There are two cantankerous old wives:

One old wife asserts it is obvious radiation occurs mainly from the middle portion of a dipole because that's where the current is strongest and the magnetic field is most concentrated.

The other old wife asserts it is obvious radiation occurs mainly from the ends of a dipole because that's where the highest voltages occur and the electric field is most intense.

Since the pair of arguments are logically identical in form they are of equal validity. But because it is impossible to reconcile the two women .... they cannot BOTH be right .... only one conclusion can be drawn ...

The SWR-meter is located in the transmission system immediately at the output of the transmitter. It is often housed in the transmitter case itself.

Immediately following the meter is an adjustable impedance transforming/matching network sometimes known as an antenna tuner. The SWR meter can also be housed in the tuner case. But in any event, compared with a wavelength, the transmitter and tuner are very near to each other with the SWR meter in between.

A transmission line either of coaxial construction or balanced-pair runs between the tuner output and the antenna.

From one frequency band to another, from one end of a band to the other, the antenna input impedance changes greatly from one complex value to another. These large impedance changes are reflected by the antenna back to the transmitter end of the transmission line sensibly unchanged in magnitude.

Transmitters are designed to work into a particular value of load resistance Ro.

Ro is the resistive load into which the transmitter is capable of developing its maximum, undistorted peak output power without exceeding any internal voltage, current or dissipation ratings. It is desirable, therefore, for the load on the transmitter to be set to the design value at the start of a transmission and continuously monitored during normal operation.

The tuner's purpose is to transform the wide range of impedances vs frequency seen looking into the transmission line to the constant resistive value of Ro required by the transmitter. In modern equipment this operation is often performed automatically, the tuner coils and capacitors being switched by relays or driven by electric motors.

The SWR meter is essentially a 4-arm, RF resistance bridge. Three of the bridge arms are integral to the meter and are fixed in value. The 4th, variable arm is the input impedance of the tuner. The circuit is arranged such that the transmitter is the signal source and the values of the bridge ratio arms are chosen such that the 4th variable arm, the input impedance of the tuner, provides the principal load on the transmitter. The values of the other 3 bridge arms are also chosen to minimise power loss within the bridge itself consistent with adequate sensistivity of detection of bridge unbalance.

Inside the meter the bridge RF unbalance voltage is rectified and displayed on a sensitive moving coil meter. When the input resistance of the tuner is equal to Ro the bridge is balanced, meter deflection is zero (a null) and the transmitter is then operating with its design value of load resistance. The purpose of the SWR meter, therefore, is to indicate that this condition has been or has not been satisfied.

It should be noted that when the bridge is NOT at balance the meter deflection gives very little information about what the actual load on the transmitter is. There is an infinite number of combinations of resistance and reactance which will give a particular off-zero meter deflection and this is further confused by the uncertainty in transmitter RF output volts.

So the meter indicates only whether or not the transmitter load is Ro. But this is all that's required of it. Brightness of a pea lamp would provide the same information just as well if the meter movement and scales were not conveniently used for other transmitter monitoring purposes (and sometimes for indicating received signal strength in transceivers).

It should now be clear an automatic tuner must make use of both magnitude and phase of the bridge unbalance voltage to maintain the bridge in a state of balance and provide the transmitter with its correct load resistance. This corresponds also to the fact that any impedance matching/transforming device must contain at least two variable components.

Note that the meter movement indicates only the MAGNITUDE of the rectified unbalance voltage. Any information regarding its phase is lost in the process.

Also note, for the meter to indicate correctly it must be designed around the same value of Ro as is required by the transmitter it is to be used with. The resistance standard universely adopted to allow interchngeability of SWR meters is Ro = 50 ohms. So meters generally differ from each other only by their through-power handling capability and operating frequency range. Internal circuitry and physical construction vary widely with frequency range but the basic mode of operation is always that of an RF resistance (not impedance) bridge.

So the SWR meter doesn't actually measure anything. It is a null detector. One thing is certain - there are no standing waves to be measured on a very short connection between transmitter output and the meter. However, the meter movement is almost always scaled to indicate the SWR, between 1.0 and infinity, which would occur on a long transmission line of impedance Ro if such a line greater than 1/4-wavelength should actually exist.

