144/430 Mhz duplexer for using dual band HT with Home Made Arrow

I've spent much time searching a portable sat antenna easy to build but at the same time with good performances. So I've found the arrow antenna made by XE1MEX a good compromise (It Was just a little bit big).

I started the construction using some aluminium tubes from an old TV antenna. The only problem was that the author uses two separate hand helds while I wanted to use my TH-D7 dual band RTX. The only way to solve the problem was to build a Duplexer.

For whose don't know what a duplexer is, I can say that it's a filter that separates two signals at different frequencies applied at the input port in two separate output port. In our case the two signals are in the VHF and UHF bands.

I've found this project in an old Ham magazine, but unfortunately I don't found it and so I can't say who is the author.

The circuit is visible in figure 1.

Figure 1

The Vhf stage is a 5 elements Low Pass Chebyshev filter. I've evaluated an average attenuation of 50 dB/octave out of the Low Pass band.

The Uhf stage is a 5 elements High Pass Chebyshev filter with an evaluated average actenuation of 50 dB/octave out of the High Pass band.

In figure 2 you can see the bands of the two filters (I've simulated it on PSpice but I don't have a spectrum analizer to evaluare the effective response...)

Figure 2

The construction data of the inductors are:

L1,2,5 : 4 turns of 1mm wire on 8mm diameter. Total inductor lenght: 8mm.

L3,4 : 3 tutns of 0.8mm wire on 4mm diameter. Total inductor lenght: 4mm.

The wire can be silver plated or enameled. Capacitors are normal ceramic capacitors.

With these components your doplexer wuould drive a maximum power of 50W.

If you want more accuracy and power, you can use ceramic trimmers for great powers (but you must have a spectrum analizer and tracking generator to set it...). I recomentd you to assembly the duplexer in a metallic box (it is not necessary to draw a printed board, I've used the 'dead bug' technique). If you think to left it outdoor you must put it in a waterproof box to prevent oxidations that causes performances degenerations.

To test it is sufficent a SWR meter: check all in/out combinations for a SWR very close to 1:1 (don't forget to apply a load - antenna or dummy - on the unused port while testing!!!).

I use this duplexer for sat traffic, but you can use it for everything you want! It is an undirectional device - you can invert in/out and it works as well - so for example you can use two of these duplexers to use a single cable with two RTX and two antennas.

Good Luck!

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