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nuckolls.bob(at)aeroelect
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 Posted: Mon Aug 19, 2013 7:49 pm Post subject: Broadcast Radio Bleedover on Transceiver |
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The demodulation of an AM signal can occur in any non-linear
resistance where the radio signal carrier is 'mixed' with the
side band energy to produce a carrier +/- SB products
which are the audio program material.
The necessary non-linearity can be found in a poorly made-up
joint where corrosion products behave like a crude rectifier
or crystal junction (rusty razor blades have been used to fabricate
"fox-hole radio" detectors). To be sure, the resulting audio
signals are very tiny . . . largely determined by the
Q of the 'tuner', length of antenna, quality of the ground,
electrical sensitivity of the transducer (head set) and
skill of designer/fabricator for optimizing impedance
matches.
It has been readily demonstrated that an artfully
crafted 'crystal set' can perform rather well and receive
even overseas short wave broadcasts. The kind of performance
was never demonstrated in the typical crystal set kits and/or
merit badge construction projects.
[img]cid:.0[/img]
If one is into SERIOUS crystal radio fabrication for optimized
performance, the lowly battery-less radio is likely to appear
more like this . . .
[img]cid:.1[/img]
Separate tuners and traps wound with Litz wire, Schottky detectors,
and matching transformer to optimize coupling of energy to the
headphones. The radio above was a Dx contest winner for the
builder.
The 'crystal set' that plagues the aircraft audio system has
antennas that are too short, tuners of un-measurably low Q, and
headset not known for electromechanical efficiency. Hence,
the un-welcomed demodulation of AM radio signals is rare . . .
often presented when a coax antenna feeder shield becomes
disconnected at a connector and the cockpit is flooded with
RF from the ship's VHF comm transmitter. This configuration
manifests in a feedback squeal only when transmitting.
The last time I heard AM demodulation in an aircraft radio
was when taxiing about 1/4 mile away from the airport's air
traffic control radar. Each sweep of the antenna put short
bursts of PRF 'buzz' in the headset as the airplane was painted
with high energy RF.
Sensitivity to terrestrial AM transmitters is rare. This case
(and that presented by the radar) probably has a root cause
in poor design of an input stage to an amplifier. The first
stage of a microphone amplifier or isolation amplifier input
is not effectively filtered for RF suppression. The strong
local signal drives the first stage transistor base into
non-linear operation and demodulates the signal which is
then amplified by downstream electronics. It's improbable
(but not impossible) that a corroded joint is root cause.
You need to troubleshoot the system to deduce which of the
ship's black boxes is affected. I doubt that it is the
radio . . . my bet is on the intercom. Shielding wires
for affected devices sometimes helps but it's a poor
bandaid . . . the elegant fix is usually addition of a
capacitor and perhaps and inductor filter to the
input circuit of the affected device.
Let us know what your findings are as to which device
is victim to the high energy RF signals.
Bob . . .
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wrmaxwell(at)bigpond.com
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| Posted: Tue Aug 20, 2013 1:56 am Post subject: Broadcast Radio Bleedover on Transceiver |
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Tue Aug 20, 2013 5:17 am Post subject: Re: Broadcast Radio Bleedover on Transceiver |
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| Quote: | It has been readily demonstrated that an artfully
crafted 'crystal set' can perform rather well and receive
even overseas short wave broadcasts. The kind of performance
was never demonstrated in the typical crystal set kits and/or
merit badge construction projects....Bob |
Indeed. In WWI soldiers had no problem building "Trench Radios" out of anything imaginable. A lump of anthracite coal made a dandy detector. Metal sheets between the pages of a book worked well for capacitors. Air-spaced coils were woven like baskets out of any available wire. Wrapping a few nails with the finest enameled wire one could get and stuffing them into a food tin worked for a headphone.
Of course they had a) Time and b) Giant spark-gap code transmitters a mile away.
They still make good science projects.
For high frequencies, joint corrosion is a concern. Surgeons use high-frequency devices to cut and cauterize surgical patients, but the slightest DC component will make the patient's muscles jump. Corroded joints are a frequent problem because the rectify the HF into DC.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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