By definition, the voltage standing wave ratio, VSWR, on a transmission lne can be determined only by measuring the RF voltage at two different places, spaced 1/4-wavelength apart, corresponding to the maximum and and minimum steady-state voltages which exist on the line. Line impedance Zo and Ro are not relevant and are not involved in the determination.

If at one end of a 160-metre band radio path there is a phased aerial array consisting of a pair of 3/8-wave vertical elements with a large number of buried radials, and at the other end of the path there is a very low, short, inverted-L antenna, with only one small earth rod in dry stony soil, then, if both stations have the same transmitter power output, both stations use the same aerial for transmitting and receiving (as is usually the case), and both S-meters have been correctly calibrated, the stations will exchange the same (reciprocal) signal strength reports.

In short, any pair of stations, both running the same transmitter output power, will exchange the same signal strength reports.

This is because overall transmission loss, including tuners's, feeders, earth connections, aerial efficiencies, aerial directional properties, ground reflection losses and the radio path itself, is independent of the direction of transmission.

(At times ionospheric and geomagnetic conditions may upset reciprocity on paths through the ionosphere. But averaged over a period of time reciprocity prevails.)

The S-meter Calibration Standard for the HF bands, which from their maintenance manuals appears to have been adopted by the principal amateur radio equipment manufacturers, is as follows:-

The signal generator should have an internal impedance of 50 ohms and an open-circuit output voltage of 100 microvolts. With the generator connected to the receiver input terminal the S-meter should be adjusted to read S9. (If the receiver input impedance is 50 ohms then the voltage at the receiver input will be 50 microvolts.)

When the open-circuit signal generator voltage is set to 10 millivolts the S-meter should be adjusted to read S9 + 40 decibels. 100 mV volts open circuit from the signal generator should then read S9 + 60 dB.

The meter scale shape should be logarithmic. All other points on the scale should correspond to 6 decibels per S-point with respect to S9. So for every S-point below S9 the signal voltage level should halve. At S-zero the signal level at the receiver input terminal should be 54 decibels below S9, or 0.1 microvlts, which is of the same order as the theoretical thermal noise level with a 50-ohm receiver IF bandwidth of 3 kHz.

In practical meters a logarithmic law is followed only down to about S5. Below S4, meter readings on modern amateur receivers are usually seriously in error - a reading of say S2 corresponding to an actual signal level of S4 and a reading of S-zero corresponding to an actual signal level of S3. These errors arise from the difficulty of providing automatic gain control over a signal level range of S9 - 60 db to S9 + 60 db without running into serious cross-modulation and related problems. (Actually this error could easly be corrected by non-linear graduation of the scale markings at the lower end of the scale.)

When both S-meters have roughly similar scale-shape errors, as is quite likely, for the same Tx power levels the above-described pair of stations will continue to exchange the same strength reports.

Note that reciprocal signal strength reports do not imply the same readability.

Readability depends on signal-to-noise ratio and interference at the receiving end of the path. Since the station with the poor antenna lays down the weaker signal with respect to noise level, it is less likely to be readable by the other station despite the other station's much more efficient antenna which increases both signal and noise to the same extent. Conversely, the station with the efficient antenna will still be readable perhaps on a receiver with an antenna consisting of just a few feet of wire.

It follows that when a new antenna has been erected, its performance compared with the old in terms of radiating efficiency can be estimated just as accurately, but sooner and more conveniently, by using it with a receiver rather than by transmitting on it and requesting signal strength reports which, as often as not, are unreliable. In fact, the first indication of a more efficient antenna+earth may be an increase in the background noise level as indicated on the S-meter.

But of course, a nice signal strength report following a test transmission is psychologically much more satisfying.

My full name is Reginald James Edwards. Since retirement I have been domiciled in Rowley Regis in the Black Country, West Midlands, England. As I was born only half mile away and have returned to live in the same house as I did at the age of seven it might be thought I'm a stick-in-the-mud. Actually I've lived in various other parts of Britain for most of my 80 years and still travel occasionally by rail, amateur radio, letter post, and of course, by Internet.

