ZL2PD Analog HF Antenna Analyser
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.
'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.
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),
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
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
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
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
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)
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
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
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
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.
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
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.
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
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
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.
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.
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
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.
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
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
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.
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
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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