How do I select a VHF antenna for ORRA?

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Family_Dog
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Sat Jun 25, 2011 8:54 pm

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

* Zara130
* Zara60


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 103 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.


Zara60 specifications
Zara 60 S5 specs.pdf
(149.87 KiB) Downloaded 120 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.


-F_D
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Royco
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Sat Jun 25, 2011 9:06 pm

Probably a noob question :tease: , but does the shorter quarter-wave antenna limit the distance that the radios can communicate?
My (uneducated) guess is that the longer the aerial the greater the distance between radios.
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Jaco Versfeld
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Sat Jun 25, 2011 9:12 pm

Hi Eric,

Thanks for the explanation.

One question, with the Hilux club licenses, we have two bands, which are a bit removed from each other. Would it be possible to cut the Zara130 so that it matches the one frequency perfect, and the other frequency will still be in the 9MHz bandwidth?

With this setup, one channel will have a nice long range, say if you are two vehicles overlanding in Richtersveld / Nam (with proper permits) etc, and the other one would be if you are driving convoy with a group and the vehicles are not likely to go over a 10km range.

Thanks,
Jaco

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Sat Jun 25, 2011 10:12 pm

Royco wrote:Probably a noob question :tease: , but does the shorter quarter-wave antenna limit the distance that the radios can communicate?
My (uneducated) guess is that the longer the aerial the greater the distance between radios.
Roy, a quarter-wave antenna radiates equally in all directions. Imagine a perfectly round ball emanating from the antenna. A gain antenna has slightly better propagation in view of its greater radiating length. Imagine placing your hand on top of the imaginary ball around the quarter-wave antenna and pressing down on top of it - the previously round shape now resembles a doughnut-shape. In other words, the sides stretch out a bit further.

In theory, it does not quite work that way. A 5/8 Zara130 would theoretically be better in the middle of the Karoo where the earth is perfectly flat but in hilly countryside, the quarter-wave antenna wins hands down.

The current two ORRA frequencies make a Zara130 a no-no for the reasons stated above in my opening post.


-F_D
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Sat Jun 25, 2011 10:26 pm

Jaco, all you require to convert a Zara130 VHF antenna to a Zara60 antenna is to remove the existing coil and replace it with a Zara60 spring (R92.00). Then, use the existing whip from your old antenna, after first cutting it to around 465mm in length.

That's it! And that setup will outperform the single frequency Zara130 antenna on both the current ORRA 1 & 2 (and also the new ORRA 3) frequencies. And the shorter whip length looks neater too.

Our vehicles at work are all fitted with quarter-wave antennas and we can trigger any of our repeaters in excess of the maximum permissible 50km radius from the repeater. The only limitation is the curvature of the Earth.

Refer the following pics for parts identification:
Antennas.jpg

-F_D
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mdp100
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Sun Jul 31, 2011 5:25 pm

Royco wrote:Probably a noob question :tease: , but does the shorter quarter-wave antenna limit the distance that the radios can communicate?
My (uneducated) guess is that the longer the aerial the greater the distance between radios.
I'm not an expert on this field but as far as I know you can damage your radio if you don't use the correct antenna's or if it is not cut to the correct spec's. Just make sure from FD if I am correct....

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Sun Jul 31, 2011 5:30 pm

Hi FD. Just want to know something from you please. If I get my ORRA license will you program my Motorola GM 340 for me with the ORRA channels?

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Sun Jul 31, 2011 8:32 pm

Marius, absolutely! Do the application in the meantime, I am awaiting details of the new ORRA 3 frequency in the interim.


-F_D
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White Fang: 1999 2.7i DC Raider 4x4
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Sun Jul 31, 2011 10:28 pm

mdp100 wrote:I'm not an expert on this field but as far as I know you can damage your radio if you don't use the correct antenna's or if it is not cut to the correct spec's. Just make sure from FD if I am correct....
Thanks Marius. I was just asking out of curiosity.
My radio and antennae (1/4 wave) comes from Mr F_D so they are perfectly matched. :yahoo:
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Mon Aug 01, 2011 9:23 am

after first cutting it to around 465mm
Hi Eric does this include the 20mm extra?i see my one is longer :problem:
Lets hope the 3rd frequency is sitting in the middle of the other 2
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Mon Aug 01, 2011 11:48 am

George, if you have the shock spring (not the Zara130 coil) then you can trim the whip length to 450mm, as we take the length of the spring into consideration as well. Remember, currently we are tuning the antenna to a 'midpoint' frequency, one for which the radio is not actually programmed. It appears that your antenna is trimmed to the old ORRA Ch 1 frequency.

All this might change slightly when ORRA releases its new frequency, but the new frequency is compatible to one of the existing frequencies so it is highly probably that we will trim the antenna whip to a total length of around 465mm including the shock spring.


-F_D
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White Fang: 1999 2.7i DC Raider 4x4
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Mon Aug 01, 2011 12:22 pm

Family_Dog wrote:George, if you have the shock spring (not the Zara130 coil) then you can trim the whip length to 450mm, as we take the length of the spring into consideration as well. Remember, currently we are tuning the antenna to a 'midpoint' frequency, one for which the radio is not actually programmed. It appears that your antenna is trimmed to the old ORRA Ch 1 frequency.

All this might change slightly when ORRA releases its new frequency, but the new frequency is compatible to one of the existing frequencies so it is highly probably that we will trim the antenna whip to a total length of around 465mm including the shock spring.


