What A Good Idea!
NOTE: These articles were taken from packet BBSs.
Copper wire conversion chart
SWG mm in
1 7.62 .300
2 7.010 .276
3 6.401 .252
4 5.893 .232
5 5.385 .212
6 4.877 .192
7 4.470 .176
8 4.064 .160
9 3.658 .144
10 3.251 .128
11 2.946 .116
12 2.642 .104
13 2.337 .092
14 2.032 .080
15 1.829 .072
16 1.626 .064
17 1.422 .056
18 1.219 .048
19 1.016 .040
20 0.9144 .036
21 0.8128 .032
22 0.7122 .028
23 0.6096 .024
24 0.5588 .022
25 0.5080 .020
26 0.4572 .018
27 0.4166 .0164
28 0.3759 .0148
29 0.3454 .0136
30 0.3150 .0124
31 0.2946 .0116
32 0.2743 .0108
33 0.2540 .0100
34 0.2337 .0092
35 0.2134 .0084
36 0.1930 .0076
37 0.1727 .0068
38 0.1524 .0060
39 0.1321 .0052
40 0.1219 .0048
41 0.1118 .0044
42 0.1016 .0040
43 0.0914 .0036
44 0.0813 .0032
45 0.0711 .0028
46 0.0610 .0024
47 0.0508 .0020
48 0.0406 .0016
49 0.0305 .0012
50 0.0254 .0010
Extending the range of moving coil meters.
Moving coil meters are ALL current indicating devices,
however, with the addition of a series resistor they
can be made to indicate voltage.
First of all, determine the amount of current required
to move the needle to full scale deflection, either by
reading the information on the scale or body of the meter,
or by empirical test.
Common full scale deflection ( fsd ) figures are:
50 microamps, 100 microamps, 1 milliamp.
Let's assume that the meter to be used has an fsd of 1 mA,
and the full scale voltage reading is to be 30 Volts,
using Ohm's Law, calculate the value of resistance required
to pass 1 mA with an applied voltage of 30 Volts.
R = ___ = ____ = 30,000 Ohms.
This figure of 30,000 Ohms is the TOTAL resistance of the
meter in series with the multiplier resistor, and so the
resistance of the meter has to be subtracted from this figure
to produce the value for the series resistor.
A typical 1 mA moving coil has a resistance of 75 Ohms,
therefore the series resistance required to give 30 Volts fsd
= 30,000 - 75 which equals 29,925 Ohms. This is not a standard
value and will have to be produced by 2 or more resistors in
series or series / parallel.
If you have a means of calibrating the meter, then by all means
make part of the resistor chain variable to facilitate adjustment.
A typical circuit would be:
Negative |___| positive
terminal 1 mA 1 k 2.2 k 27 k terminal
The resistors used would ideally have a very low temperature
coefficient and would be high stability types e.g. metal film
types. The best type of variable resistor for this application
is the "Cermet multiturn".
Rescaling the meter involves dismantling the meter and removing the
scale plate ( do this carefully in a clean area - the meter contains
a very powerful magnet, and any steel swarf or filings will find
their way into the mechanism rendering the meter useless! )
"Letraset" or "Indian Ink" are some of the methods which can be used
for changing the scale markings.
Light Emitting Diodes.
Light emitting diodes, ( commonly known as LEDs ) are CURRENT
operated devices. They are semiconductor devices which emit
light when current flows through them in the forward direction.
The colour of the light, generally ranging from infra red
to blue, depends upon the materials used for their construction.
The typical forward current of an LED is 10 milliamps, and severe
damage can result if this figure is grossly exceeded. Remember,
they are CURRENT operated devices, and any circuit devised for
illuminating an LED ***MUST*** have current limiting component(s).
The most common method of limiting the current is a series resistor.
Before we can calculate the value of the resistor, we need to know
1) The forward voltage of the LED. Like all diodes, the LED has a
voltage drop when current is passed through it. Forward voltages
of LEDS vary from typically 1.8 Volts for a red LED, 2.1 Volts
for a green LED to 3 Volts for a blue LED. Manufacturers data
sheets should supply this information.
2) The forward current of the LED. Again, manufacturers will supply
this very important parameter.
3) The maximum voltage of the circuit. This parameter is decided by
An LED is required to indicate the presence of voltage on the output
of a 13.8 Volt d.c. power supply.
Having chosen a suitable device, e.g. a red LED with a forward current
of 10 milliamps, and a forward voltage of 1.9 Volts, we now have to
calculate the value of resistor which is required to limit the current.
Starting with our supply voltage of 13.8 Volts, subtract the forward
voltage of the LED from this ( 13.8 - 1.9 = 11.9 Volts ).
