July 23, 2015
The test tool box used by field technicians and engineers to establish and maintain wireless network quality is becoming as robust as the systems themselves. Because of network complexity, selecting the proper tools for your job can save time, money, and some headaches.
One example is choosing the best possible antenna when embarking on an interference hunt. The correct one will have you on the path to quickly solving the problem. Selecting the wrong one will lead you down a series of dead ends. Today’s post will focus on the antenna types and when you should use them.
Yagi
A Yagi antenna (figure 1) is typically a very solid choice, especially in direction finding scenarios. One of the biggest pros of a Yagi antenna is that it has a very good radiation pattern, so you clearly know when it is pointing at the interfering signal. It also has a solid front-to-back ratio to reduce the likelihood that you will be misled by signals that are transmitted from behind the antenna. Low sidelobes, which help prevent you from chasing reflections that will waste time and frustrate you, are another trait of Yagi antennas. The fairly narrow frequency band of a Yagi antenna can limit its effectiveness in certain applications. For example, if you are working with a broad set of receive frequencies, you may need too many Yagi antennas for it to be practical.
Figure 1: A Yagi antenna is particularly effective in direction-finding scenarios.
Log Periodic
The biggest advantage of a log periodic (LP) antenna is that it has a very broad frequency range, with some covering frequencies from below 500 MHz to above 6 GHz. There are even LP antennas that cover up to 18 GHz and higher. LP antennas aren’t as directional as their Yagi cousin, but they do have good front-to-back ratio. If you need to work over a very wide range of frequencies, an LP antenna is a good choice, although it will take more time to find emitters due to it having a wider beamwidth than a Yagi. The best way to deal with this limitation is to move the antenna side to side and discover where the signal dips a given amount on each side of the swing. The halfway point between the two swings is the best direction estimate to the signal source.
Bi-blade
Good directivity is a hallmark of these directional antennas. The drawback is that they only have a moderate front-to-back ratio. This latter concern can be marginalized by using your body as a shield. Anritsu offers a bi-blade antenna that integrates with its MA2700A Handheld InterferenceHunter™ for highly effective interference hunting.
Omni-directional
If you are doing a mobile hunt, an omni-directional antenna may be your best bet, as it is often mounted on a car roof using a magnetic disk. It is typically used when seeking the strongest signal, such as when using the Anritsu MX280007A Mobile InterferenceHunter™.
Rubber Duck
A small, compact and mobile antenna, the rubber duck is convenient when carrying equipment in the field. Unfortunately, these antennas are not that effective. They have a short range, as well as a high Q factor, which narrows their bandwidth. Also, be careful that the antenna doesn’t pick up signals radiating from your instrument, rather than those being hunted.
Maximizing the Antenna
Another tool in your test box should be an antenna handle (figure 2), especially if you are conducting a hunt on foot. It preserves the antenna pattern and its ergonomic design puts less stress on your hand and arm, a nice advantage for long days in the field. A strap is usually available with the antenna handle and can actually free your hands to do other tasks. The trigger on the handle makes it extremely easy to take a bearing, or switch the antenna preamp on or off. Other advantages include that any small and light RF filter can easily be added before the pre-amp in the handle, and it can be easily integrated with all types of antennas.
Figure 2: An antenna handle such as the Anritsu MA2700A Handheld InterferenceHunter has a GPS receiver and antenna, electronic compass, and user-selectable preamplifier, all in an ergonomic design.
The handle also integrates a magnetic compass and GPS, so that position and direction can be easily relayed to the spectrum analyzer to simplify acquiring directional bearings. The built-in pre-amp makes it easier to send a sufficiently strong signal to the spectrum analyzer.
Conclusion
As this post shows, understanding the conditions in which you will be conducting an interference hunt will help determine the antenna to use. Selecting the proper accessory to your spectrum analyzer will help you locate the interference source faster and make your job a little easier.
To learn more about interference hunting tools, please watch this free webinar.
