3 Meter Antenna Impedance vs Diameter

radio8z · 2015-09-08T07:13:33Z

While contemplating the design for an indoor AM Part15.219 compliant antenna I found it necessary to measure the RF impedance of a 3 meter length of wire in order to spec. the loading coil. A 30 gauge wire was used and was strung horizontally in my basement lab as far away from objects as practical. The test setup consisted of an RF signal generator, scope, and a RF current transformer. The generator was connected to the input of the transformer and the voltage was measured at this point with the scope. The current signal from the transformer was connected to the second scope channel so the amplitude and phase of both the current and voltage could be observed. The ground of the current transformer was connected to the power line ground and the antenna was connected to the output of the transformer. A series inductor of 150 uH was connected in series with the antenna and the frequency was adjusted until the system resonated as shown by zero degree phase shift between V and I. The V and I were measured and from the frequency and known inductance the antenna impedance was calculated. With the 30 gauge wire the Z = 108 -j2626 ohms which corresponds to an antenna capacitance of 22 pF. This capacitance seemed rather low so the wire was replaced with a copper pipe of .8 meter length of 3/4" telescoped around a 2.2 meter length of 1/2" pipe giving 3 meters overall length. The pipe was positioned exactly where the original wire was located. The impedance with the pipe was Z = 126 - j1933 ohms corresponding to antenna capacitance of 30 pF. This experiment indicates that there may be an advantage to using a radiating element larger in diameter than a wire since it increases the capacitance and thus lowers the inductance needed for the loading coil. The loading coil is a major source of power loss in such an antenna system and the less wire used the better. Neil

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Anonymous

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Those wanting a project that might produce more interest/listeners in the general public than provided by a compliant Part 15 station might be interested in something like this -- and with no worries about FCC rules.

Posted : 19/09/2015 2:41 pm

Anonymous

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Every city should have a street organ.

Does the wheel man need to join the musician's union?

Also, in addition to entertaining in open air you could implant a part 15 transmitter on board for shut-in listeners.

Posted : 19/09/2015 3:02 pm

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