Where is the best place to mount a VHF antenna

[SKRIK NIKS]

where is the best place to mount a antenna on my bakkie ,to get the best range ?
any input will help

[CasKru]

Best... in the middel of the roof

Antenna.JPG (54.03 KiB) Viewed 3123 times

[CasKru]

Other options but less effective...

C-Pillar
B-Pillar
Magnet mount on roof

Last and least effective..
On the bullbar

[niclemaitre]

Depends what you want to do.

For transmitting and receiving over long distances a large "ground plane" (a sheet of metal under the antenna - i.e. your roof) is very beneficial. So mounted in the center of your roof gives you a nice symmetrical ground plane. If you mount to the side the ground plane will be poor in one direction but better in the other so careful orientation of your vehicle may be needed to improve transmission and reception over long distances. This is important if you are planning on using your vehicle with HAM radios or in rescue/logistics. If however you are using your radio for chatting in convoy or maneuvers on a 4x4 course it will make little practical difference.

What is of great importance is getting your radio antenna "tuned" (cut to the right length) for the frequency you will be using and having your antenna as vertical as possible as these will improve transmission and reception markedly.

[smokintyres]

What is of great importance is getting your radio antenna "tuned" (cut to the right length) for the frequency you will be using and having your antenna as vertical as possible as these will improve transmission and reception markedly.

Surely a competent radio sales person should be able to provide the correct length of antenna required? Or is that something that only a radio Guru would be able to provide?

[CasKru]

What is of great importance is getting your radio antenna "tuned" (cut to the right length) for the frequency you will be using and having your antenna as vertical as possible as these will improve transmission and reception markedly.

Surely a competent radio sales person should be able to provide the correct length of antenna required? Or is that something that only a radio Guru would be able to provide?

Correct yes. But keep in mind that an antenna is tuned to a specific frequency. So the further you move away from that frequency the less effective the antenna. Some antenna are more capable than others in handling a wider range of frequencies but it can only be tune to one frequency optimally.

[smokintyres]

Correct yes. But keep in mind that an antenna is tuned to a specific frequency. So the further you move away from that frequency the less effective the antenna. Some antenna are more capable than others in handling a wider range of frequencies but it can only be tune to one frequency optimally.

Thanks Cassie

[niclemaitre]

Standard VHF whip antennas are 2.5m long, and need to be tuned to the wavelength of the frequency that you will be using. The salesman should be able to do it for you yes. Or you can work it out yourself if you know what frequency you will be using. The equation is: wavelength = speed of light (in meters/second) / frequency (in hertz). So for 147.875 Mhz or 147875000 hz

Wavelength = 299792458 m/s / 147875000 hz
Wavelength = 2.02.... m

So your antenna should be 2.02m long.

Longer is better than shorter, if you will be using multiple frequencies, tune it for the one with the highest MHz.

[CasKru]

Easier to remember is 300 / MHz which gives you the wavelength.

[smokintyres]

Standard VHF whip antennas are 2.5m long, and need to be tuned to the wavelength of the frequency that you will be using. The salesman should be able to do it for you yes. Or you can work it out yourself if you know what frequency you will be using. The equation is: wavelength = speed of light (in meters/second) / frequency (in hertz). So for 147.875 Mhz or 147875000 hz

Wavelength = 299792458 m/s / 147875000 hz
Wavelength = 2.02.... m

So your antenna should be 2.02m long.

Longer is better than shorter, if you will be using multiple frequencies, tune it for the one with the highest MHz.

As easy as that... Thanks!

[Thunder02]

Got to through a spanner in the works,would a alli roof rack not help with the reception

[CasKru]

Got to through a spanner in the works,would a alli roof rack not help with the reception

Depending if it's a solid structure or slotted

[Thunder02]

Got to through a spanner in the works,would a alli roof rack not help with the reception

Depending if it's a solid structure or slotted

Would it not matter that it's horizontal

[CasKru]

Got to through a spanner in the works,would a alli roof rack not help with the reception

Depending if it's a solid structure or slotted

Would it not matter that it's horizontal

The rack or the antenna?

