Two Aerials?

Do all XM's have two aerials? OK I have only owned 2 XMs but both the previous and current one have had 2 aerials, one in the rooof and one built into the rear window. My First XM had a 'diversity'reciver (effcetivly 2 radio recvers in one box), which would need 2 aerials, the current one had lost its original radio, both have 2 aerial leads behind the radio. The rear screen aerail is easy to see, its looks like a heating element, but ends part way accross the screen. Give the exiatance of this I am not sure why the roof areal is there if the diversity recivers were not standard. If you do have both fitted to your car and have a leaking aerail then I'd just ditch the roof mounted one and be done with it! (make the car look even 'cleaner')

Stewart

Hi Stewart

I have read about this in a previous post but none of my XMs have had a 'diversity' radio, in fact I have never seen one. The standard one only has one aerial socket.

Peter.N.

While we are on radio/receivers, what's the likely cause of the following effect: at radio reception is as should be (stereo, clear) at one point on the road, then I move 1 meter ahead, signal becomes rubbish (loosing stereo, noise), 1 meter further ahead, good signal again, and so on. Is it something of the FMs or of the car -think I've had it on other cars as well- ?

regards
George

Can't answer that one.

I think you'll find that Series 1s had the additional aerial built into the back window - both mine had, whilst Series 2 don't. Odd thing is that my Series 2 has better reception than the Series 1s, or is that down to the radio itself?

Derek

George - that sounds like a common problem with MW that is due to how radio signals work. In MW it means that the volume becomes louder and quieter. Don't know if the same effect can be found on FM though - I'm sure it wouldn't cause volume differences due to the different modulation, but it may cause rising and falling interference like you are getting.

I can't remember my physics, but I remember there being something to do with 2 transmitters, and the waves going in and out of phase with eachother. I'm sure someone who remembers their physics can correct me on this.

I would have thought that most modern FM radios are fairly good at avoiding this unless you are right on the edge of reception though. Perhaps your aerial is not doing what it should so your reception is not as good as it should be.

Thinking about it I'm sure I used to get a similar effect when I was on the M27 (southampton) still picking up London local radio (Capital) - i.e way out of normal reception range.

I should know about this as I was TV engineer and am a radio ham, but I',m loosing my grip a bit now! But, yes, as stated it is to do with phase cancellation. As you are driving between two transmitters the relationship between the two signals is changing all the time. The 'wavelength' is actually that, in other words if you are listening to a station for instance on 250 metres from the crest of one wave to the crest of the next is, 250 metres. If you are only recieving this from one transmitter, travelling toward or away from it will not make any difference but if you are between two, the waves will go in and out of phase as you move, when in phase they will add to increase signal strength, when out of phase they cancell each other out giving you a reduction in volume or if perfectly out of phase - loss of signal. This effect only applies on medium/long wave AM signals, on VHF/FM, distortion or signal reduction can be caused by 'multi path' propagation, that is by signals being reflected by buildings or hills and causing the same adding/cancellation effect.

Peter.N.

Whilst we're on the subject then can someone explain the function of two aerials? In my limited knowledge of the electromagnetic spectrum wouldn't the signal arrive at the second aerial slightly behind the first and therefore appear at best as an echo and at worst interference?

Unless one aerial was for FM and the other AM/MW? That may explain why it disappeared as time went on. AM/MW fell out of favour.

Any thoughts?

Cheers

noz

I believe the phrase "diversity" to be the key here. As an AV Engineer I routinely use Radio-Microphone that work on a "diverstity" principle.

2 aerials with slightly different properties/locations and a CPU that picks the stronger signal in milliseconds. Very useful with FM radio mics, see no good reason why same principle should not apply to other FM signals.

Probably a prime example of engineering excellence on S1 binned to cut costs (well they're Citroen) for S2

Hi Noz

As Chadders says, diversity recievers are most well known in connection with radio mics with the stronger signal taking precedence over the weaker, so you only have one reciever operating at a time. With FM radio the problem is largely one of signal orientation as it gets bounced about. The front aerial is vertically polarised and the rear screen aerial presumably horizontally, so when you are travelling in a built up area a change of signal polarisation would result in the front or rear connected recievers being switched on according to which one was recieving the best signal. This switching would be very rapid and so would not be noticed.

As someone commented, these only seem to be found on Mk1 cars, so I can only presume the the FM coverage is now good enough to deem them unneccesary, I certainly dont have any problem with it even here in the sticks! In fact the only places I have found with no reception at all are on the M6 entering Cumbria and the A836 Lairg to Tongue road!

Peter.N.

So, there's nothing wrong with my XM or anything I could do to fix this? It's true that this occurs in cities, when I crawl on traffic lights. Strange though as it doesn't seem to happen to all cars I've been in. Nobody notices anything similar when in traffic??

George

PS. my S1.5 XM PR6336 doesn't have the double receiver either.

Hi George

If you have this problem in your car but others don't, its either down to a poor aerial connection somewhere or the radio having 'low gain' - insensitive to a weak signal. Is it working on the roof aerial? this is the best one as not only is it higher but is also the correct length to resonate at the VHF/FM frequencies. If it is, you could remove the interior light and check the connections, make sure the tag connected to the braiding is making a good connection with the roof. If this is OK and the connection to the radio is good, it must be the radio that's at fault - nothing much you can do about that except, try another one.

Peter.N.

It was My S1.5 SEi that did have the 'diversity' receiver still fitted (CItroen Badged clarion with cd changer under passenger seat) and the two aerials were there. My cuurent S1 also has both, my radio only has one aerail input but either seems to work equally well (would have liked to see the original radio to see what was on the back)

Stewart

Ah! CD player, that's why I have never seen one. If both aerials appear to work OK stationary, I am sure that you will find the front one better on the move.

Peter.N.