Tuesday 11 June 2013

NVIS: Near Vertical Incidence Skywave (Part 2)

What is NVIS?
NVIS, or Near Vertical Incidence Skywave, refers to a radio propagation mode which involves the use of antennas with a very high radiation angle, approaching or reaching 90 degrees (straight up), along with selection of an appropriate frequency below the critical frequency, to establish reliable communications over a radius of 0-200 miles or so, give or take 100 miles. Although not all radio amateurs have heard the term NVIS, many have used that mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. They may have thought of these nearby contacts as necessarily involving the use of groundwave propagation, but many such contacts involve no groundwave signal at all, or, if the groundwave signal is involved, it may hinder, instead of help. Deliberate exploitation of NVIS is best achieved using antenna installations which achieve some balance between minimizing groundwave (low takeoff angle) radiation, and maximizing near vertical incidence skywave (very high takeoff angle) radiation.
As hams, we often faithfully follow the advice: get your antenna up as high as you can get it! We do this, and other things (like choosing antennas that have a low angle of radiation) in order to maximize the distance over which we can communicate. An antenna with a particularly high angle of radiation is often somewhat disparagingly referred to as a "cloudwarmer", the implication being that if the signal isn't radiated at a low enough angle, it's being wasted. For NVIS, we ignore all this traditional advice, and select instead techniques which will maximize not our DX, but our ability to reliably communicate with other stations within a radius of 0-300 miles.
 
Not just any old frequency will work for NVIS. Successful NVIS work depends on being able to select, or find (through trial and error), a frequency which will be reflected from the ionosphere even when the angle of radiation is nearly vertical. These frequencies usually are in the range of 2-10 MHz, though sometimes the limit is higher. The trick is to select a frequency which is below the current critical frequency (the highest frequency which the F layer will reflect at a maximum--90 degree--angle of incidence) but not so far below the critical frequency that the D and/or E layers mess things up too much.
 
 
There are two main types of propagation at HF, known as "groundwave" and "skywave". Groundwave propagation occurs when the receiving station is sufficiently close to the transmitting station, and is able to receive the portion of the transmitting station's signal which clings to the ground. The range of groundwave propagation varies with the type of antenna at the transmitting station, the characteristics of the ground between the transmitting station and the receiving station, and other factors. It can be anywhere from a few miles, to a few dozen miles. Distances beyond the range of the groundwave signal are covered by skywaves. Skywaves are the waves which radiate upward at some angle from the antenna, and (we hope) are reflected from the ionosphere, to return to earth further away.
The ionosphere is a high altitude region of the Earth's atmosphere which is composed of gaseous atoms which have broken into ions. The sun is the source of the ionizing energy, so the condition of the ionosphere varies with time of day, season of the year, the 11-year sunspot cycle, and the 27-day rotation of the sun. The layers of the atmosphere that effect radio propagation are the D, E, and F layers. I won't go into much detail in outlining their roles. If you're interested in this topic, entire books have been devoted to it. In a nutshell, it's the F layer which is usually involved in reflecting our signals back to earth, while the D layer absorbs our signals. The E-layer can either help, or hinder.
Long distance propagation of radio waves is usually achieved by their being reflected from the ionosphere, and returning to earth some distance away from their point or origin. (Follow along with the diagram if you wish.) Radio waves which have been radiated at a very low angle of radiation travel a long way before finally making it up to the ionosphere, strike the ionosphere at a very shallow angle (A) and return to earth far away from their point of origin (A'). As the angle of radiation goes up, the radio waves strike the atmosphere at a more moderate angle (B), and return to earth closer to their point of origin (B'). For any given frequency and current state of the ionosphere, there may be some maximum angle of incidence at which the ionosphere will reflect signals back to earth. Signals which strike the ionosphere at a higher angle of incidence than the current maximum will not be reflected at all, but will continue on out into space, instead (C). The area of the earth to which the reflection would have occurred will be in what we call the "skip zone" (unless it's close enough to the signal source to receive the groundwave signal). The skip zone is the region consisting of areas of the earth's surface which are outside the radius the transmitting station's ground-wave will reach, and yet not far enough away to receive reflections of sky-waves.
 
