[TenTec] Bazooka antenna.. More than you wanted to know!

[Dr. Gerald N. Johnson]

On Thu, 2008-06-05 at 16:36 -0700, Jim WA9YSD wrote:
> Jerry,  Both the FD and bazooka on UHF/VHF antenna test use tubing 
> with a large diameters.  This gave the antennas band width among other things.
> The folded dipole had tubing on only one side.

Large diameter tubing alone gives wider bandwidth.
> 
> The charts on the FD went off the scale with just wire on both half's 
> and with large spaced spreaders.  Programs cannot adjust to it to give 
> you any thing accurate unless and accurate simulation is built, other 
> wise your guessing on its performance.

The only thing you can reliably compute is end effect from the fat ends
of the folded dipole. NEC programs don't handle closed spaced wires
well.
> 
> The crossed Double Bazooka will give you more band width I think 1.5 
> times more than a dipole.  Wide spaced FD about 2.0 times more than 
> a dipole.  Whats the increased band width with the 2000pf cap and 
> 3 foot cable combo compared to a dipole?

My design with 2000 pf, 3m cable, fed through 5/16" wave of 72 ohm coax
is 1.3:1 from 3.6 to 4.1 MHz, never better than 1.3. The swr gets
rapidly worse beyond that range. In a VHF experiment I got as good a
bandwidth increase with a couple 50 ohm quarter wave stubs in parallel
with the dipole feed point. I hung those stubs along the feed. My
measurements were imperfect, just in thee basement with only a couple
feet of coax to the lab type bridge so the antenna was not in free
space. I've not dug up that data from my notebook lately,  but the
authors of "Very High Frequency Techniques" from WW2 reported developing
antennas with a decent match over the entire 100 to 150 MHz frequency
range using reactive compensation techniques.
> 
> Bazooka eliminates the use of a balun that matches a balanced antenna 
> to unbalance feed line making the DB a more reliable antenna. 

Actually it does nothing for balance.

>  Normally you would use a 6:1 balanced to balanced balun 
> at the feed point of an FD and attach a 1:1 current balun 
> to it.  Too many pieces to fail for one reason or another.

Nah, 4:1. 6:1 is a hard ratio to make. And the folded dipole with equal
sized conductors on top and bottom wire makes a 4:1 impedance step up.
If the fed wire is smaller than the "radiator" wire the impedance step
up is higher.
> 
> 
>  Keep The Faith, Jim K9TF/WA9YSD
> 
73, Jerry, K0CQ