The G5RV antenna

  • G5RV antenna
    G5RV antenna

THE G5RV ANTENNA ( PART 3 ) CONSTRUCTION TIPS THE FLAT-TOP:

 

The dimensions of the G5RV flat-top are specified in Part 1. The antenna does not need to be put up as a flat-top array, but can be installed as an inverted-V. The center of the antenna should be as high as possible, of course, and the matching section should descend at a right angle to the array.

 

It is recommended that the smallest wire gauge used for the flat-top be #14, although wire as small as #18 could be used. If the antenna is raised as an inverted-V, the included angle at the apex should not be less than 120 degrees.

 

THE MATCHING SECTION:

It is recommended that the matching section be constructed of open-wire feeder for minimum loss, as it always carries a standing wave on it.

 

Due to the standing wave on it, the actual impedance is unimportant. A satisfactory construction technique for the open wire line matching section would be to make your own spreaders out of scrap lucite, or similar plastic of low dielectric loss.

The plastic strips would be cut about 2 inches long, 3/8 inch to 1/2 inch wide, and be notched on the ends to fit #14 wire.

The spreaders would be drilled about 1/2 inch in from each end for the binding (tie) wires, and the spacers would be spaced 12 inches center-to-center.

 

The next most-desirable matching section would be made from window-type open wire line, either 300-ohm, or 450-ohm. This is basically a ribbon line, like heavy duty TV-type twin lead, with #16 to #20 wire, and "windows" cut in the insulation every 4 to 6 inches.

 

The advantage of the "window" line is that the conductors won't short together if the line twists in a high wind.

 

Lastly, and the least desirable, (although it will work), is "TV- type" twin lead. The main disadvantage of the TV-type twin lead is durability. The conductors on the twin lead are usually #22 to #28 gauge, and the plastic used for the insulation deteriorates faster in the sun and/or rain. The advantage of it is that it is readily available at electronics outlets, or even most department/home improvement stores.

 

The quality is proportional to price, if a choice is available. Do not use the "shielded" twin lead. The shield will degrade the matching section, especially on 3.5 or 7 MHz.

 

MATCHING SECTION LENGTH:

The length of the matching section is an ELECTRICAL half-wave on 14 MHz. The actual physical length is determined by the following formula:

L= (492 x VF)/f (MHz), where VF is the velocity factor of the matching section.

 

The velocity factor is determined by the type of line, and the dielectric properties of its insulation. For the three types of line discussed so far, the VF is:

 

  • Open wire - .97
  • "Window" line - .90
  • "TV" twin lead - .82

 

By substituting the VF in the formula, and calculating for a center frequency of 14.15 MHz, you come up with the following matching section lengths:

  • Open wire - 34 ft
  • "Window line - 30.6 ft
  • "TV" twin lead - 28 ft

 

This matching section is connected to the center of the array, and allowed to descend vertically at least 20 ft or more, if possible. It can then be bent and tied off to a suitable post or line, and connected to the coaxial line, which is run to the shack, and the ATU.

 

THE FEEDER:

In the original article descibing the G5RV antenna, publushed in the "RSGB BULLETIN" for November 1966, it was suggested that, if a coaxial feed was used, a balun might be employed to provide the nessessary balanced-to- unbalanced transformation at the base of the matching section.

 

However, later experiments, and a better understanding of the theory of operation of the balun indicated that such a device was unsuitable due to the high reactance in the load presented at the base of the matching section. In a nutshell, DON'T USE A BALUN ON THE G5RV !!!!!

 

If a balun is connected to a 2:1, or higher SWR, its internal losses increase. The result is core heating and/or saturation. If saturated, the core can actually distort the RF wave, generating harmonics, and in extreme cases, with QRO, the core and balun can burn up (literally). An unbalanced-to- unbalanced ATU can accomodate the variable load, and cancel out the reactance present. It will also tend to reduce any harmonic energy present, which will, due to the multi-band nature of the G5RV, tend to be radiated. In general, the automatic ATU's in modern rigs will load the G5RV on all but the 10 MHz band.