From Rowley Hills, in the 'twenties', it was possible to see the Earl of Dudley's Iron Works at Brierley Hill, 3 miles away, only on Sundays when local industry shut down its shunting yards, rested the canal barge horses, braked the coal-mine winding engines, suspended blasting at granite quarries, closed down the marl-holes, brickworks, iron-pouring foundries, forges, blast furnaces, rolling mills, ground-shaking steam hammers, ear-piercing machine shops, and the hundreds of fuming, soot-belching, sky-blackening, smog-generating brick chimney stacks. However there were no petrol fumes. Some buses still had solid tires. There were petrol-driven vehicles on the roads but steam-driven lorries with trailers were quite common, loaded with the raw materials and products of the many large and small backyard factories which worked a 6-day week.

Always resourceful wildlife continued to survive amid the noise and smoke. Smaller birds, house sparrows, blue-tits, robins kept warm in winter by congregating around human dwellings all of which were warmed with roaring coal fires. And the iron works continuously radiated surplus heat through roofs and walls day and night.

Poultry and pidgeons were conveniently kept in back yards and a midweek family meal of braised rabbit was cheap and popular amongst the labouring classes. Shops and market stalls, lit with suspended acetylene lamps, were open till 9 pm on Saturday nights when joints of beef for Sunday dinner were available at give-away prices until Monday. Butcher's refridgerators were not universal.

On Sundays the miners, machine operators, iron-puddlers, pawn-brokers, cobblers, tailors, painters, carpenters, fishmongers, grocers, bank clerks, 'gaffers' and their families were called to church and chapel in their Sunday-best clothes by the peal of bells. For children there were such places as twice-a-day Sunday schools where discipline reigned.

Baird had not yet invented television and the Sunday joint of roast beef was free of BSE. Milk was delivered each day from local farms between the iron and gas works in galvanized churns via horse and cart into housewives' quart jugs, still warm from cows' udders.

There were no waiting lists for hospital beds. Every small town had its Picture House showing Charlie Chaplin, Buster Keaton or Keystone Cops. It was perfectly safe to walk about Black Country streets at night which were lit by incandescent gas-lamps - each lamp visited by the lamplighter with his pole at dusk and dawn. House doors remained unlocked - keys had been lost years before. The likelihood of such everyday circumstances being subject to sudden change never entered people's thoughts. After all - Britannia with her trident, assisted by great battle-fleets of 16-inch-gun dreadnoughts and countless smaller warships, launched in Birkenhead on the Mersey, Glasgow on the Clyde, Belfast and on Tyneside, still indisputably ruled the waves of the worlds's five oceans.

But at that time in every town high street a man was to be seen standing in the sooty drizzle, cap in hand, one foot in the gutter, assisted with a crutch, the other leg having been left behind in the gas-poisoned black mud and rusty shell-spinters of Flander's fields. My father, a surviver of the high explosives, the land mines, the shrapnel, machine gun bullets, the mud, flame throwers, mustard gas and phosgene, who had returned with all bodily parts intact otherwise I would not be at this keyboard, thought himself fortunate to suffer only from screaming nightmares, once gave me, then a child, a half-penny, probably all the cash he could spare, to put in such a deformed man's soggy, empty cap.

I recall, 65 years ago, in the middle of the night in another bedroom of the same house where I now write, hearing mother shouting to father - "Jim, Jim, wake up, wake up, it's all right now, wake up!"

My father's brother, my uncle Billy who I never knew, at the end of the war-to-end-war remained in Flanders Fields somewhere under a carpet of blood-red poppies but with no known grave, together with four million others of that generation, both friend and foe, all equally innocent of any crime.

The granite and marble memorials and cenotaphs in every city, town and village, with names, initials and rank engraved in the stone, to this present day still propagate the cretinous lie these men and boys actually gave their lives away - whereas they were deprived of life in the most horrible of fashions without any means of appeal. Some survivors were blindfolded and shot at the stake by their own comrades because they had been driven out of their minds by months of the most unimaginable horrors. They, their mothers, wives, children and families, were the victims of so-called Statesmen, thieving owners of the Cannon Industries and the much be-medalled, mentally deranged Field Marshals.

