Sunday, 3 April 2016

Design Notes on my Doublet

This is the design / measurements work I did on the doublet I have been using recently. I did not want to loose/forget this. It is more an exercise of understanding the homebrew open wire line line characteristics, the impedance seen by the tuner than anything else.

Inv V Doublet
Doublet centre at 11m above ground  [using Spiderbeam HD 12m fibreglass pole]
Doublet ends at 7.2m above ground [Tecadi mast / house end]
1:1 200W Gwhip choke balun
0.6m of RG8x between balun and tuner
LDG RT-100 remote tuner
Wire Spec 7/0.2 rapid  - diameter = 0.6mm [copper w/o insulator]
Length per arm = 8.25m



Open Wire Line design/construction
  - built with  SOTAbeams spacers
  - space between wires = 27.5mm
  - wire 7/0.2 spec / 0.6mm diameter  [copper w/o insulator]
  - length 16m



Open Wire line simulated impedance Zo = 540 ohm

Open Wire measurements [with antenna analyser]

   - Velocity factor measured  VF = 0.93
             VF=L*F /492 with  Zi = Zo @ 1/2 WL,
              {1/2 wl measurement with 100 Ohm resistor - F = 8.87Mhz, L=16m }

   - Line impedance - Zo [Ohm] = 570ohm
             Zi = Zo^2/Zl @ 1/4 WL { with Zl = 2.23k resistor /  Zi = 146Ohm}


 Dipole3.exe @ G4FGQ Simulation 
The G4FGQ dipole3 is a useful tool as allows to change parameters and see their impact as well as it gives an estimate for the overall efficiency of the system.





The table below compares the simulated values and measured values with my antenna analyser at the input of the line [balun end].

m - measurements
s - simulated

                          Rs [Ohm]  Xs [Ohm]  SWRs       Rm [Ohm]  Xm[Ohm]  SWRm
F = 8.6 Mhz      84.1           0                1.7            83                 0               1.7            *resonance
F = 3.5 Mhz      7.6             2                6.6            8                   16              6.7          
F = 7 Mhz         328            -1358         115                                                                  *out of range
F = 10.1 Mhz    361             948            58                                                                   *out of range
F = 14 Mhz       55              22              1.5            63                 38             2
F = 18.1Mhz     530            -1561         101                                                                 *out of range
F=  21 Mhz       56              39              2.1            45                 61             3.3
F=  24.9 Mhz    218            -594           37                                                                   *out of range    
F = 28 Mhz       1212          1193          48                                                                    *out of range


With the 16m length of open wire line the remote tuner and 1:1 balun are able to tune/match the aerial on all bands and the simulated efficiency is reasonable apart from 80m as one would expect.

more to follow on 4NEC2 simulations & loading coils for 80m

73 Angel M0HDF




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