Subject: “ The Triple Bazooka “ 「トリプル バズーカ」
I designed and experimented the double bazooka dipole with one more bazooka match built into the feed line, the Triple Bazooka. The result and performance of the triple bazooka are better than the double bazooka, so I would like to share these interesting results and capabilities of the triple bazooka with you.
In the summer 2008, I homebrewed a double bazooka dipole antenna (a) for 15m band because I was expecting and was hoping that the condition was becoming better. When the double bazooka antenna was completed and measured the SWR, as I expected, the readings were from 1.0 to 1.1 on my SWR meter from 21.000 Mhz to 21.450 Mhz. I became curious if the third bazooka match was built into the feed line, then what would happen. That was how I started and looked into the triple bazooka. The followings describe how to design, homebrew and install the triple bazooka on 20m band I have made recently and the results are remarkable and striking.
I use the metric units for the design calculations and the following conversion factors are used.
1 in= 2.54 cm
1 ft = 0.305 m
1m= 100 cm
1cm = 0.393 in
1m = 3.278 ft
“The Velocity Factor of coax cable “
I used all materials I needed stored in my radio shack. The key material is the coax cable and you need to know the coax cable velocity factor ( V.F. ) to determine the 1/4 wavelength for the bazooka match. If you have neither dip meter nor antenna analyzer, you simply use the V.F. given in the specification of the coax cable, normally 67% or 80% depending on the coax cable type. From my past experience I recommend check the V.F. of the coax cable for making the triple bazooka as shown in the ARRL handbook (b). I up-loaded a schematic and pictures in the album on this blog, “Triple Bazooka” on 2009/09/20, so please refer it. Click the image to enlarge it. The Figure 1, the sketch of the triple bazooka, is provided to help you understand the steps and descriptions below.
(A) The steps to determine the V.F. :
(A-1) Cut the coax cable that you decided to use as a test piece. I used a left over piece. It was printed on the cable and was said as “ Tandy Wire & Cable Type RG-8 mini-foam “. The length was measured to be 1.29 meters ( 1.29m x 3.278=4.22 ft). This length is known as the electrical length.
(A-2) I used a dip meter to measure the resonant frequency (= F Mhz) with both ends shorted and the length is a half wavelength at the measured the resonant frequency.
In my case, the length in meter = 1.29 m and F = 90 Mhz were measured.
A half wavelength = (300 x 1/2 ) / F = 150 / 90 = 1.66 m ( 1.66m x 3.278 =5.44 ft) This calculated length is known as the physical length.
(A-3) The V.F. = ( the electrical length / the physical length ) x 100 = ( 1.29m / 1.66 m ) x 100
= 77.7 % = about 78%
So I use the V.F. to be 78% on this coax cable for designing this antenna.
Looking up the specification, the V.F. is shown as 80%. I suggest you make sure the type of the coax cable you use and look up an ARRL handbook or an ARRL antenna book to check the V.F. of the coax cable you are going to use.
(B) The Design Steps:
The followings are the steps of calculations and design for homebrewing the triple bazooka. First you decide the design frequency for the antenna. I go on CW band on 20m so my design frequency is selected to be F=14.050 Mhz. When you go on the phone band for the most of time, you may choose 14.200 Mhz or 14.250 Mhz if you want.
(B-1) You need to select the coax cable to homebrew the triple bazooka.
I used the coax cable of which the V.F. was measured as described above. V.F.= 78 %
(B-2) You can also use the 75 ohm TV coax cable such as RG-59 but for us, ham operators, let us use 50 ohm coax cables such as RG-58, RG-8 etc. The design steps are the same on RG-59.
(B-3) When you select the 80% of V.F. cable, you require about 8.6 meters (=28.2 ft) plus about 8cm (=3 in). When you use the 67% V.F. cable, you need about 7.2 meters (=23.6 ft) plus about 8cm (=3 in). This 8 cm of cable length is required and used to prepare and put the cable together as a triple bazooka antenna. DO NOT cut the coax cable at this point because the cable required must have adequate and sufficient length. If it is cut too short, you can not use it for this antenna.
(B-4) To determine the total coax cable length required.
