Baluns on Log Periodic Antennas
Baluns on Log Periodic Antennas
|
|
Some antenna manufacturers place baluns at the incorrect location in LPDA arrays, or tell you to route the cable incorrectly. This can cause substantial RFI and all sorts of weird problems like RF into house wiring. If we consider what the balun does we can see how the mistake occurs. What's Wrong with the LPDA Feed?Some antenna manufacturers suggest you tape or attach your coaxial feedline back to the mast along the length of the boom. That's a very bad idea in a hot boom antenna like the Tennadyne log. The problem is the boom is electrically hot. The boom is, after all, feeding half the elements!! The very fact the boom feeds elements of the antenna requires the boom to have significant VOLTAGE along its length and current running through the boom. Any cable parallel to the boom, especially one held against the boom by tape or wire ties, 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. Feedline shields will also be excited by boom currents. The feedline really should not be within several feet of the booms, or it should be taped to the shield attachment point boom and treated like a part of that boom.
There are a few ways to correct this problem:
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. 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. This can be done by installing a barrel bulkhead feedthrough connector at the exit point and clamping the connector to the boom. Be sure to waterproof the assembly. The new balun goes between that boom connected barrel connector and the feedline exiting the antenna boom and running down the tower.
If the beam has an impedance matching balun at the feedpoint (a balun ratio other than 1:1), or if you simply want to leave the original balun at the feedpoint because you can't easily reach it, you can leave the balun at the feedpoint. Leaving the balun at the feedpoint generally won't hurt anything. The feedline has to be suspended at least a few feet below the hot booms all the way from the balun unbalanced connector to the tower or mast, or you have to add a second 1:1 current or choke balun at the mast. This will greatly reduce RFI and improve the antenna pattern and gain.
© 2004 W8JI
|
