Bazooka using 75 ohm coax?
 

How does one construct a bazooka using 75 ohm coax for the radiating
portion and 50 ohm coax for the feed (as usual)? The obvious reason
is that I have a pot full of random length, unused 75 cable I would
like to find a good use for. Most of it is RG-6 TV coax variant or
such.

I have not seen design equations for the bazooka, just articles
saying "cut it to these dimensions" (don't worry your pretty
little head implied, but not stated...)


Thanks for any help,

Phil - N4GWV

On Thu, 15 Jan 2004 16:52:30 -0600, Phillip Jockell
wrote:

How does one construct a bazooka using 75 ohm coax for the radiating
portion and 50 ohm coax for the feed (as usual)? The obvious reason
is that I have a pot full of random length, unused 75 cable I would
like to find a good use for. Most of it is RG-6 TV coax variant or
such.


Phil - N4GWV

The only thing to be concerned about is the velocity factor to obtain
the correct lengths. The impedance is not factor.

That said, before you spend you time and money I suggest you review
Walt Maxwell's evaluation of that antenna at
http://home.iag.net/~w2du/Reflection...bleBazooka.pdf

73
Danny, K6MHE

How does one construct a bazooka using 75 ohm coax for the radiating
portion and 50 ohm coax for the feed (as usual)?

Phillip, The best thing would be to short the shield and inner conductor of
the coax and use it as your typical wire dipole. The old standard 468/f(mhz)
should work as a starting point. Do an swr sweep, or trim for lowest swr at
your transmitter.
A 1:1 balun at the antenna is good engineering practice to minimize
transmission line radiation. Be aware that the magical 1:1 swr may not be
possible feeding a dipole with 50 ohm coax.
Modern literature on the subject of bazookas indicate they will give broader
bandwidth at the expense of decreased efficiency. Depends on what you want
from your antenna.
73 Gary N4AST

Since "bazooka" conjures up at least three pretty different images for
me (and that's just with respect to antennas), could you explain a bit
more just how you're thinking of building it? Is it like
http://www.qsl.net/we6w/projects/baz_std.txt? If so, I believe you
want to make the coax sections close to 1/4 wave long, accounting for
the velocity factor of the coax you use. Expect the coax part, as
transmission lines, to reflect a reactance back to the feedpoint...the
reactance they reflect as a function of frequency does depend on the
impedance of the line you use. You could model it with EZNec, or use
any of the many available transmission line calculators to find it.
What's across the feedpoint is just the two sections in series. The
radiating part of the antenna is just the outside surface of the coax
(or whatever conductors you have).

Cheers,
Tom

Phillip Jockell wrote in message . ..
How does one construct a bazooka using 75 ohm coax for the radiating
portion and 50 ohm coax for the feed (as usual)? The obvious reason
is that I have a pot full of random length, unused 75 cable I would
like to find a good use for. Most of it is RG-6 TV coax variant or
such.

I have not seen design equations for the bazooka, just articles
saying "cut it to these dimensions" (don't worry your pretty
little head implied, but not stated...)


Thanks for any help,

Phil - N4GWV

Actually, I believe it is called the double bazooka.
What three images are conjured???

thanks and 73
phil

(Tom Bruhns) wrote in
m:

Since "bazooka" conjures up at least three pretty different images for
me (and that's just with respect to antennas), could you explain a bit
more just how you're thinking of building it? Is it like
http://www.qsl.net/we6w/projects/baz_std.txt? If so, I believe you