-F_D
Thanx Eric.I will trim my antenna to 450mm.How long do you think the new channel will come out?maybe i must wait.
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Mon Aug 01, 2011 1:42 pm

Family_Dog wrote:George, if you have the shock spring (not the Zara130 coil) then you can trim the whip length to 450mm, as we take the length of the spring into consideration as well. Remember, currently we are tuning the antenna to a 'midpoint' frequency, one for which the radio is not actually programmed. It appears that your antenna is trimmed to the old ORRA Ch 1 frequency.

All this might change slightly when ORRA releases its new frequency, but the new frequency is compatible to one of the existing frequencies so it is highly probably that we will trim the antenna whip to a total length of around 465mm including the shock spring.


-F_D
Eric don't you trim using a watt box? :wth: :shock2:
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Mon Aug 01, 2011 3:53 pm

Yep, but I'm in Klerksdorp & George is in that other city deep down south somewhere ;)

We use a wattmeter, but in this case the two frequencies are so far apart that we have trimmed the antenna for the mid-frequency between them. The radio itself is not tuned to this frequency at all. So a bit of slack either way will not really matter that much as the antenna has a bandwidth of 10MHz, nearly double what we experience currently between the mid-frequency and either of the two existing ORRA frequencies.

George, ORRA will only consider releasing the third frequency - to selected dealers only - from the end of the month, once they have had their next technical meeting. Rest assured, you will all be able to utilise your existing antennas!


-F_D
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Mon Aug 01, 2011 5:36 pm

Family_Dog wrote:Marius, absolutely! Do the application in the meantime, I am awaiting details of the new ORRA 3 frequency in the interim.


-F_D
Thanks F D I'll make a turn as soon as I have everything in order :yahoo:

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Mon Aug 01, 2011 5:38 pm

Royco wrote:
mdp100 wrote:I'm not an expert on this field but as far as I know you can damage your radio if you don't use the correct antenna's or if it is not cut to the correct spec's. Just make sure from FD if I am correct....
Thanks Marius. I was just asking out of curiosity.
My radio and antennae (1/4 wave) comes from Mr F_D so they are perfectly matched. :yahoo:

Nahh... it's cool bru.... we must help and support each other here on the forum..... at least we'll learn something at the end of the day :clap:

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Mon Aug 01, 2011 9:44 pm

Oh Eric, you should also tell the guys that the 3DB 5/8th wave length antenna is long (about 1250mm) and if you mount it on or near your roof, you will keep hitting the florecent tubes in the underground parkings. So the 1/4 wave length antenna (500mm) is the better option for that reason. I have has a 3DB antenna get stuck in an extractor fan in an underground parking, made a hell of a racket and almost pulled the roof off the Land Cruiser when I tried to drive away.
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Tue Aug 02, 2011 7:10 am

I remember many years ago when I first found out about this phenomenon peculiar to long antennas... only that time it was a CB antenna. Not a pretty feeling when you suddenly experience a snowfall of broken glass... :)


-F_D
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White Fang: 1999 2.7i DC Raider 4x4
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Tue Aug 02, 2011 5:54 pm

That long aerials..... maybe 2m or longer..... cant remember exactly..... I can recall when we had those thick long ones on our bakkies, even the longer ones with those blue coils we used for the long distance radios...... those were the days......

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Thu Sep 01, 2011 12:32 pm

Hi Eric.Just a question.(I think I know the answer).I have turned my 1/4 wave antenna horisontal.This is purely for city driving so that I dont take out the lights in the parking lots :blackeye: When I do trails etc. and want to comminicate I will turn it vertical again.

But was just curious to know.Will my receiving and transmitting signal strength be severely affected by this setup? Probably will.
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Thu Sep 01, 2011 12:51 pm

Hi George,

With the antenna turned vertically, you changed the polarization of the transmitted electromagnetic waves as well. Unless the receiving antenna is also turned vertically, you will not have effective communication.

The antenna that you have is an omnidirectional antenna. The radiation pattern looks like a donut, where the antenna is in the middle of the donut. So, when you turn the antenna, you turn the "donut" on its side.
george wrote:Hi Eric.Just a question.(I think I know the answer).I have turned my 1/4 wave antenna horisontal.This is purely for city driving so that I dont take out the lights in the parking lots :blackeye: When I do trails etc. and want to comminicate I will turn it vertical again.

But was just curious to know.Will my receiving and transmitting signal strength be severely affected by this setup? Probably will.
01092011.jpg

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Thu Sep 01, 2011 12:56 pm

Thanks Jaco. That is a very good explanation. :clap: :clap:
Jaco Versfeld wrote:Hi George,

With the antenna turned vertically, you changed the polarization of the transmitted electromagnetic waves as well. Unless the receiving antenna is also turned vertically, you will not have effective communication.

The antenna that you have is an omnidirectional antenna. The radiation pattern looks like a donut, where the antenna is in the middle of the donut. So, when you turn the antenna, you turn the "donut" on its side.
george wrote:Hi Eric.Just a question.(I think I know the answer).I have turned my 1/4 wave antenna horisontal.This is purely for city driving so that I dont take out the lights in the parking lots :blackeye: When I do trails etc. and want to comminicate I will turn it vertical again.

But was just curious to know.Will my receiving and transmitting signal strength be severely affected by this setup? Probably will.
01092011.jpg
"The World is a book, and those who do not travel read only a page.-Saint Augustine"

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Thu Sep 01, 2011 6:50 pm

George, exactly as Jaco stated. But don't worry, you will have excellent communication with the Cosmonauts & Astronauts in the sky, and the Aquanauts below you ;)


-F_D
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White Fang: 1999 2.7i DC Raider 4x4
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