FM Radio interference

[jkn]

To go off at a slight tangent, for which apologies ... I know a bit about antenna theory in general, but next to nothing about commercial (FM/AM) car radio antennae. I have a Aldidl Car Radio and a nondescript roof-mounted antenna, and experience fairly poor reception and frequent fading etc. Would it be worth my while upgrading to a better aeria l, and if so can anyone suggest something 'cheap-ish' (say < £20) to try? Or is the problem likely to be due to poor intrinsic sensitivity of the radio?

Sorry for the vagueness of this...

Cheers Jon N

[Dave Plowman (News)]

I'd say most modern car radios will be (near) state of the art as regards sensitivity, since they'll likely use a 'chip' front end.

Is the roof aerial a factory fit 'bee sting' one? Many of those are also active - have a head amp inside them. Is that powered if you've changed the head unit?

A basic simple rod aerial for FM should be 1 metre long.

[Dave Liquorice]

Aerial is as supplied by the maker (Sony), about 6' of plain wire and connects to the set via a small plastic molex style connector. The instructions make no mention of using anything other than the supplied bit of wire. As that is what the maker supplies and they don't suggest any alternatives I expect it to work...

One day I might get around to putting an aerial in one of the lofts and bodging up some form of coax to "molex" connector but the average punter wouldn't be able to do that nor should they be expected to be able either.

[Dave Plowman (News)]

I'd hope a maker might just supply any radio with a standard UK external aerial connector. Or an adaptor for one.

An FM portable radio doesn't work well indoors in this part of London.

[john james]

It can't be that because the OP says that it only switches away temporarily so there is only a burst of the 'music' he doesn?t like.

[jkn]

OK, good to know. I did a quick google about the Aldilidl radios with no si gn of sensitivity problems.

I hadn't come across the term before ... it used to be a simple (I thought) half-metre or so aerial a bit like:

a--base-1911-p.asp

The roof fitting degraded and I replaced it with something similar-ish from eBay, but kept the coax. But there was some swapping of head units between cars at the time so it's hard to be sure. We have spotty reception round here an yway, I'm not expecting 100% ... but it's hard to receive R4 on a good day at the moment.

I see that (eg.) from the website above you can get amplifed (head amp) roo f mounted aerials for not much more than a tenner - maybe I'll get one of tho se and bite the bullet about changing the cable.

Thanks J^n

[Terry Casey]

100MHz is a wavelength of 3 metres so a 1/4 wave whip would be 750mm.

The centre of Band II is slightly below this (longer wavelength) but is close enough.

Mount in the CENTRE of the roof so that the roof becomes an earth plane and gives omni-directional performance.

If you make a simple base for starters with a rubber covering on the bottom and a couple of suitable neodymium magnets to ensure it doesn't move you will end up with a suitable basic aerial to experiment with.

You might want to contrive a flexible joint at the base to allow for places with restricted headroom in the permanent design!

[Dave Liquorice]

So would I but not on/with this Sony "micro HiFi". The best I've managed to get out of it from DAB was a test upstairs, on the right side of the house, by a window. All that gave was "boiling mud" but you could tell the difference between music and speech. Speech had gaps in the boiling mud noise. B-)

[Dave Liquorice]

You've forgotten the velocity factor... B-)

[The Natural Philosopher]

oh well 749.99957mm

[Dave Liquorice]

A rod is considered to be around 96% IIRC so more like 720 mm.

[Dave Plowman (News)]

Right - I was merely quoting the recommendation from the car radio handbook. Perhaps 1 metre is their best compromise for FM and AM including SW.

[Terry Casey]

No I didn't! I did consider it: (~95%) which reduces the rod length but when you add in the +2% difference for the true centre of the band, this reduces to 97%, so the precise length of the whip isn't really that important.

750mm is a lot closer to the centre of Band II than the 1000mm originally proposed!

[tony sayer]

In article , Dave Liquorice scribeth thus

Depends on the metal;!.

It is true, there are differing coefficients for differing metals and alloys thereof for use in aerial design programs..

For most s/steel whips .9 is near enough but its a bit academic anyway with regard to the needed 20 MHz band coverage. If you are going to use one then best cut for the lower end of the band, seems work better overall..

[tony sayer]

Now your asking!. Thats where peeps don't want them which is a shame as I normally have a 1/4 wave for VHF Hi-band on the rear of an estate car and thats connected to the FM radio thru a diplexer unit but when the full length FM aerial is on a mag-mount in the middle of the roof the FM band just comes alive:)..

[tony sayer]

In article , Dave Liquorice scribeth thus

The overall sound quality on DAB these days Dave low bitrates and mainly Mono isn't worth bothering with, stay with FM and the better online stations for decent reception;!..

[The Natural Philosopher]

Frankly at home most of what I listen to is on the interwebby thing anyway.

Or on digital TV MUXES

Not up to FM, but better than DAB!

Frankly I think DAB will disappear and be replaced domestically with internet and as far as cars go...well the tendency there is MP3 - you only need a radio for traffic, and thats being replaced by smart satnav.

[Dave Plowman (News)]

You may be happy with only traffic news or MP3 in your car - but please don't talk for others.

[Mark Carver]

Mine does, it has two FM tuners, and one constantly scans the FM band looking for raised TA. It's a PITA, because usually it will only come across a station when it's half way through an announcement. To add to the PITA'ness, the software totally ignores RDS EON data, so I miss most of the BBC LR announcements, that cut in via EON vectoring of PI codes from BBC national stations. All in all a pile of cack. However, the DAB section works brilliantly !

[Mark Carver]

Part of the TA switching, is to take a sniff of a station the radio's been told there's travel news on. However it has to decode what's called the PI code, to verify that station is the one required.

For instance, you're tuned to Radio 4 in the London area, when a BBC local station within the Wrotham transmitter's area has a travel flash, the frequency and PI code is sent in R4's data stream. It might however be Radio Berkshire, on 104.1. Your radio will jump to 104.1, but where you are that frequency might be occupied by a pirate (or even another legal station) The radio sits on that frequency, until it can decode the PI, which can take a few 100 ms. If the PI matches, it stays, but if not it will jump back to R4, but you'll have heard a burst of jungle music etc