On a well tuned aerial which feed line displays an SWR ratio of 1:1 all the RF energy is transmitted from the RTX final amplifier to the antenna tuner and then through the feeder to the antenna. In this configuration the current is equal at any point of the line and your antenna works properly. If the SWR increases for any reason, stationnary waves appear; in HF that means that a part of the RF energy you sent to the charge is waste as heat before reaching the antenna, mostly in the feed line. Note that using very low loss feed lines, in  HF and in a lesser extent in V/UHF, this waste due to a high SWR is very weak.

In a perfect matching system displaying a SWR 1:1, which output is constitued of a dummy load, all the power arriving to the charge in fully absorbed (converted in heat). If this load is an antenna this energy is totally radiated into space. 

If the system is not perfectly matched the larger part of the power will be absorbed by the antenna resistance but a few percents will be "reflected", in fact added or substracted in the line. The ratio of these peaks (high/low currents or induced voltage) represents the SWR.

Practically the SWR is altered when there is a mismatch of mating the 50W feed line to the higher impedance of your aerial. In such conditions the extent of the antenna radiation pattern that we will see on the next page and representing the concentration of its signals can be modified displaying a decreasing of its maximum range.

Without other consideration, the consequences of a high SWR are not as severe as we could expect and this potential loss will not interfere with TV receptors and will not create RFI as we sometimes hear or read here and there. 

Imagine an antenna system showing a SWR 6:1, thus 50% of reflected power (not the same as 50% or 3 dB of signal loss). What is the impact of this SWR on the power of your signal ? These 50% of reflected power due to the antenna system mismatch go back to the AT-tuner (adapter) and are reflected again to the antenna, in phase with the incident wave. We must then distinct two cases : the adapter is matched or mismatched.

If systems are matched, when impedances are combined, conditions of the "maximum power transfert theorem" are met in any point of the line linking the adapter to the antenna. All the reflected power going back from the antenna is resent in phase to the antenna. In this case, the only lost power is the one due to the double crossing of reflected power in the feed line (once descending, once going up again). We can demonstrate this theoretically (Cf W2DU, Walter Maxwell's book) using a no-loss feed line : for 100W sent into an adapter using a matched line showing 50% of reflected power at the feet of the antenna, 200 W are emitted and 100 W are reflected (100/200 = 0.5). Thus the emitted power is 200 W - 100 W = 100 W as well. This measurement can be confirmed using a simple Watt-meter like a Bird.

If the adapter is not matched, conditions of the maximum power transfert theorem are not met, matching is not satisfying, and the functionning point (DC) of the final tube or transistor is displaced (by "good functionning" we mean the required charge to get a minimum input current for a given feeding voltage). If matching is uncomplete, the charge will not be optimum, the functionning point will move and the current will increase (the tuning abacus follows a kind of parabolic curve). Using e.g. a Pi-adapter with a tube transceiver, we can easily note this minimum of plate current.

Many amateurs believe that when power is reflected, this one will go back up to the final amplification stage where it will dissipate,  increasing the risk of damaging electronic components. This rumor is completely false ! Telecom engineer Jacques Culot, ON5MJ remind us that in this example there is not the slightest HF dissipation in the final stage. The sole effect of the reflected power associated to a lack of matching is that the final stage will dissipate still a bit more DC. Indeed, a mismatched adapter will prevent the final stage to generate the maximum power, and thus will feed the antenna with a reduced power. 

This phenomenon is easier and more intuitive to understand using vintage tube amplifiers as we saw directly effects of the matching on the current-meter. In modern transistorised transceivers, manufacturers are no more concerned by matching impedances, and do no more include adapter in their emitter (it is cheaper and more compact) supposed linked to a 50-ohms load. But in the field this condition being far to be met, the increase of collector current had had to be limited using a device. Circuits measuring the SWR were inserted, reducing gradually the power. We can also put the adapter at the end of the emitter, the matching being established in measuring the maximum transmitted power as we can no more access to the final stage. At the end be come back to the previous system : the matching device is included in the rig. Today this tuning is automatic and loss in the adapter are important because specifications of components are too tight.

