Intermodulation

Active antenna sensitivity

The sensitivity of an active antenna is the relationship between the field strength E in V/m to the voltage output into a 50 Ohm feeder - this is sometimes referred to the efficiency of the antenna.

With a whip antenna, the efficiency can be a straight comparison of the antenna capacitance and the amplifier input capacitance. However, this comparison does not necessarily relate to the antenna sensitivity, V out versus field strength V/m. Furthermore, when making this comparison, the gain of the amplifier has to be considered.

In military and professional circles, active antenna sensitivity is usually expressed as the Antenna Factor K. An antenna K factor of 1, is 1 volt output into a 50 feeder for a field strength of 1V/m.

Normally, the K factor would range from 1 for sensitive antennas down to 0.1 (-20dB) for low sensitivity antennas used in high signal environments such as warships.

Some people would argue that the Antenna Factor is not important because, by simply having a more sensitive antenna, there will not be an improvement in reception due to the level of external noise. Whilst this is true, it is useful to know what the absolute sensitivity is, so that the user can decide the required intermodulation performance in respect to the signal strength from powerful transmitters.

Another very important consideration is the K factor or sensitivity should be lower for long wave, so that megawatt transmitters don’t overload the active antenna or the receiver. A common problem with some active whip antennas, is far too high sensitivity for long wave.

For example, the field strength of a 1MHz 500kW transmitter at 30km is 100mV/m, a 2MW transmitter at 160kHz would produce a field strength of approximately 15dB higher or 560mV/m. Conversely, the field strength of the 160kHz 2MW transmitter would still be 100mV/m at over 100km.

A well designed active antenna would reduce the Antenna Factor to approximately 0.1 at 150kHz. One other benefit of reducing the Antenna Factor is to prevent the receiver from being overloaded. In fact, most so called reception problems encounted with good quality active antennas are due to receiver overload rather than poor antenna performance.

The ALA 1530 Antenna Factor is approximately 1.0 at 30MHz gradually reducing to 0.4 at 300kHz.

Below 300kHz the Antenna factor reduces to 0.1 at 150kHz

Intermodulation products generated by some active antennas can degrade reception of weaker signals and swamp the receiver with a mush of noise and spurious signals. Intermodulation will appear as several stations chattering together in parts of the bands which are the sum and difference of frequencies normally in the Broadcast bands. For example the strong stations in the 31 and 25 metre band (9.5-10MHz and 11.5-12MHz) will seriously degrade reception in the 15m Ham band and the 13m Broadcast band. It is also possible for two very strong signals within the SW Broadcast bands to produce intermodulation products at medium and long wave frequencies. Also intermodulation caused by strong medium and long wave stations can degrade reception of the 1.7-3MHz band.

These intermodulation products fall into two categories, 2nd order and 3rd order. There is a certain amount of misunderstanding as to whether the 2nd or 3rd order intermodulation performance is the most important. Unfortunately, it is some times stated that because the 3rd order performance is usually a problem with receivers then the same is true for active antennas. This is not the case, because most quality receivers have front end filters to reduce 2nd order intermodulation, therefore only the 3rd order will be a problem. Furthermore, Broadband active antennas do not have any filtering to reduce the 2nd order intermodulation and as the 2nd order usually appears at lower signal levels before the 3rd order is noticable, hence the 2nd order intermodulation performance is critical. Unfortunately some active antennas have mediocre 2nd order intermodulation performance. In some cases, active antennas are advertised without specifying the 2nd order performance.

The intermodulation performance required, depends mainly on the signal output level of the antenna (K factor) in respect to the field strength of the strongest signals. The usual measure of the intermodulation performance is the output intercept point of which the 2nd and 3rd order are referred to IP2 and IP3 respectively. Although in some cases the input intercept point is used. The following example is used to convert the intercept point into meaningful figure:

The expected maximum output of an active antenna is 22mV or -20dBm. Then ideally the intermodulation products or spurious signals should not exceed the atmospheric and manmade noise level. However, as this varies for different locations, then a good starting point should be 1uV or -107dBm.

Therefore the required intercept point IP2 is:

IP2 = 2(max. signal output dBm) - (required intermodulation level dBm)

IP2 = 2(-20dBm) - (- 107 dBm)

IP2 = + 67 dBm

 

Therefore the required intercept point IP3 is:
IP3 = 3(max. signal output dBm) - (required intermodulation level dBm)
2

IP3 = 3 (-20 dBm) - (-107 dBm)
2

IP3 = + 23.5 dBm

The above example illustrates the importance of a high 2nd order intercept requirement and that a high third order intercept point is not really necessary. However, in some circumstances where a large single signal could cause blocking or desensitising, then a higher third order intercept may be necessary to ensure a higher dynamic range. It is important to note, that the 2nd order intermodulation varies by a 2:1 ratio of the signal output i.e. a 10 dB decrease in the signal output will reduce the intermodulation by 20 dB. Conversely a 10 dB increase would increase the intermodulation by 20 dB. The 3rd order intermodulation varies in a similar way except that the ratio 3:1 i.e. a 10dB change in the signal produces a difference of 30 dB in the intermodulation. It is also important to note, that some active antennas can have a high third order intercept point and still provide mediocre performance due to second order intermodulation.

Most reputable active antenna manufacturers will supply antennas an IP2 and IP3 greater than 66dBm and 30dBm respectively. In fact this intermodulation performance is far greater than most medium priced receivers.

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