Aircraft antennas

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...

Believe it or not (I checked with trusty wattmeter), I get good SWR
across the entire Comm band using a cheap ELT antenna I bought from
Chief Aircraft for about $35 several years ago (the one that has a very
flexible whip). I have one mounted on the top of the (leading edge)
fairing that covers the wing joint on my Corben Junior Ace The fairing
basically is the leading edge, (filling the gap between wing panels) and
goes ffrom front spar top to front spar bottom. The metal is about 6"
wide, so it would seem a bit small for a ground plane, but it works. I
can routinely talk air to air over about 70 miles with both planes at
pattern altitude. Air to ground (from 1000 feet) is 20-30 miles or so.

Scott


wrote:

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.

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

Not.

Jim



"Scott" wrote in message
.. .

Believe it or not (I checked with trusty wattmeter), I get good SWR across
the entire Comm band using a cheap ELT antenna I bought from Chief
Aircraft for about $35 several years ago (the one that has a very flexible
whip).

Sure. The subjective technical term for using a VHF antenna inside a steel
tube fuselage is "not worth a ****".

Jim


"T o d d P a t t i s t" wrote in message
...


On the related subject of mounting an
antenna inside a steel tube fuselage, have you done any
similar tests or do you know of any? That would seem to be
a much different situation, with long conductive frame
members, but real world tests can be surprising. Any
results or comments you want to share?

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

I can send you forward and reflected power readings if you wish, say
every 1 MHz from 118 to 136. Now, with that said, that is measured on
the ground, not in flight where the whip curves back from all the high
speed flying at 75 MPH

Scott


RST Engineering wrote:

Not.

Jim



"Scott" wrote in message
.. .


Believe it or not (I checked with trusty wattmeter), I get good SWR across
the entire Comm band using a cheap ELT antenna I bought from Chief
Aircraft for about $35 several years ago (the one that has a very flexible
whip).

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

First, send me the model of the instrument that you used for the
measurement. Then explain why the trap in the antenna didn't totally mess
up the VSWR for the VHF band. You DO understand that an ELT antenna is a
trap monopole, don't you? You DO understand that the top 2/3 of the antenna
is decoupled from the bottom end by an LC trap, don't you? That a properly
operating ELT antenna should be 2:1 or less at 121.5 MHz. and more than 10:1
above 123 and below 119 MHz?

My bet is that you used a CB power meter, good buddy, 10-4.

Of course, you could be measuring a 51 ohm resistor that somebody put in the
antenna to "match" the antenna across the band. That would let the antenna
radiate about as well as a limp piece of spaghetti in a copper septic tank.

Jim




"Scott" wrote in message
.. .
I can send you forward and reflected power readings if you wish, say every
1 MHz from 118 to 136. Now, with that said, that is measured on the
ground, not in flight where the whip curves back from all the high speed
flying at 75 MPH

Scott

OK Jim,
I'll clarify. I'm a ham, I work as a radio tech (up to 7 Ghz). I've
bought and built one of your com radio kits...blah blah blah. You are
entitled to your opinions. I was just offering some ideas on what may
or may not have been going on. I don't work in the aircraft building
industry. I don't know how much silver is required on a production
aircraft...

You used to be a pretty fair guy. Now you seem to immediately attack
someone on their first post. You are really coming off as completely
arrogant. Yes, you're a big engineer (you are, aren't you?), you write
a monthly column in a magazine for homebuilders (who may or may not put
10 coats of silver on their fabric) and you don't listen to anything
from anyone who doesn't share your exact thoughts. I used to respect
you in the past, but now you seem just plain abusive (and abrasive). On
one of my posts on this thread you immediately took this attitude that
I'm a hick CBer who doesn't know anything (you make the claim without
knowing any facts about me...you just make those assumptions and attack)
I admit, I DON'T know it all like you seem to, but I DID measure
forward and reflected power with my Telewave wattmeter, my Icom A-22 and
the whip ELT antenna I used on my plane. The worst reflected power I
saw was 100-150 mW, which is no more than 10% reflected power of the
1.5W forward power (ie 2:1 SWR...not perfect, but acceptable to
military standards. I was an avionics comm tech in the USAF).

I think you could work on toning down your responses and state what you
find to be true without having to resort to name calling.



Scott Littfin



RST Engineering wrote:
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