Aircraft antennas

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie

ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Please excuse my "piggybacking" your question, can anyone with an
informed opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea
at first glance...

Jim Carriere wrote:

Please excuse my "piggybacking" your question, can anyone with an
informed opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea
at first glance...

VHF COM, NAV, and ELT antennas placed inside a welded-tube fuselage
aren't worth crap!
What is the biggest complaint of anybody trying to use a Air-band VHF
hand-held transceiver inside the cabin of a metal aircraft? They cant
be heard! They all end up putting an external antenna on their
aircraft. And no, it is not just the inefficiency of the rubber-ducky
that causes this.

The fundemantal problem is that the wavelength at 120Mhz is 2.5 meters.
In order for a radio wave to pass through an opening in a metallic
structure, the dimensions of the opening need to approach a half
wavelength in diameter, and even then the wave is greatly attenuated.
Just to make the point, a half-wave at 120Mhz is 49". How many of the
openings whose edges are defined by the fuselage longerons, the
cross-brace and diagonal brace tubes are 49" across? Answer, none of
them. Even the windscreen opening is usually not that large.
A welded tube fuselage makes a real good "screen room"... Do yourself a
favor, and put the antennas on the outside where they belong...

"Jim Carriere" wrote in message
...
ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a large
plate for a ground plane or will a small plate be sufficient? If a
ground plane is required, can the copper foil tape be used on the inside
of the fabric as the ground plane?


Two comments. One is that the steel tube fuselage will be an adequate
ground plane if the attach plate is electrically connected to the fuselage
tubes.

Two, that wire whip antenna was just fine for the day in which we had 90
channel radios and didn't go much above 124 MHz.. Do you and your radio a
favor and get one of those fiberglass whips that have the word "broadband"
in their descriptor. You will be a much happier camper.



Please excuse my "piggybacking" your question, can anyone with an informed
opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea at
first glance...


Read it again. The antenna is meant for composite aircraft or metal tube
aircraft IN A FIBERGLASS WINGTIP. No VHF antenna will work worth a darn
inside of a steel tube fuselage. Google on "Faraday Cage" or "Screen Room"
to get a picture of what is happening.

Jim

I would say it isn't the best idea. The silver coat used on the fabric
has tiny aluminum pieces for uV protection. Seems to me this would
somewhat shield the antenna and limit the signals into and out of the
antenna. Of course, in the real world, it would probably work.

Scott


Jim Carriere wrote:

ccwillwerth wrote:

Hi, I am about ready to cover my Cub type airframe, but need a place
to attach a com antenna. I was considering brazing a plate to the
airframe so that it would be just under the fabric. The antenna is
the type that has a ceramic insulator on the bottom of the antenna
that insulates the stainless steel antenna from the airframe. Does
the antenna need a large plate for a ground plane or will a small
plate be sufficient? If a ground plane is required, can the copper
foil tape be used on the inside of the fabric as the ground plane?


Please excuse my "piggybacking" your question, can anyone with an
informed opinion weigh-in on this:

http://www.aircraftspruce.com/catalo...nnasystems.php

You put it inside a tube and fabric structure. Seems like a good idea
at first glance...

I would say that you are wrong. Bellanca paid me decent money back in the
early '80s to make the tests to see if we could hide their antennas inside
their wood and fabric wings. They actually shipped me a wing from Alex MN
to GV California so that I could do the preliminary work out here before I
went back there (in the dead of winter, what a mistake THAT was) to hang
antennas in a real live airframe and fly them around.

THe theory is that the aluminum powder/dust is so broken up into individual
particles insulated from each other by a dope binder that they do NOT act as
a shield.

Test: Put two antennas 30 meters apart. Radiate a signal from one and use
a field strength meter to receive at the other (spectrum analyzer).
Carefully slip a wing over the transmit antenna. Less than 0.1 dB
difference. Slip the same wing over the receive antenna. Same difference.

