Field Antenna Systems

Field antenna systems are tall, high-gain, temporary outdoor antennas. They comprise an antenna, a mast, a base, and appropriate safety measures. They are subject to all of the general antenna system concerns.

Antenna Type

The most common field antenna system used in SCCo ARES/RACES is a roll-up J-pole antenna. J-poles are essentially dipole antennas, with the same gain as a dipole (2.14 dBi gain). Because roll-up J-poles are very lightweight, they can be supported on a tall but thin telescoping mast without making it too top-heavy. Many other field antenna possibilities exist.

Many field stations use roll-up J-poles from Ed Fong*, a local RF engineer in Sunnyvale. They are low cost, well constructed, and tuned. MFJ also makes roll-up J-poles for several bands.

Antenna Mast

Many field stations use the 32′ telescoping fiberglass mast from The Mast Company*. It collapses to 44 inches in length and weighs only 5 lbs. It is very thin at the top but can easily support a roll-up J-pole, putting the antenna above the height of all single story buildings and most two-story buildings. MFJ* also makes a similar mast product.

A more substantial mast from Max-Gain Systems* can be created. These masts are also portable, although they are heavier and take up more space. But they are capable of supporting heavier antennas, such as an Austin Suburban*, regular J-poles, or ground plane antennas. Remember, supporting a larger, heavier mast and heavier antenna at 20+ feet usually requires the use of guy ropes, and those guy ropes extend the foot print and will require extra safety markers.

Antenna Base

If the station will be under a pop-up tent that has vertical (not angled) legs, strap the mast to a tent leg with some heavy-duty Velcro for a very stable support with no tripping hazard. However, make sure the tent legs are very well weighted down.

For more portable operations, tripod speaker stands such as from On Stage Stands* provide low-cost, high quality, very portable mast supports. MFJ* also makes a similar tripod. When used with a lightweight mast such as the 32′ telescoping mast from The Mast Company*, the tripod can usually be stabilized/secured with sandbags or tent stakes. With a more substantial mast (larger surface area = higher wind load) and heavier antennas at 20+ feet, guy ropes are usually required. When using tripods, and especially when using guy ropes, be sure to mark the area with safety cones and use orange safety flags/streamers on the guy ropes.

Feedline

LMR-400-UF coaxial cable is preferred in most usage. However, it is too heavy to run to the top of a thin fiberglass mast, so LMR-240UF cable is used for that.

Minimize the number of connectors and adapters; they each add signal loss.

• The radio end of your cable should terminate with the connector your radio needs.
• The antenna end of your cable should terminate with the connector that mates to your antenna.
• Prefer a single long length of cable over multiple shorter lengths of cable joined with adapters.
• When an adapter is needed, use a single adapter with the correct connectors on each side. Avoid daisy-chaining multiple adapters.

Prefer N, BNC, or TNC connectors over PL-259/SO-239 “UHF” connectors. They have less signal loss.

Keep adapters in your Go Kit to connect the antenna end of your cable to “UHF,” N, or BNC antenna feeds, male or female. You will need them to use someone else’s antenna (e.g., a roof-mounted antenna).

If you’re operating a temporary field station in the rain, you may need to apply at least some weatherproofing to any connections that are subject to water. A single wrap of electrical tape is usually sufficient. Leave a flag (short piece of folded-over tape) on the end for easier removal.

Safety

Antennas must not be placed where they can fall into power lines. Use a second person at a distance as a spotter while raising an antenna near obstructions; experience has taught us that the person raising the antenna cannot judge the distances to obstructions accurately.

Lightweight antenna masts must be weighted down to prevent falling in high winds. Heavier antenna masts must be secured with guy ropes.

Masts, tripods, guy ropes, and cables must be marked with cones, caution tape, or similar to avoid trip hazards.

An RF safety evaluation must be performed for any antenna installation, and cones, caution tape, or other barriers must be placed to keep people out of the unsafe exposure zone.

Do not operate your station while there is a chance of lightning nearby. Temporary field stations generally don’t need lightning arrestors. When lightning is threatened, shut down the station and get indoors.

* Manufacturer, product, and vendor names and images on this page are given as examples and are not endorsed by SCCo ARES/RACES.