Antenna splitter for Broadcast and 144 MHz
Driving a car owned by the firm I work, I had the choice of using a magnetic
antenna or finding an other solution. A magnetic antenna has the risk of being
removed somewhere so I started looking for the other solution. In a VHF
Communications of spring 1978 I found a project of an antenna splitter designed
by DK1OF. The Idea is to use a 2mtr. mobile antenna
with a 2 mtr. transceiver and a broadcast radio.
I could use the hole of the standard broadcast antenna to mount a 2 mtr. 1/4
lambda antenna, so this was the way to go.
Not only the schematics where described but also the theory and the formula's
to compute the components. This made it possible to recalculate the whole thing
for my proper use. The original concept used as center frequency of 95 MHz for
the FM-Broadcast there where I use to listen to stations centered around 91 MHz.
As a center frequency for 2 mtr. I used 145MHz.
The Schema
The Formula's:
R = Input impedance = Output impedance = (50 Ω)
F1 = Cut-Off frequency Broadcast (91 MHz)
F2 = Center Frequency off 2 mtr. (145 MHz)
| The calculations are automated in an Excel worksheet you can download with
the rest of the project files: Antenna Splitter.zip |
The results:
|
Calculated |
used |
| L1a
= L1b = |
5,167619E-08 |
2,5-Turns |
| L2= |
5,488101E-08 |
2,5-Turns |
| L3= |
4,072309E-08 |
2 Turns |
|
|
Coils wounded on a 6 mm coil former with 1 mm Silver plated copper
wire. The coil-formers are provided with a VHF usable core |
| C1a
= C1b = |
2,331384E-11 |
22pF |
| C2= |
3,459890E-11 |
33pF |
| C3= |
7,348927E-11 |
27pF + 47pF |
| C4= |
4,390481E-11 |
22pF + 22pF |
The theoretical frequency response:
1) The response from the antenna to the 2 Mtr. Transceiver:
| The center frequency (91 MHz) of the FM Broadcast cut-of is
more then 40 db and on 145 MHz the attenuation is quasi 0 db. the 2 meter
harmonics are extra attenuated even so the frequency range of the MW and LW
Broadcast. |
|
2) The response from the antenna to the Broadcast radio:
| Here the 2 meter frequency is suppressed more than 50 db,
the FM-Broadcast and the MW and LW are nearly not attenuated. The
impedances are all 50 Ω - (look out for the
broadcast radio, eventually one has to match the input impedance.) |
 |
The next step is to work-out the project in a practical
way.
Parts:
-
For mechanical strong ness, a car can trill and shake a
lot, I use a commercial die-cast box from 100mm x 50mm.
-
3 x 1-Hole mountable BNC connectors
-
4 x VHF Coil cores with adjustable core (it is more stable
for car-use to adjust the coils then adjusting capacitors)
-
1mm diameter - Silver coated - wire
-
4 x 22pf
-
1 x 33pF
-
1 x 27pF
-
1 x 47pF
Design of a printed circuit:
Solder-side seen from components
Earth-mask on the component side
Position of the components:
... to be continued.