The names Duplexer and Diplexer are very similar and frequently
confused.
The Duplexer is a device which separates 2
frequencies within the same band
while the Diplexer is
a device which separates 2 different bands.
The
Duplexer requires much more selective circuits while the Diplexer
normally
requires only low pass and high pass
circuits.
This DIPLEXER separates 2m from 70cm on the same coax
cable.
It allows to use 2 antennas over the same cable
and permits transmission on one band
and simultaneous
receiption on the other band, when connected on the side of the antennas.
Or use 2 equipments (one VHF and one UHF) when used on the
side of the transceivers.
The following data have been measured on 50 Ohm input and 50
Ohm output:
- Separation of the two
bands is very high (over 70 db)
-
Insertion loss is negligible (less than 0.2 db)
Its easily built into a metallic box measuring 8cm x 4cm x
2cm
or similar (E.g. TEKO 372)
Here follows the circuit diagram: 
List of components:
L1 = 1 turn
6mm diameter, 1mm enameled copper wire
L2 = same
as L1 (orientation 90 deg in respect to L1)
L3 =
3 turns 6mm diameter, 1mm silver wire
L4 = 4
turns 6mm diameter, 1mm siver wire
L5 = same as
L3
C1 = foil trimmer capacitor 15pf
(3-15pf) see text
C2 = same as C1
C3 = same as C1
C4 = foil
trimmer capacitor 40pf (4-40pf)
C5 =
same as C4
3 HF chassis plugs 50 Ohm
(e.g.BNC)
1 metal box e.g. TEKO 372
Observe spacing of coils and location on photograph:
The coils can be made of silver plated copper wire, but
enameled copper wire
serves equally well.
The coils L3, L4, and L5 have to be oriented in 3 different
orientations to avoid
coupling between them. Each one
90 degrees turned in respect to the others.
Proper adjustment before use is very important, according to
following procedure:
Before doing the adjustment,
verify that the SWR meter is calibrated properly on VHF an UHF,
that is, it reads 1.0 when terminated by 50 ohm dummy load and fed by
TX.
1. Connect a 50 ohm dummy load to the common 2m+70cm plug (make
sure that
the dummy load is
for these frequencies).
2. Connect SWR meter between 70
cm plug and 70 cm TX.
Now
adjust C1, C2, and C3 to obtain SWR of 1.0 while transmitting low power
carrier.
C1 and C3 should finally
have the same value.
3. Connect SWR meter between 2m
plug and 2m TX.
Now adjust C4 and C5
to obtain SWR of 1.0 while transmitting low power carrier.
- repeat steps 2 and 3, as they may interact.
Now your diplexer is ready for use.
Note: The power is limited by the maximum current/voltage of
the capacitors.
Foil trimmers can be
very different, depending from model, therefore
good elements are
required for higher power.
(With my trimmers I
tested the unit up to 50 watts).
Use capacitors for
higher current/voltage ratings at higher power,
e.g. good air trimmer
capacitors.