FEEDBACK: Diplexer for 2 m/70 cm
Dipl. Ing. Gyula Nagy, HA8ET
POLLAK School of Electronics (Szentes, Hungary)

It was published: AMSAT UK OSCAR NEWS Oct 2001 pp. 39-41

I was delighted to read the short translation of my article called "Diplexer for 2 m/70 cm" in the 145th issue of AMSAT-UK OSCAR NEWS. (The original article appeared in in CQ-DL 7, 1998.) Because of the repeated translations and lack of space I found the following wrong statement written by OZ1MY, Ib OM in the article. "The curve, which avarages around –10 dB, is the Return Loss at the common input to the diplexer."

Four years ago I set myself to present a simplified filter-designing for HAMs. I planned the filters of the diplexer with free working demos and low cost programs. (HP: AppCAD, ARRL: SVCfilt, MICROSIM: DesignLab Evaluation 7.1, ACCEL: Tango PCB Eval.) These programs are unable to work properly with the Distributed Microstrip Rectangular Spiral Inductors. That’s why after the theoretical inductors and capacitors values measurings they had to be modified to a little degree. So the final Layout formed in this way.

In the case of professional EDA Softwares used nowadays we don’t need such modifyings. For example: AGILENT EEsof EDA, ANSOFT: Serenade, -Ensemble, -HFSS, APLAC 7.6, CST Microwave Studio, EAGLEWARE: Genesys 7.5, IE3D, KCC, Microwave Office 4.1, OPTOTEK MMICAD, Sonnet USA, etc. These programs take both the parasite capacitors and the coupling between the turns of the inductors into consideration.

I enclose the results of the diplexer Return Loss measured on the common input port in this article. So you can produce good filters with practically free programs as well if you constantly check the measuring results, and modify the values of the elements accordingly.

I used the following Agilent Technologies (Hewlett Packard) instruments for the measuring:


Figure 1. The Return Loss at the common input to the diplexer on 144 MHz S11 >22 dB


Figure 2. The Return Loss at the common input to the diplexer on 436 MHz S11 >20 dB


Figure 3. At the mounting of the BNC-female (Shown by figure 4.) S11>24 dB!


Figure 4. Proposed mounting of the BNC-female



Other informations from author
(Assembled and tested diplexer, PCB)

Gyula Nagy, HA8ET
Muskatli u. 4.
H-6600 Szentes, HUNGARY



[POLLÁK Elektronika]



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