Help needed-combining 2 FM antennas - but splitting the frequency.....

[Eric]

...... at which they are receiving. Basically this is the scenario, I
have a car stereo system which the OEM system has 2 antennas. They're
both in the glass, front and rear. I put in an aftermarket system
that can only utilize one of the antennas. Now heres my problem, the
front antenna receives stations 95.x or so and below (including AM)
much better then the rear, and the rear picks up the higher frequency
stations much better then the front. So I need something that can
isolate the 2 antennas, so the front only picks up the 'lows' and the
rear only picks up the 'highs'.

I need it to mimic what a VHF/UHF combiner would do for a TV setup if
you had seperate UHF and VHF antennas.

Thanks.

 

[John Woodgate]

(in <[email protected]>) about 'Help
needed-combining 2 FM antennas - but splitting the frequency.....', on
Thu, 12 Feb 2004:

..... at which they are receiving. Basically this is the scenario, I
have a car stereo system which the OEM system has 2 antennas. They're
both in the glass, front and rear. I put in an aftermarket system
that can only utilize one of the antennas. Now heres my problem, the
front antenna receives stations 95.x or so and below (including AM)
much better then the rear, and the rear picks up the higher frequency
stations much better then the front. So I need something that can
isolate the 2 antennas, so the front only picks up the 'lows' and the
rear only picks up the 'highs'.

I need it to mimic what a VHF/UHF combiner would do for a TV setup if
you had seperate UHF and VHF antennas.

It's not difficult to separate VHF and UHF because of the large
frequency difference, but separating two halves of the FM band is more
difficult. Not impossible, but first see if you actually need to do it.
These car radio antennas usually use a special low-capacitance cable
that has a characteristic impedance of about 100 ohms, so you need to
make up a combiner in a small metal box:

Use Courier font
________________________
Antenna 1 | | Antenna 2
_________________| |___________________
---------------------+----100ohm------+----------------------
_________________ | | __________________
| 100ohm 100ohm |
| |________________| |
|__________ | ___________|
|||
|||
||| To radio

If this works OK, it's a simple solution. Otherwise, you have to build
high-Q filters, which is possible but much more difficult.

This delta-network combiner is electrically equivalent to the textbook
star-network combiner but has the advantage that the resistors are equal
to the cable impedance instead of one-third of it.