Using Ohm's Law, R = V / I, the resistance = 11.9 / 0.01 = 1190 Ohms.
This is not a generally available value, so go to the nearest available
value ABOVE the figure calculated. In this case it is 1200 Ohms.
Therefore a resistor of 1200 Ohms ( or 1.2 kilohms ) would be a suitable
value to place in series with the LED for operation at 13.8 Volts d.c. and
a forward current of 10 milliamps.
Reverse voltage above about 3 Volts will irreparably damage the LED, so
ensure the LED is connected the correct way. The anode of the LED goes
towards the most positive part of the circuit, likewise, the cathode of
the LED must be connected to the most negative part of the circuit.
NICAD BATTERIES - FACTS AND FALLACIES
Rechargeable nickel cadmium batteries, have, with reasons, become a
popular source of power for portable and handportable equipment. They
can provide reliable service over many years if due account is taken
of their peculiarities. Yet it remains true that many amateurs are
failing to appreciate not only the full capabilities but also the
limitations of nicad cells used in battery packs.
J.Fielding, ZS5JF, in "Nickel cadmium batteries for amateur radio
equipment" (Radio ZS september 1987,pp4-5) provides a useful survey of
the facts and foibles of nicads. The following extracts from his
article attack some of the common myths and also provide some safety
1) "Rapid charging causes a decline in cell capacity".
NOT TRUE provided that the charge is always terminated at a safe point.
2) "You should not charge only partially discharged cells as this
causes a loss in capacity."
NOT TRUE. It is not necessary to discharge fully nicad batteries
before charging. In fact, THE OPPOSITE is true. Repeated partial
charging gives an increase in the number of charge/discharge cycles
compared with full-discharged cells.
3) "White crystals growing on the tops of nicad cells mean that the
seal is faulty and the cell should be scrapped."
NOT TRUE. The electrolyte (potassium hydroxide) is extremely searching
and can penetrate the seals used in minute quantities. These crystals
are potassium carbonate, which is harmless and can be removed with
soap and water. The action of the carbon dioxide in the atmosphere
reacts with the electrolyte to form the crystals. After removing the
crystals, it is recommended that a smear of silicone grease is applied
to slow down the growth of new crystals. The amount of electrolyte
lost in this way is insignificant.
4) " I have a cell which appears to take a charge, but after the
normal charging period the open circuit voltage is very low. I have
been told I should throw it away."
NOT TRUE. The reason the cell won't take a charge is usually due to
minute crystalline growth across the internal electrodes, caused by
prolonged storage. A cure that nearly always works is to pass a very
high current for very short time through the affected cell. This fuses
the internal "whisker". Discharging a large electrolytic capacitor is
one method of doing this. But note that in a battery the faulty cell
MUST be isolated from the other cells since zapping the complete
battery will not usually result in a cure. Charge the capacitor to
about 30v and then discharge it through the faulty cell. Several
attempts may be required to clear a stubborn cell.
5) "A battery contains a cell with reversed polarity. The only cure is
to replace it".
NOT TRUE. The reversed cell can usually be corrected by a similar
technique as that given for 4). After re-polarising the cell, the
complete battery can be recharged in the normal way. Full capacity can
be regained after about five cycles.
6) "A nicad battery should be stored only in a discharged state".
NOT TRUE. It can be stored in any state of charge. Due to its inherent
self-discharging characteristics it will eventually become fully
discharged after a sufficiently long period of storage. To recharge
the battery before returning it to service, a "conditioning" charge of
20h at the normal charging rate is recommended. Afterwards charge
normally; full capacity can again be expected after about five cycles.
7) "It is not advisable to keep a nicad battery on permanent trickle
charge as this causes permanent degradation of the cells".
NOT TRUE. So long as the trickle charge current is adjusted correctly,
the charge can continue indefinitely without loss in cell capacity.
The safe current can usually be obtained from the manufacturer's data,
but 0.025C is a reasonable guide (ie. about 100mA for a 4Ah cell and
PRO-RATA). This enables the battery to remain fully charged.
ZS5JF also lists seven safety points that should be considered by users:
1) DO NOT short circuit a fully-charged battery. This if prolonged,
can cause excessive gas production with the danger of possible
rupturing of the sealed case.
2) Nicads contain a caustic electrolyte: this is perfectly safe as
long as common sense is used in use and handling of the cells.
