Aircraft antennas

[ccwillwerth]

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

[Jim Carriere]

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/catalog/avpages/antennasystems.php

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

[Orval Fairbairn]

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.

[Stuart & Kathryn Fields]

Charlie: I recently installed a VHF com antenna on the tailboom frame of my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
length of the antenna. I ran an antenna check using an MFJ 250 antenna
analyzer and the highest VSWR I got was 2.2. This was so much better than
the bent antenna that had been on the belly of the ship which did have a
large ground plane but had VSWR greater than 3 and in some cases as high as
5. One thing you can do is try out a prototype ground plane and then have a
Ham or someone with the right equipment do a check.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"ccwillwerth" > wrote in message
...
> 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
>

[Stuart & Kathryn Fields]

Oops. The installed ground plane dimensions are 16"X12".
--
Kathy Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478
(760) 408-9747 general and layout cell
(760) 608-1299 technical and advertising cell

www.vkss.com
www.experimentalhelo.com


"Stuart & Kathryn Fields" > wrote in message
.. .
> Charlie: I recently installed a VHF com antenna on the tailboom frame of
my
> Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
> length of the antenna. I ran an antenna check using an MFJ 250 antenna
> analyzer and the highest VSWR I got was 2.2. This was so much better than
> the bent antenna that had been on the belly of the ship which did have a
> large ground plane but had VSWR greater than 3 and in some cases as high
as
> 5. One thing you can do is try out a prototype ground plane and then have
a
> Ham or someone with the right equipment do a check.
>
> --
> Stuart Fields
> Experimental Helo magazine
> P. O. Box 1585
> Inyokern, CA 93527
> (760) 377-4478 ph
> (760) 408-9747 publication cell
> "ccwillwerth" > wrote in message
> ...
> > 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
> >
>
>

[[email protected]]

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.

[[email protected]]

Jim Carriere wrote:

> Please excuse my "piggybacking" your question, can anyone with an
> informed opinion weigh-in on this:
>
> http://www.aircraftspruce.com/catalog/avpages/antennasystems.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...

[XCOM]

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
>
>

Our experience is that anything inside a structure especially metal is
NOT going to work. As one other said... put the aerial on the outside
where it's meant to go.

Also we have just added a table on VSWR effects and power drop off with
a bad aerial, check http://www.mcp.com.au/xcom760/faq/faq.html for
details... right down the bottom of the page (All the way down)

Interesting if you have a bad aerial Regards Michael from XCOM

[RST Engineering]

"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/catalog/avpages/antennasystems.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

[Morgans[_2_]]

"XCOM" > wrote
>
> Our experience is that anything inside a structure especially metal is NOT
> going to work. As one other said... put the aerial on the outside where it's
> meant to go.
>

With careful reading, I take that he is asking about putting JUST the ground
plate under the fabric, with the aerial on the outside, as you suggest.
--
Jim in NC

[Scott[_1_]]

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/catalog/avpages/antennasystems.php
>
> You put it inside a tube and fabric structure. Seems like a good idea
> at first glance...

[Scott[_1_]]

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.
>

[RST Engineering]

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

[RST Engineering]

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).

[RST Engineering]

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?

[Scott[_1_]]

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
>
>
>

[Scott[_1_]]

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).
>
>
>

[RST Engineering]

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

[RST Engineering]

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

[Scott[_1_]]

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
>
>
>

[Scott[_1_]]

See my other post about the equipment used to measure power.

What LC trap? This antenna is a straight, flexible whip. There isn't
ANY coil in the whip. 51 Ohm resistor? Could be, who knows...but the
antenna radiates and receives fine.

The antenna routinely talks air to air over 70 miles or more out here in
the midwest where the hills aren't inflated like the hills and egos out
in Grass Valley appear to be.

Thanks for blowing my whole image of you. You just lost all of the
respect I used to have for you.

