Jim Weir - Rubber 'Baby Belly Baluns

For Jim (and kibitzers of all stripes :-):

I had a curious situation yesterday while flying my Fly Baby. An aircraft
complained that my radio was unreadable. Yet a buddy on the ground with a
aircraft-band receiver said that my radio was a bit scratchy, but perfectly
readable.

I think I'm getting bit by my antenna patterns. Both antennas are mounted
internal to the aircraft, with one being a horizontal coax-type dipole (with
baluns) mounted underneath the metal turtledeck (!) and the other being my
PVC-pipe job mounted vertically just a foot or two behind my manly back.

Right now, I have the horizontal dipole hooked to the receive side and the
vertical one to the transmit side. But they're hooked that way because of a
similar complaint I had *last* summer.

I know pattern is a problem with the horizontal one, since transmissions from
aircraft behind me are very garbled (directly in line with dipole). I think the
vertical one is ending up with a horrid pattern forward (due to the nearby
ugly-bag-of-mostly-water), which might explain yesterday's complaint.

(Note to newcomers in this saga: I'm using a Narco Escort II, which has
separate receive and transmit antennas. Unlike many radios, the Narco uses the
Nav antenna for Comm reception. It must have both antennas connected for
two-way operation.)

So... I'm starting to break down and am considering putting an antenna on the
*outside* of the aircraft. I was originally going to use your copper-tape
setup, running one arm down the landing gear leg and the other across the
fabric-covered belly, but I added a long metal belly inspection panel last year
that messes that up.

http://www.bowersflybaby.com/tech/belly_pan.html

The obvious thought it to install a conventional aircraft comm antenna on the
belly panel itself...it's about 1 ft x 3 ft and would make a very good ground
plane. But the panel is just ~.016 aluminum, supported along the edges. I'd
probably have to add a cross-brace in the fuselage to mechanically support a
full-size antenna. It would complicate removing the panel for routine
maintenance, and would get in my way when I want to sit flat on the bottom
longerons dangling my feet out the belly for working on my avionics box
underneath the control panel.

A big 'ol comm antenna would be a hassle...but what about mounting a
rubber-duckie antenna, instead? A BNC connector would let me quickly remove it
to pull the panel, and since it doesn't really require a massive ground plane, I
could mount it far enough aft that it could pick up the existing support
structure for the belly panel.

So...what d'ya think?

Follow-up question: I do my antenna switching on the ground, now, switching
coaxes between radio outputs. Are there any inexpensive switch boxes that would
let me take two RF inputs and select which antennas they connect to, without
switching cables?

Ron Wanttaja

First Ron you want to make sure it is YOUR problem not the other guys
receiver. A bad antenna pattern usually won't "garble" a transmitter. It
can reduce signal strength which would cause your signal to go into the
noise.
two questions how far away was the other aircraft and what was his
orientation to you?
Also what is the swr on your antenna?
My suggestion first is if you have a buddy with another plane (prefer two
seater with a helper installed in the second seat!). Have him monitor your
signal with a know good receiver/antenna and try different orientations and
distances to see it it is indeed your problem.

As far as antennas go my first impression would be an inverted 'V' type
antenna, which is basically a dipole with the feed end forming an angle
(between 45-90 degrees, not real critical) it isn't as directional as a
dipole is. Then make sure you have it matched properly and separate the
receive and transmit antennas by as much as possible to decrease likelyhood
of your transmitter feeding back into your receivers.

I hope this helps a bit!
John

On Sun, 03 Apr 2005 20:57:32 GMT, Ron Wanttaja
wrote:

For Jim (and kibitzers of all stripes :-):




So...what d'ya think?

Follow-up question: I do my antenna switching on the ground, now, switching
coaxes between radio outputs. Are there any inexpensive switch boxes that would
let me take two RF inputs and select which antennas they connect to, without
switching cables?

Ron Wanttaja

Ron, I have had pretty good luck with a stainless antenna on a .024
metal panel on the belly of my Jungster II (all wood) It resides
between the gear and in bent about 30° aft. I riveted on a doubler
around the attach point. It looks like you could attach it to the
front panel. If you leave a loop of slack in the antenna wire it makes
it easier to remove and inspect under the panel. Usually if people say
they can't receiving me it is usually their equipment or a wire has
become loose somewhere in my circuits.

