VHF ANTENNA SELECTION - WIDE BAND

Repcom Communications
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Family_Dog
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Real Name: Eric
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Wed Aug 17, 2016 7:09 am

The most common types of VHF antenna are the Zara range. There are two types that concern us here: The Zara130 and the Zara60. The Zara130 is designed as a 'gain' antenna for VHF, which means it offers a slight 2dB gain compared to a conventional quarter-wave antenna. The 'gain' is obtained by having a larger (typically ±1000mm whip length thereby offering a larger radiating surface and hence more RF into the surroundings. But it has disadvantages when there are a multitude of frequencies to consider.

"Is the Zara130 antenna not better for VHF?"
Not in this case. The Zara130 is commonly used for a single VHF frequency or two or more VHF frequencies *provided* that they lie adjacent to one another, i.e. within 1.5MHz of the centre frequency, or not more than 3MHz spacing between the lowest and the highest frequency. The antenna with the "open" spring is a Zara130 which is designed for VHF 136-174MHz. It is cut to a specific length for a frequency. The open spring is actually a coil, not a spring (although it does serve that purpose as well) and the total effective antenna length comprises the actual length of the coiled up steel as well as the antenna whip length. The problem is that it has a bandwidth of only some 1.5MHz either side of the centre frequency and is therefore totally unsuitable for the current ORRA frequencies which are wide-spaced at 8.5MHz apart from each other.

The Zara60 specifications state that it carries a bandwidth of 9MHz, so if we tune it midway between the 8.5MHz split, each frequency is only 4.25MHz away from the antenna, which is well within its specifications. This means that the antenna now radiates both frequencies much more efficiently that a Zara130 would. The spring in the Zara60 antenna functions only as a spring and not as a coil, the antenna whip length represents the full quarter-wave length of the radio signal.

Bottom line is that a properly tuned Zara60 quarter-wave antenna will outperform a poorly tuned Zara130 five-eighths antenna where a wide variation of the VHF frequencies is experienced. As far as possible, Antennas should be mounted as high as possible for optimum performance, taking care that the Whip does not come into contact with roof carriers or any other metallic object.


-F_D
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JohanW
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Wed Aug 17, 2016 10:35 am

Thanks for the Info Eric. Its a very common mistake made by a large number of people. Just because its bigger doesn't mean it works better. ;-)

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