A dipole can operate on a second band by adding a third element.  The added element will shorten the dipole 10 to 20% and make antenna rotation easier.  The practical range for the two bands is 6 meters to 15 meters… either as neighbors such as 10 and 12 meters… or separated like 10 and 15 meters. This article discusses the principles of a two-band Tri-pole antenna based on 6 and 10 meters. Instructions are given to build this or any Tri-pole antenna in the practical range.

First, about the three poles...

This antenna should be considered to be two L-dipoles with a common vertical element. Figure 1.

Figure 1       Principles of operation	Figure 2       6 and 10 meter Tri-pole antenna with choke

The antenna is fed off-center so a common mode choke must be used at the feed point for easier tuning and quieter operation. (Figure  2 insert)  This isolates the antenna from the coaxial cable shield and prevents the shielding from becoming part of the antenna.   

Vertical

The starting point of the design is to determine the frequency (length) of the vertical element. Curiously his frequency is not halfway between the desired upper and lower frequencies, but 0.47 between. In this case the two frequencies are 28.4 MHz in the middle of the 10 meter Technician band and 51.4 MHz near the middle of the 6 meter band.

Calculate the vertical element frequency based on: f = 0.47 x (28.4 + 51.4) = 37.506 MHz
Using the usual formulas where ft. = feet and f = frequency:

The starting dimension for the mid frequency quarter-wave vertical is: 6.239 ft. long.

Arms

Looking at Figure 1 it is apparent that the 10 meter horizontal arm is long and the 6 meter horizontal arm is short… too short and too long to be quarter-waves. The lengths depend on how far apart, in MHz, are the two frequencies. In this case the distance is at an extreme, 51.4 - 28.4 = 23 MHz between frequencies.

Looking at Graph 1 below you see the long and short trend lines relative to the MHz between Frequencies. To estimate Long and Short arm lengths apply the trend line K Values.

Graph 1     Trends for estimating Tri-pole arm dimensions

At 23 MHz spacing, we see that the K Value for the Long Arm (blue) is around 300.
    Length (ft) = K/f MHz = 300/28.4 MHz = 10.563 ft.

For the Short Arm (red) the K Value is around 150.
    Length (ft) = K/f MHz = 150/51.4 MHz = 2.918 ft.

We now have starting dimensions for a 10m-6m Tri-pole antenna.
    Vertical = 6.239 ft, 10m Arm = 10.563 ft, 6m Arm = 2.918 ft.

Note the total horizontal length is 13.5 ft. which is 78% shorter than the 17.3 ft. of a 10 meter dipole.

Tuning and Balance Using these starting dimensions in a 4NEC2 computer model with #14 wire over Real Ground, a sweep (Graph 2) of this antenna at a half-wave elevation (17 feet) shows that we are close to the 28.4 and 51.4 frequencies. 27.8 MHz at 1.3 SWR. 53.0 MHz at 1.7 SWR. This can be improved. The long arm adjusts the 10 meter MHz. To raise the frequency from 27.8 to 28.4 MHz it was shortened from 10.563 to 10.513 feet. The short arm adjusts the 6 meter MHz. To lower the frequency from 53.0 to 51.4 MHz it was lengthened from 2.913 to 3.148 feet.
Using these adjustments the new Tri-pole SWR values are 1.77 at 28.4 MHz versus 1.61 at 51.4 MHz. Still unbalanced. These can be brought into balance by adjusting the length of the Vertical element.		Graph 2     Tri-pole sweep using starting dimensions

Rule for balancing Tri-pole SWR’s:

If the lower (left) frequency SWR is Higher, make the Vertical longer; or shorter if SWR is Lower.

