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MG MGB Technical - Principles of radio aerials

Right - got as far as buying a good second hand Radiomobile 1070 MW/LW radio for my 1967 mgb roadster. In order to check it works, I bought one of those foot long black rubber aerials - when I hold the aerial I get a very good strong MW reception but naturally, as soon as I let go of it, the signal dissapears.

Before I go and drill a hole somewhere on the bodywork (the thought fills me with dread though), can anyone with some technical knowledge provide me with a paragraph on what the aerial needs in order to pick up signal, where it should be to get a good reception etc so I can try and make the best decision before drilling.

Out of interest, I removed the map reading light and mounted the aerial through the hole in the dash to see what would happen - when you are in a very strong signal area you can hear enough of the programme being recieved but you need to turn the volume right up - touching the aerial boosts the volume by about 500% but naturally you can't drive around holding the aerial! Would a standard extendable metal aerial be better?

Many thanks in advance.
Martin

A radio aerial composes two parts - the rubber wire or metal mast and the ground plate. It needs both to function properly - your hanging on to the aerial increases the size of mast and the car you are sitting in forms the ground.

The ground plate is ideally a flat sheet of steel at right angles to the mast. On a car this is normally either the roof or a wing.
So the best place is in the middle of the bonnet or boot on a roadster!

Most people go for the front left hand wing as it is furthest from the coil and distributor and suffers less interference. Putting it behind the under wing splash plate also means it is protected from dirt etc.

An alternative, if you do not want to drill holes, is a windscreen mounted active aerial - this has a power supply and amplifier to boost the weak signal picked up by the antenna.

Chris at Octarine Services

Would it be possible to use the windshield center rod, properly insulated of course, as a antenna?
James Johanski

What about the boot (trunk) rack as an aerial.
Bill

Chris is spot on as usual . I always fit MGB aerials exactly where the factory said to fit them . RHS wing as viewed from the rear , ( offside in UK. ) There is a grommeted hole available in the inner wing to route the feeder . I used to have some dimensional info that ensured the ae cleared the inner wing . I think I have lost it now , but could measure my car.
The lenght does not seem to be critical , they usually have coil loading in the bottom which makes them look longer , and they need the earth plate to contact the outer on the cable , I always grease the area where the earth bites into the metal , for obvious reasons. The monopole is the "reflected" in the gound plane and it looks to the reciever as though it is working into a dipole .I bought a stainless steel one last time , and it has paid off, giving years of service . Some 1070s had a polarity reversing switch on the power supply , always worth checking before you connect it up .
S Best

Bill

Many years ago Dodge imported a Mitubishi and rebadged it as a Dodge Colt. It used the entire boot (trunk) lid as the antenna, carefully insulated from the rest of the car. Don't know how well it worked.

FWIW

Larry
Larry Hallanger

If you choose to create your own "hidden" antenna, just observe what Chris and S Best have mentioned about grounding the lead-in shielding on the antenna lead. If is not grounded at both ends of its run, you are assured of better than average ignition and other noise into your receiver.

Also, if you do use something like the center rod for the windshield as the antenna, make certain that the outer shield braid is as close to the joint of the center lead wire connected to the center post as is possible. You'd be surprised how little unshielded exposure of that center lead wire it takes to pick up spark noise.
Bob Muenchausen

Hi all.

It is certainly possible to use an electrically isolated part of a car as an aerial.

One word of caution.. if the insulation between the 'aerial' and the bodywork is thin, there will be capacitance from the 'aerial' to chassis, which will effectively short circuit the 'aerial'. The capacitance increases as the surface area increases, so something like a boot lid would need quite a wide insulated gap.

Some examples :

If you have two pinheads separated by a 1mm gap, there will be insignificant capacitance between them.
If you have two frying pans separated by a 1mm gap, there will be much more capacitance between them.
If you increase the gap between the frying pans to 20mm, the capacitance will reduce to a low figure.

The more metal you have exposed to the signal field, the greater the signal it will receive, so longer is better until you reach resonance, which you certainly won't do at MW / LW frequencies (resonance at several tens of metres).
A loading coil is an attempt to bring the aerial to resonance, which is worthwhile, but is no substitute for antenna length.

Don't forget that most MW / LW radios have a trimmer to match the aerial to the receiver input.

Heated rear windows will serve as an antenna for GT's, but a coupling method would have to be devised.

HTH.. Don
Don

My wife's New Beetle convertible has an imbedded windshiel antenna. It is thinner that the imbedded wires in the rear window defroster. So to use this same approach on an antenna, I assume that a thin copper wire could be glued to the windshield. This wire would be attached to the inner wire of the coaxial cable and insulated from the remainder of the car. The outer coaxial cable (the shield) would be grounded to the body of the car. Same principles would apply to the center rod of the windshield, I presume.
Just thinking out loud and looking for affirmation, or comdenation.
James Johanski

Hi James.

The windscreen idea sounds good, I would try an 'L' shape, starting at the bottom corner of the screen, going up the screen, then along the top of the screen, with the far end disconnected.
A piece of enamelled copper wire (maybe from a scrap motor, relay or transformer) should do the job.
You can use very thin wire at MW / LW, as the aerial is very high impedance, but thicker wire will work better for VHF.
Keep the wire away from the windscreen frame, though.

Don

Don

There are a few times I use a CB radio when traveling. The antenna is mounted on the rear bumper, with the body being the ground plane.

I refuse to drill a hole in the body for a number of reasons -- but you might consider using the bumper as a mounting point for your antenna.
glg

Hi all.

glg's mention of cb sparked a thought.

A 'mag-mount' antenna may suit some owners.
These are, as the name suggests, held in place by a powerful magnet (maybe in the middle of the boot for roadsters).

Another possibility (especially for GT's) is a gutter clip mounted aerial.

Don
Don

I've thought for some time that the windscreen frame of the roadster would make a good aerial. However you would need to do some alterations. Firstly the plastic and paper packers would have to be in place at the mountings and then the mounting bolts would require to be fitted with tight fitting insulating tubes. A short piece of shrinkwrap may suffice. Then the centre stay would require an insulating plate under it and finally it would need to be bolted down with hard nylon setscrews. The aerial connection would be to one of the mounting bolts and the earth braid could be to a self tapping screw nearby.

Don't forget that you have to adjust the aerial trimmer on an AM radio for max volume on a station at the bottom of the MW.

It wouldn't work with the hood up as the clips would have to be insulated as well to achieve this.
Iain MacKintosh

This thread was discussed between 25/06/2004 and 28/06/2004

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