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:

• The SWR will be less than 2.0 if the next condition is also satisfied:

• 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.

100Re))(sin()())(cos(

))(sin())(cos()(Re25

0

00

IN

A

A ZxljZxZ

xjZxlZZ

0Im))(sin()())(cos(

))(sin())(cos()(Im

0

00

IN

A

A ZxljZxZ

xjZxlZZ

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 Length

Ladder Line Length

Lower Resonant Frequency

Lower Frequency Input Z

Higher Resonant Frequency

Higher 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:

• 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)

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

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.

• 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

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?