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gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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 Posted: Wed Jan 30, 2008 2:24 am Post subject: AM/FM radio antennas for planes (was: Dumb questions) |
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| Quote: | From: "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net (nuckolls.bob(at)cox.net)>
Subject: Re: Dumb questions
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>The antenna is another deal entirely. Vertically
>polarized, medium wave signals run along the
>ground and work really fine with short antennas on
>ground based vehicles. Once you leave the ground,
>performance for a short whip antenna degrades.
The following is AM/FM radio fun talk and links on
antennas you all might find interesting, regarding receiving
AM and FM commercial broadcast signals in a plane.
If you are determined to have a car AM/FM receiver in
your plane you will have to deal with the antenna issue.
The following info is might give you some ideas. The big
point is, AM and FM antennas are really two different
animals by a factor of 75.
FM is more line of sight. If going X-C you will only have the
radio station for a 100-200 miles. AM radio is more suited
for long range listening, but there are other issues with AM.
Satellite radio (at $13 month) is a deluxe or best solution?
Many Sat radio car receivers also have a XM / Sirius receiver
built-in. You can get small compact portable Sat radio
receivers as well; even the Garmin 496 has a XM receiver.
The Sat radio antenna is a typical small GPS looking affair.
XM of course has weather service avaiable for $50/mo.
Down side of satellite radio is the fee. If determined to have
AM/FM radio read on.
As suggested before, I'm NOT a fan of AM/FM/CD car
receivers in the panel of a plane, but to each his own.
I prefer using portable entertainment devices. My big
complaint is the weight, about 3.5 to 5 lbs. The car
radios are heavy, by plane standards, and the extra
antenna wiring is an issue. Portable devices have
their own antenna which seem to work adequately
in a small plane, especially with a bubble canopy.
Therefore I like or recommend portable iPods or
radios, like a Grundig Mini 300 AM/FM/SW (pocket)
radio
Comant makes airplane antennas for AM/FM. They
are adaptations of a VHF whip about 20" to 25" long.
The problem is AM is from 530 KHz to 1700 KHz and
FM is 88 MHz to 108 MHz, so any two-in-one antenna
is NOT ideal for either, especially AM radio. So both
Comant or automotive antennas are not ideal. The
Comant is 50 ohm. FYI, most car radios are in the
75 ohm range.
5-05.pdf]http://www.comant.com/pdfs/[ci%20222]5-05.pdf
Like anything with antennas, there is optimal
sensitivity, signal and directional characteristics.
Car antennas are compromises. The commercial
aircraft AM/FM antennas are really VHF Com
antennas warmed over, a compromise. The difference
between FM and AM Freq is a factor of 75 !
PS Engineering sells an "Aviation" AM/FM/CD
player and they sell a preamp antenna (see pg 2-5).
http://www.ps-engineering.com/docs/PAV80_IM.pdf
This antenna looks just like a commercially avaiable
one for marine use ($16). Not sure how well it works.
http://tinyurl.com/2ew9zx
*** AM antenna's ***
Bob mentioned a trailing wire antenna of a doz feet or so.
If you are going for a trailing wire AM radio antenna, than
why not go all the way, +200-300 feet would be more
optimal (but not practical). It would be fun to play with
a long wire to see what you can receive.
Trivia:
Air-force one has a trailing wire antenna 5 miles long!
Military uses 15-20Khz for talking to subs and 1/2
wave length antenna is about 26,750 feet or 5 miles.
If you fly near Arlington, Washington State, the
Jim Creek Naval Radio Station is in the foot hills
of the near by Cascade mountains. The antenna is
strung across the valley between two mountain
tops from two 200' tall towers. I am guessing the
antenna is about 5 to 6 miles long!
One popular antenna for AM radio, when you can't string
250 or 500 feet of wire outside, is a LOOP. A loop is really
a long wire antenna wrapped around in a circle or square.
They can be very effective in pulling in signals. What is
cool about a LOOP is it can pull in stations 600 plus mile
away. They are not quite as good as a long wire but more
compact for sure.
Not really practical for an airplane, but if you made a
made a small 8" loop and manipulate it in the cockpit
(with a safety pilot), you can find direction to the station.
If the loop is "balanced" you rotate the loop to find the
NULL signal. You will have to maneuver to determine
the "ambiguity" since there are two NULLS. This is
HOW the ADF works, but ADF's use 4 loops in one unit.
With the signals from the 4 loops and a sense antenna
(the long wire), the ADF receiver interprets the signals.
The direction (ambiguity) to station is solved automatically,
thus the name, Automatic Direction Finder or ADF.
Below are links on LOOPS for grins and giggles.
If you are interested in LOOPS, you could make your
own and get your old table top AM radio receiving
stations 500-1000 miles away!
http://www.mindspring.com/~loop_antenna/
Commercial AM radio indoor loop antenna
products like the link below
http://www.smarthome.com/7744ad.html
provide amazing performance It is connected
with two wires to the radio, but it also works by
just placing it near the radio. Interesting. Portable
radios typically have a "ferrite loopstick" as Bob
calls them. Often they don't have antenna hook
up scews. The oop antenna has electromagnetic
affect on the internal antenna and pulls in more
signal. If you have a fiberglass plane you could
wrap a loop in the aft fuselage..
*** FM Radio ***
You can buy slick looking coil loaded antennas for
cars but I suspect they look better than perform, like
this one:
http://tinyurl.com/37hte5
An FM radio antenna whip, min length 2.5' to 3'. A simple
FM antenna is a dipole antenna for $3 from a hardware
store. These are the "Tee" wire dipole antennas come
with most TV's in a box. They work for FM as well.
They're 58" across and 300 ohm. A 75 ohms version is:
http://tinyurl.com/cbmzj
Car radios are usually 75 ohm. Portable radios don't
usually have external antenna connections, but if they
do, it usually is 300 ohm (screw terminals) or 75 ohm,
if there's a F-connector (like your cable TV). There are
low cost balun connectors to convert the 75 Ohms to
300 Ohms. You could wrap these wire antennas under
the canopy, fiberglass fuselage or cowl area. It's only
58" long, which is 1/2 wave for FM. Will it work for AM?
Yes, but its not ideal. As I started off, AM and FM is
different. One antenna for both AM/FM is a compromise.
Also what works on a car or boat is not necessarily
ideal for airplanes. Have fun making antennas.
Cheers George ATP/CFI - RV7
Be a better friend, newshound, and [quote][b]
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BobsV35B(at)aol.com
Guest
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| Posted: Wed Jan 30, 2008 4:53 am Post subject: AM/FM radio antennas for planes (was: Dumb questions) |
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Good Morning George,
Piper Pacers and TriPacers came from the factory with a very efficient loop antenna inside the tail cone. It was supported by the longerons, had three or four turns of wire, and was about two feet in diameter. By judicious adjustment of the volume, it was easy to use that fixed loop to determine a one degree null. Appropriate maneuvering of the aircraft allowed the pilot to determine the state of ambiguity so that it could be determined if the airplane was pointing toward or away from the station.
It was, and still could be, a very low cost means of very accurate navigation.
Happy Skies,
Old Bob
AKA
Bob Siegfried
Ancient Aviator
628 West 86th Street
Downers Grove, IL 60516
630 985-8502
Stearman N3977A
Brookeridge Air Park LL22
In a message dated 1/30/2008 4:26:54 A.M. Central Standard Time, gmcjetpilot(at)yahoo.com writes:
| Quote: | If you have a fiberglass plane you could
wrap a loop in the aft fuselage.
|
Start the year off right. Easy ways to stay in shape in the new year.
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