Aircraft antennas

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

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/Aircraf..._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

"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

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