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The Norfolk Broads Forum / Broads Boat Owners Q & A / Radio Reception
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Posted By Discussion Topic: Radio Reception

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jimfin
Jul-17-2017 @ 6:34 PM                           Permalink
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We, like most of us, have a car radio fitted to our fiberglass boat.
I don't seem to get a very good signal. Sometimes I have had static and when I put my hand close to the radio the signal improves and the static goes.
Someone told me at some time that it was something to do with the areal not being earthed like on a car!!!
Can someone advise me please.

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TerryTibbs
Jul-17-2017 @ 6:44 PM                           Permalink
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What sort of ariel do you have it connected to or is it just the radios inbuilt antenna?

Dave

Je suis Charlie

jimfin
Jul-17-2017 @ 8:06 PM                           Permalink
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Cheep flexible car type.

If you don't use your boat. Sell it.

ADI
Jul-17-2017 @ 8:55 PM                           Permalink
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I have a rubber duckie car style aerial in side the front toilet compartment attached to the window frame pointing at 45 degrees, works really well hardly ever looses signal.

Regards

Adrian  Michelle

Beck  Braydon and Mere.

woodwose
Jul-17-2017 @ 10:15 PM                           Permalink
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Mardles sometimes
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Car radio aerials are a type of aerial known as a ground plane. In these aerials, the metal body of the car is part of the aerial. These aerials don't work well when they don't have this metal ground under them. This is particularly true when you feed such an aerial with co-axial cable. In this case, the cable acts as a capacitor and is probably the reason why you have poor reception.

There are various ways of dealing with this:

1. Provide some sort of artificial ground. A wire mesh would do it but, in fact, you can manage with just a few horizontal wires cut to one quarter of the wavelength of the signal you are after (about 76cms). Even one horizontal wire might be enough. Connect the inner conductor of the co-ax to the aerial and the outer to the artificial ground.

2. Remove the co-axial cable and replace it with a single wire. Connect the inner conductors only. In this case you could probably do away with the aerial altogether and just use the wire.

3. Replace the aerial with a different type which does not need a ground plane. Some kind of center fed dipole would work well.

I would try method 2 first. It has the advantage of being cheap and easy to do.

Hope this helps

Nigel
Ham radio station G4AXA





This message was edited by woodwose on Jul-17-17 @ 11:24 PM

Regulo
Jul-17-2017 @ 10:16 PM                           Permalink
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Mine is an electric car aerial. Not earthed separately, but the shield of the cable is earthed by the body of the radio. Good reception always.
Nigel just got his post in before mine. I wonder if the hefty metal body of the motor unit on mine is acting as a ground plane? Radio is not my speciality - if I plug it in and it works, that'll do!

Regards, Ray.

Whatever happens now, I'm blaming it on Brexit. Everyone else is!


This message was edited by Regulo on Jul-17-17 @ 11:19 PM

woodwose
Jul-17-2017 @ 10:51 PM                           Permalink
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I will just expand slightly on what I said.

These aerials do not need an "earth". Ground plane aerials are really vertical dipoles where the bottom part of the dipole is a reflection in the ground plane. This has the advantage of making them only half as tall since a full size dipole would be about 1.5m tall for it to be resonant in the FM broadcast band.

The outer conductor of a co-axial cable is not a shield or a screen. Rather it acts with the inner conductor to form a transmission line.

You can easily make a dipole aerial with two bits of wire about 76cms long. It is OK to feed this with co-ax. Connect one wire to the inner conductor and the other to the outer conductor. Some people would suggest you need a balun but you don't really need one. The aerial can be horizontal or vertical because the Tacolneston transmitter is mixed polarisation. Keep the aerial away from metal bits of the boat.

Hope this is further clarification.

Nigel



This message was edited by woodwose on Jul-17-17 @ 11:52 PM

jimfin
Jul-18-2017 @ 10:26 AM                           Permalink
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Thanks for the reply Nigel. OK your saying do away with the co/ax and run one single wire from the center areal output on the radio to the areal. What type on wire? Bear or covered?
You mention about making an areal or Dipole. How do I calculate the size I need for FM reception. Just a little confused.

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woodwose
Jul-18-2017 @ 12:26 PM                           Permalink
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Hi Jim

You can use any length of wire. The longer the better. It will not be resonant anyway so the length is not important. I would use insulated wire as this is generally more convenient and will not connect to any metal objects it passes. You probably don't really need the aerial on the end, but it can be connected and might add a bit extra.

If you make a dipole, it needs to be a resonant length. The correct length is between 75 and 76 cms per quarter wavelength. This website shows a simple method. https://www.electronics-notes.com/articles/antennas-propagation/dipole-antenna/fm-dipole-antenna.php

Nigel
Ludham

jimfin
Jul-31-2017 @ 1:38 PM                           Permalink
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Still a little confused. Maybe an age thing!!
OK. The length is to be 76cm. Is this 76cm the full length of the top of the T or 76cm on either side?
At the ends of the top of the T do I solder the braid to the center wire?
Think I can work out the rest!!

If you don't use your boat. Sell it.

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