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AM Receiving Antennas

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Documents » Principes de base de Technique Radio » AM Receiving Antennas
           
Jochen Bauer
Jochen Bauer
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21.Jan.13 21:49
 
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In the AM broadcast bands with wavelengths ranging approximately from 2000m to 1000m (long-wave) and 600m to 200m (medium-wave) quarter or half wavelength resonant receiving antennas are not a viable option. Instead, the short vertical wire and the small loop antenna, both with dimensions much smaller than the AM broadcast band wavelengths have earned their merits throughout the decades of radio history.

The paper presented here aims at providing a deeper insight into the performance of those two antenna types. The following topics are covered:

  • Voltage pickup from incoming electromagnetic waves
  • Theoretical power transfer of ideal, lossless vertical wire and loop antennas into a receiver
  • Losses in practical vertical wire and loop antennas
  • Power transfer of practical (lossy) vertical wire and loop antennas into a receiver

A short comparison of the performance of those two antenna types concludes the paper. For a further overview, the abstract section of the paper is quoted below.

At this point I would like to thank Joe Sousa for his valuable suggestions that greatly improved this paper.

 

Abstract

The two most common antenna types for AM reception are the loop antenna and the vertical wire antenna both having dimensions much smaller than the wavelengths of the electromagnetic waves in question. While the loop is a typical indoor antenna, the vertical wire antenna is by virtue of it's bigger size mostly an outdoor antenna. The performance of these two antenna types with respect to voltage pickup and power transfer is analyzed and compared. This is done for ideal, lossless setups as well as real, lossy setups of both antenna types. It is shown that even the theoretical upper limit for power harvesting from electromagnetic waves in the AM frequency range with lossless loop, respectively vertical wire antennas is much smaller than the power a pocket size solar panel could provide. For real, lossy antenna setups the different types of losses are analyzed and compared in magnitude. It is shown that the lossy vertical wire antenna outperforms the lossy loop antenna in typical setups when it comes to voltage pickup and power harvesting from an incoming electromagnetic wave.

 

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Cet article a été édité 21.Jan.13 21:53 par Jochen Bauer .

Jochen Bauer
Jochen Bauer
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D  Articles: 126
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24.Nov.13 22:30

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In the previous article the (untuned) magnetic loop antenna was compared to the (also untuned) vertical wire antenna. It became obvious that that vertical wire antenna typically outperforms the magnetic loop antenna in voltage pickup and power transfer into a receiver.

However, in many practical setups the magnetic loop is used as a tuned antenna connected to a  high impedance RF gain stage. This is simply accomplished by connecting a variable capacitor parallel to the magnetic loop which now serves as the inductor of the resulting tuned circuit.

 

Using the above formula for the tuned magnetic loop in the comparisons that were made in the previous paper, it can be shown that unlike the untuned loop, the tuned magnetic loop can easily keep up with the (untuned) vertical wire antenna as far as the voltage that can be provided to a high impedance RF gain stage is concerned.

 

  
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