Convert 27mhz aerial to VHF

Am I too late? 27mhz antenna come in 2 types. The ones designed for boats have a coil inside to load the antenna so it looks like it has a groundplane. Typically white fibreglass with a cable going into the side of the f/g tube or a coax connector on the side. There is no place to earth the antenna being made for f/g boats. This type have f/g rod inside with wire wrapped around it and a coil and adjusting capacitor.
To convert this type cut the outer tube near the base and remove the outer tube. Make a coaxial antenna. 17.7 inches of centre conductor with screen removed. The screen is folded back to make a 17.7 inch screen over the outer insulation of the coax. Both lengths are critical. Push this 35inch antenna up as far as poss into the f/g tube with the cable emerging at the bottom. Epoxy the tube back onto the base.

The other kind of 27mhz antenna like I use is just a whip with a metal base (usually black). The wire conductor radiator is wrapped around the f/g rod and all covered in heat shrink or similar. This type is made for a car and requires the coax outer to be connected to a large metal body. (ie mast) The centre of the coax only connects to the base. the antenna has a large chrome plated nut at the base of the rod to screw onto the separate base. ($18 at Dick Smith)
If this latter type is yours then strip all the coiled wire off the f/g rod and replace with a straight wire (or no more than 1 turn) up the rod. The lenght should be 17.7 inches or you could go for 3/4 wave length 53.1inches long. then cover again with heat shrink. A section of coax outer is often used as the conductor. Which brings us to the point that you should be able to buy from DS a VHF antenna of this type for less than $18 but it may still have to be trimmed to correct length. (and seal the cut)

My guess is that it is the former type that you have mounted on a f/g deck. Yes of course you can buy a new antenna. Are you not going to keep the 27mhz radio as well? There is not much sign of getting rid of 27mhz here in Perth. PM me if you don't understand as 27mhz story is not of much use to UK types. ....... olewill
 
Hi Dug. I am guessing you are refering to one of those white smooth fibreglass antenna designed to be mounted on deck or cabin top. It has a coax cable or connector emerging from the side near the bottom and a plastic or alloy swivel base with no connection to earth.

Cut around the f/g tube near the base and the outer tube should pull off.
Discard the f/g rod with wire wrapped around , small coil and capacitor.
if you have a spare metre of coax cable pull it through the base so that it can be pushed up the f/g tube when it is refitted. If it has a cnnector then get another 2 metres of coax 50 ohm RG58 cable. If no connector and no spare cable then buy enough to get to the set plus a metre up into the antenna.
Make a coax antenna by folding back the outer shield wire onto the insulated outer of the coax. The insulated inner remains . The inner should measure 17,7 inches as should the length of folded back shield wire. This whole thing is shoved up to the top of the tube and the tube is epoxied back onto the base.
if you have a VSWR bridge then check it but you will find it has good VSWR.
Are you getting rid of the 27meg marine band radio? You should keep both. certainly here in Perth there is no move away from 27meg.
regarding previous discussion this is a half wave antenna centre fed but presenting 50 ohm load with one half (1/4wavelength) in the bare centre wire and the other half in the folded back shield. or it could be described as a 1/4 wavelength radiator with 1/4 wavelength groundplane.
PM me if you want more details................olewill
 
Re: I certainly don\'t advise it ...

[ QUOTE ]

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./quote]

I totally disagree. To expalin reflections in RF doesn't require any understanding of electronics at all, it requires an understanding of RF transmission line theory.
 
I remember an alternative to this dipole from my ham days. Think it went like this. Cut one quarter wave element but also cut a quarter wave length of copper tube of internal diameter greater than the coax. Run the coax up through the tube and connect the screen to the end of the tube and the inner to the end of the element. Mount vertically with the element pointing up and the tube pointing down (with the coax running down the inside). I believe the idea was that the tube and the outer of the coax running through it form a quarter wave transformer and eliminate the need for a balun? (Hope I got this right - it was all a long time ago)

On the other hand it's a lot simpler just to buy an antenna.
 
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