In this case the 28.4 MHz lower frequency SWR is Higher (1.77 vs. 1.61) so a balance is achieved by making the Vertical longer. To keep the frequencies in tune, the both arms must be shortened to maintain the overall length. Note: this is tricky because both arms adjust the same direction contrary to the usual tuning methods. The Vertical was lengthened from 6.239 to 6.34 feet. To fine tune this change the 28.4 MHz arm was shortened from 10.513 to 10.479 feet and the 51.4 MHz arm was shortened from 3.148 to 3.035 feet. The antenna model now indicates a good SWR balance of 1.7 and 1.72 at 28.4 and 51.4 MHz (Graph 3). This SWR is relatively high because the frequencies are relatively far apart. Frequencies that are closer together have lower SWR’s.
We now have working dimensions for a 10m-6m Tri-pole antenna by use of wire antenna simulation. Vertical = 6.39 ft,  10m Arm = 10.479 ft,  6m Arm = 3.035 ft		Graph 2    Tri-pole sweep after tuning and balancing

Construction: Wire vs Tube vs Whip A two-band Tri-pole antenna can be made by modifying an existing vertical antenna having two radials or... by construction with wire or telescoping tubing or... for portability by using telescoping whips. Generally: If tubing is used instead of wire, the vertical radiator will be longer and the total horizontal length of the two arms will be shorter. Specifically: The changes are very small between the calculated wire dimensions and measured tube dimensions of the antenna (Figure 2) as can be seen in Table 1. The U-bolts used for mounting the antenna will add surface area to the vertical radiator and have a small affect on tuning.
		Table 1

Radiation Patterns

The 3D color pictures following are for total horizontal and vertical radiation fields. Thus the vertical component of a tall 6 meter L-antenna can skew the patterns to increase the signal level in a particular direction.

The radiation patterns for the 10 and 6 meter Tri-pole antenna can be seen in Figures 3 and 4 below.

The 3D color views are from looking down at a high angle relative to the antenna as shown.

Figure 3                  10 Meter radiation pattern at 17 feet (one � wave) elevation

On the polar graph in Figure 3 the half wave take-off angle is 25 degrees above horizontal. (blue line, vertical plane)

Antenna gain is 5.78 dBi broadside to the arms with a -7.2 dBi null towards the short arm end.

The 3D color flyover view in Figure 3 shows a typical dipole pattern slightly skewed to the long arm side.

Figure 4                               6 Meter radiation pattern at 17 feet (two � waves) elevation

Gain is 5.74 dBi broadside to the arms and 1.5 dBi off the ends of the arms.

The 3D color flyover view shows an oval dipole pattern for the 15 degree lobe.  This is favorable for DX contacts.  The 50 degree lobe from the second half-wave is slightly skewed towards the short arm. This produces the high angle null as seen in dark blue.

Elevation Effects

At the ideal dipole elevation of � wave or higher, the RF reflected from ground achieves maximum coupling with the � wave of the antenna to give maximum radiation to produce increased signal strength. This (“ground gain”) produces the classic figure-8 pattern extending from the sides of a dipole antenna.

It can be seen at the ideal elevation of 17 ft. in both Figures 3 and 4 that the Tri-pole gain is about the same on either band, around 5.7 dBi. This gain can be compared to a standard dipole at 7.3 dBi, an L-dipole at 6.6 dBi or a standard vertical at 1.6 dBi.

If possible, mount any horizontal dipole higher than � wave length because so much of the signal is lost skyward without the benefit of “ground gain”.

Construction Details

Materials List

Hardware List

Figure 5       Schematic Drawing

Figure 6      Tri-pole base,  front view		Figure 7       Tri-pole base,  rear view

The Base

• Cut off 7 inches from the no-slit end of the 6 Ft x 1.00" OD tube
• Add slits and hose clamp to the remaning long tube
  
• Cut off 11 inches from the no-slit end of the 3 Ft x 0.875" tube
• Insert this 11" tube 2-1/2 inches into the 7" x 1.00" OD tube and secure with two screws
• Insert and clamp this assembly into the long 1.00" OD tube (Figure 6)
• Complete top by inserting and clamping the 2 Ft.x 0.875" and 3 Ft x 0.75: OD telescoping tubes