Scott Littfin
Bloomer, WI

RST Engineering wrote:

> 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
>
>
>

[Scott[_1_]]

Here's a link to the ELT antenna picture (the exact antenna I am using
across the com band). You can get as abusive and abrasive and you want.
Tell me theory all day long and it still doesn't matter. The fact is,
it WORKS in my installation and has been for about 7 years now.

http://www.chiefaircraft.com/Aircraft/Antennas/Images/ATX_05-02-006.gif

Tell me more about the LC trap on this antenna.

Scott Littfin



RST Engineering wrote:

> 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
>
>
>

[RST Engineering]

"Scott" > wrote in message
...
> 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...

OK, Scott,

I'll clarify. I'm a ham (originally WB6BHI in '59, now WX6RST). I work as
an electronics engineer and college instructor (up to 24 GHz.). I designed,
produced, and troubleshot that com radio you built. I don't work in the
aircraft building industry either, but I *DO** work in the aircraft
maintenance industry and have since 1962. I have a pretty good idea how
much aluminum dope is required on any fabric aircraft.

In offering ideas (cheap silver, microthin application, "waveguide out the
wingtip") what you in essence were saying is that Bellanca and Jim were
gaming the system by either using unapproved materials OR not being savvy
enough to measure the complete radiation pattern in both E and H fields to
see if there was something funny going on. Yes, I take personal affront at
that sort of insinuation.



>
> You used to be a pretty fair guy. Now you seem to immediately attack
> someone on their first post.

Still am, mostly. And I generally wait until the second or third post
unless something completely off the wall comes down the avenue.


You are really coming off as completely
> arrogant.

Possibly.


>Yes, you're a big engineer (you are, aren't you?)

Not since I went on my diet. Now I'm a smaller engineer. Blood pressure
and all, don'cha know. Got it down to 125/75 for last week's AME physical.


, you write
> a monthly column in a magazine for homebuilders

And have for fifteen years or so.


(who may or may not put
> 10 coats of silver on their fabric)

If they do, they are damfools and I don't write for damfools.



and you don't listen to anything
> from anyone who doesn't share your exact thoughts.

Sure I do, so long as it has some foundation in reality.


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)

Don't think I mentioned anything about hick.


> 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).

Telewave makes a good wattmeter; I actually prefer them to the Bird because
of the "no slug" design. So it told you that the reflected power was about
10%, which is (as you noted) a skosh below 2:1. Let's take a look and see
how that might be achieved.

The antenna is an ATX model 05-02-006. From a brief look at the device on
page 18 of the current Chief Aircraft catalog #31, it appears from the
external view that the radiating element is a single spring steel rubber
coated wire of approximately 100 mils diameter including coating. Just for
assumption, let's say the wire is half of that diameter and the rubber
coating is the other half. That gives us a wire of 50 mils diameter and 25
mils of rubber on both sides. AWG #16 wire.

50 mil wire and 22,000 mil (22") length give us an "aspect ratio" of 440:1.
The general rule of thumb (as confirmed by Jasik's "Antennas" and the ITT
Handbook For Radio ENgineers (3rd edition)) is that the VSWR bandwidth (a
purely subjective term) is given by (6*f)/A, where f is the frequency for
which the antenna is cut and A is the aspect ratio of the antenna.
Postulating that the antenna is resonant at 121.5 MHz. gives us a VSWR
bandwidth of approximately 1.6 MHz. ... hardly the 19 MHz. of the 118-138
MHz. aircraft com band.

Now, since you didn't really specify what bandwidth you measured across, it
is possible that if you stayed within a MHz. or so of the resonant frequency
of that antenna that you will have the results you measured. However, if
you truly took it to the band edge limits, we've got to find out how that
might be accomplished.