[Thunder02]

Depending if it's a solid structure or slotted [/quote]
Would it not matter that it's horizontal [/quote]
The rack or the antenna?[/quote]
The rack.

[CasKru]

You want your base to be perpendicular to the antenna (that is how I have it) to get the best radiation pattern

[Mud Dog]

As I understand, the size of the ground-plane and its position relative to the antenna makes a difference as well. For example, if you mount the antenna on the rear cab pillar, the antenna is basically on the corner of the ground-plane (best position is in the centre, but who wants to drill a hole through the roof), then you might find that transmission / reception is better from the direction opposite the ground-plane. Then, if the antenna is mounted in such a pillar, the base of the antenna is below the level of the ground-plane and might require a few more centimetres in length than one mounted on the roof to achieve the best tuned length. Same scenario applies to mounting on a roof-rack ... usually mounted on a side. Eric can advise here but in days past, the pro's used to trim the antenna using an oscilloscope to get the best tuned length.

[Thunder02]

As I understand, the size of the ground-plane and its position relative to the antenna makes a difference as well. For example, if you mount the antenna on the rear cab pillar, the antenna is basically on the corner of the ground-plane (best position is in the centre, but who wants to drill a hole through the roof), then you might find that transmission / reception is better from the direction opposite the ground-plane. Then, if the antenna is mounted in such a pillar, the base of the antenna is below the level of the ground-plane and might require a few more centimetres in length than one mounted on the roof to achieve the best tuned length. Same scenario applies to mounting on a roof-rack ... usually mounted on a side. Eric can advise here but in days past, the pro's used to trim the antenna using an oscilloscope to get the best tuned length.

Thanks Andy

[Family_Dog]

Easier to remember is 300 / MHz which gives you the wavelength.

That formula is close enough for all practical purposes. That will result in a full-wave antenna which will be roughly 2M long, so we trim the antennas to a quarter-wave which allows a lot of leniency in respect of various frequencies (10MHz bandwidth - necessary for ORRA 1,2 & 3) and results in a length of only ±470mm. Refer the antenna on Cassie's roof.

So, for ORRA frequencies (all three of them: 300/157/4 = 470mm. Subtract about 50mm for the spring if you use a Zara60. You cannot just trim a Zara130, you will need to replace the coil with a spring.


-F_D

[Thunder02]

Thanks Eric,never a case of fitting something and off you go,but that's what's great about 4x4 ing

[smokintyres]

Easier to remember is 300 / MHz which gives you the wavelength.

That formula is close enough for all practical purposes. That will result in a full-wave antenna which will be roughly 2M long, so we trim the antennas to a quarter-wave which allows a lot of leniency in respect of various frequencies (10MHz bandwidth - necessary for ORRA 1,2 & 3) and results in a length of only ±470mm. Refer the antenna on Cassie's roof.

So, for ORRA frequencies (all three of them: 300/157/4 = 470mm. Subtract about 50mm for the spring if you use a Zara60. You cannot just trim a Zara130, you will need to replace the coil with a spring.


-F_D

So there is a bit more to it than a simple formula, especially when several different frequencies are to be used The antennas I have seen on most leisure 4x4s must have been around half a meter. Hopefully I will have the funds to purchase a two way radio some time this year, will probably get back to this topic once a frequency has been assigned to me.

Glad to have such a resourceful forum at my disposal

[Family_Dog]

Gernot, as a Club member, you can apply directly through the Club. There is more information on obtaining a radio licence in the "Club" section.


-F_D

[smokintyres]

Thanks Eric, but it won't be of much use in Namibia, or am I missing something?

We already have a license for two way radios (so far only handheld type) for use on the plot in case of fires, burglaries etc. The frequency allocated to us is for the central region only though, so for purposes of fitting a two way radio to the car; I will have to apply for another frequency... At least that's how I understood it.

