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Version:

June 14, 2010:
Revised: v2.0

ZL2PD Analog HF Antenna Analyser

A simple antenna analyser for the HF spectrum with a built-in signal generator with 3-digit LED frequency display. Results are shown on an analog VSWR meter. The design was originally published in 'Break-In', New Zealand's amateur radio magazine in Sept/Oct 2005, and is republished here with the permission of the editor.
Note: 'Break-In' is a term used in amateur radio to describe a system which allows another person's signal to be heard in the brief intervals between transmitted Morse Code symbols. 

Introduction

Like many hams, I’ve been playing with antennas for years. Much of that experimentation has been, in hindsight, somewhat hit and miss, relying mainly on SWR meters and grid dip meters. When I built an RF noise bridge (See Reference 1 - Note: All references are found at the bottom of this page), the resulting impedance measurements were a major advance, but it required a full coverage receiver and considerable analysis with a programmable calculator or PC before useful information was obtained.

Recent attempts to build a compact multi-band HF antenna required more careful measurements of antenna impedance. I briefly considered buying one of those commercial antenna analysers frequently advertised these days. Glancing at the cost, I innocently thought “Just how hard would it be to build something like that?” Casting wisdom to the winds, and with a little free time out from my regular jobthis, and a subsequent digital antenna analyser (described here), were produced.

Antenna Analysers

An antenna analyzer is a device which measures the impedance of an antenna. The first, described in this article, is an all-analog device, with not a microprocessor in sight. The second, described elsewhere on this website, is a fully fledged digital meter complete with LCD display. Both cover the full 3 – 30 MHz HF range.

The analog analyzer, as you’ll see from the photograph, is quite small. Powered with a recycled NiCd 6V battery, it contains a signal generator and a simple three-digit LED frequency counter, an impedance bridge and an automatic VSWR meter. A block diagram is shown below.
 
With this instrument in one hand, I can check an antenna literally in a matter of seconds, sweeping it from end to end of the HF spectrum, over a chosen band, or on a spot frequency, and instantly read the VSWR on the instrument’s meter.

Now, of course, it’s perfectly possible to do much the same measurement with an HF transceiver and SWR meter. In my case, I found that approach fairly inconvenient. I had to run back and forth between the antenna and my transceiver each time I made a minor adjustment to the antenna, switching between forward and reverse directions on the meter, setting the SWR meter forward power at full scale each time.

I’ve also used a combination of grid dip meter and an antennascope, the latter being described for several decades in the various amateur handbooks (See reference 2). Aside from the balancing act required with instruments in each hand, it tends to be wildly inaccurate at higher frequencies, and often pretty pathetic at lower bands too.

The device described on this webpage solved all that very quickly. Now, I can take it out by the antenna, make an adjustment to the antenna, sweep the analyser from one end to the other across each range, and see any change immediately. Or, I can simply adjust the antenna and watch the VSWR rise or fall on a spot frequency I have set the unit on. I don’t have to worry about my transceiver overheating, or being affected by a high VSWR. Accidental antenna short circuits or open circuits are no problem for this analyser.

An added bonus is that it can also be used as a compact battery powered HF signal generator with digital display at my workshop bench!

Measuring Impedance at HF

There are a variety of ways to measure RF impedance, and thus VSWR – I think I’ve tried them all in the process of developing these two analysers. The method chosen for this analyser is the bridge method. It’s shown in the diagram below.
The bridge is made from four components; Three are 50 ohm resistors, since this instrument measures the VSWR against the normal antenna system reference impedance of 50 ohms. The fourth component, ‘Z’, is the impedance to be measured. This load impedance can be purely resistive, purely reactive, or, more often, it’s a mix of both. This same circuit is the basis of a return loss bridge.

The bridge is driven by an RF signal generator. In theory, by measuring the voltages around the bridge, it’s possible to determine not only VSWR, but also, with some careful calculations, the resistive and reactive components of the impedance being measured. In practice, however, it’s not quite this simple.

