Warning: use of this antenna is not recommended if you do not use
separate antenna coil, high voltage capacitors and a fuse to connect to
high
voltage live wire. You risk death otherwise. Be careful and use all
safety precautions. Always assume that neutral wire might be live as
well, if improperly wired. Even if properly wired, it can become live
due to a fault.
Live wire / neutral
wire antenna
I was looking through some old radio books - mostly tube designs - when
it was mentioned that it's possible to use live or neutral wire in AC
household wiring as receiving antenna. Of course, this is possible only
with high voltage capacitor inserted between the receiver antenna
terminal and live or neutral wire. AC wiring antenna solution was
mentioned as some type of last resort, when no other antenna can be
installed due to space contraints. Ground conductor can be used as
Earth ground.
Old books recommend cold water pipe as ground connection, but I'm not
sure whether this is good/practical/allowed any more. They also
recommend "one good condenser" for antenna connection, not 2 in series.
I guess people were braver then :)
Receiver circuit and
antenna connection
I decided to test this system myself. I made a small 2-transistor
medium-wave AM receiver, with headphone output. To reduce main LC tank
circuit loading, HF is taken from a capacitive divider of variable cap
and 5 nF cap. Smaller voltage drop exists on larger capacitor due to
its smaller capacitive reactance Xc=1/(2*pi*f*C). Voltage taken from
main LC circuit decreases as higher frequencies are received: smaller
capacitance of variable cap takes on more of capacitive divider
voltage. This was deemed to be unimportant for fairly narrow range MW
tuning. Received high
frequency is demodulated at the base of
first transistor and then amplified as low frequency in the first and
second transistor. Calculated frequency
range is approximately 700 kHz - 2800 kHz.
To protect myself and my receiver, I
decided to use two high voltage capacitors in series, antenna coil
galvanically
separated from remainder of circuit, and fuse. If high
voltage spike is somehow able to penetrate through both 1 kV rated
capacitors,
it will blow open the fuse. Failing that, antenna coil (0.3 mm diameter
wire, 20 turns) will burn out, with no further damage done to receiver
(or user). Hopefully. :)
Simple MW AM direct receiver schematic with live wire antenna
connection shown.
Coil, variable capacitor and receiver circuit. Headphones can be of low
or
high impedance.
Testing
Several tests were done: first with 1 m wire antenna, without ground
connection. Four nearby AM stations were received, 3 were identified
for testing. Latter tests were done with live wire antenna connected to
AC outlet, neutral
wire antenna connected to AC outlet, and lastly power cable antenna NOT
plugged into the outlet.
Connection setup with live wire (black wire) connected as antenna.
Male power plug not plugged into the outlet.
Testing video with live wire as antenna. Antenna is plugged into the AC
outlet during the video. This changes main receiver LC circuit
frequency somewhat, so re-tunning is necessary for maximum sound
output. Sound is weak because it is coming from the headphones only, so
weaker stations cannot be heard in the video.
1.
1 m wire antenna, no Earth connection, Dec 24, 2:30 PM
Station frequency (kHz)
Transmitter power (kW)
Vrms (at headphones)
Reception
Distance to TX (km)
CHML 900
50
10 mV
strong
20
CKOC 1150
50
55 mV
very strong
15
CKPC 1380
25
2.7 mV
weak
30?
Clear but fairly weak signals.
2.
live wire antenna, Earth connection to grounded cable conductor, cable
plugged into AC outlet, Dec 24, 3 PM
Station frequency (kHz)
Transmitter power (kW)
Vrms (at headphones)
Reception
Distance to TX (km)
CHML 900
50
40 mV
strong+noisy
20
CKOC 1150
50
115 mV
very strong
15
CKPC 1380
25
45 mV
strong+noisy
30?
Noticeable 60 Hz hum. Measured at headphones: 2.3 mV at 700 kHz, 0 mV
at 2800 kHz.
3.
neutral wire antenna, Earth connection to grounded cable conductor,
cable plugged into AC outlet, Dec 24, 3:30 PM
Station frequency (kHz)
Transmitter power (kW)
Vrms (at headphones)
Reception
Distance to TX (km)
CHML 900
50
100 mV
strong
20
CKOC 1150
50
125 mV
very strong
15
CKPC 1380
25
80 mV
medium+noisy
30?
