term “random” might suggest that any length of wire can be used for this type of antenna, there are certain lengths that are considered more “ideal” than others. These lengths are chosen to minimize the standing wave ratio (SWR) across as many bands as possible, which in turn reduces the load on your antenna tuner and improves the overall performance of the antenna.

The ideal lengths are typically those that are not resonant (i.e., do not form a half-wave or full-wave) on any of the bands you wish to operate on. This is because resonant lengths can create high voltages at the ends of the wire, which can lead to arcing and other issues. Non-resonant lengths, on the other hand, distribute the voltage and current more evenly along the length of the wire, reducing these problems.

Here are some “ideal” lengths for a random wire antenna, given in both metric and imperial measurements:

  • 29 meters / 95 feet
  • 35.5 meters / 116 feet
  • 41 meters / 134 feet
  • 58 meters / 190 feet
  • 71 meters / 233 feet
  • 84 meters / 276 feet
  • 107 meters / 351 feet

These lengths are not resonant on any of the HF (High Frequency) bands, which makes them ideal for a multi-band random wire antenna. They provide a good compromise between performance and ease of tuning.

Safety Considerations

When installing and using a random long wire antenna, safety should be a priority. Here are some safety tips:

  1. Avoid Power Lines: The antenna should be installed well away from any power lines. A falling antenna or feedline can cause a short circuit or electrocution if it comes into contact with a power line.
  2. Lightning Protection: An antenna can act as a lightning rod. It’s important to install proper lightning protection to protect your equipment and house. This usually involves grounding the antenna and using a lightning arrestor.
  3. Secure Installation: Make sure the antenna is securely installed. A falling antenna can cause injury or damage.
  4. RF Exposure: Ensure your antenna is installed in a location where people cannot come into close contact with it while you are transmitting, to avoid RF exposure.

Conclusion

The random long wire antenna is a simple, versatile, and effective antenna for ham radio. Its ease of construction and ability to operate on multiple bands make it a great choice for beginners and experienced hams alike. With the right materials, a little time, and attention to safety, you can build and deploy your own random long wire antenna and start exploring the airwaves.

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