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The Two-Band Tri-pole Antenna
6 through 15 meters

by Dick Reid,  KK4OBI

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.

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.

Finding Dimensions

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:

Half-wave:  Length(ft) = 468/f MHz = 468/37.506 = 12.478 ft.
Quarter-Wave: Length(ft) = 234/f MHz = 234/37.506 = 6.239 ft.

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

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

The polar graph in Figure 4 is at two half-waves elevation.  This creates two half-wave radiation lobes with take-off angles of 15 and 50 degrees. (blue line, vertical plane)

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

 Quantity Item Comment 1 8-1/2” x 8-1/2” x 0.2-0.25” base Kitchen cutting board 1 Closet Rod, 1-5/16” Dia., Wood Lengths from 6 to 12 ft. available for mast 1 Tube 6 Ft x 1.00” OD, end slit Aluminum, 0.058” wall, for Vertical 1 Tube 3 Ft x 0.875” OD, end slit Aluminum, 0.058” wall, for Vertical 1 Tube 3 Ft x 0.75” OD, no slit Aluminum, 0.058” wall, for Vertical 1 Tube 6 Ft x 0.75” OD, end slit Aluminum, 0.058” wall, for Long arm 1 Tube 6 Ft x 0.675” OD, end slit Aluminum, 0.058” wall, for Long arm 2 Tube 3 Ft x 0.50” OD, no slit Aluminum, 0.058” wall, for Long & Short arms 1 Tube 3 Ft x 0.75” OD, end slit Aluminum, 0.058” wall, for Short arm 1 Tube 3 Ft x 0.625” OD, end slit Aluminum, 0.058” wall, for Short arm 1 Tube 3 Ft x 0.375” OD, no slit Aluminum, 0.058” wall, for Short arm

Hardware List

 Quantity Item Comment 2 U-Bolt, 5/16” Dia x 1-3/8” U-ID x 2-1/2” L, Zinc Clamps for 1-5/16” Mast 2 U-Bolt, 1/4” Dia x 1-1/8” U-ID x 2-1/4” L, Zinc Clamps for 1” Vertical 2 SS Round Head Machine Screw #8 x 1-1/2”, washers, lock nuts Pivots for arms 2 5/16” x 1-1/2” Clamping Knob, Star washers, Wing nuts Locks for arm angle 1 Aluminum Angle, 1-1/2” x 1-1/2” x 3” for SO-239 Bridge connecting arms 1 SO-239 Female coax connector, Panel Chassis Mount Mount with 4 screws 1 Wood screw, #8 x 2”, Vertical radiator position lock Thru base to wood mast 8 SS Hose Clamps, Size as needed for telescoping Vertical and arms

 Figure 5       Schematic Drawing

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

The Base

The vertial tube is mounted with two U-bolts and secured from slipping by a wood screw through the base and into the mast.  The U-bolts can be insulated from the tube for slightly less frequency lowering effect on tuning.

A wire electrically connects the vertical tube to the insulated center of the SO-239 socket.  The socket is mounted on the aluminum L-bracket which electrically connects the two arms at their pivot points.

To compemsate for the unequal droop in the two arms there are short slots in the Tri-pole base to allow a small adjustment by using the wing nuts in front and the clamping knobs in the rear.  This feature is not suitable for fine tuning.
As an alternative, small ropes attached to the vertical element can be used to adjust for droop.

The rear of the Tri-pole base is mounted on a 1-5/16" non-conductive mast by using two U-bolts.  Note that the mast is flattened to fit between the 1/4" diameter U-bolts.

The Three Poles

Vertical Pole
Because the SWR balance involves fine tuning of the length of the vertical pole, a short telescoping section is added at the bottom of the pole to make tuning easier.
• 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
Short Arm
For use on 6 meters two tubes must be shortened.  All four tubes are used only on 10, 12 and 15 m.
• Cut off 3 inches from the no slit end of the 3 Ft x 0.75” OD tube.  Drill holes for pivot/clamp
• Cut off 6 inches from the no slit end of the 3 Ft x 0.625” OD tube
• Finish by telescoping and clamping these with the 3 ft 0.50” and 0.375” OD tubes

Long Arm
• Start with the 6 Ft x 0.75” OD tube.  In the no slit end, drill holes for pivot/clamp
• Finish by telescoping and clamping with the 6 Ft x 0.675” and 3 Ft x 0.50” OD tubes

Choke
The off-center nature of the antenna dictates the need to keep common mode current off of the outside of the feed line.  The choking device is known variously as a 1:1 current-balun, isolation-balun or choke-balun.  The wider frequency range of the Tri-pole antenna dictates the need of a ferrite based choke, not air wound.

Various commercial chokes were tried; boxed, long and toroidal.  All work.  A choke with the shortest, flexible connection to the SO-239 coax connector is advised.  See Figure 2 insert.

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Dick Reid, KK4OBI at QSL.net

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