Testing a car aerial

I remember those trimmers. If you had a car with a rear mounted aerial, many of the motorola radios could not be trimmed enough to cover the extra cable. Brian

Not good if the roof is not metal though.

For fm, the best aerial I designed for our caravan was a full wave circle of copper tape glued to the inside of the fibreglass roof! My Mother was not impressed by the wire hanging down on the end though. Sigh. Fun times. Brian

"Dave Plowman (News)" wrote in message news: snipped-for-privacy@davenoise.co.uk...

Tuning matching slugs have not been fitted on car radios since FM came along back in the early 80's. You cannot have an aerial matched to and part of a tuned circuit for LW/MW when it also has to feed FM. Most radios nowadays have an almost resistive match with both AM and FM tuners effectively having high impedence inputs.

Until 2009 my 2000 BMW had the radio aerial in the screen and a fin with the TV aerial. This used to get satisfactory to good TV and teletext reception on the built in TV, however the receiver was analog and stopped receiving anything once Granada land went digital in 2009.

PS the screen blanks if the gearbox is taken out of 'park'

A decent extension lead had the capacitance equalised.

See elsewhere for the technical explanation. It is called Motorola after the company. They made the first car radios (before the war) and so the plug is named after them.

RCA got a plug named after them too.

I don't know who jack was...

In message , "Dave Plowman (News)" writes

I think that 'equalised' meant that it had an integral series capacitor (possibly, over part of the length, there were two insulated inners, twisted together). This allowed you still to peak the MW signal (near the HF end) - presumably at the expense of a little signal strength.

In the mid-70s, I fitted a LW/MW/FM radio to my Chevette, and put the aerial at the rear end. This entailed using an extension lead. I recall that the instructions said not to shorten it. The trimmer certainly peaked as normal, and everything seemed to work OK.

That is of course assuming it has a Motorola plug, the vast majority of manufacturer fitted kit these days don't, they use either something like the one on the right

or on anything produced in the past 10 years or so most likely this

Or maybe even one of the other types listed here

The last time I saw the backside of a DIN fitting radio it has a trimmer, but I doubt if I could remove a modern one to look, without deploying all the air-bags!

Don't we call those Phono plugs here? Odd that because we don't call record players phonographs do we?

"A plug called Wander" "The Banana Plug Show"

I once managed to slice a bit of my forefinger off whilst stripping the inner of some car radio coax. The dialectic is tough, but has a very thin wall and, the solid inner wire has a spiral of plastic to keep the wire centred. Once the Stanley knife stated to make headway there was no stopping it!

Amazingly my finger regenerated itself and you can't tell.

In message , Graham. writes

With all the radios I've seen with an aerial trimmer capacitor, it was on the front (in one of the corners).

That could be a GRP body shell and roof. If so you would have to mount the aerial somewhere near a steel roof member so you could ground the screen of the cable. But really I wouldn't recommend penetrating that sort of roof if it can be avoided. You can get stress cracks forming.

Unlikely but check by removing the fuse that feeds the screen and camera. Let's hope it isn't the same fuse for the radio.

No it wouldn't help FM. If both FM and AM reception is abnormally poor I'd say the aerial is very likely to be faulty.

As a reality check, I suggest you stand somewhere near the vehicle with an ordinary portable radio on FM. Tune through the band and count the number of signals the radio receives, for the whole band or a defined portion of it, say 92 to 98MHz. Do the same with the van radio. The van radio should receive at least as many signals as the portable. Obviously you would count every iteration of each station as a separate signal; for instance you might get three iterations of Radio 4.

Could you do a simple continuity check?

1. The inner pin of the aerial plug (the one on the end of the aerial lead that goes into the back of the radio) should read zero ohms (near enough, maybe 0.5ohms is OK) with the other meter probe/clip on the rod of the aerial.
2. The outer barrel of the aerial plug should read zero ohms (near enough, maybe 0.5ohms is OK) with the other meter probe/clip on the vehicle body.
3. With one probe on the plug's inner pin and the other on the plug's outer barrel the meter should read infinity: ie just the same as it reads if the probes aren't connected to anything.

Is the plug contacting the radio socket properly? Have you tried wiggling it?

Is the radio properly connected to the vehicle body? Try a fairly thick temporary wire between the radio chassis and the nearest steelwork.

Bill

For what it's worth I found as a teenager interested in getting the ultimate sensitivity (of my van radio you dirty bastards) the use of 75 ohm coax was to be deplored. However, I found that 75 ohm worked just fine for linking tuned loop aerials to the loop that went around the radio and coupled to the ferrite.

Bill

I do know of a centrefed FM half wave glued to the underside of the GRP canopy of a motorhome that works just fine, using 75 ohm coax.

Also I know of a vertical whip mounted on the tow hitch bracket that works well.

Bill

You had to buy a special 'compensating ' extension cable.

Bill

I had a Blaupunkt Frankfurt AM/FM radio which did have a trimmer for AM. A very long time ago.

I think you are almost completely right, although I'm sure I had an early AM/FM set with a little tuning slug under one of the controls for AM. Or did I dream it?

Bill

Yes. If you checked it for continuity using an ohm meter, you'll be disappointed. ;-)

Not being able to shorten it rings a bell.