Doing it by Design
Eric Edwards GW8LJJ has a versatile antenna analyser to build.
Eric Edwards GW8LJJ has a versatile antenna analyser to build.
This project is a development from a noise bridge that was produced for the Blackwood Amateur Radio Society and which many of the members have built. This is a standard bridge where resistors or capacitance are balanced. There are two controls, one for impedance (resistance) and another for reactance (capacitance). In the standard noise bridge, a receiver is used to find the correct ‘balance’ from the antenna, which ideally should be 50Ω to produce a 1:1 SWR.
A long-standing friend and indeed my mentor for many years, Cess GW3OAJ, suggested that I made it into a self-contained bridge that did not need a shack receiver to observe the results of the antenna impedance matching. It was duly modified with a variable frequency oscillator (VFO) replacing the noise generator and the receiver replaced with an LED. This is illuminated when the antenna is not resonant at the chosen frequency. When the SWR is at 1:1 the LED is extinguished and illuminates again when the impedance control is turned either side of resonance. A white LED is preferred because it has a higher brightness level compared to a red or green LED, which would otherwise make it difficult to see the variations of intensity in daylight. Either type of LED can be fitted. There is provision for a 5V FSD DC analogue meter to be placed across the LED anode load resistor, which follows the LED indication. The meter dips to zero when the LED extinguishes and goes to towards full scale at full brightness of the LED. A digital meter is not recommended in this position as the digits will be too slow in changing whereas an analogue panel-type meter has a fastmoving pointer. A digital meter can be used if it has an analogue readout, Fig. 1.
Taking away the Noise
A VFO was needed to replace the noise generator and it had to be reasonably stable and cover all of the HF bands at approximately the same output level but had to be simple and low cost. Transistor VFOs were tried but without much success for the requirements needed. A while back, while playing with VFO and signal generator ideas, I came across a very useful chip (integrated circuit) that is a wideband oscillator with an average output on all bands and is DC controlled. Studying the datasheet again prompted me to choose the device as the VFO for the analyser. The chip used is an Analogue Devices LTC1799 and is a ‘resistor set oscillator’ − in other words the oscillator frequency is set by a DC voltage. With this device the frequency is between 1kHz and 33MHz.
TheVFO
The VFO was discovered while searching through data on VCOs (Voltage Controlled Oscillators). It produces a very wide frequency range and is easy to use. It is a 5-pin surface mount device, which is very small but is also available on an adaptor so that it can be used as a through-hole device. The supply is 5V so a 78L05 regulator is used to obtain this from the 9V battery. The bands are selected with a rotary switch and a set of fixed 1% resistors. These are selected and used in conjunction with a 10kΩ linear potentiometer. The bands cover approximately 1.7MHz to over 35MHz. Five are used to ease the tuning and selection of the wanted amateur bands. Although 1% resistors are used throughout the oscillator design, there could be some slight variations in the band edges but it will be very close to those labelled. A counter can be connected to provide an exact frequency indication. The bands labelled on the front panel are colour-coded around the VFO control to aid identification.
Pin 4 on the LTC1799 is a selectable divider. Grounding this pin gives a division of ÷1, while leaving the pin floating (open circuit) it is set at ÷10, and by placing the pin at 5V (the supply voltage for the chip) it is ÷100. In this project we are using it at ÷1 so pin 4 goes to ground.
The Circuit
The circuit is shown at Fig. 2. This project is a standard ‘noise’ bridge design but using a VFO in place of a noise generator. This means the bridge can be used stand-alone so no receiver is needed in analysing the antenna for correct impedance matching. The oscillator is switched into five bands and with a frequency tuning control it covers the HF amateur bands. There is provision for an external frequency counter to display the exact frequency as well as the amateur band identification on the tuning control. The impedance control is labelled to show a 1:1 SWR, which means the antenna impedance is at 50Ω, and 2:1 SWR is shown at both high resistance (100Ω) and low resistance (25Ω). Others labelled are 3:1 (150Ω) and 4:1 (200Ω). The reactance is set at mid-way on the control to show a null in reactance and for further ‘dipping’ of the LED. The VFO is
connected to the transformer primary at the top end of the winding and the bottom is connected to ground. The secondary uses all three connections with the top of the winding connecting to the impedance and reactance controls. The centre of the secondary winding is connected to the antenna terminal. It also has a 50Ω resistor, which is grounded when a push button switch is pressed for a 1:1 calibration. The bottom of the winding is the output via a 100nF capacitor to the Darlingtonpair transistors.
