Figure 1. Antenna schematic
There are five ways to make a multiband GP:
A structurally complex antenna. Made to have a clean 50-ohm input impedance on all bands. An example of such an antenna. Plus: Convenient to operate. Minus: requires a complex design of precisely calibrated dimensions.
The antenna is relatively simple by design, but requires a separate LC circuit for each band, with mechanical (relay or manual) switching of the band LCs. Example of such an antenna. Plus: many bands and easy tuning. Minus: you need a lot of relays/switches and a control unit for them. Or go to the base of the GP every time you need to change the band.
A simple antenna of almost any size and a tuner in the base. Plus: easy to adapt to local conditions and many bands. Minus: the tuner is not cheap and limits power (and a tuner for at least a kilowatt would cost as much as a good transceiver).
A simple antenna of strictly defined dimensions and a dual-band LC match unit without switching. Example of such an antenna. Plus: easy to use and relatively easy to understand control matching procedure. Minus: two bands from one antenna are not enough.
It is a simple, well-defined antenna with 3 bands and no any switching, but with complex LC match circuit. Example of such an antenna. Plus: convenience in operation. Minus: difficult to make and matching.
In this article, we will try to cross the advantages of methods 2 and 4. To do this, we will find GPs of such a size, that one dual-band control system without switching it in the bands 7 and 14 MHz, and the other one in the bands 10 and 18 MHz. Then only one pair of relays would be required for the 4 bands (respectively with the simplest control: by feeding the control voltage to the central core of the feeding coaxial). And matching of each of the dual-band LCs would be relatively uncomplicated.
How do weu go about designing a combination of antenna size and LC circuits to achieve multi-band matching?
In this case, we used a dual-band antenna and with the help of excellent ImpedanceMatchin software by Andrew UR5FFR in combination with MMANA-GAL optimization we found a 10/18 MHz matching scheme and sizing correction for the GP itself to make this matching possible.
The found circuits and ratings were then built into the models and the final polishing was done by the MMANA-GAL dual-frequency optimization.
The results for the 30 mm diameter pipe vertical are shown in Fig. 1. The relays in the schematic are shown at 7 and 14 MHz. When they are switched, the antenna operates in the 10 and 18 MHz bands.
The 5 pF capacitor shown in gray in Fig. 1 simulates the mounting capacitance of the base isolator.
Model files in 7 and 14 MHz bands 7_14.maa, in 10 and 18 MHz bands 10_18.maa. The ground loss resistance is set to 15 ohms, i.e. corresponds to a system of several radials lying drex=ct on the ground.
The directional diagrams in the zenith plane over the bands (taking into account losses in the matching device made of quality parts) of the antenna standing directly on the ground in 8 counterweights at 10 m are shown in Fig. 2.
The antenna by band works as follows:
7 MHz
The transmitter of length λ/4 on the ground has a purely active input impedance of 37 ohms. With the addition of losses in the ground, it is about 50 ohms. The series circuit C4 L1 resonates in the middle of the 7 MHz band and therefore adds nothing to the impedance. Capacitor C2 because of its small capacitance has little effect in this range. The plot of the VSWR in this range is shown in Fig. 3.
14 MHz
The transmitter has an electrical λ/2 and therefore a very high input impedance. The L1 C2 L-shaped circuit matches it at 50 ohms. And the capacitor C4 has little effect because due to its relatively high capacitance its reactance in this range is small. The plot of the VSWR in this range is shown in Fig. 4.
10 MHz
The GP have an electrical 0.34λ and a highly complex input impedance. It is matched at 50 ohms by the L-shaped circuit C3 L2. And the inductance of L3 does not affect much, because its reactance is noticeably less than the large inductive component of the impedance of the pin itself. The plot of the VSWR in this range is shown in Fig. 5.
18 MHz
The electrical length of the transmitter is slightly less than 5/8λ. The series coil L3 removes the large capacitive component of the GP impedance. The L2 coil almost does not show itself in this range because of its large inductance (it behaves like a parallel choke), and the negative reactance of C2 (together with the capacitive component of the post) compensates for L3. The plot of the VSWR in this range is shown in Fig. 6.
As you can see, the bands in all four bands are wide (the VSWR is not higher than 1.5 even on the edges), which, among other things, facilitates matching. Now we are going to talk about it.
It is done down, at the reference isolator (it must be good, above 7 MHz the voltage on it will be high).
Sequence and methodology:
We start on the 14 MHz band (relays in the position shown in Figure 1). By adjusting C2 and L1 we achieve an exact match in the middle of the 14 MHz band.
Move to the band 7 MHz (relay in the position shown in Fig. 1). Without touching C2 and L1, the capacitor C4 is achieving minimum VSWR in the middle of the range of 7 MHz.
Back to 14 MHz and tuning only L1 return the resonance in this band back to the place.
Repeat with decreasing amplitude cyclically tuning items 2 and 3 until the resonances are in the middle of the bands 7 and 14 MHz.
Switching to 10 MHz band (turning on both relays). By adjusting C3 and L2 (without touching L3) we are reaching an agreement in the range of 10 MHz.
Switching to 18 MHz (both relays are switched on) and adjust only L3 match the antenna in this band.
Repeat cyclically with decreasing amplitude adjustments items 5 and 6 until the resonances are in the middle of bands 10 and 18 MHz.
Matching schemes have a good margin for possible deviations in the design of the GP.
So in the pairs of files 7_14-1.maa / 10_18-1.maa and 7_14-2.maa / 10_18-2.maa the tuned matching circuits for two different sets of telescopic pipes are shown. And a pair of files 7_14-w.maa / 10_18-w.maa shows the wire GP.
A pair of capacitors (C4 and C3) with high reactive power will be needed, such as K15U-1. At 1 kW input power (all currents and voltages are given below for this power) C4 must be able to handle at least 1kV voltage and 7kVAr of power. C3 2 kV and 10 kVAr, respectively. C2 can and must be made from a piece of thick coaxial (for example, RG-213 type). 50 Ohm coaxial cables with PE insulation have a capacitance of about 1 pF/cm. Nothing prevents you from making a cable and C3 (like 3 ... 4 short sections in parallel).
The currents in the coils are up to 7 A, so the wire is preferably not thin (e.g. electric wire with a cross-section of 4 ... 6 mm2, stripped of insulation).
Relay K1 works in the matching path, so the voltage on its contacts is no higher than 350 V, and the current is up to 7 A. F.e. RT314012.
The voltage at the pins K2 (i.e. at the pin insulator) can reach 3 ... 4 kV (maximum at 14 MHz), so you need contacts on ceramics with a large gap or vacuum relay.
So a simple 10-meter-high GP will provide good operation on 4 bands. Matching is not very complicated and is done at the bottom. The costs are low and the result is an antenna with a low zenith angles and good efficiency on the 4 bands.