Some antenna manufacturers place baluns at the incorrect location in LPDA arrays. If we consider what the balun does we can see how the mistake occurs.

In the case of antennas, a balun isolates the shield of the cable from excitation by voltage differences between the feedline's termination and ground. In one case the termination might be a balanced dipole, with each terminal connection terminal having voltage to "earth". If the dipole is balanced and has 100 volts across the feed terminals, the feedpoint would have about 50 volts to an imaginary ground at the antenna.

The above current is for a 1/2 wave dipole 1/2 wl above ground, with coaxial feed. You can see the feedline shield has significant current. In this case about 40% of the antenna's maximum current!

We can simulate a choke balun by adding a current source in series s with the shield, and setting current for zero amperes. The voltage across that current source will indicate the common mode voltage exciting the feedline.

Adding a perfect choke balun, current on the feedline shield goes to zero and the voltage across the choke balun is now found from the source menu:

Source 1 Voltage = 61.02 V. at -0.01 deg.
Current = 1 A. at 0.0 deg.
Impedance = 61.02 - J 0.009274 ohms
Power = 61.02 watts
SWR (50 ohm system) = 1.220 (450 ohm system) = 7.375

Source 2 Voltage = 37.42 V. at 173.27 deg.
Current = 0 A. at 0.0 deg.
Impedance is infinite
Power = 0 watts
SWR (50 ohm system) > 100 (450 ohm system) > 100

Source 1 is the actual terminal excitation of the dipole, while source 2 is the source that cancels common-mode feedline current. We can see the voltage across the perfect balun is indicated by source 2 as 37 volts, slightly more than 1/2 the feedpoint differential voltage. (We could also use a very high impedance load here, and do the same thing.)

What's Wrong with the LPDA Feed?

Some antenna manufacturers have you tape or otherwise attach the coaxial feedline along the length of a boom that is electrically hot, the boom feeding half the elements!! The very fact the boom feeds elements of the antenna requires the boom to have significant VOLTAGE along its length. Any cable parallel to the boom is excited by boom currents and voltages. Cables routed against the hot boom will have whatever the voltage is between the boom and the tower trying to excite current in the feedline shield.

If the log has a 1:1 balun at the feedpoint, you can simply tape the coax to the boom that the shield connects to without any balun at the log feedpoint. The best thing to do is to ground the coax shield to the hot boom at the exit point where the coax leaves the boom, and install a balun at that point! You can do this by installing a barrel feedthrough connector at the exit point and clamping the connector to the boom, then waterproofing the assembly. The balun goes between that connector and the feedline exiting the antenna boom and going to the station.

If the beam has an impedance matching balun at the feedpoint, or if you simply want to leave the balun at the feedpoint, you can leave the balun at the feedpoint. The feedline has to be suspended a few feet below the hot booms.    

since 6/21/04

© 2004 W8JI