Ed Sullivan

On Sun, 03 Apr 2005 15:34:28 -0700, Ed Sullivan
wrote:

On Sun, 03 Apr 2005 20:57:32 GMT, Ron Wanttaja
wrote:

For Jim (and kibitzers of all stripes :-):




So...what d'ya think?

Follow-up question: I do my antenna switching on the ground, now, switching
coaxes between radio outputs. Are there any inexpensive switch boxes that would
let me take two RF inputs and select which antennas they connect to, without
switching cables?

Ron Wanttaja

Ron, I have had pretty good luck with a stainless antenna on a .024
metal panel on the belly of my Jungster II (all wood) It resides
between the gear and in bent about 30° aft. I riveted on a doubler
around the attach point. It looks like you could attach it to the
front panel. If you leave a loop of slack in the antenna wire it makes
it easier to remove and inspect under the panel. Usually if people say
they can't receiving me it is usually their equipment or a wire has
become loose somewhere in my circuits.

Ed Sullivan

In order to ward off an attack from Barnyard Bob I should have said:
is bent 30° and: if people say they aren't receiving me.

On Sun, 03 Apr 2005 21:13:04 GMT, UltraJohn wrote:

First Ron you want to make sure it is YOUR problem not the other guys
receiver. A bad antenna pattern usually won't "garble" a transmitter. It
can reduce signal strength which would cause your signal to go into the
noise.

I've had enough separate cases of folks complaining about my radio. Not lately,
though...in fact I've done some fairly complex coordination with other traffic
on busy days in the pattern (e.g., "Fly Baby's on crosswind, and has the traffic
on the 45...I'll swing out and follow you") and no one complained.

two questions how far away was the other aircraft and what was his
orientation to you?

Don't know... just a voice breaking in and saying, "Hey, Ron, your radio's
unreadable." Weren't any of my "known buddies" in the air at the time, but the
FBO owner may have been flying one of his airplanes.

Also what is the swr on your antenna?

Never been checked... :-)

My suggestion first is if you have a buddy with another plane (prefer two
seater with a helper installed in the second seat!). Have him monitor your
signal with a know good receiver/antenna and try different orientations and
distances to see it it is indeed your problem.

In the past, I've set up an old Escort 110 in my hangar with a small tape
recorder and then gone and shot some T&Gs. There's definitely a higher noise
level when I transmit...but when you consider that a decimal meter reads about
109 dB chest-high, that's probably to be expected (and it's one reason I spent
the $$$ for an ANR headset...). But like my buddy yesterday, I felt my
transmissions were understandable albeit with high background noise.

Things may have admittedly changed; probably time to do this again. One email
suggested I check connectors; a good idea as I did have a BNC connector come
loose a few years back.

Or maybe I ought to pull the wind muff off my mike boom; maybe the microphone
got itself turned around.

On Sun, 03 Apr 2005 15:34:28 -0700, Ed Sullivan wrote:
] Ron, I have had pretty good luck with a stainless antenna on a .024
] metal panel on the belly of my Jungster II (all wood) It resides
] between the gear and in bent about 30° aft. I riveted on a doubler
] around the attach point.

I've done this with my transponder antenna... the builder of my plane used
0.040" aluminum in that area and it's probably strong enough. But...a) Do I
want it just a couple of inches from my transponder antenna, and B) The panel is
attached to the airplane with just a couple of wood screws. Don't know if I
want to add the twisting load from the drag of a long antenna.

And, again, I think antennas are ugly. I think the rubber duckie on the belly
would be less noticeable.

Ron Wanttaja

In article ,
Ron Wanttaja wrote:
For Jim (and kibitzers of all stripes :-):

I had a curious situation yesterday while flying my Fly Baby. An aircraft
complained that my radio was unreadable. Yet a buddy on the ground with a
aircraft-band receiver said that my radio was a bit scratchy, but perfectly
readable.

I think I'm getting bit by my antenna patterns. Both antennas are mounted
internal to the aircraft, with one being a horizontal coax-type dipole (with
baluns) mounted underneath the metal turtledeck (!) and the other being my
PVC-pipe job mounted vertically just a foot or two behind my manly back.