NVIS techniques concentrate on the areas which are often in the skip zone. The idea is to radiate a signal at a frequency which is below the critical frequency, at a nearly vertical angle, and have that signal reflected from the ionosphere at a very high angle of incidence, returning to the earth at a relatively nearby location.  Of course, no antenna radiates all its signal at exactly one angle, so the best we can get is a range of angles, ranging from perfectly vertical, to nearly vertical. The portion of the signal which is radiated at a vertical, or nearly vertical, angle reflects back to earth over some radius, which is determined by the lowest angle at which the antenna radiates much signal. Absorption by the D layer, and other factors, determine some minimum frequency below which the signal will no longer be usable, and usually some distance beyond which signals will no longer be usable.
For areas which are within the groundwave range of the transmitting station, the ground-wave's presence may interfere with the reflecting skywave. It may very well help, too. It all depends on whether the groundwave and the skywave arrive in phase, out of phase, or somewhere in between, and their relative strengths. If the groundwave arrives at about the same strength as the skywave, and the two are out of phase, the signal will disappear. Since the height of the ionosphere varies with time, phase alignment may drift from in phase, to out of phase, resulting in signal fading. For this reason, it's best to minimize groundwave radiation when using NVIS techniques, so that it will be less likely to interfere with the skywave.
Although this discussion has focused mainly on the transmission of signals, there is a corresponding advantage of using NVIS techniques in reception, and a trick or two that are useful mainly for reception. The corresponding advantage is that if your antenna favors high angles for transmission, it will also favor high angles for reception. An antenna optimized for radiating at the high angles used for NVIS will also be optimized for receiving the skywaves which will be arriving at a high angle from the ionosphere. An antenna which does not radiate much groundwave signal will also probably not receive groundwave signals as strongly. When both stations are using antennas which are optimized for NVIS, the mode is favored both in transmission and reception, and those advantages add together, increasing the chances of reliable communication.
There is also an advantage inherent in the use of NVIS style antennas which applies only to receiving. The frequencies which are useful for NVIS (usually 2-10 MHz) are the same frequencies which are most susceptible to atmospheric noise. A major source of atmospheric noise is distant thunderstorms. Nearby thunderstorms are the worst, of course, but the noise from all possible sources adds together. Unless there is a nearby thunderstorm, most noise will be the sum of the noise from distant sources which are all propagated to the receiving antenna. Since an antenna optimized for NVIS is listening mostly to signals propagated from relatively nearby areas, and does not favor the reception of signals, static crashes, and other sources of noise and interference from more distant sources, it will not hear as much noise or interference as an antenna optimized for DX operation. The result is a better signal/noise ratio.
Often, taking measures which optimize a station's NVIS capabilities will drop the noise level substantially. Sometimes, the drop in noise can be maximized at the expense of some signal strength, and result in a communication circuit which has lower signal levels, but even more dramatically lower noise levels, for an even better signal/noise ratio than could be achieved by focusing only on maximizing signal levels.
So, selecting a frequency below the critical frequency, but not too far below it, and selecting an antenna which will radiate skywaves at a high angle, and minimize groundwaves and the reception of noise, are the essential tricks of establishing reliable communication in the 0-200 mile radius which is so often a challenge for HF operation.
What are the advantages and disadvantages of NVIS?
Among the many advantages of NVIS are:
  • NVIS covers the area which is normally in the skip zone, that is, which is normally too far away to receive groundwave signals, but not yet far enough away to receive skywaves reflected from the ionosphere.
  • NVIS requires no infrastructure such as repeaters or satellites. Two stations employing NVIS techniques can establish reliable communications without the support of any third party.
  • Pure NVIS propagation is relatively free from fading.
  • Antennas optimized for NVIS are usually low. Simple dipoles work very well. A good NVIS antenna can be erected easily, in a short amount of time, by a small team (or just one person).
  • Low areas and valleys are no problem for NVIS propagation.
  • The path to and from the ionosphere is short and direct, resulting in lower path losses due to factors such as absorption by the D layer.
  • NVIS techniques can dramatically reduce noise and interference, resulting in an improved signal/noise ratio.
  • With its improved signal/noise ratio and low path loss, NVIS works well with low power.
Disadvantages of NVIS operation include:
  • For best results, both stations should be optimized for NVIS operation. If one station's antenna emphasizes groundwave propagation, while another's emphasizes NVIS propagation, the results may be poor. Some stations do have antennas which are good for NVIS (such as relatively low dipoles) but many do not.
  • NVIS doesn't work on all HF frequencies. Care must be exercised to pick an appropriate frequency, and the frequencies which are best for NVIS are the frequencies where atmospheric noise is a problem, antenna lengths are long, and bandwidths are relatively small for digital transmissions.
  • Due to differences between daytime and nighttime propagation, a minimum of two different frequencies must be used to ensure reliable around-the-clock communications.