Some 60 Hz hum.
4.
live wire antenna, Earth connection to grounded cable conductor, cable
NOT plugged into AC outlet, Dec 24, 4 PM
Station frequency (kHz)
Transmitter power (kW)
Vrms (at headphones)
Reception
Distance to TX (km)
CHML 900
50
90 mV
strong
20
CKOC 1150
50
120 mV
very strong
15
CKPC 1380
25
8 mV
weak
30?
Clear signals.
Fourth station received and understandable, but not used for testing,
was 820 kHz CHAM, 15 km distance to TX, 30 mV at headphones, 50 kW TX.
Measured with live wire antenna, Earth connection to grounded cable
conductor, cable
NOT plugged into AC outlet.
Discussion
Regular 6 feet (1.8 m) power cable, even when not plugged into the
outlet, provides surprisingly good reception, much better than just
single 1 m wire. I suspect this has something to do with the fact that
antenna and ground wire are identical in length, which makes it into a
crude non-resonant dipole antenna (but with parallel arms instead of
180 degree arms). Or not.
Live wire / neutral wire antenna connections provide a lot of usable
signal, so my judgment is that this is a viable way to have some
antenna if nothing else is available. Main problem: 60 Hz hum that
masks weak stations. It could be reduced in two ways:
1.
Use smaller number of turns for antenna coil. 20 is way too much, 5
will do just fine. It will provide better impedance match to main
tuning circuit, and reduce inductive reactance (less 60 Hz voltage
induced into antenna).
At 1 Mhz inductive reactance of 20 turns antenna coil with inductance
of 40 microH:
XL = 2*pi*f*L = 6.28*1000000*0.00004 = 251 Ohms
At 60 Hz inductive reactance is:
XL = 6.28*60*0.00004 = 0.015 Ohms
Voltage drop on antenna coil is sum of voltage drops on XL and antenna
coil resistance. Resistance is approximately 0.4 Ohms.
2.
Use smaller antenna capacitors. Test setup used two 250 pF 1 kV
capacitors in series. This gave total capacitance of 125 pF. Capacitors
in 10-50 pF range should be sufficient. They will weaken 60 Hz hum as
well being of higher capacitive reactance at 60 Hz (smaller cap -
higher capacitive reactance).
At 1 Mhz capacitive reactance of 125 pf antenna capacitors:
Xc=1/2*pi*f*C = 1/(6.28*1000000*0.000000000125) = 1273 Ohms
At 60 Hz capacitive reactance is:
Xc=1/(6.28*60*0.000000000125) = 21.23 MegaOhms
How much voltage is
60 Hz power line voltage inducing into the antenna coil?
At 60 Hz we have, in essence, series R, L, C connection. Impedance of L
being very small, we have to practically deal only with series
connection of impedance Xc and coil resistance Rcoil. Voltage induced
is then:
Vinduced = Vac * (Rcoil / Xc) = 120 V * (0.4 / 21 230 000) = 2.26
microVolts
which agrees well with measured values (see table 2, live wire antenna).
Okay, but how do I
use this in my own radio?
Most people will not be building their own receivers to test this. Here
are a couple of suggestions:
1.
If you just have one connector labeled antenna, ANT or similar, you can
try to connect to neutral/live wire with two high voltage capacitors
and fuse in between your connector and neutral/live wire.
2.
If you have two connectors, labeled antenna, ANT and Earth, -||| or
similar, connect to neutral/live wire with two high voltage capacitors
and fuse in between your ANT connector and neutral/live wire. Connect
Earth connector to grounded conductor in your power (turned into
antenna) cable.
3.
Cheap transistor radio with no external antenna connections? You can
open the radio, and wind one turn of insulated wire onto radio's
ferrite rod antenna. One end of wire goes to 2 high voltage caps + fuse
and
then to live/neutral wire. Other end goes to grounded conductor.
Conclusion
Neutral wire antenna was overall the best, so for safety reasons
it would be wise to use neutral conductor as antenna. Your local
conditions may vary however, in which case you may find that live wire
antenna is better.
Literature
1. Radio-praksa za sve, Beograd, 1962. Translated into Serbian from
German
original: Radiopraxis für alle, Praxis durch selbstbau:
Normalempfänger und UKW, Sechste Auflage, Heinz Richter
