Ground Plane for Bands 14, 18, 21, 24, 27 and 28 MHz

From a detailed study of gamma matching (γ-match) it follows that γ-match can be used not only for resonant radiators, but for vibrators of almost any length, with the exception of severely shortened ones.

A close examination we can discover a very interesting property of gamma-matching: the physical length of the γ-match tube almost does not change when changing the height of the grounded ground plane from 0.15λ to 0.21λ and from 0.28λ to 0.6λ.

It follows that if you choose a grounded tube size that falls within these limits in all the desired bands, it is possible to get the active part of the antenna input impedance Ra close to 50 Ohm with the physical dimensions of both the GP itself and the tube γ-match unchanged. Of course, the reactive part of the input impedance of JXa will vary with frequency and the capacitor γ-match capacitor will have to to adjust as the band changes. But in this case we'll settle for that. It is not difficult to tweak one variable capacitor. The resulting antenna is shown in the following figure.

This grounded vertical transmitter is 4.5 m high with a diameter of 20 mm and The matching tube is 1.32 m high with a diameter of 8 mm at a distance of 150 mm from the vertical. All dimensions are relatively critical, except for the diameter of the matching tube, which it can have a diameter of 6 to 10 mm.

By a variable capacitor C1 10...70 pF, the antenna can be tuned in resonance on amateur bands 14 MHz, 18 MHz, 21 MHz, 24 MHz, 27 MHz and 28 MHz with SWR   < 1,35.

And in the 18...29 MHz band, the antenna can be tuned to any frequency with SWR < 1,35.

In the 15 ... 17 MHz it is impossible to obtain a low SWR, because the height of the of the antenna is close to a quarter wave and this does not fall within the aforementioned allowable for such matching vertical heights. But in the band 14 MHz the GP height is reduced to 0,2λ and tuning is again possible.

Tuning the antenna when changing bands is very simple: rotate the variable capacitor C1 to a minimum of SWR. I did this by hand. But nothing prevents you from using Variable capacitor C1 without the and a small geared motor to rotate it.

With a 1 kW transmitter power, the distance between the C1 plates should be at least 3 mm.

SWR by bands:

Directional diagrams by band:

An example of this antenna implemented by Sergei, UB1QBD, is shown in the photo below:

 

 

Antenna modifications

 

Adding band 10,1 MHz

A trap 14,18 MHz 20 uH \ 6,3 pF is added to the top of the mast, and a piece of thin wire 51 cm long is pulled away from it with a fishing line. The variable capacitor C1 must be increased to 130 pF.

The SWR of such an antenna in the 10 MHz band is shown in the following figure:

The trap is designed as a single layer spaced coil. Since the calculated capacitance is very small, depending on the design of the coil and its own capacitance, the additional capacitance of the circuit should is only 2...5 pF. This capacitance can be achieved simply by placing the coil leads of the coil next to each other (by passing them inside the frame). The trap must be tuned to 14.18 MHz before mounting it on the antenna.

The bandwidth in the 10 MHz band is narrow. Therefore, when installing the antenna, it is necessary to adjust either the trap coil or the length of the auxiliary wire or the angle of that wire to the mast.

The other bands are almost unaffected by the 10 MHz addition, only a slight the minimum SWR at 21 MHz increases.

  p> 

Doubled version of the antenna for 7, 10 and 14 MHz bands

The mast is 9.25 m high is made of a telescopic set of pipes with diameters from 30 mm at the bottom to 14 mm at the top. The gamma coaxial loop is connected at a height of 2.7 m and is made of 2 mm diameter wire.

SWR by bands:

 


First published: 24.08.2005

Latest revision: 01.05.2023

Main page