Antenna ground planes for composite aircraft

On Sun, 20 Dec 2009 16:03:47 -0500, rich
wrote:

Here's the article: http://www.express-builder.com/docs/tip1/tip1.pdf
written by Bob Archer. It's the 5th paragraph down.
what he says is: "I do not recommend any antenna on the market that
has a little black box in the center of the antenna. This device is a
ferrite transformer which provides a very good VSWR and a very good
bandwidth but at the cost of being a very lossy (absorbs energy)
device.

Irregardless of the fact that we've been using them for about fifty
years as a way of going from coax to balanced inputs on TV sets. They
work just fine. However, I don't use them as I don't need to. The
center impedance of an infinitely small wire used as a dipole is 72
ohms. However, as they get fatter, the center impedance drops until
they fairly resemble a 50 ohm load with fat copper strip.


.. Also if you were planning to go with Jim
Weir of RST's designs don't bother with the ferrite beads. At these
frequencies the beads don't do anything that I could detect in the RF
lab.

Then with all due respects, your RF lab isn't very well equipped. The
beads do the same thing at RF that a "clamp-on" alternator filter
does...it does nothing for the noise at the source, but it strips off
the noise on the wire preventing it from radiating all over creation.
The ferrite beads simply act as a "noise filter" stripping off any RF
that gets reflected back down the outside of the coax outer conductor
(shield).



A good balun would work better as a dipole feed because it
balances the currents on the elements and matches the impedance at the
same time and it doesn't
absorb RF energy. My antenna designs do not need a balun because I use
a modified version of a feed called a Gamma match that feeds the
antenna at the fifty-ohm point and automatically balances the currents
on the elements."

And all a gamma match does is introduce a tuned circuit into the
mixture which by definition reduces the bandwidth of the device.

Jim

On Sun, 20 Dec 2009 11:13:46 -0500, rich
wrote:

I'm installing it on the belly panel. What I'm
wondering is how big to make the diameter of the ground plane, and how
to make contact with it to the blade type transponder antenna. I would
assume it somehow needs to connect to the outer portion of the BNC
connector? And what about the GPS antenna, does it need a ground
plane?

Make both of them out of aluminum pie pans and stand them off from the
fiberglass surface so that you don't have any agitated porcupine
quills sticking out from the airframe.

Transponder diameter about 5.5" diameter; GPS about 3.75" or 11" if
the 3.75 is too small to work with.

Jim

On Sun, 20 Dec 2009 13:33:55 -0800, jim ham
wrote:


This is a description of the design Jim Wier promoted years ago.

Weir


His
design included a couple of ferrites slipped around the coax to keep the
shield from ringing (introducing a lossy element). I'm not sure if these
count as ferrite transformers.

They do not.



Everybody talks about transmit power, but 1W transmitted from the
antenna is plenty for aircraft. The real difference you will see with a
good antenna is receive sensitivity. A good transmit antenna is a good
receive antenna and vice-versa.

The effective range of a 1 watt transmitter on one end and a 1
microvolt receiver on the other end is something on the order of 2000
miles, which is why we can talk to Spacelab with a handie-talkie quite
easily.

Jim

On Mon, 21 Dec 2009 11:17:54 -0500, rich
wrote:

You know, I can't find those articles Jim Weir wrote in Sport Aviation
about ground planes. My Sport Aviation on CD set only goes up to 2001,
so those articles must have been written after that. I'll check the
EAA website, they've got later issues available online for members.


Probably because I don't recall writing anything about ground planes.
And I haven't published anything in Sport Av since I found out in the
late 1970s that other magazines actually PAY their authors for
articles.

Jim

RST Engineering is still alive and well.
http://www.rstengineering.com/index.html They don't seem to mention
the early articles from Sport Aviation, but they do still sell antenna
kits. Jim Weir used to hang out on this forum, but I guess not any more.

snip

Yeah, he do. Just nothing interesting to comment on until an avionics
question comes up.

