How to select a VHF Radio Antenna

Communication & Navigation Equipment: VHF, UHF, GPS, Satellite phones ETC.
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Family_Dog
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How to select a VHF Radio Antenna

Post by Family_Dog » 10 Mar 2018 19:53

This is a subject that bears some discussion - and the reasons for selecting the correct antenna for any VHF radio.

The following applies mainly to the two ORRA frequencies currently in use.

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The most common types of VHF antenna are the Zara range. There are two types that concern us here:

* Zara130 (commonly known as a 5/8 gain antenna)
* Zara60 (commonly known as a quarter-wave antenna.)
The Motorola quarter-wave antenna referred to in the price list is a recommended alternative for the Zara60 if you want a roof-top mount antenna.

The Zara130 is designed as a 'gain' antenna for VHF, that 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.


"My dealer recommended that I use the Zara130 antenna for VHF"

Yes, normally that would be correct - but only for one 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 the current ORRA frequencies which are 8.5MHz apart from each other.


Refer the Zara130 specifications in the attached PDF chart.
Zara130 Specs.pdf
(115.41 KiB) Downloaded 22 times
Now if you look at the Zara60 specifications, you will see 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.


Refer the Zara60 specifications PDF file.
Zara 60 S5 specs.pdf
(149.87 KiB) Downloaded 28 times
Bottom line is that a properly tuned Zara60 quarter-wave antenna will outperform a poorly tuned Zara130 five-eighths antenna where a wide variation in the VHF frequencies is experienced.


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Re: How to select a VHF Radio Antenna

Post by Jorrie » 10 Mar 2018 19:59

Eric

Thanks for the info. It was really helpful!
Jorrie
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