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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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 Posted: Sun Mar 24, 2013 6:53 am Post subject: Bazooka Dipole Antenna |
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I made a Bazooka Dipole Antenna and tested it in my workshop using a RED DOT 1050A SWR bought on eBay.
Overall length = 46.75"
Braid length in center of antenna = 31"
136.525 Mhz, 2.8 SWR
127.525 Mhz, 1.31 SWR
118.525 Mhz, 2.4 SWR
Will that SWR range give acceptable performance?
I understand that the location and orientation in an aircraft will greatly affect antenna performance.
Links to Bazooka Dipole Antenna:
http://www.k3dav.com/buildadoublebazookaant.htm
http://www.nsw.wicen.org.au/technical/projects/coaxial-folded-dipole-antenna
Joe
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Joe Gores |
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nuckolls.bob(at)aeroelect
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| Posted: Sun Mar 24, 2013 9:36 am Post subject: Bazooka Dipole Antenna |
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At 09:53 AM 3/24/2013, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
I made a Bazooka Dipole Antenna and tested it in my workshop using a RED DOT 1050A SWR bought on eBay.
Overall length = 46.75"
Braid length in center of antenna = 31"
136.525 Mhz, 2.8 SWR
127.525 Mhz, 1.31 SWR
118.525 Mhz, 2.4 SWR
If the SWR does not change significantly when the antenna is installed in an aircraft, will the antenna performance be acceptable? |
If you can find a means by which this antenna
can be mounted vertically in the airplane, then
it's almost a sure bet that it will perform
as well as say, at 1/4-wave whisker sticking
out of the fuselage. The only 1/2 wave antennas
I recall were some vertical dipoles stuck on
masts out in front of the wing on both sides
of an airplane. This antenna pair was used on
VHF direction finding systems to locate downed
aircraft homing on their ELT or VHF comm transmissions.
I'm curious as to foundation for statements
in the article. Further, there are no empirical
data offered for the trade off between this and
any other design. This antenna and similar designs
are touted for their increased bandwidth. But
I'm wondering if this isn't simply a function of
smaller length/diameter ratios for the conductors.
I'm suspicious of the term "leaky coax" used to
re-enforce the notion that foil-wrapped (and I
presume double braid) coaxes won't work as well
(or not at all?).
The idealized, infinitely small conductor dipole
has a very high Q (small bandwidth) compared to
it's practical cousins built from say 1" diameter
tubing with smaller l/d ratios.
http://tinyurl.com/akb37r9
I'll have to
dig out the antenna manuals but I think the classic
folded dipoles found on many TV antennas having
lengths of several feet divided by spacings of 2 or
3 inches were similarly blessed with large bandwidths.
The author speaks to compatibility with either
50 ohm coax or 400 ohm ladder line (twin-lead)
which is specious. The antenna will present an
impedance at the feed point that will have the
greatest compatibility with SOME kind of feed line
but the commonly available commercial offerings
are 50, 75, 93, 300 and 450 ohms . . .with the
higher values being limited to balanced feeders
that are NOT compatible with our world of 50
ohm transmitters and coax-wired airframes.
The bottom line is that the literature I was
able to find (admittedly hurried search) on
this design offered no compelling arguments for
its superiority over other dipole designs.
As to probable performance on an airplane, we
fly in a cloistered world of short-range,
line of sight communications and navigation.
For exceedingly relaxed constellation of
requirements, radios like this found their way
into light aircraft of the 1940s.
[img]cid:.0[/img]
Powered with dry cells (A and B batteries!) this
transceiver listened on 200-400 KHz (radio range
and locator beacon frequencies) and talked on
120 Mhz VHF. The antenna was a short piece of
wire strung outside the cabin. in some practical
manner . . .
This transmitter was probably good for 100 milliwatts
or so, the receiver was probably in the 10-100 microvolt
class. If you had asked the designer about importance
of SWR for receive (antenna WAAAY too short)
and transmit (WAAAY too long) the response
would probably have cited a quantum leap in
system performance bove light guns, megaphones and
signal flags. Mounted to the panel of Luscome or J-3
in 1948, this radio would allow the TALENTED
pilot to fly an non-precision approach to an
airport using nearby A-N Radio Range transmitters.