[Family_Dog]

Yep, sorry, I had not noticed you were in Windy Corner. The normal VHF licence is is granted for a specific area, usually 50km around your home address. At least that applies to SA, but most likely to Namibia as well. You could ask them to extend the area coverage, i.e. you wish to use the frequency in other parts of the country. They may agree to this or they may offer you a new frequency, both of which are at additional cost.


-F_D

[niclemaitre]

Ground plane:
With a centrally mounted antenna (black circle):

center.jpg (39.24 KiB) Viewed 2968 times

Off center antenna:

off-center.jpg (35.19 KiB) Viewed 2968 times

Longer arrows indicate better reception/transmission directions.

[SKRIK NIKS]

so the centre mounted is better

[Cleaner]

Easier to remember is 300 / MHz which gives you the wavelength.

That formula is close enough for all practical purposes. That will result in a full-wave antenna which will be roughly 2M long, so we trim the antennas to a quarter-wave which allows a lot of leniency in respect of various frequencies (10MHz bandwidth - necessary for ORRA 1,2 & 3) and results in a length of only ±470mm. Refer the antenna on Cassie's roof.

So, for ORRA frequencies (all three of them: 300/157/4 = 470mm. Subtract about 50mm for the spring if you use a Zara60. You cannot just trim a Zara130, you will need to replace the coil with a spring.


-F_D

Apologies if I high-jacked the thread with the below - Mods are welcome to move to a new thread if required -

Based on the above - if I understand correctly - and looking at the below -

Zarra Type Antenna

Zara Antenna Types.JPG (41.75 KiB) Viewed 2948 times

Lenght

I have a Zarra 60 Antenna and it should be about 420mm long (just the whip - including the spring about 470mm) - Do I understand correctly? Because my Antenna is currently 1250mm long - including the spring! It might explain why I get very clipped reception from other mounted radios, though it seem that from handhelds I have no issue.

Would it be recommended to cut the whip to 420mm? I have an extra whip and can do this to see if the reception improves.

[MOFASA]

I have a Zarra 60 Antenna and it should be about 420mm long (just the whip - including the spring about 470mm) - Do I understand correctly? Because my Antenna is currently 1250mm long - including the spring! It might explain why I get very clipped reception from other mounted radios, though it seem that from handhelds I have no issue.

Would it be recommended to cut the whip to 420mm? I have an extra whip and can do this to see if the reception improves.

I would say cut it and see what happens but what radio do you have??
Best you speak to Eric and see cos he is the expert....

[CasKru]

Easier to remember is 300 / MHz which gives you the wavelength.

That formula is close enough for all practical purposes. That will result in a full-wave antenna which will be roughly 2M long, so we trim the antennas to a quarter-wave which allows a lot of leniency in respect of various frequencies (10MHz bandwidth - necessary for ORRA 1,2 & 3) and results in a length of only ±470mm. Refer the antenna on Cassie's roof.

So, for ORRA frequencies (all three of them: 300/157/4 = 470mm. Subtract about 50mm for the spring if you use a Zara60. You cannot just trim a Zara130, you will need to replace the coil with a spring.


-F_D

Apologies if I high-jacked the thread with the below - Mods are welcome to move to a new thread if required -

Based on the above - if I understand correctly - and looking at the below -

Zara Antenna Types.JPG

Lenght.JPG

I have a Zarra 60 Antenna and it should be about 420mm long (just the whip - including the spring about 470mm) - Do I understand correctly? Because my Antenna is currently 1250mm long - including the spring! It might explain why I get very clipped reception from other mounted radios, though it seem that from handhelds I have no issue.

Would it be recommended to cut the whip to 420mm? I have an extra whip and can do this to see if the reception improves.

Jou Antenna moet nader aan 500mm (seker so +-495mm) wees volgens my somme.

[Family_Dog]

Ernst, we cut them at anything between 455-470mm, allowing for a bit of leeway. With the length of the spring to the base of the antenna added, it comes very close to the 500mm length Cassie mentioned , correct for the mid-frequency of the ORRA range. Ideally, one should measure for reflected power after the antenna has been mounted. 420mm is too short.


-F_D

[Cleaner]

Thanks all! I'll trim a whip and test to see if it then works better!

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