Testing a car aerial

Our transit based motorhome has radio reception problems. Okay in strong signal areas but unusable anywhere else (like on the M5 round Birmingham).

Transits are known to have problems with their door mounted aerials so we're currently trying a stick-on screen mounted one which is better, but still crap.

We're beginning to wonder if it's the radio that's the problem rather than the aerial but it certainly behaves like a radio with a duff aerial.

Is there any way of testing a car aerial short of trying another radio?

Tim

Reply to
Tim+
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Assuming a basic aerial with a standard cable length, the centre of the co-ax is connected direct to the tower part of the aerial. The screen to the bodywork where the aerial mounts. However, if the cable is much longer than the norm, it may not be able to test the centre in this way.

And, of course, many aerials these days have an amp built into the base.

FWIW, I've found screen mounted aerials rubbish. If you do need a new one which isn't a direct replacement for the original, fit it on the roof.

Reply to
Dave Plowman (News)

Take out the radio and test it with another aerial. Just a metre or two of wire [outside the car] will do, but you will need a Motorola plug.

Reply to
R. Mark Clayton

The radio may well be a bit duff, but stick on aerials are probably the worst thing sinced sliced beread, indeed a slice of toast might work better.

Even a rear window heater works better!

A decent roof mounted aerial has a lot going for it. Its high and normally can be angled a bit and when it hits things many seem to bounce back again, though the noise can be disconcerting.

If that fails then try the radio, but at least in most of theones friends have bought, the ones with amplifiers in them overload near transmitters and screw up fm and am reception big time with warbles and other sundry noises. Don't know what dab is like.

They ask me for some reason they seem to think I know... I've never driven a car in my life, and I bet you are glad!

Brian

Reply to
Brian Gaff

It might come to that but I'd just like to be certain that it is an aerial problem before making any unnecessary changes.

I'd like to just replace the Transit one but the way it's mounted involves destructive disassembly of the door mirror surround just to get at the mount and then extraction of headless shear bolts. Easier to just fit a new one!

Tim

Reply to
Tim+

Which Maplin sell. Not easy to solder apparently though. Why did they go with this odd plug I wonder? Brian

Reply to
Brian Gaff

Well it's working better than the Transit one! ;-) Oddly, reception is better with the aerial mounted horizontally rather than vertically. Don't understand that.

Tim

Reply to
Tim+

Note a slightly bent coathanger end can be gently connected to these sockets, but be very careful not to strain the socket. some are on short fly leads not on the radio. Presumably this is due to the cramped conditions in the back of the radio mounting position.

Many motor caravans seem to include a kind of entertainment system with a remot in the front these days and usually a completely useless aerial on the top that claims to also work with tv.

well....

Funny nobody ever gets back to me about how their shiny new mobile home was on Coronation street in the welsh valleys...

Brian

Reply to
Brian Gaff

It is possible that you may have a heat coated windscreen which is also a very effective signal blocker! Have a look at a reflection of the windscreen from outside preferably with polarised sunglasses. If it looks a pink or purple colour all over then it has the heat coating. Also electrical heating of the windscreen (very fine wires embedded in the glass top to bottom) which is common on Fords have the same effect even when switched off.

You say it is bad around Birmingham but station are you listening to, FM or MW? FM is broadcast from Sutton Coldfield about 10 miles north of the city (R2=88.3) MW is broadcast from Droitwich which is close to M5 J5 - the two big masts are a marker - R5L=693KHz. Both of them should give a near saturation signal - certainly do in my experience of the M5/M6/M42.

Reply to
Woody

RF is a black art.;-)

It could be the edge of the roof is acting as a reflector.

Reply to
Dave Plowman (News)

Funnily enough, this follows on from my replying to Brian's post in the preceding thread "What does PACW stand for in coax descriptions".

I forgot to mention, for LW and MW where a Hi-Z coax connection to an 'electric field probe' antenna forms part of the first tuned circuit's capacitance, such radios have a padding trimmer capacitor to compensate for variations in this external capacitance.

This is usually, via a small hole to allow a small screwdriver access to a mica compression capacitor, often located by the tuning knob on the front panel or else via a hole near the antenna socket on the rear panel.

