FAQ's on Printed Circuit Board (PCB) Loop Antennas.

  
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FAQ's on Printed Circuit Board (PCB) Loop Antennas.

Postby radiomadeeasy » Fri Jan 11, 2013 5:22 pm

Printed Circuit Board (PCB) Loop Antennas

Many low power radio devices use Printed Circuit Board (PCB) “Loop Aerials”. These antennas are compact, efficient, rugged, easy to fabricate and are inherently low cost. For optimum performance, however, they must be tuned to resonance. This note provides some guidance and hints on how this can be accomplished.

Firstly it is necessary to have some means of measuring the radiated power output of the transmitter. As this only needs to be a relative measurement (more or less) a receiver with an RSSI (Received Signal Strength Output) output will suffice. This output should be connected to a suitable indicating device such as an analogue moving coil meter or ‘scope.. This allows trends (the more or less) to be more easily observed. The tests should be performed in “open field” conditions with both transmitter and receiver placed on non-conducting surfaces (wooden tables) about 1 metre above ground. For initial tests a variable tuning capacitor should be used to tune the loop. Whilst variable tuning capacitors can be used on production devices most manufacturers prefer “fixed” tuning as it simplifies production testing and prevents tampering. Modern component tolerances and production methods make this method entirely practical.

A non-conducting jig or fixture should be used to hold the transmitter (for example) in a fixed and constant orientation to the receiver. The distance and orientation between the transmitter and the measuring receiver should be adjusted to bring the receiver into a “linear region” of its RSSI output. This ensures that the signal is neither buried in the noise nor “saturating” the receiver. The tuning capacitor should be varied throughout its full range and a definite “peak” in the output should be observed. Ensure that sudden “dips” or “peaks” are not caused by the tuning capacitor going “over the end stops” from maximum to minimum. As the value of the capacitor is known (e.g. 2-6pF) some guess can be taken as to its value at resonance by its mechanical position. This is the starting point for the substitution of fixed value capacitors.

As standard component values provide too coarse a range of adjustment it is usual to use two capacitors in series to provide intermediate values. The table below gives all the values that can be obtained with the combinations of standard values. The method used is to start with an estimated value and then measure and record the power output. The next values up and down must then be placed in circuit and a note taken of the power output. This will give the direction in which to proceed. This tedious process is then continued until a definite peak is recorded that matches or betters the value obtained with the variable capacitor. Whilst roughly “tagging’ in capacitors may be quick, it may not be accurate and it is recommended that the capacitors be neatly soldered into the board at each stage. There are considerable benefits to using surface mount capacitors as they generally have a better tolerance and can also be obtained with values of less than 1p8.

Hints & Tips

• Use the dimensions and track width recommended in the data sheet.
• Do not make the loop too small or too large. Small loops will not radiate efficiently whilst large loops will only resonate with minute values of capacitance. Such values may be difficult to realise practically.
• The main parameter that affects performance is area enclosed, shape is relatively unimportant.
• Keep the loop away from conducting objects. E.g. batteries, metal cases. If using a double sided PCB leave the opposite surface clear of all copper.
• Do not put objects or components in the middle of the loop or be tempted to run the loop around the perimeter of the board.
• Use a generous area of ground plane on the rest of the board and keep the loop well away from it.
• Receiver loop aerials obey exactly the same laws of physics as transmitter aerials.
• Do not be misled by the thought of “only a few puff”. Consider the effect on a timing circuit in which a 150nF capacitor was replaced by 220nF capacitor !



Values obtained by Series Combination of Preferred Values

C1 C2 Result Step

1.8 1.8 0.90
1.8 2.2 0.99 0.09
1.8 2.7 1.08 0.09
2.2 2.2 1.10 0.02
1.8 3.3 1.16 0.06
2.2 2.7 1.21 0.05
1.8 3.9 1.23 0.02
1.8 4.7 1.30 0.07
2.2 3.3 1.32 0.02
2.7 2.7 1.35 0.03
5.6 1.8 1.36 0.01
2.2 3.9 1.41 0.04
1.8 6.8 1.42 0.02
1.8 8.2 1.48 0.05
2.7 3.3 1.49 0.01
2.2 4.7 1.50 0.01
1.8 10.0 1.53 0.03
2.2 5.6 1.58 0.05
2.7 3.9 1.60 0.02
3.3 3.3 1.65 0.05
2.2 6.8 1.66 0.01
2.7 4.7 1.71 0.05
2.2 8.2 1.73 0.02
3.3 3.9 1.79 0.05
2.2 10.0 1.80 0.02
2.7 5.6 1.82 0.02
2.7 6.8 1.93 0.11
3.3 4.7 1.94 0.01
3.9 3.9 1.95 0.01
2.7 8.2 2.03 0.08
3.3 5.6 2.08 0.05
2.7 10.0 2.13 0.05
3.9 4.7 2.13 0.01
3.3 6.8 2.22 0.09
3.9 5.6 2.30 0.08
4.7 4.7 2.35 0.05
3.3 8.2 2.35 0.00
3.9 6.8 2.48 0.13
3.3 10.0 2.48 0.00
4.7 5.6 2.56 0.07
3.9 8.2 2.64 0.09
4.7 6.8 2.78 0.14
5.6 5.6 2.80 0.02
3.9 10.0 2.81 0.01
4.7 8.2 2.99 0.18
5.6 6.8 3.07 0.08
4.7 10.0 3.20 0.13
5.6 8.2 3.33 0.13
6.8 6.8 3.40 0.07
5.6 10.0 3.59 0.19
6.8 8.2 3.72 0.13
6.8 10.0 4.05 0.33
8.2 8.2 4.10 0.05
8.2 10.0 4.51 0.41
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Re: FAQ's on Printed Circuit Board (PCB) Loop Antennas.

Postby denizli » Mon Jun 24, 2013 3:54 pm

Hi, thank you for sharing. This is a really interesting and much debated topic. Since I’m more of a digital electronics guy who has recently started studying RF, I have a lot of learning to go through. If we change the area of the ground plane, will that effect the transmission of the loop antenna? Secondly, how about multiple loops or an inward spiral on a PCB, in my opinion, it should have better radiation characteristics than a single loop antenna.
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Re: FAQ's on Printed Circuit Board (PCB) Loop Antennas.

Postby Antman » Wed Jun 26, 2013 4:44 pm

Hi Denizli,

Thank you for contributing.

PCB Loop antennas and spiral antennas properties will change with differing sizes of ground plan etc. These are quite complicated subjects, however there are excellent online sites that offer much better advise and examples.

TI offer some very good advise http://ww1.microchip.com/downloads/en/A ... 00831b.pdf
Our distributor Wireless world have another useful link. http://www.wirelessworldag.com/appnotes ... tennas.pdf

As well as websites there are some of very useful books associated with the foundation, intermediate and advanced amateur radio license (please see the link below).

http://www.rsgbshop.org/acatalog/Online ... .html#a438

Books by Alan Betts are clear and well written, with lots of practical examples. If later you want to get involved in design work, you will probably want to invest in a text book, which provides a more fundamental treatment of the topic. The following books are good:

1) Antennas - for all Applications, by John D. Kraus and Ronald J. Marhefka
2) Antenna Theory – Analysis and Design, by Constantine Balanis
3) Antenna Theory and Design, by Warren L. Stutzman and Gary A. Thiele

The book by Balanis is well regarded in the field, but it is quite a heavy from a mathematical point of view. Kraus is a bit lighter.
This webpage provides some book reviews http://www.antenna-theory.com/intro/books.php. It mentions "The ARRL Antenna Book".
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