TheToroid
The toroid is shown in Fig. 3. There are four lengths of 26-gauge copper wire that have been twisted to form a quadfilar winding. Wind six turns through the hole of the yellow toroid (T50-6). Start the windings by passing the wire through the hole to leave sufficient to fit onto the board. I suggest leaving 25mm protruding. There has now been one turn on the toroid (wire passing through the hole is one turn). Carry on passing the wire through for another five turns making six in all. Spread the windings so that they fill (or nearly) the toroid. Separate the wires and at both ends scrape with a knife to clean away the enamel, then lightly solder to produce tinned leads. There are four leads and two sets of two are required to make the transformer. Check the continuity of the leads and pair off two sets. The four ends of the leads on the left are the start and the ones on the right are the end windings. Of course, that is for reference because the start and end can be the reverse (left or right).
However, with each set there will be a start and an end. When fitting onto the PCB it will form a centre-tap transformer. The PCB tracks make the centre connections. If you have correctly separated the windings, you will have produced a 1:1 primary and secondary transformer with centre taps. The inductance of each side will be approximately 1.1µH (end to end, as measured at 10kHz on my bench LCR bridge).
Choice of LED
The LED used is 5mm and can be a standard red type but for good visibility in sunlight it will be better to use a white LED. Both will be in the ‘picking list’ sent upon request. The white one is very bright and both dip down to almost extinguished when at 50Ω resonance (1:1 SWR). The white LED can
not go completely out because it is in series with the transistor so cannot be absolute zero. The red LED looks extinguished at 50Ω as it is lower in brightness and on dip it appears to be completely out. So, it’s a tradeoff – you choose. The intensity of the LED can be adjusted by changing the value of the resistor from the base to ground on the first transistor of the Darlington pair. The bias of these resistors is set with the 1MΩ resistor from the base of the first transistor to the supply voltage (9V) and a 150kΩ resistor from the base to ground. The 150kΩ resistor was chosen to set the intensity of the LED and also the extinguishing of it. A red LED may not be quite bright enough, especially in daylight, which is why the white one was chosen. If a red one is preferred, this resistor may need to be changed to a slightly higher value such as 180Ω to provide sufficient illumination. A little experimentation can be carried out with the resistor values between, say, 120kΩ and 180kΩ depending on the LED used.
Adding a Meter
A voltmeter with a 5V FSD (Full Scale Deflection) can be connected to the pins as marked on the PCB, which is connected in parallel with the LED anode resistor (1.2kΩ). Any analogue voltmeter or multimeter such as an ‘AVO’ can be used but a suitable meter is shown in the reference section. As explained earlier, a digital meter is of little use as the object is to see a voltage ‘dip’, which is much more easily seen with an analogue type.
Mounting in an Enclosure
While this is not a full kit (no case or control knobs), I have recommended a suitable enclosure in the reference section. Control knobs can be what you have or can obtain and will be your own choice. The variable capacitor and bandswitch have short spindles and there may not be sufficient length to take a standard control knob when the PCB is fitted inside a case. A push-on control knob with a screw thread on top of it to tighten onto the spindle can be used for the controls with shorter spindle lengths. The types used can be seen in the main photo, and are the two right-hand control knobs. Another method I have used is to enlarge the holes in the case to take standard control knobs and fit the knobs on the spindles before pushing the whole PCB through the case.
