Entries from January 2014 ↓

Double Bazooka Dipole

There is a lot of information about this antenna on the internet. Lots of offers to sell. Lots of construction information with formulas to calculate lengths for various bands. Very little information on why it works, how it works, and where best to use it.

The bazooka is a dipole made up of two quarter wave transmission lines connected in series and wire tails connected to the ends of the transmission lines to resonate the antenna to the desired frequency.

As the transmitter frequency is varied the antenna impedance (real and reactive) also begins to vary. The impedance of the quarter wave sections will also vary but in the opposite direction of the antenna impedance. So the variation in quarter wave impedance offsets the variation in antenna impedance and we have an antenna that maintains an overall impedance over a larger span of the band. We have a broadbanded antenna. About twice as broad as a normal dipole for the same band.

Now, where is more bandwidth really needed? Although more bandwidth would be welcome everywhere it is most useful on 80 meters and 2 meters. Lets take a look at the 80 meter version.

First, the quarter wave sections. The two quarter wave sections are constructed from coax. Most of the construction articles on the internet assume that solid dielectric RG58 will be used. This coax has a velocity factor of .66 and is not suitable for high power levels. We could substitute RG8 but it is expensive and heavy. So lets use RG8X instead. But wait. RG8X has a velocity factor of .84 so the formulas designed for RG58 will not work. They will not get us the quarter wave sections we need and without those quarter wave sections we do not get a broader banded dipole. We still get a dipole, it will just not be broader bended.

Something else to notice is that quarter wave sections for 80 will be quarter wave only for 80 ( and odd multiples of the 80 meter frequency they are cut for i.e x3, x5 etc.). This makes the bazooka a single band antenna. Those claims of multiband bazooka dipoles do not tell the whole story. Yes, you can have a multiband antenna that incorporates a bazooka dipole but it will be a broadband bazooka only on one band. Acually, that is probably okay if the bazooka effect is on 80 meters. You don’t need a broadbamd antenna on 40. A normal dipole is plenty good on 40 meters.

If we want to construct a bazooka dipole for 80 from RG8X, we can use the formula for a half wave wire dipole and apply the .84 velocity factor. 468/Fmhz X .84. This comes out to a little over 100 feet. Good deals on RG8X are available for lengths of 100 feet ($40 on Amazon). 100 feet of RG8X will make two quarter wave sections for 3.932mhz. This should allow 1.5:1 SWR from 3.78 to 4.082 mhz or the entire 75 meter band.

Take that 100 feet of RG8X, Short each end of the coax, center to shield, find the center at 50 feet and cut the outter covering and shield being careful NOT to cut the center conductor, fray the cut ends of shield to make two pigtails for connection to the feedline to the rig.

(The 100 foot roll of rg8X from Amazon comes with coax connectors attached to both ends. You can avoid cutting these off by terminating them into female coax connectors and shorting them at the new connector ends.)

The part of the bazooka that does the radiating is the shield of the coax we used to build the quarter wave sections. The quarter wave sections are a quarter wave due to their relationship to the center conductor and the insulation in between that conductor and the shield. The shields are too short to be a quarter wave alone. They are too short by the velocity factor of .84. To make this a resonant dipole we have to add wire to the ends of the coax. The overall length of the antenna should be a half wavelength or 468/Fmhz. Some articles show 300 ohm twinlead added to the ends. This may help the broadbandedness but ordinarry wire will do too.

The bazooka has an additional advantage over a standard dipole. It is quieter. No static buildup because the antenna is a short circuit to DC and can be grounded at the feedline. Use a 1:1 balun at the feedline to prevent radiation from the feedline.