What kind of antenna works well for NVIS?
 
Dipole
Once again, the dependable dipole antenna proves itself useful. One of the most effective antennas for NVIS is a dipole positioned from .1 to .25 wavelengths (or lower) above ground. When a dipole is brought very close two ground, some interesting things happen. The most interesting thing, from an NVIS perspective, is that the angle of radiation goes up. In the range of .1 to .25 wavelengths above ground, vertical and nearly vertical radiation reaches a maximum, at the expense of lower angle radiation (which we'd like to minimize, anyway, for NVIS). A dipole can be used at even lower heights, resulting in some loss of vertical gain, but often, a more substantial reduction in noise and interference from distant regions. Heights of 5 to 10 feet above ground are not unusual for NVIS setups, and some people use dipoles as low as two feet high with good results (relatively weak signals, but a very low noise floor).
Another interesting thing that happens with very low dipoles is that their feedpoint impedance goes down. An acceptable SWR with 50 ohm coax is likely. Plan to bring your tuner along just in case, but you may get by just fine without it.
Yet another fortunate thing about low dipoles is that they are easily erected. Finding a tree which will serve as a support is often easy, and it's not hard to get a line in a branch which will suffice. Masts made of PVC tubing are practical at these heights. Very low dipoles can be supported by traffic cones with a notch cut in the top, or a simple tripod made from short sections of PVC pipe or wooden dowels, and bungee cords.
With the exception of the very lowest dipoles, most dipoles will gain an extra 2 db or so of vertical gain if you allow the center to droop a few feet. Allowing the center to droop means that the end supports don't have to be as sturdy, which makes installing a good NVIS dipole that much easier.
 
Inverted Vee
The dipole's close cousin, the inverted vee, is another good NVIS antenna, which can be even simpler to support. An inverted vee will work almost as well as a dipole suspended from a slightly lower height than the apex of the inverted vee, so long as the apex angle is kept gentle--about 120 degrees or greater. An inverted vee is often easier to erect than a dipole, since it requires only one support above ground level, in the center.
Counterpoises
The high angle radiation of a dipole (or inverted vee) can be enhanced by adding a counterpoise wire below it, about 5% longer than the main radiating element, to act as a reflector. The optimum height for such a counterpoise is about .15 wavelengths below the main radiating element, but when the antenna is too low to allow for that, a counterpoise laid on the ground below the antenna is still effective.
A knife switch at the center point of the counterpoise can be used to effectively eliminate the counterpoise from the antenna system. This technique is useful for using a dipole for NVIS and longer distances, too. A counterpoise is installed at ground level, or as high as the switch can easily be reached, and a dipole is mounted .15 wavelengths above the counterpoise. When the switch is closed, the vertical gain will increase, and the noise levels will drop. When the switch is open, lower angle gain will increase, improving the antenna's performance for non-NVIS use.
 
How do I select a frequency for NVIS operation?
The selection of a optimum frequency for NVIS operation depends upon many variables. Among the many variables are time of day, time of year, sunspot activity, type of antenna used, atmospheric noise, and atmospheric absorption. To select a frequency to try, one may use recent experience on the air, trial and error (with some sort of coordination scheme agreed upon in advance), propagation prediction software, near real-time propagation charts (available on the Internet) showing current critical frequency, or even just a good educated guess. Whatever the strategy used for frequency selection, it would probably be best to be prepared with some sort of "Plan B" involving communicating through alternate channels, or following some pre-arranged scheme for trying all available frequency choices in a scheduled pattern of some sort.

THE CLOUD WARMER NVIS BEAM

AIM FOR THE CLOUDS AND GET BETTER "LOCAL" SIGNALS!
AN NVIS STYLE "BEAM" ANTENNA FOR BETTER "LOCAL" AREA COVERAGE ON HF




Some of you may recognize this design as nothing more than a half wave dipole, but upon closer examination, you will see that there is a reflector at the bottom of the antenna spaced at about .15 wavelength or less from the driven, (dipole), element.
This in fact, makes this antenna a 2 element wire "beam" aimed
straight up at the clouds!

Hense the name "Cloud Warmer Beam".
NVIS style antennas work best below about 8mhz as confirmed by the U.S. military.

