Convert 27mhz aerial to VHF

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snoozydude

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Gidday

Is it possible to use simple handyman tools (hacksaw and hammer -multimeter) to convert one of the standard a 1.8 metre 27mhz whip aerials a VHF aerial.

Will the 27MHZ aerial work to some degree of satisfaction on a VHF radio without modification?
 
Technically you will get experts answers all saying wave length .... etc. etc.

In a previous office I used a CB base loaded aerial on a VHF set .... fixed on the roof. It actually gave us more reception than the original correct VHF one.

Only mod we made was to remove the magnetic base and fix to a L bracket ....
 
It may be possible on some CB aerials but would be technically most difficult.
The reason is that most CB aerials have a loading coil built in (often hidden in the base) to increase the effective length to that required to resonate at 27mHz.

This loading coil on its own would be too long to enable the aerial to resonate at the much higher than the 150-160 mHz required for marine VHF.

It may be possible to get some sort of match by stripping a CB aerial all down and essentially manufacturing a VHF aerial from some of the parts.
But, without the required technical knowledge and the test equipment the result is more likely to be a bad mismatch and a damaged VHF transmitter.

Iain
 
I certainly don\'t advise it ...

As an antenna should be matched to use ...

But in ignorance used the CB antenna on the VHF set - a standard Midland 5600 ..... that set is till working 10 years later - albeit with correct antenna now - the CB one took a lightning strike and blew it to bits !! Funny thing is the VHF set survived ....

Unlike the Sailor set that melted itself down a ships bridge bulkhead after a strike in Port Harcourt Nigeria ......
 
Re: I certainly don\'t advise it ...

I wouldn't try it without a VSWR meter (if you don't understand VSWR you shouldn't be operating the radio at all)

Modern RF transistors have a much lower die area than a twenty year old set and thus take a lower power return to over heat. So a CB antenna may have worked with little dameage on an old set, but could fry, or set of the protection mechanism on a newer set.
 
Yes. You have all you need to make a perfect resonant dipole.

With the hacksaw cut the whip into 2 lengths each 0.47m long.
Join them with an insulating material or clamp them to some plastic with the ends a few mm apart (last time I did this I used the barrel of a biro) Connect braid of your coax to one arm, centre of your coax to the other. This connection should be made in the middle of the dipole onto the ends of each arm.

If you are feeling creative you could incoroporate a so239 socket into the connection in the middle.

Operate vertically.

Should give excellent results.
 
My 1st response to this was a single word beginning in b and ending in x, with ll in the middle!


However, on mature reflection:

Strictly speaking you are correct that it 'should' be required. However, at VHF the effect of a balun is negligable. Don't ask me why, but it is. I have been using an antenna like this (cobbled together from an old VHF TV antenna) on 2m for over a year and the SWR is tiny. It is very widely done, and no-one uses a balun.
 
Yes that's theoretically correct as a true dipole at resonance, has an impedance of 75ohms...however in practice with cable losses, K factor, and a bit of tweaking at the ends they very often work out nearer to 50 Ohms than 75 and give something like a 1.2 or 1.3:1 match which is fine.

In my PMR days it was more common to use folded dipoles which although better are more difficult to match to 50 Ohms.

Steve.
 
At the risk of this spiralling down into a radio geek thread....

Impedance matching is a separate issue. A 1:1 balun would seem like a logical way to go to connect an unbalanced feed to a balanced dipole - it just does not seem to help in practice -presumably the losses in the balun are greater than the benefits of the conversion.

As for impedance, 75 Ohms is close enough! There are a number of tricks to improve the match, but realistically it is not an issue.
 
Without a balun the balanced (antenna side) to unbalanced (coax side) transformation will result in the transmission line radiating thus becoming part of the antenna. The result will be a reduction in the horizontal gain of the antenna (which may not be a bad thing on a yacht which heels, but is not so on a power boat).

The dipole being vertical will also result in coupling into the transmission line further increasing radiation from that.

It will work, but not as well as a proper antenna installation, and as long as matched will not likely be a problem at the low power output of a pleasure boat's VHF. But in radio terms it could only be regarded as a lash up and is not something I would do myself considering my life might rely on it.

John
 
[ QUOTE ]
The dipole being vertical will also result in coupling into the transmission line further increasing radiation from that.

[/ QUOTE ]

Good point, I should have said that the dipole should be stuck on the end of an arm and the coax led along it so that it does not dangle down close to the antenna.

However I think the answer to the original question is yes! Would I rely on a lash-up like that as a permenant fitting on a boat ? Absolutely not. But I don't think that was the question.
 
Also it should be remembered that mounting a Dipole off a metal structure (mast) then it should be at least 1/4 wavelength away otherwise quite wierd and unhelpful radiation patterns can result.

Steve.
 
Re: I certainly don\'t advise it ...

[ QUOTE ]
(if you don't understand VSWR you shouldn't be operating the radio at all)


[/ QUOTE ]

Are you serious?

Should people be able to listen to Radio 4 if they don't know how a radio works?

Will you explain to me about VSWR so I can use my radio, I do not want to throw it away?
 
Re: I certainly don\'t advise it ...

A broadcast band FM receiver for Radio 4 shouldn't be radiating.

If a VHF transmitter is being used, for what is essentially Safety of Life operations, surely the OP should know what a sudden jump in VSWR means. I was once on a yacht where the owner complained of having to use 25W at all times and couldn't use the radio for long periods. His VSWR was about 18 (i.e. 90% power reflection).

VHF sets need to work, and the operator needs to know how to check they're working.
 
Re: I certainly don\'t advise it ...

I understand the concept.

What you are saying is that everyone should spend time in college learning RF.

To get to the point of being able to explain reflections in RF means a fundamental understanding of electronics. It is a good thing if you do have understanding, but not essential. There are many boaters out there who do not know how to wire up an NMEA circuit, explain in simple terms how high VSWR conditions can occur.

The fact you can buy off the shelf and antenna with cable and connector to plug straight into the radio does mean people do not need to understand the finer intricacies of RF theory.

I can't make sails, should I buy a Mobo?
 
Re: I certainly don\'t advise it ...

So to summarise and add a bit of my own.

If you need to ask the question, don't bother. Buy a Marine VHF antenna. You'll need to strip the CB antenna and re-manufacture with the bits.

You can make anything radiate if you have the right knowledge and equipment. How efficiently it will radiate is another question all together.

And of course you can operate a marine VHF if you don't know about VSWR. That's because you buy it and the antenna, follow the installation instructions and then use it. You don't have to have any test gear to measure VSWR according to your licemce. So, if you can't measure it, how can you tell if it's good or bad. Even if you could you just replace the antenna and/or cable anyway. To operate a marine VHF, you don't have to design it or the antenna first. In fact if you do, it won't be CE marked and you'll be breaking the terms of your ships licence.
 
Re: I certainly don\'t advise it ...

No, perhaps they don't need to understand how the reflections occur, but they need to understand that if you do buy the kit off-the-shelf, then it's important that the install is good, and it's important that the OP knows what the signs of it going wrong are. In the example I gave the Op was outputting 25W but radiating 2.5W.

What I would like to see is the VHF SRC course extended by an hour to include a basic DIY antenna, matching, and what to look for faultwise.
 
Gidday
Thanks for all you replies - I still have to get my VHF licence and from your answers, it looks like I should do some more study into radio operation.

I would not like to blow up my new radio through ignorance.
 
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