Sunday, August 1, 2010

I am using a Hustler 4-band trap antenna fed with coax. At the antenna the coax is attached not by a connector ...

Q KT4SP asks, “I am using a Hustler 4-band trap antenna fed with coax. At the antenna the coax is attached not by a connector, but by the shield and center conductor to screws. I assume that this is proper, since the antenna does not have an SO-239 connector. The antenna is mounted about three inches above ground with no radials. I have been using this setup for a number of years, but my power output on phone is only about 45 W from a 100-W transceiver. Would the addition of a 1:1 balun to the system improve or degrade the output? Is this type of vertical considered a balanced or an unbalanced antenna?”

A While a vertically oriented half-wave dipole would be a balanced antenna, a quarter-wavelength vertical—with its “missing bottom half” made up using an image antenna reflected in the ground plane—is indeed an “unbalanced antenna.”

However, just because this is an unbalanced antenna fed with an unbalanced (coax) feed line doesn’t mean that everything is hunky-dory in the installation. The real clue to the nature of this problem is that your system doesn’t have ground radials. Two concerns immediately arise: (1) power reduction in the radio due to common-mode currents and (2) poor radiation efficiency.

If the ground plane or radial system is inadequate under the vertical, then there’s a really good chance that common-mode currents are being radiated onto the shield of the coax cable running on the ground under the vertical. Such common-mode currents can fool a transceiver’s SWR sensor and cause it to reduce the output power. Putting a balun at the feed point may, or may not, reduce the level of common-mode currents, but this isn’t really the proper approach—putting down radials is what is really required here!

Consider this: a quarter-wave vertical with a perfect ground system should show a feed-point impedance of about 36 Ω, and this is only a 50/36 = 1.388:1 SWR—not enough to cause an SWR shutdown unless common-mode currents are involved. Indeed, because of the lack of radials, the feed-point impedance due to losses might be even closer to 50 Ω, even if the radiation efficiency is poor. So, unless common-mode currents are involved, the SWR alone wouldn’t cause a power reduction to protect the radio.

Adding ground radials would improve the radiation efficiency and increase your output power (by eliminating common-mode currents that are falsely activating your radio’s SWR sensor).

From QST October 2000