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Auxiliary Loop Antennas For AM Reception

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Papers » Basic principles of radio technique » Auxiliary Loop Antennas For AM Reception
           
Jochen Bauer
Jochen Bauer
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02.Jun.14 21:32

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A common way of improving the reception of weak stations by an AM pocket radio with a relatively small build-in ferrite rod antenna is to place a large tuned loop antenna near it and tune this auxiliary loop antenna to the same reception frequency as the pocket radio. Examples for such a setup can be found in the following (German language) threads:

The following article aims at providing a more in-depth understanding of such setups using auxiliary loop antennas. For further reference, the abstract section of the article is quoted below:

A common way of improving the reception of weak stations by an AM pocket radio with a relatively small build-in ferrite rod antenna is to place a large tuned loop antenna near it and tune this auxiliary loop antenna to the same reception frequency as the pocket radio. Obviously, this amounts to an inductive coupling between the tuned auxiliary loop antenna and the tuned build-in ferrite rod antenna and hence, this setup is similar to a tuned transformer. However, unlike the standard tuned transformer that is treated extensively in the literature, we are here dealing with a tuned transformer where the primary and the secondary side are driven synchronously by an incident electromagnetic wave. In this paper, we shall therefore re-derive some of the equations governing the tuned transformer and augment them to include the case of a synchronously driven secondary side. From the resulting equations, an expression for the optimal mutual inductive coupling factor between the auxiliary and build-in antennas is derived and it is shown that the optimal coupling depends on the voltage pickup ratio between the auxiliary and build-in antennas. It is also shown that the auxiliary loop antenna needs to meet certain performance criteria to actually improve reception. Furthermore, it is shown that reversing the direction of the build-in ferrite rod antenna with respect to the auxiliary loop antenna (turning the pocket radio by 180°) will have no effect on reception.

 

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Jochen Bauer
Jochen Bauer
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06.Jun.14 13:10

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One of the implications of the results obtained in the previous article is that there is on optimal size of the loop antenna around the pocket/portable radio if it is placed at the center of the loop, which is the most common setup. This is due to the fact that the distance of the ferrite rod at the center to the loop wire around it will have a crucial impact on the mutual inductive coupling factor k between the loop and the built-in ferrite rod antenna of the radio. Hence, the optimal coupling factor can be reached by choosing the size of the loop appropriately.

At this point, we need to get an idea of the magnitude of the mutual inductive coupling factor for a typical, not overly large loop, with the receiver at the center and how it compares to the typical
magnitude of the optimal coupling factor for reasonable setups. For this purpose the following setup
has been built:

The loop is actually a rectangular frame with a side length of 35cm and 18 turns of litz wire resulting
in an inductance of approximately 370uH. The ferrite rod antenna is 17cm long and has 50 turns of litz
wire resulting in an inductance of approximately 385 uH. A ferrite rod antenna with these parameters
would typically be used in a portable AM radio.

Using the "short circuit method" that has been presented here and analyzed in greater detail here, the mutual inductive coupling factor between the two coils is found to be k=0.077. While at first, this does not appear to be an overly high coupling factor, the reader is reminded that the optimal coupling factor between the tuned loop antenna and the ferrite rod antenna is in most cases much smaller. In the previous paper examples were given were the optimal coupling factor is typically in the range of k=0.01 to k=0.015.

Therefore, if in this case the ferrite rod were built into a pocket/portable radio, to reach an optimal
improvement of reception, we would need to significantly decrease the coupling between the loop and the ferrite rod antenna. While this could be done by moving the radio away from the loop a better yet more laborious method is to increase the loop's diameter while at the same time adjusting the number of turns to keep it's inductance approximately constant. This has the benefit of an increased voltage pickup from the incident electromagnetic wave (a more detailed analysis can be found here) resulting in an even better improvement of reception in the pocket/portable radio.

In practice, calculating the optimal geometry of the loop is not feasible since the parameters of the
radio's built-in ferrite rod antenna are usually not known. And even if they were, this would be a very
complicated endeavor. Instead, if time is available one might start with a relatively small loop, test
the improvement in reception, increase it's size, test it again, and so on, until the optimal size of the
loop is reached.

 

 

  
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