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)) ImZ IN 0 ImZ 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)) ReZ IN 100 25 ReZ 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?
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