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 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:

4. Transmission Line Transformers The input impedance, load impedance and line length are related by the following equations: Where –Z 0 is the line impedance –Z A is the antenna impedance, which depends on the antenna length –f is the frequency –x is the length of the transmission line –f v 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 ψ(x) is known, there is no closed form functional representation for Z A (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 Z A (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 BandsDipole Length Ladder Line Length Lower Resonant Frequency Lower Frequency Input Z Higher Resonant Frequency Higher Frequency Input Z 75/40144 ft 10 in89 ft 6 in3.87 MHz89 Ω7.25 MHz32 Ω 30/1754 ft 9 in36 ft 2 in10.12 MHz88 Ω18.12 MHz39 Ω 20/1777 ft 8 in76 ft 2 in14.13 MHz33 Ω18.11 MHz83 Ω 20/1551 ft 0 in50 ft 8 in14.17 MHz53 Ω21.27 MHz41 Ω 20/1268 ft 0 in46 ft 8 in14.15 MHz33 Ω24.92 MHz82 Ω 20/1048 ft 3 in50 ft 6 in14.08 MHz34 Ω28.40 MHz50 Ω 17/1228 ft 7 in46 ft 8 in18.11 MHz77 Ω24.95 MHz75 Ω 17/1033 ft 4 in62ft 6 in18.08 MHz88 Ω28.42 MHz87 Ω 15/10102 ft 0 in70 ft 6 in21.25 MHz48 Ω28.32 MHz64 Ω 10/616 ft 6 in33 ft 5 in28.40 MHz69 Ω50.10 MHz64 Ω

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) BandsFreq. RatioDipole LengthLine Length 20/171.2877 ft 8 in76 ft 2 in 20/151.5051 ft 0 in50 ft 8 in 20/121.7668 ft 0 in46 ft 8 in

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 Z B 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?