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.