ADF Antenna Length

[John Sofia]

Can someone tell me what the optimum antenna length should be for a
Cessna (ARC) ADF installed in a 1977 Skyhawk. At my home field, there
are so many different lengths in use on other aircraft, that it seems
random? There must be an optimum?

Thanks
John Morley

[John Prickett]

In article <32550B...@analysistech.com>,

well, the loop antenna should be about six inches :-)

the sense antenna should be 1/4 wavelength of the frequency.
2000 KHz would be about 150 meters, so 1/4 of that would
be, uh, lets see, 37.5 meters, or 123 feet. hmmm, i guess
i need a longer fuselage.

seriously, the antennae are made to appear longer
electrically with some arrangement of inductors and
capacitors. at least one of the capacitors (or inductors)
is variable, allowing the installer to tune the antenna
at installation time. so, it's probably not really
critical. generally speaking, the longer the better,
but the improvement of a 9 foot sense over an 8 foot
sense would probably not show up in the cockpit.

hth,

john

--
'i'd rather be flying'
john.p...@amd.com

[brian whatcott]

In article <32550B...@analysistech.com>, in...@analysistech.com
says...

>
>Can someone tell me what the optimum antenna length should be for a
>Cessna (ARC) ADF installed in a 1977 Skyhawk. At my home field, there
>are so many different lengths in use on other aircraft, that it seems
>random? There must be an optimum?
>
>Thanks
>John Morley

Looking over the manual for the tagged Narco ADF-31A that's been
sitting forlornly on my shelf for the last fourteen years (going
cheap!) I see it recommends 10 to 12 feet as the optimal length for the
sense antenna.

It notes that automatic gain control is used to control the signal
ratio between sense and loop antennas.

Recalling that the usual top frequency for ADFs is 1.8 MHz it's easy
to see that a quarter wave tuned antenna would be at least 300/1.8
divided by four meters = 42 meters or 138 ft long.
Hence 11 feet is a 'random wire' or 'long wire' i.e untuned antenna.

Regards
brian whatcott <in...@intellisys.net>
Altus OK

[Robert Scott]

In article <32550B...@analysistech.com>, in...@analysistech.com
says...
>
>Can someone tell me what the optimum antenna length should be for a
>Cessna (ARC) ADF installed in a 1977 Skyhawk. At my home field, there
>are so many different lengths in use on other aircraft, that it seems
>random? There must be an optimum?
>
>Thanks
>John Morley

ADF receivers use such low frequencies that "optimal" in the sense
of "tuned" antenna lengths is just impossible. The purpose of the
wire antenna is to resolve the ambiguity between the two possible
bearings to the station as determined by the coil antennas. These
antennas measure the magnetic field component and the wire antenna
measures the electric field component. The RF phase relationship
between the two antennas resolves the 180 degree ambiguity. For this
reason, a wire sense antenna long enough to be tuned is actually a bad
idea. As the antenna approaches a quarter wavelength, the phase of
the signal undergoes drastic changes. I think the optimal length of
the wire antenna is as short as possible consistent with getting
enough signal strength to work.
-Bob Scott

[Bill Hale]

The antenna looks like a capacitor.

You pick whatever the specified length for it is. That is figured
with low capacitance coax of a certain length. Not RG-58.

There is an adjustment in the ADF to allow it to be peaked for
your exact setup.

True for most ADFs that use a separate sense antenna.

+Regards, Bill Hale; ha...@howard.fc.hp.com
+Thru ether-K0QA; In ether-BE33 N33HP CFIAIME A&P; Twisted pair 1-970-229-3037
+Snail: c/o HP UTD MS73 3404 E Harmony, Ft Collins, CO 80525 Incoming:40 44.2N
+105 33.1W; Pictures 303-970-6176 M1 bigot ABS BPPP instructor

[Angelo Campanella]

In article <32550B...@analysistech.com> John Sofia <in...@analysistech.com> writes:
>From: John Sofia <in...@analysistech.com>
>Subject: ADF Antenna Length
>Date: Fri, 04 Oct 1996 09:03:57 -0400

>Can someone tell me what the optimum antenna length should be for a
>Cessna (ARC) ADF installed in a 1977 Skyhawk. At my home field, there
>are so many different lengths in use on other aircraft, that it seems
>random? There must be an optimum?

Some simple radio physics will settle this one.

1/ The sense antenna is needed only to get a sense of the phase of the radio
wave's ac electric vs its ac magnetic field. (The "loop" picks up the
magnetic field. They used to be air-core loops - like a small hoola-hoop -
but now they use a ferrite core to get an adequate magnetic sens signal
strength and said assembly is small. )

2/ There is so much static (noise) in the air in these low radio frequencies
that sensitivity (being able to detect tiny signals) is not a problem.

3/ Only a sample of the radio wave is needed. Ant short wire stub will do
this for the electric field. The only requirement is that the electric field
signal be stronger than the electronic noise (transistor or thermal noise) of
the radio input amplifier circuit.

4/ Longer wires will give more electric signal to the radio, but this becomes
useless. The same situation exists for your car AM radio, in virtually the
same radio frequency range as ADF signals.

5/ So the challenge is to find the smallest whip or stub that will supply
enough ac electric signal for the data processing circuitry of the ADF radio
to do its job, but little more.

NB: Now, if on the other hand, you are trying to TRANSMIT an adf signal into
the air, then you need a really BIG antenna. But we pilots don't find need to
transmit radio signals in these very long wave length bands. So it's a
non-problem for aircraft in flight. I suppose that there are those that would
want to do all that, too, but real world physics makes it impractical...

Ang.

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