A Two-Band Antenna Wire Loaded with an L & C Parallel-Tuned CircuitAuthor: R.J.Edwards G4FGQ ©25th September 2004
The LC parallel-tuned circuit is constructed exactly like an antenna trap. It is located in the antenna wire and used as L or C loading at two other resonant frequencies. Parallel-tuned circuits have a reactive impedance on either side of resonance. On the low frequency side there is a +ve inductive reactance. On the high frequency side there is a -ve capacitive reactance.
The purpose of the LC loading components is to detune the antenna wire from its natural 1/4-wave resonant frequency. When the loading circuit is behaving as an inductance the antenna resonant frequency is reduced. When the loading circuit is behaving as a capacitor the antenna resonant frequency is increased. The pair of resonant frequencies depend on the LC ratio, on wire lengths, and on the location of the LC loading circuit along the wire.
For a given loading location along the wire and a given pair of resonant frequencies this program calculates the L and C loading-component values. The pair of frequencies lie above and below the unloaded resonant frequency of the wire. The LC circuit behaves as an ordinary trap at its own resonant frequency which, by experiment, can sometimes be arranged to fall into a third amateur band. But the wire length to the trap location will not be a 1/4-wave at the trap frequency.
"Measurement" of Antenna Input Impedance
The program contains an independent test frequency generator which can be swept over a wide range at 500Hz intervals. This enables the antenna input impedance versus frequency to be 'measured' by either slow or fast sweeps. Behaviour in the vicinity of resonant frequencies can be investigated. Information can be used for choosing a feedline and designing impedance matching arrangements.
Validity of Calculation of L and C
Certain settings of the loading position along the wire are impossible. There may be no values of L and C at which two resonant frequencies can be obtained. (A warning message appears on the screen when impossible conditions exist.) This occurs most frequently when the loading position is too near to the far end of of the antenna. Also, impractical values of L and C often occur, values being either too large or too small or are otherwise unsuitable/unavailable.
Basic Program Operation
Select the required pair of resonant frequencies, one above and one below the natural unloaded resonant frequency of the antenna. Then vary the loading position along the antenna until the most convenient values of L and C appear. Do not load the antenna near to the far end. Prefer keeping to the middle third.
Resonant frequencies in the frequency response are recognised by observing that antenna input reactances pass through zero as the sign changes. At an ordinary resonance input resistance passes through a minimum. At an antiresonance input resistance passes through a maximum. There is always an antiresonant frequency between the selected pair of frequencies. The antenna is unusable at antiresonances because input impedance is too high to be efficiently coupled to a line.
The SWR, normalised relative to 50 ohms, is calculated only to indicate the SWR bandwidth in the vicinity of resonance. The shorter the antenna relative to the wavelengths at the two resonant frequencies the smaller the bandwidth.
Choosing Wire Lengths and Frequencies
To ensure valid calculations the pair of resonant frequencies must always be arithmetically or geometrically disposed about the natural resonant frequency of the unloaded antenna wire. Overall antenna length, therefore, is decided by the average or geometrical mean of the pair of frequencies, or vice-versa.
The difference between the two resonant frequencies, or their ratio, is subject to calculated values of L and C being feasible and practical. Usually it will be required for both frequencies to fall into amateur bands. Self-resonant frequency of the LC loading circuit is of use only for testing during its construction.
The following combinations are listed in order of overall antenna length. They may be used as starting points for selecting L & C component values.
Note: The ratio FHigh/FLow lies in the range 1.4 to 3.7, averaging about 2.3.
Initially the best location for the loading circuit is half way along the wire. At this point L & C values are nearly always valid and can be varied later to obtain more convenient values. Varying overall antenna length also produces interesting changes in L & C and can be used to keep the loading position well away from the far end of the antenna. The most efficient location is when the L & C position is in the middle third of the wire and the two resonant frequencies are between the ratios 1.5 and 3. Too close together and they interact with the intervening antiresonance and with each other.
An important figure is the series loss resistance of the L & C circuit. It is related to the Q of the coil as the circuit becomes nearer to self-resonance. When the value increases to a few tens of ohms at the test frequency a large fraction of transmitter power may be dissipated in the loading coil as can occur in an ordinary tuner. It is suggested values above a few ohms should make associated L & C values INVALID. It depends on the transmitter power. There's no way of distinguishing coil dissipation from radiated power when the antenna is in use.
The length in wavelengths of the two wire lengths at the higher resonant frequency are displayed for the interest of experienced experimenters.
Small values of loading capacitor C MUST take into account the stray circuit circuit capacitance. Trim capacitance by resonating it against the inductance of the loading coil at the calculated theoretical resonant frequency. But in general, small C values are best avoided.
Sometimes the antenna's actual resonant frequencies, as determined from Zin at the test frequency, will not coincide precisely with the chosen frequencies. This is due to the small effect of coil Q and related effects of antenna wire resistance. The program assumes a typical coil Q of 300 at all frequencies, whereas, for a given physical size of coil, Q increases with frequency, and also size.
Actual values are displayed of the L and C loading reactances inserted in the antenna at the upper and lower frequencies. Values of a few thousand ohms may be affected by an antenna's environment. Detuning may occur when an antenna wire swings about in the wind. So avoid very small values of loading capacitance.
The results of this program can be applied also to centre-fed dipoles by connecting two endfed antennas back to back with an L & C circuit in each half.
The advantage of an L & C antenna over an ordinary trapped dipole is that the whole of the antenna is used as a radiator at both frequencies. With a trapped dipole, at the higher frequency, the length beyond the trap is not used and radiating efficiency may be lower. But this depends on the feedlines used.
A disadvantage is the difficulty during construction with trimming both L and C values to be exact in conjunction with two antenna wire lengths. Trimming a wire length affects the values of L and C needed for the other frequency.
At the lower resonant frequency the antenna overall length due to loading is less than 1/4-wave. At some QTH's a short antenna is a desirable necessity.
There may be no strong advantage over any other type of a simple loaded wire antenna. But precise L & C loading values are now available via this program and dedicated experimenters may be induced to go ahead. Please let me know!
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