Jim

On Sun, 20 Dec 2009 17:50:02 -0600, brian whatcott
wrote:

rich wrote:
Here's the article: http://www.express-builder.com/docs/tip1/tip1.pdf
written by Bob Archer. /snip/ Also if you were planning to go with Jim
Weir of RST's designs don't bother with the ferrite beads.
/snip/
My antenna designs do not need a balun because I use
a modified version of a feed called a Gamma match that feeds the
antenna at the fifty ohm point...

Hmmmm. so this is a fellow with another antenna design, bad-mouthing the
opposition. It makes about as much sense as my giving you this advice:

Yeah, I know. Daddy taught me that if you can't say anything nice
about somebody, then don't say anything, which is why I've never
commented on the Archer designs.



"If you were planning to go with Bob Archer's design, don't bother with
the gamma match lengths he uses..."

What DOES make sense, is that if you really do follow Archer's advice to
rig a dipole hooked directly to the coax - you guarantee standing waves
on the outer (sheath ). A balun is what matches this dipole
arrangement to coax...

But since we aren't trying to squeeze the last fraction of a dB out of
the arrangement, and even though a balun still guarantees SOME
reflected power except at the precise center frequency of the antenna,
then simply connecting the dipole ears to the coax and stripping the
reflected power from the sheath with single-turn chokes in the form of
ferrite beads slipped over the coax right at the antenna does a
credible job of making a simple, lightweight, and nearly bulletproof
antenna.

Jim

RST Engineering wrote:

But since we aren't trying to squeeze the last fraction of a dB out of
the arrangement, and even though a balun still guarantees SOME
reflected power except at the precise center frequency of the antenna,
then simply connecting the dipole ears to the coax and stripping the
reflected power from the sheath with single-turn chokes in the form of
ferrite beads slipped over the coax right at the antenna does a
credible job of making a simple, lightweight, and nearly bulletproof
antenna.

Jim

Which reminds me, one old time method of matching a balanced antenna to
a co ax feed was with an extra coax matching section so the center
conductors connected to each half of the dipole, then the coaxes were
joined appropriately.

Probably too mutch fiddle factor, but rather than stoppering the base of
the dipole with ferrites, what would happen if you placed the stoppers a
quarter wave back on the coax? I never tried this out, but it's just
possible some of that extra gain might get into the coax?
Did you ever try this arrangement?

Brian W

Here is an antenna I built last year for a friend's PIK-20 sailplane. The glider is one of the early fiberglass designs. (http://www.soaridaho.com/Antenna/index.html)

Later when doing a minor repair the owner was surprised to find an antenna of the same design inside the vertical stabilizer. It was also is a 1/2 wave length center fed; however, made out of two sections of 3/4 inch aluminum tubing. The coax was routed up through the lower tube to the feed point. The coax only extended about 6 inches below the bottom antenna section and was terminated by a BNC fitting.. There wasn't any indication that the antenna had ever been used.

Wayne
HP14 N990
http://tinyurl.com/N990-6F



"brian whatcott" wrote in message ...
RST Engineering wrote:

But since we aren't trying to squeeze the last fraction of a dB out of
the arrangement, and even though a balun still guarantees SOME
reflected power except at the precise center frequency of the antenna,
then simply connecting the dipole ears to the coax and stripping the
reflected power from the sheath with single-turn chokes in the form of
ferrite beads slipped over the coax right at the antenna does a
credible job of making a simple, lightweight, and nearly bulletproof
antenna.

Jim

Which reminds me, one old time method of matching a balanced antenna to
a co ax feed was with an extra coax matching section so the center
conductors connected to each half of the dipole, then the coaxes were
joined appropriately.

Probably too mutch fiddle factor, but rather than stoppering the base of
the dipole with ferrites, what would happen if you placed the stoppers a
quarter wave back on the coax? I never tried this out, but it's just
possible some of that extra gain might get into the coax?
Did you ever try this arrangement?

Brian W

Thanks for sharing the pix.

The device I had in mind was built in a similar sheath folded back way -
but with more fiddling:
the shield is disconnected at the fold back point. The center conductor
goes to one quarter wave leg: the INNER shield goes to the other
quarter wave leg. The OUTER shield is left open at its end, but a
quarter wave back, it is connected to the regular shield.

