Antennas

[Terry]

I'm wanting to put an antenna inside the fuselage of my Aeronca Champ.
I've read about the folded dipoles, and wondered if this was
something that would be feasible. I want it for communications with
my handheld. Any thoughts?

Thanks!

Terry
Gordonville, MO

[UltraJohn]

Terry wrote:

> I'm wanting to put an antenna inside the fuselage of my Aeronca Champ.
> I've read about the folded dipoles, and wondered if this was
> something that would be feasible. I want it for communications with
> my handheld. Any thoughts?
>
> Thanks!
>
> Terry
> Gordonville, MO


Terry the major advantage of a 'folded dipole' over a standard dipole is a
wider bandwidth. I really don't see much of a need for it, that being said
they're easy to make so I see know problem. I don't see where it would
really be any more advantages with respect to installing it inside your
Champ.
John

One possibility is a VHF DDRR antenna basically an electrically shortened
antenna that mounts horizontally but has vertical radiation pattern like a
1/4 wave antenna. It consists of approximate 10" square double sided pc
board material with a loop mounted about 3 inches above or below it the end
bends down and attaches to the pc board. The loop is 5/8 tubing about 6 - 7
inches loop diameter. About 2 - 3 mhz bandwith. You could mount it inside
like you said pc on underside of top inside fuselage with loop under it or
on outside of bottom of fuselage with loop under it and make a fiberglass
radome for it. Below is a link for a 2 meter version you will need to scale
it slightly for the aviation band. Also he goes though his development
routine on it what you want is near the bottom.

http://www.antennex.com/preview/2mddrr.htm

[RST Engineering]

One big problem right off the crack of the bat. "Folded" dipoles have a
characteristic feedpoint impedance of something on the order of 300 ohms. A
standard dipole at the center of resonance will have a characteristic feed
point impedance of something on the order of 72 ohms.

The mismatch for a 50 ohm radio (without some sort of a transformer) to 300
ohms is a hell of a lot more than 72 ohms.

Somebody posted that a folded dipole has a broader bandwidth than a straight
dipole. I do not believe that to be true. The main use for folded dipoles
without transformers was in the 1950s when we made them out of twinlead and
fed a TV set with them. TV sets in those days were designed for 300 ohm
antennas.

Get yourself a 3/8" diameter wooden dowel 48" long. Get two pieces of
copper tape 21.5 inches long. Glue the tape to the dowels so that there is
a 1/4" gap in the center of the dowel between the pieces of tape. Run the
tape out as far as it will go towards the end of the dowel. You should have
about 2" of bare dowel at each end of the dowel.

Drill an 1/8" hole into each of the pieces of tape right at the center ends
of the tape. Take a piece of 50 ohm coax cable (RG-58 from the spool in the
back of the Radio Shack store) and peel back the black outer jacket for an
inch. Now fish the center white/clear insulator through the braid by means
of an icepick or similar tool. Twist the braid tightly and poke it through
one of the 1/8" holes. Solder the braid to the copper foil. Strip the
white/clear insulation from the center conductor for about 1/4". Poke the
center conductor/insulation through the remaining 1/8" hole. Solder the
center conductor to the copper tape as it exits the hole.

As close to the braid as possible, run a tie-wrap around the black outer
sheath of the coax and fasten it securely to the dowel. Now run that coax
at right angles for at least a foot or two before snaking it around through
the airframe.

You've got two inches on either end of the dowel to nail, screw, glue, or
otherwise fasten your dipole antenna to one of the fuselage formers aft of
the baggage compartment. Keep the TIPS of the antenna as far away as you
can from any metal.

Some refinements: A balun is always a good idea when connecting a BAL anced
antenna (dipole) to UN balanced coax cable. You can do this with little
tiny chunks of powdered ferrite called toroids, or you can do this with a
coax "split-tube" design. Both are a little beyond the scope of a newsgroup
article.

Since I absolutely refuse to use the newsgroups to pimp my own goods, will
somebody else please tell this fellow where he can get copper tape, toroids,
and the illustrated book that shows how this all works?


Jim




"Terry" > wrote in message
om...
> I'm wanting to put an antenna inside the fuselage of my Aeronca Champ.
> I've read about the folded dipoles, and wondered if this was
> something that would be feasible. I want it for communications with
> my handheld. Any thoughts?
>
> Thanks!
>
> Terry
> Gordonville, MO

[Blueskies]

"RST Engineering" > wrote in message ...
> One big problem right off the crack of the bat. "Folded" dipoles have a characteristic feedpoint impedance of
> something on the order of 300 ohms. A standard dipole at the center of resonance will have a characteristic feed
> point impedance of something on the order of 72 ohms.
>
> The mismatch for a 50 ohm radio (without some sort of a transformer) to 300 ohms is a hell of a lot more than 72 ohms.
>
> Somebody posted that a folded dipole has a broader bandwidth than a straight dipole. I do not believe that to be
> true. The main use for folded dipoles without transformers was in the 1950s when we made them out of twinlead and fed
> a TV set with them. TV sets in those days were designed for 300 ohm antennas.
>
> Get yourself a 3/8" diameter wooden dowel 48" long. Get two pieces of copper tape 21.5 inches long. Glue the tape to
> the dowels so that there is a 1/4" gap in the center of the dowel between the pieces of tape. Run the tape out as far
> as it will go towards the end of the dowel. You should have about 2" of bare dowel at each end of the dowel.
>
> Drill an 1/8" hole into each of the pieces of tape right at the center ends of the tape. Take a piece of 50 ohm coax
> cable (RG-58 from the spool in the back of the Radio Shack store) and peel back the black outer jacket for an inch.
> Now fish the center white/clear insulator through the braid by means of an icepick or similar tool. Twist the braid
> tightly and poke it through one of the 1/8" holes. Solder the braid to the copper foil. Strip the white/clear
> insulation from the center conductor for about 1/4". Poke the center conductor/insulation through the remaining 1/8"
> hole. Solder the center conductor to the copper tape as it exits the hole.
>
> As close to the braid as possible, run a tie-wrap around the black outer sheath of the coax and fasten it securely to
> the dowel. Now run that coax at right angles for at least a foot or two before snaking it around through the
> airframe.
>
> You've got two inches on either end of the dowel to nail, screw, glue, or otherwise fasten your dipole antenna to one
> of the fuselage formers aft of the baggage compartment. Keep the TIPS of the antenna as far away as you can from any
> metal.
>
> Some refinements: A balun is always a good idea when connecting a BAL anced antenna (dipole) to UN balanced coax
> cable. You can do this with little tiny chunks of powdered ferrite called toroids, or you can do this with a coax
> "split-tube" design. Both are a little beyond the scope of a newsgroup article.
>
> Since I absolutely refuse to use the newsgroups to pimp my own goods, will somebody else please tell this fellow where
> he can get copper tape, toroids, and the illustrated book that shows how this all works?
>
>
> Jim
>
>


I've seen all this stuff here:
http://www.rst-engr.com/rst/catalog/airplane_antenna.html

;-)