My design frequency is F= 14.050 Mhz
The formula: The 1/4 coax cable wavelength of dipole =the left side half coax cable Ⓐ= the right side half coax cable Ⓑ = the third bazooka match coax cable built in the feed line Ⓒ
=( ( 300 x 1/4) / F ) x V.F. = (75 / 14.050 ) x 0.78 = 4.16 meters = 416 cm (= 13.6 ft )
The total main element length of coax cable required
= 2cm + the left side half Ⓐ+ the center feed point + the right side half Ⓑ+ 2cm
= 2cm + 416cm + 4cm + 416cm + 2cm = 840 cm = 8m 40cm ( = 27.5ft )
So now I am ready to cut my cable, RG-8 minifoam. I measured the cable CAREFULLY, not too short and cut it for 8m 40cm.
(B-5) To determine the overall length required.
I used the AWG 14 wire on hand. You can use any type of wires as long as it is strong enough for using antennas such as heavy enamel wires, vinyl speaker wires, antenna stranded wires and so on.
The overall length = 1/2 wavelength = (300 x 1/2) / F = 10.67 m
(B-6) To determine the wire length of right side half and the wire length of left side half.
The wire length of right side half = the wire length of left side half.
= (The overall length – (The 1/4 coax cable wavelength x 2 ) ) x 1/2
= (10.67m – ( 4.16m x 2) ) x 1/2
=1.18 m ( = 3.87 ft )
In summery, using the 78% V.F. coax cable, I need one piece of 8m 40cm long coax cable, one piece of 1/4 wavelength of coax cable Ⓒ, 4.16m long calculated on (B-4) and two pieces of 1.18 m long wire on calculated (B-6).
Please note that if you use 67% V.F. or 80% V.F. coax cable, the coax cable length required is different from this calculated length. You need to calculate the length required as I did above.
When you have an access to internet, VE3SQB has a website (c) to calculate the length required on the coaxial dipole, double bazooka using the V.F. of 67% or 80% on the specification. Your calculation can be compared to make sure the length required on the coax cable and wires.
(C) The construction:
The figure 1 shows the wire connections.
(C-1) I cut the coax cable for 8m40cm long for the length you calculated at (B-4) above.
(C-2) Mark 4cm (=1.47 in ) at the center of the cable. Remove the outside jacket ONLY for this 4cm portion by a sharp knife and try NOT to cut the copper shield inside.
(C-3) Then use a sharp knife to cut the copper shield only at the center position which is the right at the center of the coax cable. Make sure NOT TO CUT both the dielectric material and the center conductor.
(C-4) When the copper shield is cut at the center, prepare the copper shield so as to solder as shown on the detail 1 of figure 1.
(C-5) The both ends of the coax cable need to be prepared and solder as shown in the detail 2.
Note that the wires cut in the calculated length are soldered as shown in the detail 2.
(C-6) The third piece of the coax cable Ⓒ needs to be connected at the feed point as shown in the detail 1 of figure 1. At the other end of the third coax cable Ⓒ is connected to the feed lineⒹ to the transceiver as shown in the detail 3 of figure 1.
First, I took the assembled antenna as a double bazooka at my backyard and hung it down at about 1.8m ( 6 ft ) above the ground. Using the MFJ-259B antenna analyzer I have, the antenna was trimmed on both ends to tune at my design frequency, 14.050 Mhz.
The table below shows the measurement as a double bazooka.
Freq.(Mhz) SWR Impedance(ohms) R X
14.000 1.4 38 39 12
14.050 1.5 42 40 17
14.100 1.6 45 42 22
14.150 1.8 50 43 27
14.200 1.9 52 45 32
14.250 2.0 58 47 35
14.300 2.2 60 49 41
14.350 2.4 64 51 45
14.400 2.5 70 53 48
The Table below exhibits the measurements as the triple bazooka tuned and completed.
Freq.(Mhz) SWR Impedance(ohms) R X
14.000 1.2 77 61 0
14.050 1.1 76 59 0
14.100 1.0 65 56 0
14.150 1.0 52 53 0
14.200 1.1 58 49 6
14.250 1.2 43 45 8
14.300 1.3 40 41 10
14.350 1.4 35 37 10
14.400 1.6 33 33 11
As you notice from the Tables above, the triple bazooka is capable of absorbing and adjusting these unwanted factors by itself to give you the better SWR readings over the wide frequency range over the double bazooka.