Right now, I have the horizontal dipole hooked to the receive side and the
vertical one to the transmit side. But they're hooked that way because of a
similar complaint I had *last* summer.

I know pattern is a problem with the horizontal one, since transmissions from
aircraft behind me are very garbled (directly in line with dipole). I think
the vertical one is ending up with a horrid pattern forward (due to the nearby
ugly-bag-of-mostly-water), which might explain yesterday's complaint.

(Note to newcomers in this saga: I'm using a Narco Escort II, which has
separate receive and transmit antennas. Unlike many radios, the Narco uses
the Nav antenna for Comm reception. It must have both antennas connected
for two-way operation.)

So... I'm starting to break down and am considering putting an antenna on the
*outside* of the aircraft. I was originally going to use your copper-tape
setup, running one arm down the landing gear leg and the other across the
fabric-covered belly, but I added a long metal belly inspection panel last
year that messes that up.

http://www.bowersflybaby.com/tech/belly_pan.html

The obvious thought it to install a conventional aircraft comm antenna on the
belly panel itself...it's about 1 ft x 3 ft and would make a very good ground
plane. But the panel is just ~.016 aluminum, supported along the edges. I'd
probably have to add a cross-brace in the fuselage to mechanically support a
full-size antenna. It would complicate removing the panel for routine
maintenance, and would get in my way when I want to sit flat on the bottom
longerons dangling my feet out the belly for working on my avionics box
underneath the control panel.


A big 'ol comm antenna would be a hassle...but what about mounting a
rubber-duckie antenna, instead? A BNC connector would let me quickly remove it
to pull the panel, and since it doesn't really require a massive ground plane, I
could mount it far enough aft that it could pick up the existing support
structure for the belly panel.

Range with a rubber-duckie is 'medium lousy' compared to f full 1/4 wave
with a good ground plane.


So...what d'ya think?

Follow-up question: I do my antenna switching on the ground, now, switching
coaxes between radio outputs. Are there any inexpensive switch boxes that
would let me take two RF inputs and select which antennas they connect to,
without switching cables?

A switchbox for receiving antennas is relatively "trivial".
Switching for xmit antennas is somewhat more involved, especially at high
power levels.

_Many_ ham radio operators have switching relays that work with transmitters
rated for power in the kilowatt range. Used to alternately connect a
transmitter or receiver to a single antenna. Almost all integrated
two-way "transceivers" also have a T/R switch (usually a relay, sometimes
solid- state) for switching the antenna between the transmit and receive
sections of the box.

Switching 2 radios among 2 antennas, i.e. "A-1 / B-2" or "A-2 / B-1" is
more problematic, *if* one side would be transmitting. Shielding in the
switching assembly becomes a significant concern -- using a 'standard' DPDT
switch is out -- although that approach does work for receive-only antenna
switching.

You _can_ do the cross-switching, by using _4_ T/R relays. There are
'fail safe' issues to contend with, to make sure that the transmitter and
receiver don't both simultaneously connect to the same antenna. It's
definitely _doable_, but "inexpensive" is probably not an operational
qualifier. suitable T/R relays are probably $20+ each, on the used
market.

However, probably the _least_cost_ way to accomplish in-flight switching
would be to mount 4 BNC connectors 'somewhere convenient' -- one to each
antenna, and one to each radio -- and a pair of patch-cables, to connect
'antenna X' to 'radio Y'. It wouldn't be "pretty", but it would provide
the functionality.

On Mon, 04 Apr 2005 15:25:45 -0000, (Robert Bonomi)
wrote:

Range with a rubber-duckie is 'medium lousy' compared to f full 1/4 wave
with a good ground plane.

Which means it'll probably be at least as good as I'm getting now. :-)

However, probably the _least_cost_ way to accomplish in-flight switching
would be to mount 4 BNC connectors 'somewhere convenient' -- one to each
antenna, and one to each radio -- and a pair of patch-cables, to connect
'antenna X' to 'radio Y'. It wouldn't be "pretty", but it would provide
the functionality.

Actually, this is very much like what I have now. I've got 3' cables coming
from the radio and terminate in male BNC connectors by the seat. The coaxes
from the antennas terminate in male BNC connectors at the same location. The
little BNC "bullets" connect the sets. I could certainly replace the bullets
with a small set of bulkhead mounts on a plate....

Ron Wanttaja