Center Fed Zepp
Written by Linda KD7NGZ on Friday, 10 September 2004
The center fed zepp is a non-resonant antenna fed with high impedance balanced line to a "remote balun" with a short piece of high quality coax to the antenna tuner. This antenna will work very well on all bands from 160 meters to 6 meters. It will perform equally well in the standard flat top or inverted "V" configuration. The length of the antenna is adjustable to the lot size. For best performance, each leg should be in the order of 1/4 wave length at the lowest frequency to be used.
Note: For 160 meters, the length should be greater than 75 feet on a leg, the closer to 1/4 wave the easier for the antenna tuner to generate the conjugate match. Some tuners may not be able to generate sufficient reactance with shorter antennas on 160 meters to achieve a good match.
The golden rule for antennas is as high as possible and in the clear. For operation on 75 meters and up it is recommended that the antenna be greater than 45 feet above the average terrain for best results. The height above ground will determine the value of the conjugate match and the directivity of the antenna. From 45 to 90 feet, the pattern will resemble an omni-directional antenna. The reflection from the ground creates many minor lobes which fill in the pattern to give the omni-directional effect. As the antenna is raised above ground the minor lobes slowly decrease to the point of 1/2 wave above ground where the figure 8 pattern remains dominant.
Typical dimensions:
Inverted V, the angle of the V should exceed 100 degrees. Less than 90 degrees coupling from the 2 ends takes place and cancellation of the fields, acting like a transmission line.
The center support should exceed 45 feet for best results, the ends elevated 20 feet or greater and terminated in 2 series insulators spaced approximately 6" apart. For best results on 75 meters and above, 130 feet or greater - although 100+ will give good results though harder to match dependent on the tuner.
The feed line can be any high impedance balance line from 300 to 600 ohms. The recommended line is the 450 ohm windowed balanced line. The length is not critical other than it must reach from the center of the antenna to the "remote balun" located near the shack. The recommended installation is to mount the balun just under the roof eaves outside the window of the shack.
From the "remote balun" a coax feeder to the antenna tuner is used. The coax should be of the RG/8 type but of good quality and not to exceed 15 feet in length. For best results the coax should be 10 feet or less. When used in multi-band conditions, this coax can have high SWR resulting in high voltages appearing on the coax so quality coax is essential.
Over the years this antenna has been duplicated more than 100 times and has given better than expected performance in every case.
Some of the positive points are:
- It can be used in a large or in a limited space.
- No test equipment is necessary to tune and resonate the antenna.
- All parts are readily available for construction.
- It is a single antenna that can be used on multiple bands very efficiently.
The down sides are:
- It uses balanced line for low loss but must be strung in the clear of conducting objects.
- An antenna tuner of adequate power rating must be used.
- A specific 4:1 balun (remote) by "Radio Works" must be used.
My thanks go out to Orv K6UEY for the above article originally made in a post in our old forum.