The antenna impedance can be measured using modern RTX that often provide a built-in Antenna tuner. Otherwise you can use an external reader know as a Noise or SWR bridge to fit on the coaxial line.

The SWR-meter

After your transceiver and your antenna, it is more than necessary that you own an external SWR-meter or wattmeter. It will allow you to know exactly what power radiates your antenna system. A dummy load suited to your emitting power is also interesting to get to make some emission tests without perturbating the bands with useless vocalizes and tunes...

In addition peak-readings SWR-meters can be active or passive. "Active" means that the peak reading is electronic, and amplified before to be displayed. These meters must be powered, usually in 12V. As expected they are the meters the most accurate, contrarily to passive meters that use an unamplified meter-damping circuit to read the peak value. Most need however to be powered, not because they are active but simplier to light the cross-needle display.

Some old meters still include additional knobs to adjust the load reactance and resistance. They are very accurate but a bit longer to set. Some desk models have also to be calibrated according your peak emitting power (100W, 1kW, etc) but most are today autocalibrated and they request no adjustment. You read and appreciate the values... 

At last some SWR-meters are very small, in both size and readings, suited for QRP operations, other are as large as a laboratory device, attracting, suited to operate on the amateur's desk.

Some amateurs complain sometimes of forum to the fact that most "customer SWR-meters" are unaccurate. They are wrong. In fact most passive readers are accurate for what they cost and all are able to read the power and give the SWR within 3-10% of error. I think that in practice amateurs are not really interesting in knowing if they are emetting 80 or 88W but rather if all their input power is well radiated by their antenna. The active readers (e.g. Palstar WM150) are of course the most accurate but here also the combined HF/VHF readers suffer of a lack of accuracy over 150 MHz.

Antenna tuner

Instead of purchasing an external SWR-meter or a wattmeter you can buy an external antenna tuner that will help you to match the load according the working frequency. It will not tune your antenna at all, but transform the impedance to provide your transmitter with the proper load of 50W.

Although more expensive than a SWR-meter, an external antenna tuner is really useful because modern transceivers have a narrow range of impedance matching close to 50W, and are unable to match the high impedance of many wire antennas that are ranging between 10-600W or more. Even a so-called 70W dipole antenna can see its impedance change as soon as you modify its length, its height above ground or the length of the parallel-wire feed line. Weather conditions also affect these values. Hence the interest of using an antenna tuner in all circumstances.

If your transmitter built-in tuner is able to match SWR mismatches up to 2.5 or 3:1, beyond this value you couldn't use your emitter, and surely not if you use an amplifier. Therefore today we find an external antenna tuner into most shacks at one time or another. Some have to be manually tuned but others are automatic, analog or digital, and offer you in all circumstances the lowest SWR and losses. Some are able to handle 1, 2 or 3 kW output or even more. Some tuners can read settings of your RTX and save them in memory. The best models tuned your antenna system in about 5 seconds whatever the band or the antenna design.  How that works ? An antenna tuner is far different from a SWR-meter. This is an active device that needs to be manually tuned (at least the manual models).

The antenna tuner AT-100, now discontinued, from Kenwood. This is a manual model but very efficient.

Opening the case of an antenna tuner you will see two large capacitors and an inductance switch (or a roller inductor) connected outside to a rotary switch. Labeled "ANTENNA" and "TRANSMITTER" on the front panel, these capacitors must be adjusted until you hear the loudest signal on your receiver. This is simple and efficient.

