The Quarter-Wave Vertical on a Metal Roof

Over the years, many Amateur Radio operators have successfully used a metal roof as the ground plane or radial system for their quarter-wave HF vertical. This is possible with a metal roof on a home, barn, garage, carport, RV, porch and metal sheds or awnings, and even on commercial buildings. The actual height of the metal roof above ground is not especially important for an HF quarter-wave vertical when using a metal ground plane or radial system.

In order to take advantage of a metal roof as a ground plane, you cannot elevate a quarter-wave vertical high above a metal roof. The key is that the antenna feedpoint, which is usually at the mounting bracket, should be the same physical level as the metal roof to offer the best ground plane effect. The feedpoint of the vertical should not be any more than 4 to 8 inches above the roof, even if the mechanical mounting would allow for a higher installation. In other words, raising a quarter-wave vertical antenna many feet above the metal roof does more harm than good with regard to impedance matching and tuning. Mounting the HF quarter-wave antenna at the top of a tall mast, without a feedpoint level radial system, does not function electrically, and is mechanically very difficult. If you must use a tripod for mechanical mounting reasons, use one that is as short as possible. With a tripod mount, you must use a basic wire radial system that emanates from that slightly elevated feedpoint and drapes down over the metal roof to make use of the coupling effect described below. You cannot make a direct connection of the coax shield to the metal, due to the distance of the feedpoint above the metal.

There are two ways that a quarter-wave vertical can be installed on a metal roof or large metal awning as a ground plane.

The potentially less efficient installation uses direct grounding of the coax shield and antenna bracket to the metal roof. This might work well for transceiving, and would probably "appear" to work well due to excellent receiving results. However, you may find that you aren't easily heard. This would be an indication that the sections of your metal roof are not well bonded. Remember, we are trying to provide the same low impedance RF current collection and distribution that a ground mounted system of 30, or more, straight copper wire radials that are about one-quarter wavelength long. Just because the metal panels of a metal roof are connected with a bunch of screws or nails, and even when a multi-meter shows DC continuity between the panels, does not mean that you have a good radial system for RF. You can try it and believe that it is working great, but you may not really know how efficient the directly connected metal roof is working for you. Measured at the antenna, a frequency range of over 400 kHz under a 2:1 SWR across and beyond the 40 meter band (7.0 to 7.4 MHz) would be a general indication of radial system or ground plane inefficiency, depending upon the size of the vertical. Conversely, a very efficient ground plane will narrow the SWR bandwidth of a vertical.

The preferred installation for your quarter-wave vertical using a metal roof or large awning as your ground plane is to connect the coaxial cable shield to several (6 to 16) insulated radial wires that are placed on, under, or are draped out over the metal roof. The radials must be attached to the antenna feedpoint, never at the bottom of a tall mast or tripod. Capacitive and inductive coupling between the radial wires and the metal roof accomplishes the collection of the RF currents. The number of these "coupling radials" that you need will vary from one installation to another, but you may need fewer than would be required for a high performance elevated mount or ground mount where there is no metal roof. They may be resonant lengths, unless space is a concern. Many short radials may get the job done for the lower frequency bands, too. In this installation, the antenna mounting bracket, the coax shield, and the radial system may need to be kept insulated from the mast and metal roof, to allow the coupling effect to work properly. Then again, actually connecting the metal roof to the radial system may work better for your installation. This is another example of the fun of HF; where changing a simple electrical antenna connection may dramatically improve your antenna system performance.

Ideally, a quarter-wave vertical is physically mounted in the center of a metal roof ground plane. This would provide the best omni-directional low angle radiation pattern. However, mounting a structurally sound vertical antenna at the center of a roof is not always possible. It is true that mounting the vertical at one end of the roof will affect the omni-directional pattern. However, the signal reduction of of a few dB in the direction away from the metal roof, or any very good radial system, will not prevent you from enjoying the low angle pattern, and resulting Dxing benefits of quarter-wave vertical!

Consider the DX Engineering Vertical Feedline Current Choke kit model DXE-VFCC-H05-A, to make up for deficiencies in the metal "ground plane" or radial system. Replacing the ineffective choke made of a coil of coaxial cable, choke and its properly mounted shelf kit isolates the feedline, balances currents on the system for maximum signals, and offers the additional advantages of reduced noise and RFI/TVI.

The use of the DX Engineering Tilt Base, model DXE-TB-3P, is designed specifically for the Hustler mounting bracket. The DXE-TB-4P  Tilt Base will work for other quarter-waves. Both are intended for ground mounted quarter-wave verticals, allowing  easy, one man, lift-and-lay, and walk up or down convenience for tuning, stealth or bad weather. For your roof top installation, the Tilt Base will only be helpful if you can safely walk on the roof, perhaps along the peak, to tilt the antenna up and down. The antenna must then be supported in the horizontal position at a point away from the Tilt Base, while tuning adjustments are made, as the Tilt Base does not allow the antenna to tilt below horizontal.

Some vertical antenna manufacturers indicate their antennas do not need guying. During times of high winds or ice loading, some of these verticals may sustain damage or fail altogether. With the small amount of effort needed to install a four point guying system, the risk hardly seems worth taking. A four-point guying scheme provides the best mechanical advantage to prevent wind stress, regardless of direction. When using an optional DXE-TB-3P or -4P Tilt Base, a four point guying system is recommended for use with DX Engineering vertical antennas because just one of the guy ropes has to be loosened when you tilt the vertical down. The remaining guys help stabilize the vertical in three directions when being raised. The DX Engineering Vertical Guying Kits have been designed to be used with ground mounted vertical antenna systems. The guying kits are ideal for fixed or portable installations.

You can expect your large area metal ground plane or radial system to make a quarter-wave vertical resonate especially low in frequency and exhibit an impedance below 50 ohms at the feedpoint. IF that happens on your system, congratulations! You are the proud owner of a highly efficient quarter-wave vertical antenna system! Now, adjusting the antenna for resonance at the desired frequency and re-matching the feedpoint to the feedline may require some additional adjustments or a simple matching network. These are not difficult to accomplish and are discussed in texts that cover the topic of feedpoint matching a quarter-wave vertical antenna system.