If you already have a half wave dipole up and running, then you have been using this type of antenna to some extent without knowing it, however, yours is not as effective in getting your signal to the "local" area out to a few hundred miles due to the properties of the ground underneath, your present dipole, and the nature of the dipole pattern.

This design gives you the ability to more closely match the ideal situation for your dipole to perform much better in the close in range, (a few hundred miles radius), from your station and give you a little added"gain"!!!!

The military uses the NVIS configeration while operating mobile for better "local" coverage on their low bands by laying down their whips in a horizontal position on their mobile units.THERE IS NOTHING SPECIAL ABOUT THIS ANTENNA CONSTRUCTION OTHER THAN THE ADDED REFLECTOR AT THE BASE OF THE DIRECTOR (DIPOLE)!

By adding the reflector, which is 5% longer than the driven element, and spacing it .15 wavelength  or less below it, you turn your dipole into a beam type antenna projecting your signal up to that big reflector in the sky where it is bounced back down into a sort of upside down cone pattern extending out several hundred miles!
THIS IS NOT A DX ANTENNA!



The standard formula can be used for calculating the length of the director....468/freq mhz
Reflector length = director length + 5% longer.
Spacing = aprox 140/freq mhz


See further experimentation concerning spacing below~


Example:
Design for middle of the General Phone Band around
3.925mhz

468 / 3.925 = 119.24 FEET FOR DIRECTOR (DIPOLE)
REFLECTOR = 5% LONGER THAN DIRECTOR = 119.24 X .05 = 5.96 FEET ADDED TO 119.24 = 125.20 FEET
SPACING = 936 / 3.925MHZ = 238.47 FEET X .15 = 35.77 FEET FOR SPACING
(See further experimentation concerning spacing below)
If your starting this project from scratch, start with the director, (the dipole), a little longer and prune to lowest swr for middle of band as with any other antenna project!
If your dipole is already up with low swr, then just add the reflector at the proper spacing distance.
The distance from the reflector and the ground should not make any difference.
You will note by the calculations above that the distance from the driven element and reflector would require that the director be at least 35.77 feet from the ground!
If you can't get the formula spacing for installation reasons, then just do the best you can. Some experimenters state that even much lower overall dipole height above the reflector work even better. See below.

UPDATE! Spacing Experimentation


More recent experimentation by Pat Lambert, W0IPL and others conclude the distance from the antenna and the ground can be lowered considerably with much better results.

Here is a teaser comment made by him:
"While 1/8th wave works reasonably well, better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground. A second advantage of lowering the antenna to near 1/20th wavelength is a lowering of the background noise level. At a recent S.E.T. communication on 75 Meters was started with a dipole at approximately 30 feet. We found communication with some of the other participants to be difficult. A second 1//2 wave dipole was built and mounted at 8 feet off of the ground. The background noise level went from S7 to S3 and back when we switched back the antennas, plus communications with stations in the twenty-five and over mile range were greatly enhanced."
See the complete article here with lots more on NVIS NOTE:

ANTENNA SUPPORTS MUST BE NON-CONCUCTIVE

 
 
FOR BEST RELULTS!
USE A GOOD HEAVY WIRE SIZE SUCH AS # 12 OR 14.
OTHER TYPES OF ANTENNAS CAN BE USED NVIS STYLE BY JUST ADDING THE CORRECT LENGTH REFLECTOR AT THE BOTTOM OF THE ANTENNA
.

About NVIS antennas


NVIS propagation is simply sky wave propagation that uses antennas with high-angle radiation and low operating frequencies. Just as the proper selection of antennas can increase the reliability of a
long- range circuit, short-range communications also require proper antenna selection. NVIS propagation is one more weapon in the communicator’s arsenal.
To communicate over the horizon to an amphibious ship or mobile on the move, or to a station 60-190 miles away, the operators should use NVIS propagation. The ship’s low take-off angle antenna is designed for medium and long-range communications. When the ship’s antenna is used, a skip zone is formed. This skip zone is the area between the maximum ground wave distance and the shortest sky wave distance where no communications are possible.
Depending on operating frequencies, antennas, and propagation conditions, this skip zone can start at roughly 12 to 18 miles and extend out to several hundred miles, preventing communications with the desired station.
NVIS propagation uses high take-off angle (60° to 90°) antennas to radiate the signal almost straight up. The signal is then reflected from the ionosphere and returns to Earth in a circular pattern all
around the transmitter. Because of the near-vertical radiation angle, there is no skip zone. Communications are continuous out to several hundred miles from the transmitter. The nearly vertical angle of radiation also means that lower frequencies must be used. Generally, NVIS propagation uses frequencies up to 8 MHz.
The steep up and down propagation of the signal gives the operator the ability to communicate over nearby ridge lines, mountains, and dense vegetation. A valley location may give the operator terrain
shielding from hostile intercept and also protect the circuit from ground wave and long-range sky wave interference. Antennas used for NVIS propagation need good high take-off angle radiation with very little ground wave radiation.


