Antenna ground planes for composite aircraft

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

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

jim ham wrote:
rich wrote:
I'm using pre-made di-pole antennas for the comm radios in my Glasair.
But I need to install antennas for the transponder and marker beacon.
I've found some pre-made dilpole marker antennas, but for the
transponder, I'm using one of the blade type, and for that one, I'll
need a ground plane. 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? I've seen lots of pictures of ground planes made with strips of
copper foil tape radiating from the center. Which looks adequate, and
should work as well as a big piece of solid copper foil, which I have
no idea where to purchase.
Copper tape is available from electronics suppliers such as Digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=3M1181B-ND

Jim Weir of RST Engineering wrote a series of articles in Sport Aviation
many years ago on how to construct these antennas and ground planes. He
sold kits of materials at the time. You could email him to see if these
kits are still available. Otherwise you wind up buying a whole roll of
tape.
I also read an article by Bob Archer about
di-poles. He said if the dipole has a little black box in the middle
of it, not to use it, as that contains ferrite beads which greatly
reduce it's effectivness. Unfortunately I already have that type
burried inside the leading edge of my verticle stabilizer.
I'd get a second opinion about this. I'd like to see the article.
My understanding is that the ferrite beads are necessary for impedance
match from the coax to the antenna. One can check how good an antenna is
with an SWR meter. The better the SWR the better the antenna. Anything
better than 2:1 will work, but 1.2:1 makes an superior antenna. It's
always a good idea to check an antenna installation with a SWR meter in
any case. Check right at the connection to the radio. It's too easy to
introduce problems at connectors or other coax connections.
I know of several airplanes with home-made copper tape antennas that
followed the Weir design. They include the ferrite beads and they work
fine.

Rich


Actually, the SWR meter should be used right at the antenna. That said,
you won't find many people that have SWR meters that work at the
approximately 1000 MHz range of the transponder. Plus, I believe they
only transmit when they receive an "interrogation" from the radar site
transmitter, so it would be hard to "key up" the transponder to read the
SWR meter.

snip


Actually, the SWR meter should be used right at the antenna. That said,
you won't find many people that have SWR meters that work at the
approximately 1000 MHz range of the transponder. Plus, I believe they
only transmit when they receive an "interrogation" from the radar site
transmitter, so it would be hard to "key up" the transponder to read the
SWR meter.

Good point. I was only thinking in terms of the nav/com antenna. 100MHz
is not to hard to measure.

Also a good point about the placement of the SWR meter. It depends on
what you want to measure. In a typical homebuilt I was thinking only of
qualifying the entire antenna installation. This definitely includes the
coax along with everything else including the antenna. But if you get a
bad reading you have some detective work to find out where lies the problem.

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

Boy, those are a lot smaller than I thought they had to be. The pie
pans sound like a good idea since they aren't so thin like foil is
and easier to keep from damaging. But solder doesn't stick to
aluminum, so how to make a good contact that won't come loose to the
pie pan? I may just get that antenna kit you sell at RST with that
copper foil.
I had started to think that you didn't write any ground plane articles
in SA, since nothing comes up about that online or in the CD
collection.

That belly panel on a Glasair is fiber glass with two layers, and a
half inch foam core between them. Do the ground planes need to stand
off from the inner skin if they are seperated from the lower/outer
skin by a half inch? The space between the upper skin of the belly
panel and the lower wing skin it covers is less than a quarter inch,
so there's no room for the ground plane to stand off. I could mount
them back in the tail cone if that is needed.

I had thought about installing one of those pre-made di-pole
transponder antennas. But to mount it vertically, about the only place
I could put it would be on the back side of the spar in the wheel
well. And that would put the microwave energy only a half inch away
from the fuel cell. Not sure that would be kosher. On a Glasair 3, the
entire leading edge of the wing from the spar forward and from tip to
tip is all fuel.

It makes sense to me that the ferrite beads are needed, as without
those, I can't see how the antenna would know where to start and stop.
It seems without those, the antenna and the coax would all become the
antenna, and be totally out of tune for just about any frequency.
Rich


On Tue, 22 Dec 2009 10:28:31 -0800, RST Engineering
wrote:

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 Wed, 23 Dec 2009 14:02:11 -0500, rich
wrote:

Boy, those are a lot smaller than I thought they had to be. The pie
pans sound like a good idea since they aren't so thin like foil is
and easier to keep from damaging. But solder doesn't stick to
aluminum, so how to make a good contact that won't come loose to the
pie pan? I may just get that antenna kit you sell at RST with that
copper foil.

The transponder and GPS antenna bolt directly to the ground plane.
That makes both the mechanical and electrical connections. Copper
tape is not being used; the transponder can be the little whip
monopole with the BB on the end. The GPS can be any one of a number
of commercial units that bolt directly to the ground plane. Copper
tape dipoles are used where you need isotropic (whole orange)
radiation pattern. Ground plane antennas are where you want
hemispherical (half an orange) radiation. Transponder radars are
always down. GPS satellites are always up. Unless you fly inverted
{;-)


I had started to think that you didn't write any ground plane articles
in SA, since nothing comes up about that online or in the CD
collection.

That's true.


That belly panel on a Glasair is fiber glass with two layers, and a
half inch foam core between them. Do the ground planes need to stand
off from the inner skin if they are seperated from the lower/outer
skin by a half inch? The space between the upper skin of the belly
panel and the lower wing skin it covers is less than a quarter inch,
so there's no room for the ground plane to stand off. I could mount
them back in the tail cone if that is needed.

I do antennas. I don't do mechanicals. That is left to the
homebuilder.



I had thought about installing one of those pre-made di-pole
transponder antennas. But to mount it vertically, about the only place
I could put it would be on the back side of the spar in the wheel
well. And that would put the microwave energy only a half inch away
from the fuel cell. Not sure that would be kosher. On a Glasair 3, the
entire leading edge of the wing from the spar forward and from tip to
tip is all fuel.

Transponder antennas should NOT be dipoles. They should be monopoles
on ground planes for the half-orange radiation as described above.


It makes sense to me that the ferrite beads are needed, as without
those, I can't see how the antenna would know where to start and stop.
It seems without those, the antenna and the coax would all become the
antenna, and be totally out of tune for just about any frequency.

That's not true. The antenna stops where the pigtails converge into
coaxial cable. The beads are only there to keep unwanted reflected
power from coming back down the outside of the coax cable.

Jim


Rich


On Tue, 22 Dec 2009 10:28:31 -0800, RST Engineering
wrote:

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