Jim



"Scott" wrote in message
.. .
I would say it isn't the best idea. The silver coat used on the fabric has
tiny aluminum pieces for uV protection. Seems to me this would somewhat
shield the antenna and limit the signals into and out of the antenna. Of
course, in the real world, it would probably work.

Scott

OK, maybe so. BUT...I STILL vote for putting the antenna outside.
Maybe the wing was acting like a waveguide and the RF was coming out
holes at the root end Or maybe Bellance used cheap silver or only
put on a layer a micron in thickness

Scott


RST Engineering wrote:

I would say that you are wrong. Bellanca paid me decent money back in the
early '80s to make the tests to see if we could hide their antennas inside
their wood and fabric wings. They actually shipped me a wing from Alex MN
to GV California so that I could do the preliminary work out here before I
went back there (in the dead of winter, what a mistake THAT was) to hang
antennas in a real live airframe and fly them around.

THe theory is that the aluminum powder/dust is so broken up into individual
particles insulated from each other by a dope binder that they do NOT act as
a shield.

Test: Put two antennas 30 meters apart. Radiate a signal from one and use
a field strength meter to receive at the other (spectrum analyzer).
Carefully slip a wing over the transmit antenna. Less than 0.1 dB
difference. Slip the same wing over the receive antenna. Same difference.

Jim



"Scott" wrote in message
.. .

I would say it isn't the best idea. The silver coat used on the fabric has
tiny aluminum pieces for uV protection. Seems to me this would somewhat
shield the antenna and limit the signals into and out of the antenna. Of
course, in the real world, it would probably work.

Scott

ExCUSE ME. I don't mind carrying on a technical discussion, but to suggest
that we didn't use standard manufacturing procedures OR that a certificated
airplane used "cheap silver" whatever the hell that is or spread it on a
micron in thickness and still expected to pass the inspector's muster is
just plain stupid.

Nor do you give me the credit for knowing how to take polar plots of
antennas to meet FAA expectations for certificated aircraft antenna
installations. You DO understand dB/relative angle plots, don't you?

Quite frankly, I think we have a self-anointed CB radio expert with us who
doesn't have a freakin' CLUE about aircraft antennas.

Good by, good buddy, 10-4?

Jim




"Scott" wrote in message
.. .
OK, maybe so. BUT...I STILL vote for putting the antenna outside. Maybe
the wing was acting like a waveguide and the RF was coming out holes at
the root end Or maybe Bellance used cheap silver or only put on a
layer a micron in thickness

Scott

In article ,
"ccwillwerth" wrote:

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie

The ground plane should extend an antenna length around the antenna base.

Copper or aluminum tape will serve the purpose very well. This is
outlined in CAM 18. Make sure that your ground has a good electrical
connection with the elements of the ground plane.

Orval Fairbairn wrote:
In article ,
"ccwillwerth" wrote:

Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe.

Charlie, you will get several opinions, but here is mine.

If at all possible, get a "broadband" VHF Com antenna, not a wire whip.
The "broadband" fiberglass antennas have a VSWR of less than 2:1 across
the range of 118 to 136MHz, while the metallic whip will have an
bandwidth of only about 5Mhz where the VSWR is below 2:1. At the band
edges, the VSWR will be high enough to cause the VSWR protection
circuitry in transistorized transmitter to shut the output power down
to nearly zero. Although the wire-whip can be cut&tuned to just Unicom
frequencies (122.7 to 123.6 Mhz), it could be marginal for transmission
at some ATC frequencies. The whip will work ok for receiving even at
the band-edges, because the receiver doesn't care about the VSWR.

If your fuselage is anything like my Piper PA20, there are enough metal
tubes to act as a ground plane without adding any additional conductive
material, other than a mounting plate. I would put the antenna base on
a metal plate which is just below the plane formed by the fabric.
Radius the edges of the plate so that the fabric doesn't ride on a
sharp edge. The plate could be long enough to bridge between two
fuselage cross-brace tubes, but only about 4 to 6" wide. It must be
electrically "bonded" to the cross-braces, so to avoid drilling holes
in the cross-braces, your idea of welding some attach "ears" to the
cross-braces to mount the plate is good.