Autor Tema: Spajanje 3 antene u sistem  (Posjeta: 553 )

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Offline 9a6c

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Spajanje 3 antene u sistem
« : 17. Kolovoz. 2015, 05:10:28 »
Ponekad je vrlo teško podići 4 antene u vertikalnom stacku, mjesta na stupiću ima samo za tri. Jedno od praktičnih rješenja je koristiti kabele od 50 i 75 za prilagođenje da bi se postigla impedancija od 30 oma za prijelaz od 17 na 50 oma. Kabeli se spoje paralelno uz što kraće slobodne izvode, a poželjno je sve isprobati prije dizanja u zrak, da ne bude suza :D. Kabeli se mogu smjestiti u kvadratnu aluminjsku cijev na koju se montiraju konektori, malo su prekratki za savijanje ako su 2m u pitanju. Taj je prijedlog prihvatio i DK7ZB pa objavio na svojoj web stranci:

Radiation resistance Z of the Yagi    

Lambda/4 Coax-cable
   

 Impedance of the line

12.5 Ohm
   2x50 Ohm parallel    25 Ohm
18 Ohm    50+75 Ohm parallel    30 Ohm
28 Ohm    2x75 Ohm parallel    37.5 Ohm

12.5-Ohm: Good for high-gain  yagis with small bandwidth in the 6-m- and 2-m-Band, losses tenable

18-Ohm: Can be used on 6m and 2m, medium bandwidth

28-Ohm: Best balance for all antenna parameters on 2 m and 70 cm
   

Update: The 18-Ohm-Match in practise

It is possible to use 50- and 75-Ohm-cable in parallel. The condition is the same VF. That means we can use cable with Full-PE (VF=0,67) or Foam-PE (VF about 0,82) with the same lengths. I have tested this method in various antennas with excellent results.

For example two impedance matches for 50-MHz- and 70-MHz-Yagis:

 
   

Here two cables RG58 (50 Ohm) and RG6 (75 Ohm) in parallel. The two cables have different diameters,  but the same VF of 0,67. The 70-MHz-Yagi has a perfect match from 18 to 50 Ohm.
   This is a 50-MHz-Yagi with cables of VF=0,82. 50 Ohm with Aircell-5 (5 mm diameter) and a CATV-cable 75 Ohm with 6,5 mm diameter. Perfect match from 18 to 50 Ohm in a 50-MHz-Yagi.