PCB
The PCB is shown at Fig. 4 and is a singlesided FR4 type with a ground plane on the copper side. Pins are fitted to allow connections to the on-off switch, external counter (frequency meter), antenna socket (SO239) and the calibration push button. There are also pins fitted to enable the potentiometers to be fitted onto the PCB. The method of attaching the potentiometer tags to the pins is shown in Fig. 5. The tags drop onto the pins and are then soldered. The photo also shows the variable capacitor with the tags
pushed through and soldered on the copper side. The rotary switch (band select) is mounted but the tags have to be connected to the pins next to it using suitable gauge, 22swg (0.7mm) or similar, hookup wire. One important point to remember when wiring this switch is to observe the pin numbering. The Common on SW1A is at the bottom with Pin 1 as the next one up and as it increments it should be noted that pin 4 is at the top and pin
5 is below it. A set of small pins, Mac 8, Black Terminal post (RS part number 5006508) are used to secure the transformer connections. This is better than pushing the wires through holes in the PCB. The pins look like split pins with a hole on top and a spacer. The (two) ends are pushed through the PCB holes and slightly spread then soldered. The transformer wire can be fed through the small hole at the top of the pins and soldered.
The LED is mounted on the copper side, Fig. 6, with the leads pushing through to the other side and cropped, until the two notches on the LED pins are reached and soldered. This allows the LED to stand off the board 4mm, allowing it to just protrude through the case. A battery holder is fitted to enable a PP3 battery to be used for portability. The complete PCB fitted in the case is shown at Fig. 7.
Impedance Matching
The impedance of an antenna might contain resistive and reactance components and the aim is to obtain purely resistive impedance, albeit it may not be possible to achieve that but it can be close. The SWR can be calculated by taking the ratio of the impedance to 50Ω. An antenna with 50Ω impedance is 50/50 or 1:1 and with an impedance of 100Ω is 100/50 or 2:1. For impedances less than 50Ω, the SWR is calculated using SWR = 50/R (impedance). An antenna with an impedance of 25Ω will be 50/25 or 2:1. There is a reactance control alongside the impedance control, which is adjusted to minimise the reactance when searching for an SWR of 1:1 (see setting up).
Calibration
The calibration button is used to set the analyser to 50Ω (1:1 SWR). This is done with no antenna connected. With the unit switched on press the ‘cal’ button and this will show the LED illuminated. Turn the impedance control knob until the LED is extinguished or go as low as possible if it has a white LED fitted. Adjust the reactance control for any further ‘dip’ in the brightness. The impedance control knob should be pointing to 1:1. If not, then adjust the control knob until it is pointing at the correct mark. Check also that the reactance control is set mid-way. Adjust the control knob if not. The calibration is now complete for measuring the antenna impedance. Any frequency can be used for this test. The actual frequency can be calibrated with a (calibrated) frequency meter (counter) and the control knob for the frequency can be set for the band required. It is suggested that the band centre frequency be used to set the frequency control knob.
Setting Up
(A useful guide by Cess GW3OAJ)
Let us assume we will check the antenna for 80m.
1. Set the band switch to position 2.
2. Set the VFO to the 80m section.
3. Set the capacitor to mid-position (from now on we will call that the Reactance pot)
4. Set the bridge zeroing pot to 1:1 (50Ω).
Switch on the analyser and note the LED brightness, adjust the bridge control potentiometer to extinguish the LED. If it only dims it, adjust the VFO for best results and then turn the Reactance potentiometer up or down to cancel out any reactance. If this is not enough, readjust the VFO.
Note: It is important to set the band setting to avoid finding a dip at a frequency that the antenna is capable of resonance at a non-amateur radio frequency and giving a false reading. The absence of a digital reading of frequency could cause some confusion to inexperienced amateurs.
Reference
• Test Equipment for the Radio Amateur. Second Edition, RSGB.
• Test Equipment for the Radio Amateur. Fourth Edition, RSGB.
• LTC1799 datasheet.
• Voltmeter: 5V FSD. 85C1/85L1 Analogue Panel Meter.
• Suitable case: Farnell Part number 1426563.
• Picking list: GW8LJJ.
• An A4 size PCB layout will be supplied with the PCB.
• An A4 size circuit can be supplied on request.
Acknowledgements
Cess Davies GW3OAJ for testing, suggestions and evaluation along with the guide notes. In memory of Dave Newell G0AKS (SK) for his enthusiasm for this project. Ray
Koster G7BHQ for checking the text and contents.