Unidirectional V Beam Antenna

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Long Wire / End Fed Wire Antenna Includes:
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One of the big drawbacks for the basic V wire beam antenna is the fact that it is bi-directional - its main lobes are along the axis of the antenna in both directions.

Often, especially for receiving, a uni-directional antenna is needed to remove unwanted interference and the like.

Fortunately, with a simple change to the basic antenna, it is possible to make it uni-directional in a direction along the axis and away from the feed point.

Unidirectional V beam antenna basics

The unidirectional V beam antenna differs from the bidirectional version in having a terminating resistor to a virtual ground or low voltage point.

This makes it into what can be termed a travelling wave antenna where the direction of the wavefront and hence the radiation is towards the termination at the remote point.

In theory, the termination would be to a ground point, but in reality, this is not always feasible if the remote end of the antenna is to remain high.

To overcome this a quarter wave section is placed beyond the terminating resistor.

This works because the quarter wavelength section is open at the far end or end away fromt he resistor. As an open circuit at this point will be a high voltage point, a quarter wavelength away from this becomes a low voltage, high current point.

Using Ohm's Law, a low voltage and a high current means that the point at the resistor is a low impedance, i.e. a simulated ground.

The value of the resistor is normally taken to be around 500Ω. Naturally the resistors should be non-inductive otherwise they will not act as resistors.

They should have a power dissipation capability of 20% of the anticipated maximum power, and obviously they need to be weatherproof, or contained within a weather-proof casing.

Feed impedance

The feed impedance of both the standard bidirectional and unidirectional antennas is high. Typically a 600 open wire feeder may be used, although some have used 450Ω ribbon / twin feeder. If this is used, make sure it is the black weatherproof variety and not the opaque stuff which is not intended for this type of application and absorbs water.

Typically the antenna has a surprisingly wide bandwidth and may be used over, possibly the top bands in the HF portion of the spectrum, for example, dependent upon the length - possibly a minimum of 30 metres or more for frequencies of 15 MHz and above.

As the feed impedance will change with frequency, it is very wise to use not only a balun, but also an antenna tuner to ensure any transmitter does not see a very high standing wave ratio.

Likely performance

The unidirectional V beam antenna works very well. Typical gain figures can be well above those for an antenna such as a Yagi, although the size is obviously larger.

Very good fron to back ratios are often experienced. Figures of 20dB and more have been noted.

Having terminating resistors on both legs of the antenna does mean that the forward gain is less for the unidirectional antenna than for the bidirectional one because of the dissipated power.

This can be mitigated if required by having the capability to short out the resistors when required. For example to preserve the transmitted power, the terminating resistors could be removed from circuit while transmitting to ensure the strongest signal, even though it radiates in two directions.

Then for receive to remove interference and reduce the noise level, the terminating resistors can be introduced.

Alternative arrangements

The original layout had the quarter wavelength section in line with the rest of the antenna. To save space the the quarter wavelength section can b incorporated into the guying system for the antenna masts.

Provided the antenna is over a quarter wavelength high, which it normally would be, this scheme can work very well, enabling the quarter wavelength wires to have a dual purpose.

The unidirectional V beam antenna can be an ideal system for anyone with plenty of space and masts on which to erect antennas. Typically the centre or feed point would normally be made the tallest, and the others coud be lower to meet the needs and budget for any system like this.

However, for most of us, the size might be too large to fit into the space that might be available, but it is good to understand the concept and to be able to contemplate it should the opportunity arise.

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