Firstly, the two RF voltages shown on the diagram above, the forward and reverse voltages, just as in a normal SWR meter, must be measured accurately with RF detectors. These need to be able to measure a wide range of voltages, typically over a dynamic range in excess of 30 dB, say from 10 mV to several volts. For accurate results, the detectors must also be linear across this 30dB range, and reasonably well matched across the HF spectrum. Diodes fit the requirement well if carefully selected, provided other side effects can be minimised.

Germanium diodes are nearly perfect for this, but have become hard to find, and expensive. Hot carrier diodes, essentially a silicon diode with low forward voltage drop, are now the device of choice. Unfortunately, not every hot carrier diode will do, but the regular parts suppliers can supply suitable diodes.

This meter uses OA91 or OA60 diodes. These may appear to be the same part number as the well-known germanium diodes of old, and even be described in the supplier catalogs as such, but a quick check with a meter will quickly disprove that fact. The ones I purchased were all, without exception, hot carrier diodes.

Other detectors were tested, but many proved to have poor linearity or limited frequency response, or involved hard to find, near-obsolete or expensive parts. Examples of ideal detectors include the Analog Devices AD606 or AD8307, or the now obsolete Motorola MC3359. I could build a great detector with these parts, but it's getting increasingly hard for many of us to duplicate such designs. Here, in this meter, the emphasis is on simplicity.

Operating RF Oscillators on 6V

A relatively high output level is required from the signal generator source used by the bridge due to the use of diodes as the detector and the losses in the bridge. In this case, a minimum of 3Vpp into a 50 ohm load is necessary. Nothing less will do. For example, using signal generator voltages of around half this level prevent VSWR measurements below 1.3:1.

The design of the signal generator is driven by the need to cover as wide a frequency range as possible while delivering an almost constant output level across this range, and using readily obtainable parts. In addition to this, the output must be extremely pure. Any sign of distortion on the waveform can quickly lead to large measurement errors.

An early design decision was to use a 6V supply rail. Probably, it was a combination of available batteries, a box on hand which was ideal for that number of NiCd cells, keeping the weight of the instrument to a minimum, and a desire to avoid lossy regulators. In the process of the design, however, my batteries proved to be faulty and the box too small. But, regardless, I kept pursuing the idea of a 6V supply rail!

In the various circuits tested, I found I could get either a wide tuning range, or high output levels, or a flat output across the spectrum, or low distortion. Combinations of these factors were more elusive. The circuit described was one of only two found to be satisfactory. (The other, although simpler, required a minimum supply voltage of 9V)

The main device used in the prototype oscillator is a Motorola MC3346 transistor array. This IC contains five good RF transistors. It is perfectly feasible to use other variations of the same device made by other vendors, such as LM3046 and CA3046, or to replace the IC with five discrete RF transistors. The excellent and low cost PN3563 is a good choice, and some may still be found in odd corners. Other alternatives from a well stocked junk box might include 2N3563, 2N918, 2N5770, PN5770, and the 2SC1906. I noted that using the PN3563 or 2SC1906 in the oscillator gave less output rolloff above 30 MHz, a useful improvement.

Important features of the oscillator include the lack of any capacitors in parallel with the main oscillator coil and tuning capacitor. This maximizes the tuning range. The other feature is the amplitude feedback (via D5, D6 and IC6c) which provides amplitude control. The RF output is picked off the gate of Q1 with another FET, sharing a common gate resistor. It looks a little odd, perhaps, but it works, minimising oscillator loading.

The toroid used for T1 and T2 in the generator's output stages were wound on small 9mm diameter ferrite cores recycled from cordless phones. These were used for RF filters on the battery leads in the phones. Amidon FB-43-2401 cores are suitable replacements. These are not critical, and even pairs of miniature ferrite beads, which were also tried, seemed to work reasonably well.

The Frequency Counter

It is perfectly feasible to build the analyser without this digital display, relying on front panel calibration to set the oscillator frequency. It’s also equally possible to build a frequency counter for this device using a microprocessor. I have three or four different 8051-based frequency counter designs scattered around my bench. (Several are also described elsewhere on this website) But these pose significant extra costs and hassles for potential builders lacking a computer, a microprocessor programmer, and all of the required software.

