Technical information by PA3FXO


Following page gives some interesting information about items I have used or matters that ocurred while I was making RF things.


The first part will give information about rebuilding my EME antenna situation. As mentioned on my website I have used a 10 element CueDee and later a 15 element CueDee.
As Martin DK7ZB has great designs on his website I had contact with him and was convinced that I should rebuild my former 10 element CueDee to his design.
Due to the boom length it was to be a 8 element DK7ZB with 28 Ohm dipole.
First I measured the length of the boom and element positions and lenghts to put in the software program MMANA to see what the CueDee antenna pattern would be:
Plot of the 10 element CueDee in MMANA:


As you can see the backlob is ausome and that is not what you want when doing EME DX.
I have used the information from DK7ZB 8 element Yagi and resized the CueDee elements and replaced them on the old boom. The former gamma-match dipole was removed and a 28 Ohm version replaced.
A photo of the refurbished CueDee to ZB Yagi will be uploaded to this page soon. Some testing with tropo and EME compared with the 15 element CueDee will also be done.
The design of DK7ZB uses a 10mm spacing between the two dipole halves. It's best to use non conducting material like teflon as I did.



As I wanted to build a double antenna system with 2x 10 element DK7ZB, I needed to think about combining the two 50 Ohm coax cables. There are two ways to do this:
We have two coax cables of 50 Ohms each that come together and give 25 Ohms (Zo) at the connection point.
This has to be transformed back to 50 Ohms otherwise the mismatch will cost you the final PA of your rig.
The formula Zo = squareroot (25 * 50) = 35,35 Ohm. With this impedance you transform 25 to 50 Ohms.
First is the easy and cheap way. You take two pieces of a quarter wave each of 75 Ohm coax as used with cable TV and put them parallel. 2x 75 parallel gives you 37,5 Ohms (near to 35,35 Ohms).


Better is to make your own 3dB power divider or impedance matcher as you like. Using freeware software from HP the program APPCAD gives you the possibility to calculate what is needed;


The sizes of aluminium 25x25mm and copper 12mm are standard values which can easily be purchased in PA-land. Zo outcome is 36,6 Ohms ...near enough to the desired 35,35 Ohms....as coax gives you 37,5 Ohms.
The reflection coefficient I measured was around 23dB which gives a swr of around 1.6 good enough for our use.
Another option is the use of 30x30mm aluminium square tube with inner tube copper of 15mm what will come a little bit closer to the perfect Zo ... but I will leave the Zo outcome for you to calculate with Appcad...have fun !





73's Cees PA3FXO

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