Emergency VHF antenna

[PuffTheMagicDragon]

I know that some years back the was a feature in PBO about how someone made an emergency VHF antenna.
Does anyone remember roughly when this was? It would save me going through endless issues (well, almost; I have most issues since #1). If I start to do that I would probably end up reading long forgotten articles and not getting anything done!
Thanks.
Fair Winds!

<hr width=100% size=1>Wally

 

[tugboat]

Try August '99, page 112. I'm not sad - I have indexes (sp) /forums/images/icons/smile.gif

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[PuffTheMagicDragon]

Thanks tugboat!
I wish I were well organised and had indices like you!
;<)
Fair Winds (and no condensation)!

<hr width=100% size=1>Wally

 

[Talbot]

hardly rocket science. Keep the old antenna that you replace at the mast and secure it to an old broom handle with a bit of gaffer tape, job done.

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[William_H]

If you send me a PM I will copy to you an article for PBO which was lost in the system or just not to their taste. Any way Talbot is not quite right in that you can stick your old antenna on a broomstick with the antenna rod connected to the inner of the coax however it needs a groundplane. Normally the mast is the groundplane connected to the sheath of the coax. Cars use the metal body of the car as the groundplane. Now you can make a very nifty groundplane by folding the braid of the coax back over the outer insulator to a length equal to the rod antenna (1/4 wavelength) or you can use another piece of braid or even a piece of copper water pipe However the far end of the braid or pipe (usually the bottom end ) is high impedance (high voltage but not dangerous) and must be insulated.
So a simple emergency antenna will have 17.67 inches of the centre conductor with iunsulation of a legth of RG58 coax exposed. The braid should be folded back over the outside insulation of the coax so that 17.67 inches are folded back. (expose more then trim to length)This makes about 35 inches from the tip of the inner to the end of the folded back braid. This will work as an excellent antenna which would be best fittted inside some plstic pipe or even an old fibreglass fishing rod which is mounted as high as possible say on a sternrail. You will need to fit a connector of course to the radio end.
This arrangement can workwell on a MOB of fibreglass as a main antenna. It is well worth making up for use if you lose a mast or the main antenna fails. Here in Oz we use 27mhz for marine com and I use this type of antenna mounted near the transom with a commercial whip antenna outside and the braid folded back inside which comes to around 8 ft long and it works a treat. This type of antenna is what is used in handheld GPS only much smaller due to highrer frequency.
good luck will

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[ccscott49]

My masts are wood, where's the groundplane there? What about ones for motorboats, are they built as you describe, with a dipole inside? If mine is a masthead type on wooden masts, amybe its not working too well? Need to have a look at this! Any advice sort!
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[Talbot]

some of the antenna stand-offs for mast mounting are made of plastic, others are made of coated metal. In neither case will they provide a groundplane. IMHO you only need a ground plane for HF TX where you are using the ground plane as a means of tuning the aerial to the correct length. For VHF, the aerial is already the appropriate length for the transmision

Where is the groundplane on a hand held?

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[ccscott49]

Funnily enough thats what I thought!

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[Talbot]

I have an old aerial, and a spare brrom handle, on eof these days I must give it a try.

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[Oldhand]

Isn't 17.67 inches a bit short? 1/4 wave for channel 16 (156.800MHz) is 20.58 inches, nearly 3 inches for end effects seems a bit excessive. I would have though something around 19.5 inches would be about right.

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[webcraft]

If it's a bit of emergency kit you want to be sure it works when you need it. You can buy commercial emergency aeriels for between £30 and £40.

Before we replaced our rigging two of our first safety purchases were cable cutters and an emergency VHF antenna - about £70s worth of kit to enable recovery from a rig failure was not bad I though.

Have used one of these emergency aeriels in earnest on a charter boat with a dodgy ariel connection at the masthead and it worked fine held above the head in the cockpit.

