DIY Car aerial splitter?
 

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?

http://www.gns-gmbh.com/en/ Quick Find: 4037735101388
http://www.navigon.com/site/int/en/s.../universal/102

"matt" wrote in message
. uk...
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?

http://www.gns-gmbh.com/en/ Quick Find: 4037735101388
http://www.navigon.com/site/int/en/s.../universal/102


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

In article ,
Huge wrote:
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.

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

--
*INDECISION is the key to FLEXIBILITY *

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:

In article ,
Huge wrote:

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.


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


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

On Fri, 6 May 2005 20:16:46 +0000 (UTC), Dave
wrote:

Dave Plowman (News) wrote:

In article ,
Huge wrote:

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.


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


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.

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

--
Frank Erskine

Frank Erskine wrote:


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

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

Dave

In article ,
Frank Erskine wrote:
IIRC, cars use 300 ohm aerial systems - domestic 75 ohm. So splitters
would be slightly different for maximum efficiency.


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.

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

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

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

--
*Out of my mind. Back in five minutes.

Dave Plowman London SW
To e-mail, change noise into sound.

On Fri, 06 May 2005 10:23:34 GMT, "matt" wrote:


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).

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.

--
Peter Parry.
http://www.wpp.ltd.uk/

On Fri, 6 May 2005 20:41:30 +0000 (UTC), Dave
wrote:

Frank Erskine wrote:


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

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

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).

--
Frank Erskine

Dave Plowman (News) wrote:
In article ,
Frank Erskine wrote:

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


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.


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


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


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


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.


--
Mark
Please replace invalid and invalid with gmx and net to reply.

In article ,
Mark Carver wrote:
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.

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.

--
*Support bacteria - they're the only culture some people have *

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
Mark Carver wrote:

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.


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.

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 ?

troll mode

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

/troll mode

--
Mark
Please replace invalid and invalid with gmx and net to reply.

In article ,
Mark Carver wrote:
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.

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 ?

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

troll mode

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

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

--
*I went to school to become a wit, only got halfway through.

Dave Plowman London SW
To e-mail, change noise into sound.

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).

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.

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.

On Fri, 06 May 2005 10:23:34 GMT, "matt" wrote:

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?

http://www.gns-gmbh.com/en/ Quick Find: 4037735101388
http://www.navigon.com/site/int/en/s.../universal/102


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