Let's examine the TSO specifications for an ELT. The PERP (Peak Effective
Radiated Power) as specified in RTCA document DO-183 (which is the technical
document that the TSO relies upon) is 50 milliwatts at 121.5 and 243.0 MHz.
I draw your attention to PEAK power, not average power. Just like PEP on
SSB is the PEAK of the modulating envelope, this is the PEAK of the AM
envelope, which for an 80% depth of modulation is about 3:1. The
transmitter, then, is only required to put out about 20 milliwatts. Hm.

The TSO also requires the antenna to have an EIRP (Effective Isotropic
Radiated Power) of >0 dB. If the antenna is truly a quarter wave whip over
a ground plane, it has an intrinsic EIRP of 2.14 dB for starters, which
reduces the amount of power that we need to radiate another factor of 1.5 or
so. Gee, this is getting real easy.

Now I have no idea what is in that rubber cone on the base or what is
between that input connector and the wire radiating element, but if it was
up to me to meet those specifications, I'd sure as hell put in a 5 dB
resistive pad made up of quarter watt (and quarter-cent) resistors...and
guess what, that would be a 10 dB worst case return loss which will give me
a 2:1 VSWR from DC to daylight.

I don't say that's what they did. I actually don't have a CLUE what is in
the rubber base without buying one and cutting it apart. But that's the
only way I know of (other than the aforementioned trap self-resonant coil
shown on antennas 3 and 4 of that Chief page) of getting a single element
antenna to resonate at both 121.5 and the SECOND harmonic 243.0, where the
single wire is exactly ANTIresonant.

Would that lossy pad let you transmit and receive over a 70 mile range?
Let's take a look at that. If the pad is truly resistive, and if they used
quarter watt resistors in a pi-net pad, I'd expect the resistors to fry
fairly quickly. ELT antennas of this design weren't meant to be used as COM
antennas; that's why they sell COM antennas elsewhere on the pages.

Or, if they just said "to hell with it, we don't have to meet the TSO, the
ELT has to meet the TSO" then you do have a decent antenna spot on 121.5
MHz. and for about a MHz. on either side. If you are doing a legal chat on
122.75 MHz. you will probably get away with it because the ICOM has a pretty
good VSWR protection built in. I'd really like to know what the VSWR is at
243.0 MHz., though, where the single element wire is antiresonant.

Just for grins, that 1 watt transmitter and a 1 microvolt receiver has a
theoretical free-space range somewhere in the vicinity of a thousand miles,
so even if you throttle the 1 watt down to a hundred milliwatts, you still
have well over a hundred mile range, horizon not being a factor. (Radio
horizon is given in miles as (sqrt(2*h)) where h is your altitude in feet.)
At pattern altitude of 1000', this would be 44 miles; if the other guy is at
pattern altitude also, this doubles the range to 88 miles, which is about
what you are seeing.

Howzat?

jw

[Scott[_1_]]

Well, like I said, it works for me. I can double check wattmeter
readings since I did them 7 or 8 years ago...maybe I did only go up to
125 or so since I never really saw myself calling on 133.6 or something
like that. Regardless of that outcome, the fact remains (at least in my
case)...it works. That may, in fact, be due to the fact that 99, no
make that 100% of the time I am between 118 and 126 and 99.9% between
122.7 and 123.0. I did NOT want to get in any sort of ****ing contest
with anyone, I just wanted to give the original guy another option and
post on my experience with it. And for $39, it would be a pretty cheap
experiment. If it failed, you can always use it for its intended purpose.

Scott


RST Engineering wrote:

>
> Just for grins, that 1 watt transmitter and a 1 microvolt receiver has a
> theoretical free-space range somewhere in the vicinity of a thousand miles,
> so even if you throttle the 1 watt down to a hundred milliwatts, you still
> have well over a hundred mile range, horizon not being a factor. (Radio
> horizon is given in miles as (sqrt(2*h)) where h is your altitude in feet.)
> At pattern altitude of 1000', this would be 44 miles; if the other guy is at
> pattern altitude also, this doubles the range to 88 miles, which is about
> what you are seeing.
>
> Howzat?
>
> jw
>
>