DIY Car aerial splitter?

[matt]

A appreciate that this maybe slightly OT, but it is DIY and I am sure others here will be able to help.

I have a GPS reciever that also recieved RDS broadcasts (i.e on a standard FM frequency)

The current GPS unit uses a single wire with a phono type plug as an antenna.

As an alternative there is a "Y Adapter" avaliable that plugs inbetween the radio and aerial which gives the GPS unit the use of the main car antenna (improving reception). See links below. The official splitter costs around

35 Euro, but I am sure that there is not much to it and a DIY solution would work.

Plugs and bits are avaliable from maplin, but can any recommend the best way to go about this?

Quick Find: 4037735101388

[Huge]

Resistive splitters are relatively easy to make, but in all honesty, there's no reason why you shouldn't just use a UHF/VHF antenna splitter, the resistive ones work over several orders of magnitude of frequencies. It will cost less than you can buy the parts for.

For example;

inc VAT.

Just change the plugs on the various cable for ones that suit.

[Rusty]

I'm bit confused by your antenna arrangments "..single wire with a phono type plug as an antenna". Hand held gps units tend to use MCX connectors, which Maplin don't do though I have got them elsewhere. So are car gps connectors different. I am interested as at one time I tried to connect my redundant cellphone antenna on the car rear window to my gps but problems getting a connector for a GSM phone (Nokia).

rusty

[Dave Plowman (News)]

IIRC, cars use 300 ohm aerial systems - domestic 75 ohm. So splitters would be slightly different for maximum efficiency.

[Dave]

I have never come across 300 Ohm ribbon cable in a car, so I would tend to think that the coax for a car radio would be close to 50 Ohms.

Dave

[Frank Erskine]

No - the "coax" on a car aerial is special "low capacitance" cable, and is indeed about 300 ohms.

[Dave]

Very interesting Frank. Thanks for that. However, when I google for 300 Ohm coax, I get nothing.

Dave

[Dave Plowman (News)]

You can make cable what you want it to be - balanced ribbon isn't necessarily 300 ohms.

It's got a strange spiral centre conductor last time I looked. But I'm no expert in Rf cables.

[Peter Parry]

A car antenna is not going to improve GPS reception.

The "splitter" you are referring to allows you to connect to a car antenna incorporating an active GPS antenna in its base. A normal antenna is utterly useless.

[<me9]

That's an ancient relic. Haven't they realised that most car radios are VHF these days?

[Frank Erskine]

Car radio cable, although it IS technically "coaxial", isn't really very "coax": Find yourself an old car aerial and carefully strip down the cable. You'll find that the outer conductor, which is normally either a braid and/or metallic foil, is usually just a spiral of a few strands of wire. This reduces the capacitance of the inner/outer and means that, although there is a bit of screening of the cable if it passes through the engine compartment where it may pick up ignition interference, there is less "tuning" effect of the length of the aerial, so it is effective from long wave up to Band II (FM).

[Mark Carver]

It's really of no relevance. The characteristic impedance of the aerial itself will not be 300 ohms , IIRC that's the impedance of a folded dipole ? and how many of those do you see on a car ? !

There's mismatches everywhere, not least because the aerial is usually just a length of conductor, you'd need to apply chaos theory mathematics to calculate them, however essentially a car radio takes an unbalanced input so just use any RF grade co-ax that mechanically fits the bill.

[Dave Plowman (News)]

This will work for VHF, but not AM. Modern radios have automatic matching for AM - older ones had a preset. When extending an aerial lead you use a pukka extension which is matched already by IIRC the addition of series capacitance.

[Mark Carver]

There used to be a tiny trimming capacitor normally on the front panel for this matching. On modern sets I don't know how this is achieved automatically ?

Who bothers to listen on crappy old AM these days anyway ?

[Dave Plowman (News)]

I don't know how it works either - but since the trimmer has disappeared it must be automatically achieved?

When driving the length of the UK listening to R4 you still need LW in places.

[john]

Agreed if the antenna was trying to receive gps however the splitter is not for GPS reception but FM/RDS

The unit in question is a combined GPS & TMC/RDS

The TMC system receives broadcasts from the FM frequency (classic FM) containing traffic info.

[Steve Pearce]

Be aware that some cars have "active" aerials and a DC voltage is sent by the radio via the co-ax.