3) A nicad can supply a very high current for a short period (a 4Ah
cell can supply over 500A for a few seconds). Sufficient thought
should be given when selecting a fuse between the battery and the
equipment. The connecting wire should be capable of passing enough
current to ensure the fuse blows quickly in the event of a short
4) DO NOT use partially-discharged cells with fully-charged ones to
assemble a battery. Assemble the battery with all the cells discharged
and then charge them as a battery.
5) DO NOT carry a fully or partially charged battery on an aircraft
without taking proper safety precautions. A short-circuited battery
pack can be a time bomb in such situations. Consult the relevant IATA
regulations or ask at the airline check-in.
6) DO NOT subject battery packs to very high or low temperatures.
Never dispose of a battery pack in a fire or throw it out with
domestic waste. If it cannot be disposed of properly it is probably
best to bury it in the garden in a safe spot.
7) DO NOT discharge battery packs below about 1V per cell, otherwise
there is a possibility of cell reversal.
ZS5JF provides a good deal of other information on charging nicad
batteries, and gives as a reference a Varta publication of 1982
"Sealed Nickel Cadmium Batteries" from which some of his notes may
have been derived.
Toroidal transformer ratings.
For the majority of toroidal transformers operating at 50Hz, the
rating can be determined from the physical size of the transformer,
including the copper windings, but not any external casing or
Rating VA dia (mm) height (mm) approx regulation
15 62 37 19%
30 70 37 15%
50 80 43 13%
80 95 43 10%
120 90 56 10%
160 110 50 8%
225 110 55 6%
300 120 65 6%
500 135 65 4%
625 140 75 4%
1000 160 82 3%
These figures are from a typical manufacturer's data sheets and can be
used as approximations for 99% of toroidal transformers operating from
a supply frequency of 50Hz only.
The regulation figure is obtained from the formula:-
( Open circuit secondary voltage )
( --------------------------------- -1 ) x 100%
( Fully loaded secondary voltage # )
# All secondaries fully loaded.
Power loss due to vswr
| TX POWER TO ANTENNA
VSWR | 100W 50W 25W 10W
1.0 | 0 0 0 0 \
1.1 | 0.2 0.1 0.1 0.12 |
1.2 | 0.8 0.4 0.2 0.08 |
1.3 | 1.7 0.9 0.4 0.17 |
1.4 | 2.8 1.4 0.7 0.28 |
1.5 | 4.0 2.0 1.0 0.40 |
1.6 | 5.3 2.7 1.3 0.53 |
1.7 | 6.7 3.4 1.7 0.67 |
1.8 | 8.2 4.1 2.0 0.82 > REFLECTED POWER IN WATTS
1.9 | 9.6 4.8 2.4 0.96 |
2.0 | 11.1 5.6 2.8 1.11 |
2.1 | 12.6 6.3 3.1 1.26 |
2.2 | 14.1 7.0 3.5 1.41 |
2.3 | 15.5 7.8 3.9 1.55 |
2.4 | 17.0 8.5 4.2 1.70 |
2.5 | 18.4 9.2 4.6 1.84 /
THIS ASSUMES NO POWER LOSS IN THE TRANSMISSION LINE OR ANTENNA ITSELF
MORE ON VSWR
Let's first take the comment that VSWR can only be measured
accurately at the load, or at half wave intervals back from the
load. The first part of this is correct, owing to line losses.
It has to be said, though, that it needs a rather lossy cable to
make a major difference. Take the case of using 50ft of H100
cable to feed a 2m aerial. A 3:1 VSWR at the load would be
indicated as 2.4:1 at the input; and 1.5:1 at the load would show
as 1.4:1 at the input. Given the general accuracies of the
measurement concept, these errors are not particularly a matter
When it comes to the 'half wave intervals' concept, one needs to
think about what is actually being measured by the 'VSWR meter'.
The first thing to note is that the one parameter most positively
NOT being measured is the VSWR! In order to make a direct
measurement of VSWR it is necessary to probe at intervals along
the line and measure the total voltage amplitude. Assuming that
the VSWR is not 1:1, it will be possible to find a point where
the voltage is a maximum and, 1/4 wave from this, where the
voltage is a minimum. The VSWR is the ratio of these two
voltages. It is pretty obvious from this that VSWR cannot be
measured directly at a single point on the line.
So if 'VSWR meters' are not measuring VSWR, just what are they
measuring? First we need to understand what happens when a line
is not correctly terminated. When power is first applied, the
voltage and current organise themselves according to the
characteristic impedance of the line and flow towards the load.
On arriving at the load, the forward waves find that it cannot
accept them in their existing relationship. The load accepts
what it can and sends the remainder back up the line towards the
source. With a normal transmitter arrangement, when the
reflected waves appear back at the source they find a very large
mismatch and are reflected back up the line towards the load.