I’d always wanted to build a "plain vanilla" CMOS frequency counter ever since I’d seen the circuit originally in one of Pat Hawker’s books (Reference 3). Since it only had to display frequency to the nearest 10 kHz, I believed it might also be possible to avoid the need for a crystal as the counter's reference clock. And so it proved.

This counter is very simple to build, and suitably accurate for this use. Close scrutiny of the circuit will reveal that the CMOS is powered from 6V. This voltage is perfectly OK for most CMOS ICs, but it is on the edge of the acceptable supply voltages for the 74HC4060 divider IC. Actually, with a freshly charged NiCd, this supply voltage can rise to almost 7V. Probably not so good, but no problems have been noted with the arrangement shown, even after many months of use. Those with a more nervous disposition can add a 5V regulator (78L05 or LP2951 etc), and the necessary higher battery voltage to allow for the regulator voltage drop, if desired.

Do not use a Fairchild 74HC4060 chip in the counter, nor any standard CMOS CD4060. They have unacceptable performance, the latter only able to reliably operate up to about 5 MHz. A Philips 74HC4060 is strongly recommended. It’ll cheerfully work up to 80 MHz in most cases while the Fairchild struggles to divide frequencies above 24 MHz on a bad day. It’s all down to the silicon process used in the chip foundry.

Note: The above design uses a CD4047 as the counter's main clock reference oscillator. This chip can be quite hard to find. An alternative arrangement is shown in the Digital Dial schematic elsewhere on this website.

The Analog VSWR Meter Circuit

With the oscillator, counter and bridge design resolved, the last remaining section to design was the VSWR meter. This must take the forward and reverse voltages from the resistor bridge and convert them into a DC voltage proportional to VSWR.

Other analog ‘VSWR Calculator’ or ‘VSWR Computer’ designs typically need many op amps and a myriad of fine adjustment presets. Again, it would be possible to adopt the A/D converter and microprocessor approach, but that temptation was once more firmly rejected.

For this section of the design, I was able to reuse some circuitry developed as part of a previous design for an automatic HF antenna tuner. At its heart is the simple and elegant VSWR meter design using a circuit originally designed by Udo, DL2YEO (Reference 4), which is possibly based on the same principles used in an earlier published design. (Reference 5). The circuit converts forward and reverse VSWR bridge voltages into a pulse-width modulated square wave. The mark and space outputs generated by the two input voltages results in an average DC voltage proportional to VSWR.

This antenna analyser uses a somewhat modified arrangement from that developed by Udo, DL2YEO, with several op amps added to amplify the forward and reverse bridge voltages with the correct phase relationship. It’s a neat trouble-free circuit, and only requires a single full scale meter adjustment to allow for a variety of different meters. This circuit is ideal for use as a QRPP VSWR meter, given its sensitivity. It
is also a great VSWR detector to use with a microprocessor, especially those without A/D converters. It's easy to measure the resulting pulse widths very accurately. These can then be used to calculate VSWR using the cheapest of microprocessors. This approach also tends to be more resistant to RF interference.

Odds and Ends

The instrument could be powered either with dry cells, preferably C or D size, or NiCd cells. Current drain is around 100mA. The prototype used five 1.2V NiCd cells recovered from dead or discarded cordless phones. The circuit permits the battery to be recharged only when the instrument is switched off to avoid the higher charger voltages reaching the circuitry. Recharging requires 8.5V to 15V at 50mA. A red LED (D3) lights when the battery is charging. The circuit around Q3 is designed for the usual 14 hours recharge time.

It is not possible to use NiMH or lithium cells with the charger circuit shown. These batteries require other charging arrangements.

A tiny 3mm diameter ferrite bead wound with at least three turns (RFC2) is used to feed voltage to the LM555 in the VSWR circuit. Do not forget to use this. All standard 555 chips deliver massive pulses to the DC rail when the output in the IC changes state, and using this ferrite bead is the only proven method to prevent problems.

Construction

The prototype was built partly on a PCB, with the RF oascillator built in 'Mahattan' or ‘dead insect’ style, this featuring ICs soldered legs-up directly on unetched PCB material. For one-off designs like this, it’s arguably the cheapest method. It also benefits RF circuit performance as a result of the large copper mass at earth potential under all of the circuitry. The photographs show the main, but gruesome, details of my prototype.