This arrangement is intended to provide a quarter wave match to coax
over the band of interest. Stoppers might well help at the junction of
the two sheaths. Getting the lengths right is the name of the game.

Brian W

Wayne Paul wrote:
Here is an antenna I built last year for a friend's PIK-20 sailplane. The glider is one of the early fiberglass designs. (http://www.soaridaho.com/Antenna/index.html)

Later when doing a minor repair the owner was surprised to find an antenna of the same design inside the vertical stabilizer. It was also is a 1/2 wave length center fed; however, made out of two sections of 3/4 inch aluminum tubing. The coax was routed up through the lower tube to the feed point. The coax only extended about 6 inches below the bottom antenna section and was terminated by a BNC fitting.. There wasn't any indication that the antenna had ever been used.

Wayne
HP14 N990
http://tinyurl.com/N990-6F



"brian whatcott" wrote in message ...
RST Engineering wrote:
But since we aren't trying to squeeze the last fraction of a dB out of
the arrangement, and even though a balun still guarantees SOME
reflected power except at the precise center frequency of the antenna,
then simply connecting the dipole ears to the coax and stripping the
reflected power from the sheath with single-turn chokes in the form of
ferrite beads slipped over the coax right at the antenna does a
credible job of making a simple, lightweight, and nearly bulletproof
antenna.

Jim
Which reminds me, one old time method of matching a balanced antenna to
a co ax feed was with an extra coax matching section so the center
conductors connected to each half of the dipole, then the coaxes were
joined appropriately.

Probably too mutch fiddle factor, but rather than stoppering the base of
the dipole with ferrites, what would happen if you placed the stoppers a
quarter wave back on the coax? I never tried this out, but it's just
possible some of that extra gain might get into the coax?
Did you ever try this arrangement?

Brian W

Brian,

I built one of these fold back antennas, roll it up and carry it in my survival kit. When used with the ICOM, I throw it over my shoulder and let it hang down my back. It sure outperforms the handheld's "rubber ducky."

Wayne
http://tinyurl.com/N990-6F


"brian whatcott" wrote in message ...
Thanks for sharing the pix.

The device I had in mind was built in a similar sheath folded back way -
but with more fiddling:
the shield is disconnected at the fold back point. The center conductor
goes to one quarter wave leg: the INNER shield goes to the other
quarter wave leg. The OUTER shield is left open at its end, but a
quarter wave back, it is connected to the regular shield.

This arrangement is intended to provide a quarter wave match to coax
over the band of interest. Stoppers might well help at the junction of
the two sheaths. Getting the lengths right is the name of the game.

Brian W

Wayne Paul wrote:
Here is an antenna I built last year for a friend's PIK-20 sailplane. The glider is one of the early fiberglass designs. (http://www.soaridaho.com/Antenna/index.html)

Later when doing a minor repair the owner was surprised to find an antenna of the same design inside the vertical stabilizer. It was also is a 1/2 wave length center fed; however, made out of two sections of 3/4 inch aluminum tubing. The coax was routed up through the lower tube to the feed point. The coax only extended about 6 inches below the bottom antenna section and was terminated by a BNC fitting.. There wasn't any indication that the antenna had ever been used.

Wayne
HP14 N990
http://tinyurl.com/N990-6F



"brian whatcott" wrote in message ...
RST Engineering wrote:
But since we aren't trying to squeeze the last fraction of a dB out of
the arrangement, and even though a balun still guarantees SOME
reflected power except at the precise center frequency of the antenna,
then simply connecting the dipole ears to the coax and stripping the
reflected power from the sheath with single-turn chokes in the form of
ferrite beads slipped over the coax right at the antenna does a
credible job of making a simple, lightweight, and nearly bulletproof
antenna.

Jim
Which reminds me, one old time method of matching a balanced antenna to
a co ax feed was with an extra coax matching section so the center
conductors connected to each half of the dipole, then the coaxes were
joined appropriately.

Probably too mutch fiddle factor, but rather than stoppering the base of
the dipole with ferrites, what would happen if you placed the stoppers a
quarter wave back on the coax? I never tried this out, but it's just
possible some of that extra gain might get into the coax?
Did you ever try this arrangement?

Brian W