Antenna Facts and Myths

1) Relays

There seems to be a belief that relays used to switch RF along the length of an antenna must be very large. The fact is that the current near the center of a 50 Ohm antenna is only about 5.5 amps at 1500 watts. The current is only 7 amps at 2400 watts. This current changes very little as the frequency moves away from the resonant frequency of the antenna. The current increases to 5.9 amps as the frequency is lowered to the 3:1 SWR point. The current decreases to 5.1 amps as the frequency is increased to the other 3:1 SWR point.

The current along an antenna element becomes less away from the center and reaches zero at the end of the element. For the MacTenna Simple Beam, the first relay is approximately half way between the center and the end of the element. At this point the current is less than 80% of the current at the center. At 1500 watts into 50 ohms the first relay must carry only 4.4 amps. This current is split between 2 relays that each carry only 2.2 amps. The relays used in MacTenna antennas are rated at 10 amps. The relays that are located further from the center must carry progressively lower currents than the relay closest to the center. In the case of the Simple Beam the currents in the director and reflector are even less than in the driven element. Because the current is so low the 1500 watt rating of MacTenna antennas and switching modules is a continuous "key down" rating.

The real challenge for the relay is the voltage that the contact must withstand when it is open. The voltage on an antenna element is lowest near the center and highest on the end of the element. The voltage is highest at the center frequency of the antenna and becomes less at higher SWR levels. The voltage increases with the square root of the power level. This results in only a 41% increase in voltage each time the power level is doubled.

When the relay closest to the end of the antenna is open, there is still a wire segment between the open relay contact and the end of the antenna. Due to capacitance between this wire segment and the active end of the antenna, a voltage divider is formed which reduces the voltage across the relay contact. This rather complicated "end effect" makes it very difficult to calculate the actual voltage across the relay contact with any accuracy, and it is virtually impossible to measure. This situation has been resolved through extensive testing at power levels well above the published rating.

2) Circuit Survival

We have seen comments on forums that the circuitry driving the relays on the antenna elements is sure to fail due to static discharge or EMP (electromagnetic pulse) caused by nearby lightning strikes.

I have seen several antennas totally destroyed by direct lightning strikes. The whole world seems to recognize that lightning is a powerful force of nature that is not easily tamed by man. This is why lightning is not a warranty issue but an insurance issue.

Please watch our video of a high voltage generator discharging into the MacTenna circuits.

In normal operation the circuitry on the module closest to the end of the antenna has approximately 1800 volts of RF connected to it when operating at 1500 watts. This voltage is connected to both the positive side and the negative side (called rails) of the circuitry. The RF voltage across the components is essentially zero. A voltage that is applied equally to both sides of a circuit is called a "common mode" voltage and has no effect on the circuitry. Damage to the circuitry can only be caused by a difference in voltage between the positive and negative parts of the circuitry. This type of voltage is referred to as a differential or "normal mode" voltage. It is almost impossible to create a differential voltage across a circuit hanging in the air, but just in case, there is a transient surge suppressor across the positive and negative rails of every switching module.

EMP or static discharge can easily damage matching networks. It is very difficult to protect components that must operate at significant differential (normal mode) RF levels. Transient surge suppressors can not be used due to their high capacitance which forms a virtual RF short circuit. Inductors can be used to drain away a slow static discharge, but they provide little protection against fast rise time events. Note that the MacTenna Simple Beam does not require a matching network because of the basic 50 ohm design.

3) Wide Bandwidth and Losses

We have seen comments on a particular forum that the Simple Beam must have high losses since the 10M bandwidth is so wide. The wide bandwidth is actually caused by the action of the unconnected antenna segments when all relays are open. Because of the end effect capacitance of each unconnected segment, the segments actually increase the bandwidth and provide a small additional collinear gain. This effect is most pronounced on 10M and becomes less on the lower frequency bands. At 20M there are no unconnected segments beyond the end and the bandwidth is the same as any full size 20M beam.

This same effect is present on the MacTenna wire dipole antennas.