- Nick

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[LadyInBed]

I know that my £5 2nd hand boat jumble arial mounted on its 3ft plastic pole works ‘cos I've used it. I can talk to Solent and Portland from Studland. I'm not sure what the distance is though as I don’t know where the nearest coastguard arial is, it might be up on Studland /forums/images/icons/blush.gif

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[andy01842]

"Isn't 17.67 inches a bit short? 1/4 wave for channel 16 (156.800MHz) "

I have made many aerials as described by william_h and they work vary well
19 ½ inches works well on the 2m band (145Mhz) so would say 17 ½ inches is right for marine band (156Mhz). The formula I use is 143 divided by the freq in Mhz gives a ½ wave in meters. Divide by 2 for a ¼ wave. I get 18 inches so cut the 0.4 inches off to get the best swr.
andy


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[William_H]

Thanks Andy I was begining to panic.Was my calculation correct? I have never actually used VHF FM marine band though familiar with 144mhz ham band and 120 mhz aircraft band. Now people sell antennas for boats here in Oz (27mhz remember) which have a coil inside them and a tap on the coil for the centre of the coax while the braid goes to I think the bottom of the coil while the top is connected to the spiral wrapped wire on the pole about 2 metres long. The design is specifically for no groundplane and seems to give an acceptable SWR although I don't know how.
Possibly your antenna is designed in that manner but if it is just a piece of rod then you need a groundplane.
The problem for those who talk of my old aerial on a broomstick works fine is that the strength of a received radio signal is perceived in proportion to the square root of the radiated power. A logrythmic sort of effect so that halving the power is not very noticeable. This coupled with the fact that FM transmissions as heard are not indicative of the strength until it is too weak to receive means that your rough antenna could be radiating just a few percent of its intended power and you may not notice the difference until you are on extreme of range. Also in early transistor transmitters it was possible to damage the output amplifier withy a bad antenna although I think modern transistors are more robust and sometimes the amplifiers are designed to throttle back the power to avert damage. It just seems to me that while a piece of wet string may work you should have it working properly. good luck to all and a happy Christmas. will

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[Oldhand]

You should be using 150 as a true starting point for the calculation based on the fact that 1m wavelength is 300MHz. I expect your 143 is to allow for end effects but this is not constant with frequency and also varies with the antennas' physical properties, such as "thickness" of the elements. As marine VHF is not a single frequency you also need to consider the properties necessary for it to give satisfactory propagation over the full frequency range, such as using "fat" elements, which in turn changes the end effect properties. So using 143 is likely to succeed more by luck than design.

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[gjeffery]

Wavelength of emr in free space = c/f
= 2.998 * 10^8 / 156.8 * 10^6
= 1.912 m

c velocity of em radiation in m/s
f frequency Hz

1/4 wavelength = 0.478m (18.81 ins)


But velocity of signal in copper is about 0.9 of velocity in air, so tuned 1/4 wave in copper is 0.430m (16.94 ins)


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[PuffTheMagicDragon]

Thanks William!
Pm on the way.


<hr width=100% size=1>Wally

 

[andy01842]

Luck! Luck I should coco! Hi Hi

Light and radio waves travel at about 300,000,000,000 km per second i.e. 1hz has a wave length of 300,000,000,000 km. Therefore 1Mhz has a wave length of 300m. However full wave antennas don’t work and rf travels slower on the surface of metal than in free space. I use a velocity factor (fiddle factor) of 95%. A larger diameter dose increases the bandwidth. Having said all the above I have found that theory really works in practice. I think 90% is the velocity factor for coax.
p.s did I get the decimal point in the right place?
pps it was a long time ago that I did the rae.
Andy m0cnm


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[LadyInBed]

Re: formula

When I was taught arial theory (God! it was 1965) we were taught 234 / Freq = ¼ wave, but I think the frequncy was in Kcs /forums/images/icons/blush.gif Oh alright, KHz not MHz.

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