Amateur Radio Asked on November 27, 2021
I’ve seen these antennas built out of coax that are called a bazooka vertical. It’s a kind of 1/4 wave vertical, or perhaps a 1/2 wave dipole. Constructing such an antenna is to roll back the outer braid of coax to expose 1/4 wavelength of the center conductor, the braid being folded back over the braid towards the radio to make the other 1/4 wave of this dipole. The interaction between the two layers of the braid makes a kind of balun to keep common mode RF from feeding back into the radio, or picking up noise.
This sounds like a simple variation of a balun made of a few loops of coax at the center of a dipole. I keep seeing claims that a properly constructed dipole will be close enough to 50 ohms that a balun is not needed. But then I see real world demonstrations on how a lack of a balun is bad because a balun is not just an impedance match transformer, it’s matching a balanced dipole to an unbalance coax.
The reason I ponder this is because I’ll see tape measure Yagi antennas with no balun as being quite routine. Would not this bazooka vertical balun effect happen if only the coax was run on top of the element fed by the coax shield? Does it take more than being close, does it have to surround the coax?
If it must surround the coax to get the balun effect then what of a Yagi were the driven elements were made of small copper pipe, just big enough to easily slip some coax inside? Then the element with the coax inside would be soldered to the shield of the coax at the center, and the other element soldered to the center conductor.
Then ponder a dipole made similarly for 10 meters, 12 meters, or perhaps 15 meters. A common 10 foot length of copper pipe could be cut to be 1/4 wavelength (adjusted for velocity factor), feed the coax through the pipe and solder the shield to the pipe, then solder on a 1/4 wavelength wire to the center to complete the 1/2 wave dipole. This would become a kind of end fed half wave dipole.
I left out a lot of finer details, as I’m sure there is much left for interpretation on how to hang this, weather proof it, and so forth.
The questions I have are… Has this been done? I’m sure it has somewhere so perhaps, how practical is this? How well would this work? The bazooka vertical seems to be all over. I’m not seeing bazooka dipoles for HF, or as part of a tape measure UHF/VHF Yagi.
I keep seeing claims that a properly constructed dipole will be close enough to 50 ohms that a balun is not needed.
The necessity for a balun has nothing to do with how close the feedpoint impedance is to 50 ohms. Rather, a balun is required because a dipole does not have equal impedance from each half to the coax shield. Consequently, the current will not split equally among the two dipole halves, with the remainder being made up by common-mode current on the shield. See Using a balun with a resonant dipole.
Does it take more than being close, does it have to surround the coax?
It has to make a transmission line, somehow. In the typical construction where the shield is folded back over the coax, what you're doing is making another piece of coax over the coax: the coax shield that you didn't strip is the center conductor, and the shield that was folded back is the shield. See more at antenna-theory.com - Bazooka Baluns.
The transmission line doesn't need to be coaxial, it could be a balanced transmission line too. In that case you could have to conductive things nearby and parallel, instead of concentric.
what of a Yagi were the driven elements were made of small copper pipe, just big enough to easily slip some coax inside?
I guess you could do this, but then you'd have the coax exiting off the side of the Yagi, which isn't especially convenient. How will you get the coax back to the tower so it can run down to the ground and back to the radio? What advantage is there of having the coax stick out the side of the Yagi?
Then ponder a dipole made similarly for 10 meters, 12 meters, or perhaps 15 meters. A common 10 foot length of copper pipe could be cut to be 1/4 wavelength (adjusted for velocity factor), feed the coax through the pipe and solder the shield to the pipe, then solder on a 1/4 wavelength wire to the center to complete the 1/2 wave dipole.
I don't see any reason that wouldn't work, though it does sound a little heavy and challenging to make mechanically robust. If I wanted a rigid dipole, it's mechanically easier to support it in the middle since that means a significantly reduced bending moment. A wire strung between two trees works pretty well too: I'm not sure what advantage there would be to the bazooka construction.
Answered by Phil Frost - W8II on November 27, 2021
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