Basic Wire Antennas

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					Two Bands from One Dipole

    Marc C. Tarplee Ph.D., N4UFP
    ARRL South Carolina Section
        Technical Coordinator
    Drawbacks of Existing Two-Band Dipoles
• Multiple dipoles on a common feed.
    – Spreaders are required to separate the two
      sets of wires
    – Proximity of the dipoles makes tuning
    – The additional weight of the spreaders makes
      the antenna heavy and cumbersome to erect
• Addition of a second parasitic radiator.
    – Spreaders are required to maintain proper
    – No simple design rule exists for this antenna;
      much experimentation is necessary to get a
      workable design
    – Tuning can be difficult
• Trapped dipoles.
    – Weather resistant, high-Q traps are not easy
      to construct.
    – Traps add weight to the antenna.
    – Traps increase losses in the antenna.
        Transmission Line Transformers
• When a transmission line in terminated in an impedance not
  equal to its characteristic impedance, the impedance at the input
  to the line depends on the line’s electrical length
• A transmission line can be used to transform a load impedance
  into a more desirable value.
• Example: quarter-wave sections used to match loops.

• The input impedance, load impedance and line length are
  related by the following equation:
        Transmission Line Transformers
• The input impedance, load impedance and line length are
  related by the following equations:

• Where
   –   Z0 is the line impedance
   –   ZA is the antenna impedance, which depends on the antenna length
   –   f is the frequency
   –   x is the length of the transmission line
   –   fv is the velocity factor of the line
    A Two-Band Dipole Using a Transmission
             Line Transformer
• An antenna system made up of dipole antenna of length l, fed with a
  transmission line of impedance Z0 and length x, will have a resistive
  input impedance when the following condition is satisfied:
              Z (l ) cos( ( x))  jZ 0 sin( ( x)) 
                  Z cos( ( x))  jZ (l ) sin( ( x))   ImZ IN   0
          ImZ 0  A
                                                     
              0                   A                 
• The SWR will be less than 2.0 if the next condition is also satisfied:
               Z A (l ) cos( ( x))  jZ 0 sin( ( x)) 
                   Z cos( ( x))  jZ (l ) sin( ( x))   ReZ IN   100
      25  ReZ 0 
                                                       
               0                     A                
• Although the function ψ(x) is known, there is no closed form
  functional representation for ZA(l), so these equations must be solved
• The problem can be solved by using antenna simulation tools to
  create a table of values for ZA(l) which is put into mathematics
  software such as MathCAD® along with the transmission line
  equations. Variables x and l are varied until the antenna has a low
  SWR at the two design frequencies.
                  Two-Band Dipole Designs
• These designs are made from #14 copper wire and 450 ohm ladder
  line with a 0.9 velocity factor

 Bands   Dipole         Ladder Line   Lower       Lower       Higher      Higher
         Length         Length        Resonant    Frequency   Resonant    Frequency
                                      Frequency   Input Z     Frequency   Input Z

 75/40   144 ft 10 in   89 ft 6 in    3.87 MHz    89 Ω        7.25 MHz    32 Ω

 30/17   54 ft 9 in     36 ft 2 in    10.12 MHz   88 Ω        18.12 MHz   39 Ω

 20/17   77 ft 8 in     76 ft 2 in    14.13 MHz   33 Ω        18.11 MHz   83 Ω
 20/15   51 ft 0 in     50 ft 8 in    14.17 MHz   53 Ω        21.27 MHz   41 Ω

 20/12   68 ft 0 in     46 ft 8 in    14.15 MHz   33 Ω        24.92 MHz   82 Ω

 20/10   48 ft 3 in     50 ft 6 in    14.08 MHz   34 Ω        28.40 MHz   50 Ω

 17/12   28 ft 7 in     46 ft 8 in    18.11 MHz   77 Ω        24.95 MHz   75 Ω

 17/10   33 ft 4 in     62ft 6 in     18.08 MHz   88 Ω        28.42 MHz   87 Ω

 15/10   102 ft 0 in    70 ft 6 in    21.25 MHz   48 Ω        28.32 MHz   64 Ω

 10/6    16 ft 6 in     33 ft 5 in    28.40 MHz   69 Ω        50.10 MHz   64 Ω
                        Design Comments

• 450 ohm ladder line (vf = 0.9) was used for these designs because of
  its low cost, low loss, and wide availability. It is possible to redesign
  the antenna systems to use other parallel lines.
• As the ratio of the two design frequencies approaches an odd multiple
  of 1/2 , the length of the dipole is a minimum. For example:

       Bands     Freq. Ratio   Dipole Length     Line Length
       20/17     1.28          77 ft 8 in        76 ft 2 in
       20/15     1.50          51 ft 0 in        50 ft 8 in
       20/12     1.76          68 ft 0 in        46 ft 8 in

• In general, as the ratio of the design frequencies becomes close to 1.0,
  the electrical length of the antenna and matching section becomes
  very long.
• In general, the dipole portion of the antenna system will not be
  resonant on either band (even though the system as a whole is)
                  Design Comments

• These designs have less bandwidth on a given band than a
  single band dipole.
• All designs except the 75/40 m design have been tested. The
  resonant frequencies and SWR were close to that predicted
  by simulation of the design.
• For antenna systems whose ratio of resonant frequencies is
  less than 2.0, the radiation pattern will be similar on both
• The antenna system is fed with 50 ohm coaxial cable that is
  connected to the input of the antenna system (the ladder
  line) through a choke balun.
       Use of Other Types of Feed Lines as
               Matching Sections
• Coaxial cable is not used because it is relatively lossy when used at
  high SWR.
• Other types of ladder line could be used (300 ohm, 600 ohm, etc.),
  but the design of the dipole must be reworked.
• Other line impedances can give an antenna system with a smaller
• 440 ohm ladder line may be used in place of 450 ohm ladder line
  without problem
• Certain frequency ratios cannot be matched when 450 ohm line is
  used, necessitating the use of a different type of ladder line.
                Putting up a Dipole

• A dipole may be erected
  between 2 supports or with one
• A dipole antenna using a
  single support is known as an
• The legs of a dipole may also
  be bent to form an inverted U.
  The bend should be at least
  half way to the end of the wire
                    Closing Comments

• This is about the simplest and least expensive multi-band
  antenna that one could construct.
• There is room for further experimentation:
   – Is it possible to vary l, x, and ZB so that there is a good match on 3
   – Is there any advantage to using thicker elements?
   – Can this technique be adapted to vertical antennas?

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