If the car radio uses permeability tuning for the LW and MW bands and your problem relates to these bands, it's worth trying to locate the padding trimmer to 'tune' the front end to the car aerial's actual capacitance.

If it's a VHF / DAB reception issue, I suspect this fix probably doesn't apply (but what do I know?).

Reply to
Johny B Good

I shall start practicing a few incantations. Would a hexagram drawn on the windscreen help? ;-)

The only place to put it vertically is near the end of a windscreen blind that has a magnetic strip along its edge. Could this affect things?

Tim

Reply to
Tim+

We were driving from Ross-on-Wye to SW Scotland and the radio was almost unusable for the whole journey on FM. Little joy on AM either. Oddly though, in my home town (Ayr) it seems to work pretty well, although not without some hissing and signal loss at times.

Tim

Reply to
Tim+

If the screen mounted aerial is better than the rod aerial than the rod aerial is faulty. For confirmation use a short length of wire, attach the bared end to a small screwdriver, gently touch the screwdriver to the inner terminal of the Motorola socket in the back of the radio. With the wire outside the car that will work quite well as an aerial.

It isn't a big deal to buy a new aerial and fit it. Best place is on top of the roof (assuming a metal rather than GRP roof). But angle it backwards so it doesn't get snapped off (quite so often!).

Have you checked that the problem isn't interference from something in the motorhome? LCD screens, digital display modules, inverters, various types of light, can all give a greatly raised noise floor which seems like a duff radio or aerial.

I don't suppose the radio has the old type of trimmer adjustment?

Bill

Reply to
Bill Wright

To distinguish it from the more usual Lo-Z co-ax feeder connection (50 to 90 ohms versus a few hundred ohms for the car aerial 'co-ax' feeder - in reality, an electric field shielded bit of wire connected to the electric field probe antenna).

This harks back to the days of LW and MW only car radio design where the extremely short antenna (a tiny fraction of a percent of the wavelengths of interest) meant that it had to be treated as an electric field probe with its capacitance and that of the 'feeder' had to form part of the input parallel tuned circuit which, out of necessity had to use permeability tuning (iron dust slugs initially then ferrite tuning slugs in the later years). It would have been impossible to achieve the 10:1 capactor tuning ratio in the presence of the shunt capacitance due to the co-ax and antenna probe.

Such permeability tuned front ends will include a variable padding capacitor to compensate for variations in the car aerial's 'stray' capacitance. Unless you're familiar with this type of tuning method and the reasons for its use or have actually "RTFM"ed, it's easy to overlook the need to adjust this trimmer on a new installation.

Reply to
Johny B Good

I think most radios do this automatically these days and have done for some time. Or everything is made to a fixed standard, so no adjustment needed

Not with FM. DAB usually needs a separate aerial - or a special one with two feeders. Or rather my aftermarket ones does. Not sure about maker's installations.

Reply to
Dave Plowman (News)

I'll try that.

It a bit like this.

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Dunno how easy it is to fit an aerial to this type of roof.

Well there is a screen for the rear view camera above the radio on the dash. I've tried turning it off which makes no difference. Is it possible that it could cause interference even when in "standby" mode? It's a "soft switch" so I suppose there's always power to the monitor.

I could have a look. Would that help FM reception?

Tim

Reply to
Tim+

Bill's also mentioned this. I'll have a look.

Tim

Reply to
Tim+

In message , Johny B Good writes

In the 'radio shack', I've use a Cancer Shop £3 Panasonic RD526LEN car radio / tape player. The aerial is a 100MHz quarterwave vertical wire in the loft, connected to 75 ohm coax (braid to cold water pipe entering the plastic water tank). The run is maybe 20'. There is absolutely no sign of the traditional aerial tweaking capacitor.

Of course, it works fine on FM - but it also performs well on the LW and MW. I would have expected the 75 ohm coax to be a killer. How does it do this?

Reply to
Ian Jackson

How old is the radio? Last time I saw a trimming capacitor was on a 1980s radio. Not seen it on a recent one. Of course I've not seen them all. ;-)

Reply to
Dave Plowman (News)

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