[img]cid:.1[/img]
The return on investment for seeking greater
antenna performance in terms of radiation efficiency
and directivity (so called gain) will have
to be collected in points of academics and/or
sense of craftsmanship. The probability that you and the
guy you're talking too will hear any substantive
difference between antenna-A and antenna-B is
pretty low.
By the way, if anyone stumbles across one of these
radios offered for sale, I'd like to know about
it. It was offered under Mitchell, Motorola and
Gavin trade names but the term "Airboy" seems to
have persisted across all the manufacturing venues.
Bob . . .
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Mon Mar 25, 2013 6:10 am Post subject: Re: Bazooka Dipole Antenna |
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Thanks for the reply, Bob.
I thought the same as you, that the electrons would oscillate in foil just as easily as in braid. So I tried to use some foil and braid wrapped RG-6/U for greater bandwidth. But it was very difficult to remove the foil. It was glued to the foam insulation surrounding the center conductor. So I gave up using the RG-6/U and used RG-58 instead.
I think using the Bazooka Dipole Antenna would be an option in a non-conductive fuselage like wood or fiberglass. The antenna could be glued to the fuselage skin so that the ends of the antenna are vertical.
The claims of improved bandwidth and performance may be unfounded. But I like it that no balun is required, unlike some other dipole antennas.
Joe
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nuckolls.bob(at)aeroelect
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| Posted: Mon Mar 25, 2013 6:44 am Post subject: Bazooka Dipole Antenna |
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| Quote: | | I think using the Bazooka Dipole Antenna would be an option in a non-conductive fuselage like wood or fiberglass. The antenna could be glued to the fuselage skin so that the ends of the antenna are vertical. |
Keep in mind that it's the high current conductors
of the antenna that do the radiation. These happen
at the center. It's the portions around the feed line
connection that need to be 'most vertical' . . .the
ends can wrap around the fuselage.
| Quote: | | The claims of improved bandwidth and performance may be unfounded. But I like it that no balun is required, unlike some other dipole antennas. |
That's the really phoney feature of this article.
The antenna is obviously a form of dipole which is
a balanced antenna . . . simply attaching a coax
feeder to the center as depicted produces the same
conflict as for any dipole.
In terms of measurable performance, doing the
balanced to unbalanced mambo isn't all that
big a deal. For a plastic/wood airplane, you
might consider 1" wide copper foil tape stuck
to the inside surface of the fuselage. See
the foil antenna series of pictures at:
http://tinyurl.com/btbu4cm
The elements for VHF comm would start at
22+ inches each and then get trimmed for
lowest SWR at 126Mhz. The width of the
foil goes to bandwidth.
As suggested before, orientation in the
aircraft should have as much of the center
vertical with the ends wrapping around
the upper and lower surfaced of the
fuselage.
Bob Archer's SA-006 antenna is one example
of a physically shortened design with an
unbalanced feed point.
If you're inclined to experimentation, you
could do some more work on the bench with
a design that makes a bit more sense than
the 'bazooka' dipole.
Bob . . . [quote][b]
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Mon Mar 25, 2013 7:57 am Post subject: Re: Bazooka Dipole Antenna |
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Bob,
Thanks for sharing your knowledge. Sometimes I believe what I read on the internet even though I should not.
Joe
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nuckolls.bob(at)aeroelect
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| Posted: Mon Mar 25, 2013 10:59 am Post subject: Bazooka Dipole Antenna |
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At 10:57 AM 3/25/2013, you wrote:
| Quote: |
Bob,
Thanks for sharing your knowledge. Sometimes I believe what I read
on the internet even though I should not.
Joe
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Better to have read, considered and accepted/rejected
for logical reasons. Education is not cheap. It takes
time, thought, experimentation, study and in some
cases assumption of risk.
Over my lifetime, I've probably pitched $thousands$
in poor return-on-investment or failed experiments . . .
but it's just as useful to know what things do work
as for those that do not. I'm pleased that you
brought this conversation to the List!
Bob . . .
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