Docstoc

Basic Wire Antennas

Document Sample
Basic Wire Antennas Powered By Docstoc
					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
      difficult
    – 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
      spacing
    – 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
  numerically.
• 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
  bands.
• 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
  dipole.
• 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
  support.
• A dipole antenna using a
  single support is known as an
  “inverted-V”
• 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
     frequencies?
   – Is there any advantage to using thicker elements?
   – Can this technique be adapted to vertical antennas?

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:1
posted:5/2/2013
language:Unknown
pages:11