Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 313
Instead, here are some commonsense guidelines that may be helpful in planning
your installation:
• As a broad generality, lower-frequency beams (larger dimensions) will have a
greater effect on higher-frequency beams (smaller dimensions) than the reverse.
• Try to avoid stacking antennas with full-size or trapped elements for harmonically
related frequencies (such as 10 and 20 m). Especially avoid third harmonic
combinations such as 40 m and 15 m. Instead, combine 15- and 20-m beams, or
10 and 15, or 12 and 17, etc. This prohibition is often not necessary for a beam
with loading coils (such as the Cushcraft 40-2CD or XM240, or the Force 12
Delta family) or linear loading (such as the M 2 40M2LL) because a lumped inductance
that makes the elements resonant or nearly so on the design band will
likely make the same element nonresonant on the harmonics.
• If it is absolutely necessary to stack two beams that interact, consider rotating
one of the beams 90 degrees with respect to the other. However, be aware that
the boom of one beam may well be resonant in the frequency range of the other;
addition of a current breaker or other form of boom detuning may help.
• Choose the overall height of one or both beams, if possible, to create a ground
reflection null in the elevation pattern(s) at 90-degree elevation (straight up). If
your antennas are for long-distance work, you should already be striving for
this.
• Accept the fact that some models from some manufacturers are inherently more
susceptible to interaction than others.
• Put the lower-frequency beam above the higher-frequency one whenever the
tower and mast strength allow it.
• Consider using a multiband beam (having traps or interlaced elements on a
shared boom) in place of separate beams.
Examples of some stacking combinations that one of the authors has had good luck
with over the years include:
• Hy-Gain TH-6DXX (20-15-10) and 402BA (40) separated by 5 ft at 62 ft and 67 ft
• Cushcraft 40-2CD (40) and A-4S (20-15-10) separated by 5 ft at 92 ft and 97 ft
• Cushcraft 20-4CD (20) and 15-4CD (15) separated by 5 ft at 125 ft and 130 ft
• Cushcraft 20-4CD and 40-2CD separated by 5 ft at 92 ft and 97 ft
• Cushcraft 20-4CD and M 2 40M2LL (40) separated by 5 ft at 92 ft and 97 ft
In one case, the two beams (for 20 and 15) were not harmonically related; in all other
cases, one of the two antennas employed loaded elements that kept it from being resonant
on other bands.
Rolling Your Own
In years past, this book might have contained a greater number of charts displaying
dimensions for a variety of different Yagi design philosophies and optimization prefer-