"NVIS techniques concentrate on the areas which are often in the skip zone. The idea is to radiate a signal at a frequency which is below the critical frequency, at a nearly vertical angle, and have that signal reflected from the ionosphere at a very high angle of incidence, returning to the earth at a relatively nearby location." 

de WB5UDE
 

EXPLORING RECHARGEABLE BATTERIES

by Peter Parker VK3YE
( first appeared in Amateur Radio, December 1999)     
  

Rechargeable batteries:
They're used everywhere, and there's many different brands and types. Almost every amateur has their own opinions on the merits of different types and the best ways to look after them. Here we examine the main types available and their suitability for various equipment amateurs use.

How rechargeable batteries work
Batteries convert stored chemical energy into electrical energy. This is achieved by causing electrons to flow whenever there is a conductive path between the cell's electrodes.

Electrons flow as a result of a chemical reaction between the cell's two electrodes that are separated by an electrolyte. The cell becomes exhausted when the active materials inside the cell are depleted and the chemical reactions slow. The voltage provided by a cell depends on the electrode material, their surface area and material between the electrodes (electrolyte). Current flow stops when the connection between the electrodes is removed.

Rechargeable cells operate on the same principle, except that the chemical reaction that occurs is reversed while charging. When connected to an appropriate charger, cells convert electrical energy back into potential chemical energy. The process is repeated every time the cell is discharged and recharged.

Different cells use different electrode materials and have different voltage outputs (1.2, 1.5, 2 and 3.6 volts for the types discussed here). Higher voltages are possible by connecting cells in series. A set of several cells connected together is called a battery. However, because lay people do not distinguish between a 1.5 volt cell and a 9 volt battery (which comprises several cells), the term battery is widely used for both batteries and cells.

The capacity of cells is expressed in amp-hours (Ah) or milliamp-hours (mAh). The approximate time that a battery will last per charge can be found by dividing the battery pack capacity (normally written on the battery pack itself) by the average current consumption of the device. Thus a 600 mAh battery pack can be expected to power a receiver that takes 60mA for 10 hours.

Cells can be visualised as consisting of a cell with a resistor in series. You won't find an actual resistor should you split open a battery pack, but the effect is the same. Some battery types have higher values of internal resistance than others. High internal resistance doesn't matter if powering items that draw fairly low currents (eg a clock or small receiver). However, if running something like a 5-watt handheld transceiver, a battery with a high internal resistance will not deliver the current asked of it.

Having explained some of the characteristics important to all batteries, we will now look at each cell type in turn.

Nickel-cadmium (NiCad)
Nickel-cadmium cells are the most commonly used rechargeable batteries in consumer applications. They come in similar sizes to non-rechargeable cells, so they can directly replace non-rechargeable alkaline or carbon-zinc cells. NiCads have a lower voltage output than non-rechargeable cells (1.2 vs 1.5 volts). This difference is not important in most cases.

NiCad battery packs have voltages of 2.4, 3.6, 4.8, 6, 7.2, 9, 10.8 volts, etc. This corresponds to 2, 3, 4, 5, 6, 7, 8 and 9 cells respectively.

NiCads perform best between 16 and 26 degrees Celsius. Their capacity is reduced at higher temperatures. Hydrogen gas is created and there is a risk of explosion when cells are used below 0 degrees.

NiCad batteries have a low internal resistance. This makes them good for equipment that draws large amounts of current (eg portable transmitting gear). However low internal resistance means that extremely high currents (as much as 30 amps for a C-sized cell!) will flow if cells are short-circuited. Short-circuiting should be avoided as it can cause heat build-up and cell damage.

Most portable transceivers come with NiCad battery packs where the cells are welded to metal connecting straps. There is good reason for this. In high-current applications, the unknown (and varying) resistance between cells and battery holder contacts can result in erratic operation. This is especially so when the transceiver is used in a salt-laden environment. An encased battery pack overcomes these difficulties and provides more reliable operation.