Black Country streets and houses are now, by comparison, spotlessly clean. The external stones and brickwork of a few of the very old buildings still remain blackened with the grime of ages. Some factories have become museums, visited with well-earned respect and appropriate reverence by our friends from the Land of the Rising Sun and other long-distance tourists. The steam hammers are silent and the ground no longer trembles as if in fear of their massive blows. The few remaining brick chimney stacks are now smokeless monuments to a bygone age.

New factory chimneys are of steel and appear to emit, if anything at all, only clean white steam. But the granite of Rowley Hills, long extinct volcanos, widely known as Rowley Rag, is still quarried by blasting, crushed into graded chips and used for road-making as it has been for 2000 years ever since perceptive military engineers of the disciplined Roman Legions first visited Rowley Hills and recognised the stone's extremely hard and durable qualities. The skyline has been changed in the process but unlimited Rag remains available for new motorways and repair of the old.

The night clouds are no longer tinged orange by reflection from the bright glow of yet more hundreds of tons of molten iron pouring from blast furnaces like lava from volcanos. They are now tinted a sickly yellow by reflection from the glow of 100,000 sodium electric street lights.

Some chapels still open on Sundays but only by co-operating and taking turns to do so. Other old chapel and church buildings, now colourfully redecorated inside and out, welcome people through their doors to listen to the Word of God from ministers of other religions originating in ancient civilisations and cities such as Jerusalem, Mecca and holy places further towards the Eastern sunrise.

Local industry is still busily concerned with casting, forming, shaping, joining and assembling things made of metal. But nuts, bolts, rivets, screws, spanners and soldering irons seem to have gone out of fashion. There is now a whole battery of arc-welding machines 500 yards away from my QTH which blankets the 160 and 80 metre bands with intense radio noise most of the working day. If, for a few seconds, just by chance, none of them happens to be arc-ing, I then come under attack from another battery at a range of 1000 yards. It is so strong I have been unable to detect television timebase interference during factory working hours for ages. Pollution of one sort or another, it seems, must always be with us.

When still a boy I amused myself with basket-weave plug-in coils, Hertzite crystals, cat's whiskers, spaghetti resistors, and eventually a PM1HF 2-volt filament valve with which I could use a reaction coil and condenser. These components were provided by my father (who made his own variable, air-spaced tuning condensers) with threats of dire consequences if any should ever be ill treated. Seventy years later I still flinch at the thought of consigning unwanted radio components to the domestic rubbish bin. I try to find a younger deserving radio amateur to donate them to.

In the 1930's the Australian Broadcasting Company could be heard as clear as crystal using one valve with reaction (now referred to as positive feedback) and 50 feet of aerial wire suspended from mother's clothes-drying posts. I heard prime minister Neville Chamberlain's, 3rd September 1939, declaration of yet another war over the wireless on a pair of S.G.Brown's iron-diaphragm headphones. It was noon, Sunday. I felt frightened. I went out into the road. There was nobody about. Father was silent as he forced down his Sunday dinner.

The World, yet again, was about to change. The air-raid sirens were soon to be heard above the background noise from local factories which continued to produce things made of iron, steel, brass and copper as they had done for the previous 150 years.

At nearby Birmingham University Randall and Boot were working in haste on the final development stages of the Cavity Magnetron - shortly to be used in airborne radar to guide fire-storm machines to their many thousands of defenceless victims.

As a long-serving seargent in the St.Johns Ambulance Brigade father was later to spend his nights in the burning streets during air raids. Mother, at the age of 43, went to work in a local factory making nuts, bolts and rivets - vital components of war-machines. My brother Frank also joined the RAF killing squadrons. He returned safely to become an arc-welder. Father was eventually decorated with "Serving Brother of the Distinguished Order of Knights of Jerusalem". He took piano lessons and never returned to his between-wars radio construction experience.

Long after WW2, father having rejoined his brother, mother gave away the headphones, together with a BTH moving iron horn loudspeaker and other sentimental, nostalgic antiques to the rag and bone man who announced his weekly presence in the street by blowing on an ex-army copper and brass bugle. She declined his offer of recompense in the form of a live goldfish because she hadn't an empty Hartley's golliwog jam jar in which to keep it. I forgave her only because she was my mother - God bless her.