It is remarkable improvement and unique capability on the triple bazooka. The resonant frequency of my triple bazooka can be considered to be between 14.100 Mhz to 14.150 Mhz with the impedance of about 60 ohms.
Also the tuning can be made with a SWR meter with the following steps.
Assuming you have assembled the triple bazooka as I described above, you need a transceiver with a CW key and a SWR meter as the instrument. You may have to take the assembled triple bazooka and the instrument to set up outside and hang the antenna at 1.5 m to 1.8 m ( about 5 ft to 6 ft ) above the ground so that the tuning and adjustment can be made easily.
(D-1) Set up and connect starting from your transceiver then your SWR meter, then the antenna in this order.
(D-2) Set the power level on your transceiver at 5 to 10 watts, relatively small power and set your transceiver for CW mode at the frequency of 14.005 Mhz. Then switch on.
(D-3) You are determining whether the main element is too long or too short and what is your resonant frequency. Press down the key.
Write down the SWR readings on a piece of paper at 14.005 Mhz, 14.100 Mhz, 14.200 Mhz, 14.300 Mhz and 14.340 Mhz. The least number reading is the resonant frequency of the antenna.
(D-4) When your resonant frequency is lower than your design frequency or your aiming resonant frequency, then you need to trim the both sides of wire elements with a small and equal length or without trimming BEND BACK the wire with a small and equal length at each end. You keep doing this on (D-3) until the antenna is tuned at the resonant frequency you want.
(D-5) When your resonant frequency is higher than your design frequency or your aiming resonant frequency, then you need to lengthen the both wire elements, say, 30 cm ( about 1 ft ). You solder this length of wires. Then do the step (D-4) till you reach the resonant frequency you want.
(D-6) If the SWR readings do not reach 1.0 or 1.1 level, you might have something too close to the antenna such as a house wall or big trees, or metal objects such as an antenna tower, etc. You need to make sure there is nothing around and relatively ample space around the triple bazooka for tuning.
(E) The installation and operation
When the tuning is completed, the water-proof needs to be made on the joints by using water-proof caulking materials that are available at any hardware stores.
I recommend reinforce the center of the main element to increase the strength when the antenna is pulled and installed. I used an antenna wire insulator and tied with the multi-purpose plastic ties purchased at a dollar store in town.
I put up the triple bazooka like an inverted V shape because of the space I had. The antenna can be installed as a dipole. The height I achieved was about only 4 meters above ground (13 ft ) and the wire ends were tied onto the wooden fence at 6 feet high. I had neither ample space nor height.
On July 8, 2009, I have made my first QSO as testing with WB2AKP, Lou, at Rockville, CT. with RST of 579. I was running a Kenwood TS-450s with 5 watts. Then the second QSO was made with KD0V, Merlin, at Waseca, MN. with 559. It was pretty good with 5 watts of power.
On July 17, 2009, my first DX QSO was made with PY2EYE, Nilson, at Sorocaba near Sao Paulo, Brazil, with RST of 549. My power was 20 watts. And on July 18,2009, two contacts with the Japanese stations were made with JF1SQC and JA2XYO with RST of 579 and 599 respectively. My power was 70 watts. I confirmed that the triple bazooka has worked well on DX also.
(F) My comments:
I feel great for using the triple bazooka when I transmit because the needle of my SWR meter shows almost nil movement of the reflection power. I have homebrewed three triple bazookas in the past a year. These are 6m, 15, and 20 m band. These show typically low flat SWR readings over the most of the ham band frequencies. Of course this design can be used on 40m and 80 band as well.
Perhaps the triple bazooka can perform well also as an attic antenna although I have never used and experimented it in my attic. I would like to hear the result and comment from someone for the experiment.
The triple bazooka antenna is not too difficult to homebrew and it works well. Why don’t you make one for your preferred frequency with some left over materials you may have in your radio shack.
Good luck and 73.
Hiro Hayashi, VE3CGC
(a) The ARRL handbook, 1977 issue, page at 599.
(b) The ARRL handbook, 1989 issue, page at 17-20
(c) VE3SQB website http://www.ve3sqb.com/