The normal charging rate is 10 per cent of a battery's capacity for 14 hours. For example, if a battery pack has a 600 mAh rating, its correct charging current is 60 mA. Because the charging process is not 100% efficient, the charger needs to be left running for about 14 hours instead of 10 hours. Higher charging currents are possible, but the charging time needs to be proportionally reduced. NiCads can be left on a trickle charger indefinitely if the charging current is reduced to 2% of the battery's amp-hour rating. Avoid the build up of heat during charging for long battery life.

NiCad batteries require a constant current charger; ie one where the current provided to the battery is fixed over the entire charging period. Such a charger can be something as simple as an unregulated DC power supply with a series resistor to limit the charging current into the cells. If the charger's voltage and the battery's desired charging current is known, Ohm's Law can be used to calculate the correct series resistor value. Because NiCads have a low internal resistance, proper charging can occur with several cells in series.

For best life, do not discharge NiCads to less than 1.0 volt per cell. When charging, NiCads should read 1.45 volts per cell. If the cell voltage is higher during charging (eg 1.6 or 1.7 volts), the cell is faulty and should be discarded.

You'll often hear discussions about the so-called 'memory effect' exhibited by NiCad cells. This refers to the claimed tendency of cells not to deliver their rated voltage when placed in a charger before being fully discharged. Belief in the existence of the 'memory effect' is widespread amongst users of NiCad batteries. However, textbooks and data from battery manufacturers make little or no mention of it. Believers say that to prevent it batteries must be discharged to 1 volt per cell before charging. Non-believers say that this discharging merely reduces cell life.

Evidence suggests that true 'memory effect' is rare. It was first noticed in communications satellites where cells were discharged to precisely the same discharge point every time. In casual amateur use batteries are most unlikely to be discharged to the same point after every use. Much of what is mistaken for the 'memory effect' is voltage depression, which is caused by long, continuous overcharging, which causes crystals to grow inside the cell. Fortunately both the 'memory effect' and voltage depression can be overcome by subjecting the battery to one or more deep charge/discharge cycles.

Another term you will hear is 'cell reversal'. This can occur when a battery of cells is discharged below its safe 1.0 volt per cell. During this discharge, differences between individual cells can lead to one cell becoming depleted before the rest. When this happens, the current generated from the remaining active cells will 'charge' the weakest cell, but in reverse polarity. This can lead to the release of gas and permanent damage to the battery pack.

NiCads can short circuit due to the build up of crystals inside the battery. The use of a fully-charged electrolytic capacitor placed across the cell can effect a temporary cure. Over-discharging of batteries invites short circuiting. Batteries should be stored charged. A lifespan of 200 to 800 charges is typical for NiCad batteries.

Nickel metal hydride (NiMH)
Like NiCads, nickel-metal hydride cells provide 1.2 volts per cell. Battery makers claim that NiMH cells do not suffer from the 'memory effect' and can be recharged up to 1000 times.

NiMH cells are not as suitable as NiCads for extreme current loads, but do offer a greater capacity in the same cell size. A typical AA NiCad may have a 750 mAh, but a NiMH may provide 1100 mAh - 45 percent more. This makes NiMH cells a good choice for applications where long life is desired but current demands are not high - eg portable receiving equipment.

NiCad chargers can be used to charge NiMH batteries, but the charging time needs to be lengthened to take NiMH's typically larger capacity into account. The main enemy of rechargeable cells is heat. If cells get hot during charging, reduce the charging current to no more than that recommended.

Rechargeable alkaline manganese
Unlike the preceding two battery types, rechargeable alkaline manganese (RAM) cells give a full 1.5 volts each. They are therefore suitable for applications where the substitution of 1.2 volt NiCads for 1.5 volt dry cells results in degraded equipment performance.

RAM cells are cheaper to buy than NiCads. They can be recharged between 50 and 750 times. They also have a greater capacity than do NiCads - 1500 mAh is typical for size AA cells. RAM cells are good for use with outdoor and solar equipment as they will work efficiently at temperatures up to and exceeding 60 degrees Celsius.

RAM cells have a much higher internal resistance than NiCads (0.2 ohms vs 0.02 ohms). This means that they cannot supply high peak values of current. For this reason they are unsuitable for use with standard amateur HTs. However, their high capacity and long shelf life (5 years) makes them suitable for low powered or emergency-use applications, such as clocks and emergency torches.