During WW2 I joined the RAF and worked on microwave radar and similar top-secret airborne devices. I received a high-tech education, with long holidays abroad on tropical islands with palm trees and surfing-beaches, all generously paid for by His Majesty King George 6th. After the war it took another 30 years to directly return again to radio by becoming a licensed radio amateur and building a solid-state 160-metre portable SSB rig. I also taught myself another language - Morse!

Around 1962, still in the forefront of technology, I attended a 3-day computer programming course. The input and output device was a teleprinter. No floppy disks - just reels of ticker-tape. I used for a few days a discrete transistor machine, 6' by 4' by 2' which was almost as good as a modern pocket calculator and even did something useful with it but did not sit at a computer keyboard again for 20 years when I bought a pre-IBM Osborne computer - the very first portable - with a green 3-inch screen. I still have it. It still works but I don't use it. Since then I have taught myself three more languages - BASIC, MsDOS and Pascal.

I do not know how to use a computer to control a transceiver or even how to use one for maintaining the station log. The two sets of hardware, computer and transceiver, have never been together in the same room. (2002, they now are.) The pair of hobbies have peacefully co-existed independently of each other for years - with one important exception: I have always used self-composed computer programs to assist with radio construction and experimentation. There's been a sort of cross-fertilisation. The pair are now respectably wedded and decently co-habiting these appropriately-named Home Pages.

Their offspring is a collection of small, simple to use, high-quality computer programs the clones of which are available free to radio amateurs, professional engineers, teachers, or anyone who is prepared to provide a good home. In the process I have acquired yet another language - HTML. Sometimes I dream in it!

1st Nov 2000.

Program GRNDWAV2 contains an arithmetical error and is no longer available from this site.

The true field strength at the receiving site in microvolts per metre is Sqrt(2) = 1.414 times greater than the value predicted. However, the general behaviour in decibels of signal strength versus distance and type of terrain is modelled as I intended.

The error arose because my old (1930's) theoretical reference book used peak values of field strength when deriving the field strength formulae from first principles. Later on, when writing the program, this lead me to convert from peak to rms values when I should not have done. On looking back at the reference text I now see the author, 3/4 of the way through, switched from the symbol Eo for peak values to Ea for rms values, between one line and the next, with hardly a mention.

Anyway, that's my excuse.

Unfortuately I cannot correct the program because of an accident 18 months ago with my hard drive which caused me to lose the source code for ever. Should I re-write it under a new name the Eo/Ea = 1.414 bug will be excised.

An improved, error-corrected version of the above program was eventually written and made avaible sometime in 2001. New mame: GRNDWAV3.

17th December 2000

The original version of Program PI_TANK contained an error in the operating instructions. The user was asked to insert a value of the network phase delay between 0 and 90 degrees. But the true network phase delay always lies between 90 and 180 degrees. However, as the user has little or no interest in phase delay and a value is required only to indirectly set the value of the network's operating Q, and all computed results were correct, no damage was caused.

The actual phase delay is 180 degrees minus the value which was entered.

The error has now been corrected and the operating notes amended. The program user is asked to enter an angle between 90 and 180 degrees which is the true phase delay. The name of the program has not been changed. On the first program screen the date of issue has been changed to 15th December 2000. You are invited to down-load the amended version and use it to overwrite the original.

1st July 2002

There was an error in the issue of program PADMATCH dated 29th June 2002. The power dissipated in each of the resistors, stated to be a percentage of attenuator input power, was in fact the percentage of total power dissipated in the attenuator. Within a few hours the error was corrected. Anyone who downloaded PADMATCH between 28th and 30th June is recommended to download again and use it to ovewrite their existing issue.

Program TOPHAT2 replaces TOPHAT1 which is no longer available. The earlier program covered a frequency range less than the range of frequencies accepted. The program aborted too frequently. This was corrected. A minor change has been made to the model. The introductory notes have been partially rewritten.

Program FEEDPOWR is intended to estimate the unwanted power radiated from an unbalanced (to ground) coax feedline of given length when connected to the centre of a balanced (to ground) dipole without a balanced-to-unbalanced impedance matching transformer. (A balun). An estimate of power radiation from feedlines is useful to decide whether or not a balun is needed before erecting an antenna.

