"Walter Maxwell" wrote in message
...
On Mon, 4 Apr 2005 16:02:04 -0500, "Ken Bessler"
wrote:

My friend says that a double bazooka is 98% efficiant
and that a dipole is only about 70% efficiant.

Is he right? Will a double bazooka outperform a dipole
enough to notice a difference on 40m?
Hi Ken,
Those who tout the double bazooka have been misled for eons. That antenna
is
simply a resistance-loaded dipole and the increase in BW is due to the
resistance loading of the dielectric in the coax that forms the dipole.
The
shunt reactance of the shorted quarter-wave sections forming the dipole
does
nothing to increase the BW. The principle is ok, and was used on military
equipment during WW2, but for the shunt reactance to provide the increase
in BW
the feedline Zo needs to be at least two times the resistive component in
the
dipole input impedance for the concept to work. Consquently, the double
bazooka
as misused by the amateur community has been misengineered.

I published a long and detailed expose of this antenna in Ham Radio,
August
1976, with a shortened version in QST, Sept 1976. I explain mathematically
why
it doesn't work as many claim. It appears as Chapter 18 in Reflections 1
and 2,
and is available for downloading from my web page at www.w2du.com. Please
review
this document before wasting your time and energy on a dud.

Walt, W2DU

Hello Ken
We beat this one to death some time ago on this group.
A perfectly tuned bazooka (I had to build seven of 'em for 40 meters 'til I
got it nailed.)
exhibits some interesting SWR reduction effects right around resonance,
where the SWR is already so low it doesn't matter,
but any increase in the 1.5:1 SWR bandwidth is due to loss as Walt proved
decades ago.
In the mean time you lose 10-20 percent of your signal if you're lucky.
And if the antenna's not perfectly tuned, you lose more than that.
The equivalent circuit is a series-resonant network (the dipole) in parallel
with a parallel-resonant network (the stubs).
The (driven) parallel-resonant network oscillates at it's driven frequency
when it is driven close to it's resonant frequency, causing the already
small reflected power to nearly vanish. Move very far from resonance (where
the SWR on a dipole 1.2:1) and the parallel resonant circuit stops
oscillating.
I posted quite a bit of data here for bazookas made for 160, 80 and 40.
Don't waste your time, I already wasted mine.
Buy a balun from Walt or make your own and tune your dipole carefully.
There ain't no free lunch.


73
H., NQ5H