Chargers intended for NiCad and NiMH cells will not charge rechargeable alkalines. This is because rechargeable alkaline cells require a constant voltage source of between 1.62 and 1.68 volts to charge. RAM cells should be connected in parallel rather than in series when charging several cells at a time. Unlike other rechargeable batteries, RAM cells are pre-charged and do not require charging before first use.

Lithium ion
Lithium ion cells are the most recent of the battery types discussed here to come onto the market. They offer higher cell voltage (3.6 volts) and greater capacity for a given volume. This makes them especially suitable for handheld equipment where long operating times are important, such as mobile phones.

As an example of what Lithium ion battery packs can do, a typical lithium ion battery pack is 55x45x20mm but provides 7.2 volts with a 1100 mAh capacity. Lithium ion batteries are still quite expensive, but are coming into amateur use through their inclusion in handheld transceivers such as Yaesu's VX-1R and VX-5R models.

Sealed lead acid
Sealed lead acid batteries (or 'gel cells') are less popular than NiCads in handheld equipment, but find widespread use as back up batteries in security systems and for amateur portable operation. Per-cell voltage is 2.3 volts when charged, and 1.8 volts when discharged. This equates to 13.8 and 10.8 volts respectively for a battery of six cells. For best use of the full battery charge, equipment intended to operate with '12 volt' sealed lead acid batteries should operate well (if not at full power) at voltages of 10.8 volts or less.

Gel cells are cheap, rugged and reliable and should last several years at least. If you want a battery to run a QRP HF station or a VHF/UHF handheld for several hours, they are the ideal choice. They are also widely used with small solar systems.

Sealed lead acid batteries can either be used on a cyclic charge regime (battery connected to charger for a specific time) or continuous float use, where the battery is across the charger any time it's not in use. Cyclic chargers should charge at 2.4 or 2.5 volts per cell and be current limited to prevent overcharge. In contrast continuous float charging (or trickle charging) requires a charging voltage of only 2.3 volts per cell (13.8 volts for a '12 volt' battery). With both types of use the charger voltage is held constant. Connect batteries in parallel if charging two or more from the one charger.

Chargers for sealed lead acid batteries are available commercially or can be made at home. Special gel cell charger ICs exist to provide the necessary voltage and current regulation. Alternatively chargers can be made from the more common regulator chips such as the 723 or LM317. These chargers can be used to directly trickle charge the smaller '12 volt' gel batteries. No damage is done if the charger remains on, even when the battery is fully charged. This is because as the battery voltage approaches 13.8, the charging current will fall to negligible levels.

Sealed lead acid batteries should not be charged at voltages higher than those indicated as safe above. This is because high charging voltages (eg 2.6 volts per cell) will endanger the battery due to the production of excess gas. At a 13.8 volt charging voltage the production of gas is low, and the battery should give years of service. Charging current should not exceed 20 per cent of the rated amp hour capacity of cells. If using a high current 13.8 volt power supply as a charger, some form of current limiting is desirable to stay within the battery's limits.
Conclusion
This article has examined the characteristics of all major types of rechargeable batteries used by amateurs. We learned that NiCads and sealed lead acid cells were best for high current applications, while other varieties, such as rechargeable alkaline and nickel metal hydride work well for low current applications. The charging of batteries varies too - Rechargeable alkaline and sealed lead acid required a constant voltage, but nickel cadmium and nickel metal hydride cells needed a constant current to charge properly. In all cases over-charging, through excessive voltages, currents or charging periods can cause heating, gas build-up and possible cell damage. However, if you treat your batteries well, you should have many years of successful operation from them, whichever type you choose.
Acknowledgments
I wish to acknowledge the people and organisations who have contributed to the writing of this article. These include:

The late Bill Trenwith VK3ATW for suggestions on the manuscript and imparting of knowledge gained through many years as a mechanics teacher, model engineer and radio amateur.
Peter Wegner from Coorey & Co, distributors of BIG rechargeable alkaline cells.
Danielle Cvetkovic from Invensys Energy Systems Pty Ltd for material on Hawker sealed lead acid batteries.
Adeal Pty Ltd for information on Varta's range of NiCad and NiMH cells.
References

1. Hawker P G3VA, Technical Topics Scrapbook 1990-1994, RSGB, pages 1, 16, 142

2. ARRL Handbook 1988, ARRL, pages 6-25, 27-32

3. Gruber N WA1SVF, QST November 1994, ARRL, page 70


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