MFJ-1778 G5RV Multiband Antenna
1
MFJ-1778 G5RV
160 through 10m Antenna
Introduction
The G5RV antenna is a multi-band center-fed dipole antenna capable of 1500 Watts. It was originally
designed to operate as a 3/2 wavelength center fed antenna for 14 MHz, but amateurs soon discovered
it would offer reasonable performance on many other bands. The MFJ-1778 (G5RV) consists of a 102
foot flat top fed with a 32.5 foot 450 ohm matching section ending in an SO-239 coaxial connector.
The 450 ohm balanced transmission line serves as a 1:1 transmission line transformer on 14 MHz. The
SWR is approximately 2:1 at 14 MHz. The same balanced line section acts like a transmission line
impedance matching section on the other HF bands. This antenna generally requires the use of a
suitable matching network (antenna tuner) since the SWR of the G5RV is almost certainly not 1:1 on
any band. The use of a tuner will guarantee maximum performance from solid state exciters.
The G5RV is a balanced antenna that is fed by an unbalanced coaxial line. This feed system causes
unwanted parallel RF currents to appear on the shield of the coaxial feedline. A choke type balun
should be used with this antenna to eliminate or reduce the undesirable parallel feedline currents.
Failure to use a choke balun may result in RFI, RF feedback, or other symptoms of RF "in the shack".
The Balun Requirement section describes how to construct a choke balun for this antenna.
WARNING:
Always mount this antenna so that it is out of the reach of adults and children.
Contact with any part of this antenna can cause RF burns or other injuries.
Theory Of Operation
A description of how the G5RV functions on each band follows:
80 meter band:
On 80 meters the MFJ-1778 (G5RV) functions as a slightly short
half-wave dipole. The current distribution of the antenna and
matching line is shown in figure 1. The horizontal radiation
pattern is the same as a half-wave dipole on this band. With
horizontal polarization at heights below 100 feet the antenna is
practically omni-directional and has a high takeoff angle.
40 meter band:
On 40 meters the MFJ-1778 (G5RV) functions as "two half-
waves in-phase" fed with a quarter-wave impedance matching
transformer. The current distribution of the antenna and
transmission line impedance matching section is shown in figure
2. The horizontal radiation pattern is somewhat narrower than the
pattern of a half-wave dipole and is broadside to the radiator on
this band. The G5RV has a small amount of gain and radiates at
medium wave angles on 40 meters.
Figure 1
Figure 2
