No. 49: The Basic 10-Meter Monopole

Over the last 12 years of this column, we have pretty much overlooked one of the most basic of all 10-meter antennas: the ¼ wavelength monopole with a ground plane. The antenna is simple to build, fairly compact, and performs reasonably well, so we had better fill in this gap.

We shall not start with a ground-mounted antenna, simply because the losses are too high. However, when we elevate the monopole, we make a discovery. If we set the ground-plane radials at right angles to the monopole, the feedpoint impedance drops into the 20-25-Ohm range. Instead of adding a matching system for our 50-Ohm coaxial cable, we shall take a different tack. We shall let the radials bend downward at a 45-degree angle. This measure does two good things. First, it raises the feedpoint impedance to the 50-Ohm ballpark. Second, the radials, to the degree that they are partially vertical, add to the overall radiation from the antenna.

Fig. 1 shows the outlines of a same monopole with drooping radials. The vertical section is about 8'-1" long if we use ¾" outside diameter tubing for the element. Of course, you can combine two sizes of tubing that nest together so that you can adjust the length for the lowest SWR at whatever frequency you prefer. The model used here is centered on 28.4 MHz. However, the operating bandwidth will generally cover all of 10 meters.

The radials are 8' long when you use quarter-inch diameter rods. If you use thinner materials make them slightly longer, and if you use fatter tubes, then make them slightly shorter. Remember that you can make final adjustments to the main vertical element. The monopole uses 4 radials in a symmetrical pattern. Adding further radials to an elevated monopole will not improve performance, but it will add unnecessary weight.

I shall not give any detailed construction advice, since there are as many ways to build a monopole with radials as there are builders. Use available materials and your own skills to work out the assembly. Check various handbooks and magazine articles for construction methods that match up with your preferences. Just be certain that the radials have a common connection and that the feedline connector has its center pin going to the vertical section, with its shell going to the radials.

The next question is what I might mean by "elevating" the monopole. The answer to that question can be any base height from 10' on upward. We normally define the base of the antenna as the junction between the vertical element and the radials. Fig. 2 provides some selected elevation patterns at 10' intervals. Examine these patterns together with the table that follows them. Together, they will give you some idea of what to expect from the monopole a various heights you might use.

Between a base height of 10' and 20', the lowest lobe is dominant. In fact, at 10' up, it is the only lobe. However, by the time that we reach 20', the upper lobe is just as strong. It is at an angle that is good for very little but noise and possibly some sporadic E-skip.

From 25' up to 35' up, the lowest lobe is not the strongest lobe. The next higher lobe (as shown by the 30' pattern in Fig. 2) dominates. Nevertheless, as we continue to raise the antenna height, the gain in the lowest lobe continues to climb. Also notice that each new height shows the emergence or the rapid development of a new lobe. With vertical antennas, these lobes tend to merge so that we can only detect individual lobes by their peak values. (In contrast, a horizontal antenna would show very distinct lobes with very deep nulls between them.)

By the time we move the antenna base to a height of 40', the lower lobe once more dominates the elevation pattern. The upper lobes have significant but not overriding strength. The progression of lobe development is not an accident. It is a function of the way in which the direct radiation from the antenna mixes with radiation reflected from the ground. When the antenna base is between about 5/8 wavelength up and 1 wavelength up, the combined direct and reflected radiation favors higher angles. Above 1 wavelength up, the combination of direct and reflected radiation again favors the lower angles.

To a degree, you can translate these general pattern tendencies to almost any vertical antenna, including vertically oriented dipoles and Yagis. The key element in the translation is the height equivalence: the base height for the monopole is approximately equal to the center of the dipole and the boom of the Yagi.

The general trend of the gain and TO angles (where the TO or Take off angle is the elevation angle of maximum radiation) suggests that the higher you mount the monopole, the better the performance. Since the feedpoint impedance shows remarkable stability, you may generally adjust the antenna for best SWR at a fairly low height and then finish the installation with no further adjustments.

Not everyone has a handy tower to use for a highly elevated mount. The antenna is light enough to accept a chimney mount intended for an outdoor TV antenna (assuming in this age of cable that we remember such things). Mounting the antenna on top of your living space does call for some practical considerations.

First, it is likely that the radials may droop so that the roof comes between pairs of them. If the attic space is clear of significant metal, the roof will not usually detune the antenna. However, if the attic rafters have metal-faced insulation, it may have an affect. In such cases, you may wish to extend the mounting pole high enough to clear the roofline.

Second, the vertical element may gather static charges. Fig. 3 shows a cure for this potential problem.

Across the coax connector--or, what amounts to the same thing, between the vertical element and the junction of the radials--connect an RF choke. A value of 10 uH is usually sufficient, and 100 UH will work as well. At 10 meters, the choke blocks RF energy, so you will not be shorting out the antenna relative to operation. However, the choke does provide a DC path to discharge static energy in the vertical element as it builds. So all of the parts of the antenna will be at the same potential.

As an adjunct to this measure, where the coax would enter the house, connect a ground lead to the braid and the other end to a ground rod. Make this a stout lead and rod, since the monopole becomes a lightning rod. Remember that lightning rods tends to bring the ground potential to the rooftop, making it a less likely target for lightning than other nearby objects--such as trees--that can store and hold a charge that is more opposite to the cloud bottom charge. As a result, the other objects become more attractive than your antenna to cloud discharges. (However, if your antenna is the only tall object within a large area, then all bets are off and you need a strong discharge path for any strikes.) It is always wise when a storm approaches to disconnect your coax out of doors and reconnect the antenna lead to the earth-ground system.

The monopole with drooping radials can be an effective vertical antenna. The principles of installation that apply to it also apply to any of the commercial trap verticals that you might use for multi-band operation.

Updated 10-20-2005. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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