ANTENNAS FROM THE GROUND UP
A common all-band doublet for smaller ham properties is the inverted-Vee, usually cut to either 135' or to 67' (or thereaboutsþsince the exact dimensions are not critical for an antenna fed with parallel line and an antenna tuner). The question that recurs is whether the Vee þworks as wellþ as the level or flat-top doublet of the same length.
The answer depends on 2 factors. One factor is the frequency of use. The other factor is the angle of the Vee wires relative to the level doublet. To see what difference each of these factors make, letþs look at the doublets in Fig. 1.
The basic length of the doublet will be 67' for a fundamental frequency of 40 meters. We shall look at both azimuth and elevation patterns for 40, 20, 15, and 10 meters, on frequencies about mid-band in each case. However, the patterns will not change from one end of a band to the other.
We shall look at patterns for the level doublet as a sort of basline against which to measure the patterns of the 2 Vee versions. One version will be a moderate Vee that slopes downward only 30 degrees relative to the level doublet. The other Vee version will slope downward 45 degrees. Together, the two Vees will give us a picture of the trend in pattern change for intermediateþor even more radicalþslopes.
In some cases, Vee patterns will be weaker, that is, have no lobes as strong as the strongest lobes of the doublet. Does that make the Vee a poorer antenna? Not necessarily. In each case, look also at the shape of the pattern and the strength of the lower angle elevation lobes. It will be the evaluation of all of the pattern features that will tell you which version of the doublet is best for your operation. This assumes, of course, that some kind of doublet is best in the first place. On 40 meters, there is really very little to choose between a flat doublet and a Vee. The elevation angle of maximum radiation climbs upward as we make the Vee slope more radically. It changes from 38 degrees for the flat-top to 46 degrees for the 45-degree Vee. However, in all three cases, the elevation lobe is so vertically broad that the differences are unlikely to make a detectable difference in performance.
Likewise, there is a slight difference in the strength of the main lobe broadside to the antenna, which is 50' up at the center in these patterns. A difference of 1-2 dB is not likely to be detected by the user without high-cost lab equipment. (That is why we rarely detect reduced performance due to the lack of antenna maintenance until the antenna falls down.) So on 40 meters, A Vee and a flat-top are about equal.
On 20 meters, where the doublet is about 1 wavelength long, we begin to see significant differences. The elevation angles of maximum radiation climb from 19 degrees for the flat-top to 25 degrees for the 45-degree Vee. As well, the gain drops by over 3.5 dB as we increase the slope of the Vee, although most of that drop occurs in the move from 30 degrees of slope to 45 degrees.
However, notice the shape of the azimuth patterns. The flat-top shows extremely deep side nulls (off the ends of the wire. The user can look at these nulls as QRM fighters or as directions in which almost no QSOs are possible even under the best propagation conditions. The lesser nulls of the two Vees offer some hope of contacts, although condition might have to be very good to get them.
On 15 meters, the situation becomes a good bit more complex, perhaps even more complex than the maze of lobes and nulls in the combined sets of patterns. The flat-top has the strongest lobes by far, although the four best are fairly narrow in width. If these lobes happen to go exactly where you want them to go, then all is well. If not, then they may radiate where no one lives. Antenna orientation is important.
The broader patterns of the Vee antennas offer slightly weaker but more uniform propagation in most directions. However, nothing is perfect. Note the elevation patterns, which show almost all radiation to be at higher angles in the 23 to 30-degree range on a dx band that does best when radiation is at much lower angles. The flat-top take-off angle is 13 degrees, just about right if the lobe points at a target.
On 10 meters, we have a similar situation to the one on 15 meters. Everything is just a bit more extreme. The flat-top lobes are narrower, but at a nice low 10-degree elevation angle for DX. Unfortunately, the two Vee antennas have lost virtually all of their low- angle radiation: their lowest elevation lobes correspond to the secondary lobe from the flat-top. Hence, the DX potential of the Vees on 10 meters is somewhat dismal.
Indeed, it might be better for 10 meters to erect a simple dipole, even one that can rotate. At a 16' length, it can be hidden if need be. It pattern is likely to be superior to either the narrow flat-top lobes of the high-angle Vee radiation. In other words, the idea of 1 antenna for everything may not be best for everyone.
Updated 09-28-2002. © 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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