After all of the transients have died down, there will be
constant amplitudes of forward and reverse waves flowing along
What the 'VSWR meter' is doing is deriving representations of the
magnitudes of the forward (VF) and reverse (VR) voltages. To use
the meter it is first necessary to set the sensitivity control
so that the forward wave representation is full scale on the
meter, then switching to the reverse position yields a direct
indication of VSWR. If the meter were to be calibrated with a
linear scale of 0 to 1 then the reverse reading would actually
indicate the fraction of the forward voltage being reflected.
This fraction is called the reflection coefficient. The VSWR can
be determined from the relationship:
VSWR = (1+P)/(1-P) where P is the magnitude of the
Ignoring line losses, the magnitudes of VF and VR and of the
reflection coefficient are constant along the length of the line.
The phase between the two voltages, and hence of the reflection
coefficient, does, however, change along the line - there being
a 360 degree phase rotation every half wave travelled. At the
points where VF and VR are in-phase the total voltage on the line
is VF+VR; and when VF and VR are in anti-phase the total voltage
is VF-VR. These are the peaks and troughs mentioned earlier when
talking about probing along the line to measure the VSWR.
Since the 'VSWR meter' derives values representing the magnitudes
of VF and VR, the phase between them is not relevant. In other
words it doesn't matter where in the line the meter is installed,
the readings are just the same (line losses apart). There is
certainly no need to restrict the placement to half wave
intervals from the load.
If a line is terminated with a load impedance not equal to its
characteristic impedance, there will, as described in part 1 of
this screed, be a standing wave on the line. The question is:
does this matter? There are two problems likely to result:
firstly, the transmitter may not be able to provide its rated
output power; and secondly, any line losses will be increased.
The first of these problems can be cured, if necessary, by the
use of an ATU; the second is a characteristic of the line, so one
is stuck with it. Let's examine the loss problem first. Fifty
feet of H100 coax will have an attenuation of about 0.8dB at
144MHz if the VSWR is 1:1. If the VSWR is 3:1, this loss will
increase by about .25dB; with a 2:1 VSWR, the loss will increase
by about 0.1dB. A 3:1 VSWR for a 2m aerial is pretty horrendous,
and 2:1 is not very much better. Even with these high VSWR
figures, though, the additional loss is hardly worth bothering
This leaves us with the problem that the transmitter may not be
able to deliver its rated power. The reason for this is that the
transmitter's output stage is not being presented with the
correct load impedance. An ATU at the transmitter output will
transform the actual load impedance into that required by the
transmitter, and full rated power will be restored. At this
point one only needs to worry about the increased line losses
resulting from the VSWR and, as mentioned above, this need not
be a problem if the appropriate cable for the application is
There is certainly no need to locate the ATU at the feed point
to the aerial unless the feed point impedance is miles away from
the characteristic impedance of the cable being used. The 5/8
vertical aerial so often used on 2m is a good example of where
an ATU at the aerial is employed. The feed impedance of such an
aerial is well away from 50ohms - apart from anything else, it
is highly reactive. It is normal with such aerials to include
a simple arrangement to transform the feed impedance to something
close to 50ohms resistive. With a short line between the aerial
and the transmitter, though, the ATU could just as well be at the
transmitter end with no noticeable loss of performance.
Perhaps it needs to be said that an ATU of sorts is always in
use! The output device of a transmitter, whether a valve or a
semiconductor, requires a specific load impedance in order to
deliver its rated power. This load impedance will be much higher
than 50ohms for a valve and much lower for a semiconductor. The
PA stage includes a network to transform the 50ohm nominal load
into the value needed by the output device. An external ATU can
be considered as being merely an extension of this internal
It is interesting to consider the readings on an 'SWR meter' when
it is inserted in different places in the system. Assume that
a transmitter feeds an ATU via a short length of coax and the
output of the ATU feeds a line with a VSWR of 2:1 on it. With
the 'swr meter' between the transmitter and the ATU, the latter's
controls can be twiddled so that the meter reads a VSWR of 1:1.
Now set the transmitter's output power so that the forward power
reading on the meter is 100W, the reverse power reading will be
zero. Move the meter so that it is between the ATU and the line
to the aerial. A 2:1 VSWR will be indicated, as expected, but
the forward power reading (assuming no ATU losses) will be 112.5W
and the reverse power will be 12.5W. The power in the load is
the difference between these, 100W.
The conclusion is that the introduction of an ATU allows the full
rated power to be delivered to the load, wherever it is fitted
in the system - the best place for an ATU is actually where it
is easiest to get at to twiddle the knobs.
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