VHF antenna length

[jmp]

Just noticed the top of my Vitronix has been broken off .A friend has told me that it will not work unless I adjust the length to some multiple .He says there are various set lengths in harmony with VHF wavelengths.

There is still 36" left above the electronic mount .

Any help /advice would be appreciated.

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[jeanne]

There is a factual basis for the comment of your friend, but it may not apply in this case. Because the VHF band covers a range of frequencies, no length will be the right proportion of a wave length for all of them. The manufacturers usual way of dealing with this is to put a matching circuit in the base of the aerial, which makes the aerial 'look' like the right length. This may still be coping with the mismatch.If you have any 'Ham' friends, a SWR meter can measure the efficiency of an aerial in a couple of ticks. Otherwise I would listen for calls which you know are distant, and must be weak, and if you can receive them, then try tx on low power first. Older radios blew their output stages if transmitting without a good aerial. I would hope [but don't know]that a modern set would just shut it down, with no damage. On the other hand, a new aerial is a lot cheaper than a new VHF.

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[paulrossall]

I asked afriend of mine who had some sort of electronics qualification a similar question and his considered reply was that if a chap talked bollarks then he would talk bollarks no matter how long his ariel was! I would not worry too much but make sure you seal the end of the ariel with sikaflex or something similar. Hope this helps.

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[jmp]

Re: VHF antenna length rePaulrossal

Just returned from PUB .At least Jeanne came up with some useful info.
Had some problems with my 36" last season but obviosly bollARks got in the way of bollOCks.Will see if after buying all the Bombadier tonight I can afford a new antenna to save my ICOM ! unless I get more help from experts in the field.

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[trays]

I thought that the ideal is 1/4, 1/2, of full wavelength. However, I presume it is not too critical becaue of the range of frequencies involved. Am I right?

Ray

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[alpha]

Yep, you'll get away with 'just any length', as you'll still be producing some sort of signal - you'll still be able to call the marina from two miles off, for example...

BUT, to get the best range you need a good antenna, and the time to wish you had replaced the broken one is NOT when you are putting out a MAYDAY because you're sinking fast, nor when you sit retching in the raft waiting for assistance which is not coming because the boat which would have helped you is out of range of your broken antenna.

My advice? Relace the broken antenna with a new one, of quality. I know it's a pain, but there it is.

All the best,

alpha

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[byron]

<font color=blue>Buy a new aerial and be done with it. I will say this, my boat already has two 9' aerials and I need yet another one, so to prevent my boat looking like a pin cushion I bought a 'rib' aeria. About a foot high with a sort of springy base. It works better than my flashy whip 9' aerials.

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[summerwind]

As I understand it, several of the above posters are right in that different frequencies require different length antennas. However, the VHF antenna that most of us use is the correct length for channel 16. The further the frequency gets away from channel 16, the less efficient the antenna becomes.

It follows then that having lost a bit of your antenna, your ch 16 trtansmission will not be maximised and you might therefore seriously consider replacing it.

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[dickh]

I thought the normal victronix whip aerial for masthead is about 36" long - how much has broken off? - compare it to a new one. - unless you have one of those white motorboat type aerial.

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[jmp]

Thanks all who have responded,or will in future .

Shows how other views concentrate the mind.Obviously I should buy new ! was thinking of avoiding the work.
Alpha has to be right.

Is there a BEST antenna for a yacht mast height 38'. Built a boat in 80's and used a short bendy coil type similar I guess to Byrons suggestion,which seemed to work,although things may have come a long way since then .
Any suggestions with details of a good source appreciated.

PS The mast comes down each year to bring boat home with mast,and vulnerability is some consideration .

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[jmp]

Re: VHF antenna length re Alpha ,Byron & others

Thanks all who have responded,or will in future .

Shows how other views concentrate the mind.Obviously I should buy new ! was thinking of avoiding the work.
Alpha has to be right.

Is there a BEST antenna for a yacht mast height 38'. Built a boat in 80's and used a short bendy coil type similar I guess to Byrons suggestion,which seemed to work,although things may have come a long way since then .
Any suggestions with details of a good source appreciated.

PS The mast comes down each year to bring boat home with mast,and vulnerability is some consideration .

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[byron]

Re: VHF antenna length re Alpha ,Byron & others

<font color=blue>The one I described which is made for use on a RIB they are built to take a whacking. Cheap too.

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[Strathglass]

Re: VHF antenna length re Alpha ,Byron & others

I will try to answer your Best aerial question. A little bit long winded.

The radiation pattern of a low gain aerial (say 3dBi) looks a bit like a doughnut with a tiny central hole around the aerial. If you take a slice of this doughnut you get two circles just touching each other touching with the aerial running vertically between them.
The transmitted power is represented by the distance from the junction of the two circles and the circumference of the circles. In other words at 90 degrees to the aerial the maximum signal is obtained and at 45 degrees about half the power is available. This drop in power is not significent and would only just be noticed at extreme range.

This means that if you use a low gain aerial on a yacht then there would be no significent change on VHF range when sailing upright or on your ear at 45 degrees.

You don't get anything for nothing. As you increase the gain of an aerial then the doughnut progressively expands but it also compresses and you end up with higher power at right angles to the aerial and a dramatic loss of signal at any angle of heel. This is fine for some powerboats and perhaps cats but a high gain aerial has no place on a yacht.

A good compromise for a yacht is to go for an aerial with a relatively short stainless whip that is quoted by the supplier as having 3dBi gain. This will give good performance at most angles of heel.
If you go for one quoted at 0dBi (such as a short helically wound aerial) then the maximum range will be slightly reduced but the signal will remain constant even with the tip of the mast is almost touching the water.
If you go for one quoted at 6dBi this would increase the maximum range slightly IF you are sailing upright but as you heel the range could drop dramatically.

On the question of length the most usual end fed aerial is slightly shorter than a quarter of the wavelength in metres.
This is normally optimised for channel 16.
A half wave aerial is normally centrally fed and the next end fed aerial is aproximately 5/8 of a wavelangth.

Sorryabout being so long winded.

Iain


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[Oldhand]

The marine VHF frequency band is very narrow and is thus relatively easy to design an antenna to cover the range. Any reduction of its length will tune it to a higher frequency range and it must be replaced. Most transmitters expect to be looking into a load (the antenna) with a VSWR of 2:1, which maens that at least 50% of the transmitter power is radiated. The energy not radiated is reflected back down the feed cable into the transmitter output stage. This causes heating of the oputput transistor and if excessive, it will blow up.

Even if your transmitter can withstand the poorer VSWR of your "modified" antenna, the radiated power will be reduced and thus your transmissions may not be decipherably received at longer ranges. Reception efficiency will also be reduced but this will be less obvious. The life of your transmitter is also likely to be reduced.

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[dickh]

Some are better than others - there was a comparison some time ago in PBO but can't remember the result except that more expensive did not necessarily mean better. Some have a fixed wire into the aerial some have a screw in type fitting, so if you take your mast down each year maybe this is the best. However when I did this, all I did was reversed the antenna so it lay vertically downwards and just a piece of tape to stop it getting caught.
If you are interested I'll see if I can dig out the report.
V-Tronix are a well known make as well as Shakespeare.

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I'd rather be sailing... /forums/images/icons/smile.gif