Active Antennas KB6RWX
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Revision as of 12:31, 31 March 2016 by QDP2012 (updated categories)
Active Antennas - Friend or Foe?
Application note 100 -Lowell Beezley KB6RWX
Radio enthusiasts have a love hate relationship with active radio receiving antennas. They are generally a topic of hot debate with many good arguments both pro and con and equally as many experiences. The reason for these diverse opinions is that active antennas are not clearly understood and more often misunderstood.
Active antennas will generally not perform better than a full size (resonant) antenna or even a long wire. Even so, active antennas are useful to those desiring to achieve adequate performance, but do not have the space for a long wire or other full sized antenna. Another major advantage is that is that an active whip is omni-directional which a long wire is definitely not. In fact, long wire antennas are fundamentally bi-directional and can be extremely directional depending on its physical length with respect to the signal frequency involved.
Active antennas are good alternatives at a cabin or trailer or some other get-away place where erecting a long wire is not practical or desired. They are especially useful in a cruise ship cabin or an apartment! Active antennas do have performance limitations. With this in mind, you can probably understand that using an active antenna is largely a matter of preference with the main benefits being relatively compact size and portability.
Why active antennas?
Many people have misconceptions about active antennas. Some think that their radio receiver system will be transformed into a highly sensitive “spy grade” radio that will hear everything that is broadcast around the world. This is simply not so. With HF, propagation will largely dictate what can be heard and when and the atmospheric noise will determine the basic band sensitivities. Propagation at VHF and UHF frequencies is different and requires a shift of thinking in terms of the location, techniques and results.
The preamplifier gain in most active antennas is only designed to overcome the coax loss and provide a small margin of net gain to the receiver. This allows the antenna to be located remotely with the convenience of a coaxial cable feed line. The low noise figure of the active antenna preamplifier is generally far better than the front end of the radio and that can improve the ability to hear really weak stations, but it will normally not increase the signal level to any great extent especially when being compared to a full sized antenna. The reason for this is that the active antenna is electrically short, providing a relatively small capture area that relies on the preamplifier for its performance.
Many people employ active antennas due to lack of space or ability to erect a full sized antenna. Others have local CCR’s prohibiting the erection of outdoor antennas of any type. Being small active antennas are “stealthy” and much less conspicuous. At VHF and UHF frequencies the radio horizon distance determines the receiving range of your radio, so mounting height is more important. However, at HF the signals are coming in at higher angles and the antenna will do as well in your bushes as it will on a 50 foot tower. Both will generally perform well in an attic even into the UHF frequency range.
The performance benefits and tradeoffs of an active antenna or preamplifier must be understood to be appreciated. Receiver sensitivity is a key performance factor. It is true that a larger antenna will help make up for low receiver sensitivity. A high end receiver with impressive sensitivity specifications may work very well with a modestly small antenna. Noise and sensitivity are directly related. We’ll talk more about the details of sensitivity in a subsequent note.
External noise is either atmospheric, QRN and man made QRM. There are methods to reduce QRN with antenna design and that too will be addressed later. For now, proper receiver grounding is normally the simplest remedy for reducing external noise. The internal noise of the receiver is related to the sensitivity of the front end. There are methods to improve this too, but receivers are generally balanced in terms of the performance of the front end RF preamplifier, mixer and IF stages. Adding external gain can actually make the receiver more sensitive, but at the cost of dynamic range which is perfectly acceptable under certain conditions. If the atmospheric noise level is relatively high, even the most sensitive receiver in world will not be able to pull a signal out of the noise. That is where filters come in and that too will be another topic.
When the propagation conditions are good so is reception. When the band is dead there is generally not much that can be done to pull in a signal even with a full size beam antenna. For the time being we need only consider that there are two basic types of noise, external and internal to the receiver. As far as sensitivity goes, if you can hear a signal it is above the noise.
There can be problems
Active antennas and preamplifiers have one common weakness. Being broadband, the amplifiers are susceptible to overloading in the presence of a strong local broadcast signals. Antenna overload or preamplifier compression causes the amplifier to become non-linear and the strong signals become the equivalent of local oscillators in a mixer. Then the weaker signals mix with the strong ones producing images or intermodulation signals that appear to the receiver as being real. This will cause your receiver to “hear” the same signal in three or four places on the dial.
The key to this problem is in the quality of the amplifier circuits in the active antenna or preamplifier. In order for the antenna or preamplifier to be sensitive amplifier should have a low noise figure, but most important it must have an exceptionally high linear dynamic range. Your receiver has filters in the front end and IF stages to reduce the bandwidth that the detector sees. This is a tremendous advantage allowing excellent performance even in the presence of a high level signal on an adjacent frequency. The antenna preamplifier does not have a filter so it sees every signal in its bandwidth simultaneously. This is why one active antenna works better than another and why some are more expensive than others.
There are other issues concerning the use of active antennas, full size antennas, preamplifiers and receivers in general. These will be addressed in future application notes.