Two Bands from One Dipole

1
Two Bands from One Dipole

• Marc C. Tarplee Ph.D., N4UFP
• ARRL South Carolina Section
• Technical Coordinator

2
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.

3
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 lines 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

4
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

5
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
• The SWR will be less than 2.0 if the next
  condition is also satisfied
• Although the function ?(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.

6
Two-Band Dipole Designs

• These designs are made from 14 copper wire and
  450 ohm ladder line with a 0.9 velocity factor

7
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
• 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)

8
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.

9
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.
• Certain frequency ratios cannot be matched when
  450 ohm line is used, necessitating the use of a
  different type of ladder line.
• 440 ohm ladder line may be used in place of 450
  ohm ladder line without problem

10
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

11
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?