However, be aware that the coil used in an antenna tuner cannot withstand much power. As the load impedance increases, losses will get higher. If you system can still work with 50% of 1 kW in the antenna tuner, 500W will be dissipated as heat in the tuner coil. A high-power antenna tuner will probably supports such losses, but be careful working with high SWR and ordinary tuners because a tuner failure can always happen. In the worst cases, in presence of high SWR, high power (current) and and moisture you can even burn the coaxial terminals on the line.

Note that many antenna tuners of the old generation (say before 2000) are unbalanced. If you work with a balanced antenna system you probably use a balun to match impedances that can vary quite much according to the frequency.

Like for SWR-meters, the price of an antenna tuner is proportional to its performances, some being for example unable to match SWR over 2:1 on 160 m. For a HF cross-needle model supporting 3 kW PEP the price is ranging between $330 (MFJ) to $600 (Ameritron). A "laborarory" grade SWR-meter can be as expensive.

These two gears being very important to check your emitting conditions, here is a list of very appreciated SWR-meter and antenna tuner manufacturers. Most models sold in the past by Icom, Kenwood or Yaesu are today discontinued but they are always available on second-hand. A must for your shack !

Low SWR but no efficiency

Imagine that you want to work an extra band on which your antenna in not tuned because it is not cut for. Take a simple example with the well-known 31.1m (102') G5RV multi-band dipole using a ladder line and a balun 4:1 to match the impedances. In this length and without modifications you cannot use it on the 160 m band. Why ? What's the matter if we try... (To never do in the field please). Here is the answer. Using a good antenna tuner you can reduce the SWR on that band until to reach SWR 1:1 or so as displayed at right on the transceiver S-meter. At first sight all is fine, the antenna resistivity is not absorbing the power, and you might transmit a strong RF signal on 160 m. But in the field you observe that nobody answers to your call... If you try to transmit in such conditions, this is unfortunately misinterpreting how work an antenna and a resonant circuit.  In fact you have indeed a SWR 1:1 between your transceiver final amplifier and... the antenna tuner, often built-in, but not further. The SWR-meter built-in in your RTX  has not read the SWR on the feed line to the antenna where your signal should radiate in the air. If you do this measurement you will discover than the SWR reaches huge values, say over 200:1 if not infinite like on the picture displayed at right trying to work on 160m with a multiband dipole 33m long, not suited for this band. Why so ?

Trying to work on a band on which your antenna is not tuned, you can get SWR 1:1 and full power at the tuner output (read on the RTX SWR-meter) but you will get a sky high SWR on the feed line to the antenna (read on the external SWR-meter) with a risk to burn your transceiver PA due to the heat dissipated in circuits...

To read : Input impedance and SWR

On the contrary as soon as you extend your aerial to reach 1/2l on the working frequency (in this case extending its length to 84m long or working on the 80m band) you could read R = 75W, X = 0W. Your antenna is purely resistive, offering in best cases 100% of efficiency; you have a resonant circuit and you can feed it without loss ! But if you calculate the efficiency of a dipole 31.1m long on 160 meters (1.9 MHz), you will get an overall radiating efficiency of... 1.4 % only !

For dipoles, you can also simulate other lengths and impedances downloading the excellent and free program DIPOLE3 created by G4FGQ. This routine running in a DOS box analyses performances of any dipole, of any length, any wire diameter, placed at any height, and for any length of any type and combination of feed lines. Once you get a solution you can do varying these factors. The resulting values displayed on screen are the efficiency, SWR, loss, and their counterparts in dB or S-units (S-points). Also calculated are L and C component values of L-network to match to 50W. At last, since 2005, Timewave  sells an antenna impedance analyzer model "TZ-900 AntennaSmith" ($999.95) displayed at right. It is a small box operating between 0.5 and 60 MHz able to simulate on its small TFT screen SWR graphs and Smith charts in full color. It is a pity that it is so expensive.

For more information

SWR-meters & wattmeters : Ameritron, Array Solutions, Autek, Bird, Daiwa/NCG, Diamond, MFJ Enterprises, PalStar, Ten-Tec, Yaesu, ...