Although the computed estimates were correct the notes and data descriptions accompanying the first issue of this program were ambiguous and (thanks to a perceptive user) could be misleading. The notes, etc., have now been largely rewritten. The issue date displayed on the opening screen has been changed. The program name has not been altered. Users are recommended to download the improved version to overwrite the original file.

Program SOLNOID2 was reissued on 6th May 2003 to correct an error in calculating wire diameter and pitch for very small numbers of turns combined with small wire diameter/pitch ratio.

Program L_NETWK: Very minor error corrected in the calculation of working Q. Issue date changed.

I am grateful to Icelandic radio amateur, Villi, TF3DX, for informing me of a serious error in program GRNDWAV3. The available power input to the receiver is exactly 6dB greater than the true value although the computed field strength at the receiving site is correct. GRNDWAV3 has been withdrawn and replaced by GRNDWAV4 and minor amendments have been made to the program notes. The error arose due to a misunderstanding of vertical antenna gain relative to an isotrope when mounted above a groundplane. Some learned text books are unclear on this point.

Program RADIALS2. A few minor amendments made. Accuracy slightly improved. Issue date changed.

Program RADIALS2 deleted from website. Improved version named RADIAL_3 uploaded. Calculations hardly changed but program notes and results screen amended.

In the author's opinion the name "SWR meter" should be abolished.

In what follows 'measurements' apply to the radio amateur's usual SWR meter.

The instrument can't measure SWR. It is a fixed-ratio RF resistance bridge.

Any conclusions drawn from SWR 'measurements' are ambiguous.

Entirely different values of transmitter load impedance give the same SWR.

The meter's usual location does not allow it to indicate SWR on any line.

A line which can be measured does not, in any case, usually exist.

The only way to measure SWR is by two independent voltage measurements on an actual line.

The line must be long enough to make two measurements spaced 1/4-wavelength apart.

SWR measurements tell you nothing about line Zo or terminating impedances.

If a line's impedance is unknown then SWR measurements cannot proceed.

It is just a voltage ratio between two different points spaced 1/4-wave apart.

The meter wastes at least half of the information available to it.

The angle of the reflection coefficient is discarded in the measurement process.

The so-called SWR meter only indicates whether or not the transmitter load resistance equals its design value of 50 ohms. If not equal to a resistive 50 ohms it cannot tell you what it actually is.

However, as a load indicating device it is a valuable, almost essential instrument.

SOLUTION: Change the meter's name to TLI (Transmitter Loading Indicator).

Programs currently
available:

RJELOOP1      RJELOOP2
RJELOOP3      SOLNOID2
DIPOLE3       HELICAL3
TOROID        VERTLOAD
RADIOETH      EARTHRES
GRNDWAV2      ENDFEED
TANT136       LINEAR1
BALUN4        RJELINE1
RJELINE2      COAXPAIR
RJELINE3      SWR_FREQ
TRAPDIP       PHASENET
TPINET        SOILSKIN
COAXRATE       ZL_ZIN
MAGLOOP4      MIDLOAD
 LCR          SWRMETER
VARNEY        LOADCOIL 
PI_TANK       ZOC_ZSC
PI_L_NET      TETRODE1
TOPHAT2       RXZCALC
RJELINE4      T_TUNER
GRNDWAV3     DIFFRAC1
DIFFRAC2     LINOSITE
STUBTUNE     MATCHSEC
BOTLOAD       L_TUNE
WIRESKIN     SELECT_1
TWOCOILS     WIRECAGE
ADDALOAD     TRIODE1
MULTILAY     WINDOM2
PADMATCH    FEEDPOWR
COAXTRAP    TWINTRAP
OSCTRACK    BOTLOAD2
STUMATCH    STUBTUNE
LPF_HPF     BANDPAS1
FEEDPOWR    INV_VEE
WHIP_1       TWOHOPS
HARM-GEN     R_X_SWR
CHOKEBAL    NOTATRAP
TRAP3       SELFRES3
RJELOOP4    COAXTUNE
TWINTUNE    TUNEHALF
VERTUNE     SWRARGUE
METALFLM     QL_QC
L_NETWK     HYPERGEO
C_POISE     OP_ANGLE
CONNECT     TRANCO_1
INPUT_Z     TRANCOIL
SKYTRIG     PROPGATE
HELIX_2     LINELEN