The bridge diodes should ideally be matched using a volt-meter to select diodes with similar forward voltage drop. One of each pair goes in the bridge, the other into the subsequent matching op amp stage.

Coil details are not shown for the oscillator. Most people have a wide range of coil-related components in their junk box and in this case, it’s best to spend a quiet hour or two simply winding half a dozen coils and selecting the three which best provide coverage across the HF spectrum. I built the prototype coils using a couple of rewound 455 kHz IF transformers and a coil "liberated" from an old shortwave radio.

For this reason, it’s best to build the frequency counter section of the circuit first, and calibrate it using a known crystal oscillator or another frequency counter. Then, build the oscillator and wind the coils, using the counter to help select a suitable set of coils. In some cases, and particularly if your variable capacitor has less range than the one used in the prototype, it may be necessary to wind four coils. The variable capacitor is a cheap plastic unit obtained from a local retailer, with both FM and AM capacitors connected in parallel. You can also use a recycled variable capacitor from any cheap AM/FM portable radio.

I’ve also provided my front panel layout artwork and a meter scale in the Download section below. These can be copied onto plain paper. The front panel artwork can then be covered with clear self-adhesive plastic to form a reasonably durable front panel for the instrument.

Alignment

The combination of the variable and fixed resistor (RV2 and R43) connected to the VSWR meter (M1) allow the full scale setting of the meter to be adjusted to suit a wide range of different meters. The prototype used a lvery cheap 5 mA FSD meter.

Leave the meter disconnected initially. Turn the analyzer on, and set the oscillator variable resistor (RV10) for an output level of at least 3Vpp with the analyzer connected to a 50 ohm load. (Two 100 ohm 1/4w resistors connected in parallel is a perfect load for this test)

Then, connect the meter into circuit, and, with no load connected, and with the oscillator set to the lowest frequency, adjust RV2 for full scale meter deflection (FSD). In some cases, you may need to adjust the value of R43 to suit your meter.

Now set the oscillator to the highest frequency, around 30 MHz, and check that the meter does not fall more than one or two widths of the meter needle from FSD. Any more than this indicates your oscillator output is falling at high frequencies, and you may need to check the oscillator amplifier stages for correct operation, or replace the oscillator transistors with better RF devices.

Meter alignment, to set the spots on the meter for specific VSWR values, can be achieved using a set of resistors connected to the input port of the meter. I used 10, 25, 50, 100, 150, 200, 300 and 500 ohm resistors, some made from several resistors. My meter artwork is found in the Download section below, and it may suit your meter too.

You can expect some slight variation in meter readings as you tune across the frequency range with a specific load, but this should be less than 10% FSD. If you see a greater variation, carefully check the oscillator output purity and double check that you have used the correct diodes, and that they are matched.

Using the Analyser

Simplicity itself! Connect the antenna of choice to the instrument and tune to determine VSWR at any frequency.

The only issue I’ve noted is to avoid using the instrument close to nearby transmitters. The meter is relatively sensitive and induced voltages on the antenna being tested can cause some confusing results.

A Digital Version

For those wanting even more details about their antenna’s impedance, the next antenna analyser I went on to design may be suitable. It's complete with a 8051-family microprocessor and LCD, it displays frequency, resistive and reactive impedance components, as well as VSWR. It covers the entire HF spectrum too.

You can find the details here.


References


1.    R.A. Hubbs, “Improvements to the Rx Noise Bridge”, Ham Radio, Feb, 1977, p10-20
2.    One example: W.I. (Bill) Orr, “Radio Handbook”, 23rd Edn, Howard Sams, 1981, p 31.18
3.    Pat Hawker, “Amateur Radio Techniques”, 6th Edn, RSGB, 1978, p327
4.    The original, along with an extensive description of the circuit, can be seen on DL2YEO’s website at www.qrp4u.de
5.    D.B. Lawson, “A Simple Computing VSWR Indicator”, Ham Radio, Jan 1977, p58-63


Downloads

Click here here to download the front panel artwork.

Click
here to download the artwork for the meter. This may not suit your meter, however, so test your  unit first before using this artwork.
 



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