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Ham radio antenna hints and tips
- some essential hints and tips for radio amateurs or radio = hams about=20 deciding upon how to install the best radio antenna or aerial for=20 operating on the amateur radio bands.
A radio antenna is the crucial link in any = receiving or=20 transmitting station whether it is used for ham radio, short wave=20 listening, or for commercial or professional use. The overall = performance=20 of the radio station depends upon the performance of the radio = antenna. A=20 good radio antenna will enable the performance of the whole radio=20 communications station to be maximised, whereas a poor antenna = will=20 degrade the capabilities of the transmitter and receiver = regardless of how=20 good they are.
Radio stations used for professional or = commercial=20 applications have a large degree of flexibility and they will site = antennas where they will give a suitable level of performance. = However for=20 ham radio enthusiasts and short wave listeners it is necessary to = install=20 the best antenna around the house. Very few ham radio operators = are able=20 to utilise a field or other large area, and often the radio = antenna will=20 be something of a compromise. Nevertheless, by following a few = guidelines,=20 it is possible to make the best of any radio antenna installation = and=20 ensure that its performance is as good as it possibly can be. In = addition=20 to this it is worth mentioning that it is usually necessary to = undertake=20 some experimentation to find out what type of radio antenna works = best for=20 a given location and style of radio operation.
Some hints and tips are given below, but these = can only=20 be general guidelines, and they are not exhaustive. However they = form a=20 good starting place when thinking of installing a ham radio = antenna=20 system
General antenna situation
One of the most important aspects of setting up = any radio=20 antenna is its location. The location of the antenna will govern = many=20 aspects of its operation, and therefore the location of the = antenna must=20 be determined along with the type of antenna to be used. A number = of=20 points associated with the antenna should be considered:
- Choose a location where the radio antenna can "see" =
all=20
around: In order for to operate at its best it =
must be=20
able to "see" all around it. To be able to achieve this it =
should be=20
kept away from nearby objects that might act as a screen. In =
this way=20
the maximum amount of signal can be reach or leave the antenna =
without=20
being absorbed in nearby objects.
.=20 - Remember that nearby objects can "detune" an =
antenna:=20
When considering the location of a radio antenna it is =
worth=20
remembering that nearby objects can detune an antenna even if =
they do=20
not affect the all round visibility. Nearby objects can cause an =
antenna=20
to operate away from its resonant point and become less =
efficient. This=20
is very important for antennas that are cut to a particular =
length and=20
do not have a means of being tuned in situ. Many items can cause =
this to=20
happen - metal items as well as electrical wiring are =
particularly bad=20
but even trees can degrade the performance of antennas in this =
way.=20
Generally the effects are noticeable within distances of a =
wavelength or=20
two, the closer the object and the greater the conductivity the =
greater=20
the effect.
- Consider suitable points for anchoring =
antennas:=20
Horizontal antennas need anchor points at either end. It =
is worth=20
considering whether there are any suitable anchor points already =
in=20
existence. Chimneys or other points on the house can provide one =
suitable point. Trees may also be located conveniently, although =
pulley=20
schemes are required to enable any movement in the tree due to =
wind to=20
be taken up without snapping the antenna wire. Also it may be =
possible=20
to erect a pole or antenna mast and consideration can be given =
to this=20
possibility and its location. Whatever option is decided upon, =
this must=20
be considered at the outset.
- Inside or out: In many instances the =
use of an=20
internal radio antenna may have to be considered. External =
antennas=20
operate better because they can be further away from objects =
that will=20
introduce loss or detune the antenna. It is very difficult to =
estimate=20
the amount of loss which having an antenna inside the house has. =
The=20
roof or brickwork will cause the signal to be reduced, =
especially when=20
it is wet. The amount of loss will also depend on the frequency. =
For VHF=20
and UHF signals this will be much greater.
Antenna height
Although the height of a radio antenna could be = considered under the general situation of the antenna, the height = is very=20 important and can make a marked difference to its performance. As = a result=20 the various points are considered separately.
- The higher the antenna, the less it will be =
screened:=20
The higher an antenna is situated, the less will be the =
effect of=20
screening by nearby objects as it will tend to become higher =
than many=20
of them and thereby reduce their effect.
- The higher an antenna the greater its distance to the =
radio=20
horizon: In the same way that standing on a high =
hill=20
increases the distance to the visual horizon, so increasing the =
radio=20
antenna height increases the distance to the radio horizon. This =
is=20
particularly important for signals at VHF and above which travel =
almost=20
in a direct line and are not reflected by the =
ionosphere.
- Increasing radio antenna height can reduce the angle =
of=20
radiation: For short wave signals the height of =
the=20
antenna is also important. Long distance signals are generally =
received=20
at a low angle of elevation. If the antenna is higher it enables =
these=20
signals to be received more easily.
Interference considerations
Interference caused to ham radio stations and=20 interference cause by them is obviously of great importance. The = location=20 of the antenna for the ham radio station can have a significant = impact on=20 both aspects of interference. As a result this should be kept in = mind when=20 choosing the location for the radio antenna.
- Keep the radio antenna away from sources of =
interference in=20
the house: Most houses contain many items which =
are very=20
good sources of noise. Although televisions and computers are =
very much=20
better nowadays, some interference is still generated =
particularly as=20
many computers now have networks associated with them. =
Interference is=20
also generated form a variety of other electrical items around =
the home.=20
Vacuum cleaners, electric drills, electric mixers and a host of =
similar=20
utensils all contribute to the level of electric noise =
generated. This=20
radiates in and around the house, reducing in intensity the =
further away=20
from the source one moves. As a result it is best to try to keep =
the=20
antenna as far away from the house. This may not always be =
possible the=20
level of interference can be minimised by keeping the antenna =
away from=20
particular sources of noise.
- Ham radio transmitters (or any radio transmitters for =
that=20
matter) can cause interference if they are close to domestic=20
appliances: Despite the fact that domestic =
equipment is=20
very much more resilient to interference than it used to be, =
there is=20
still the possibility that a ham radio transmitter can cause=20
interference. One of the best ways of reducing the possibility =
of any=20
interference is to ensure that the ham radio antenna is situated =
away=20
from any other equipment. This is best achieved by keeping the =
antenna=20
away from any domestic premises. Obviously the risk is less for =
low=20
power transmitters, but for high power amateur radio stations =
this can=20
become more of an issue, especially if directive antennas are =
used which=20
could be beamed towards any domestic premises.
Antenna matching
Matching the antenna to the feeder is an = important factor=20 if the radio transmitter and antenna system are to operate = effectively.=20 Radio frequency systems such as antennas, feeders, sources (i.e.=20 transmitters) and loads (i.e. receivers) all have a characteristic = impedance. For the system to operate effectively these must match, = otherwise standing waves are generated and the power transfer is = reduced.=20 To ensure that the overall system is properly matched and is = operating=20 correctly a number of measures can be introduced.
- Use a VSWR meter to ensure that the radio antenna =
system is=20
operating correctly: Antennas usually operate =
only over a=20
relatively narrow bandwidth, and many antennas are only able to =
operate=20
on a single band, and sometimes adjustment is needed to enable =
them to=20
operate even at different ends of the band. This ensures that =
the=20
antenna is matched to the feeder, and the maximum amount of =
power is=20
transferred. To check whether the antenna is operating correctly =
a VSWR=20
meter can be inserted in the line. By keeping one in circuit at =
all=20
times the operating of the system can be monitored.
- Consider the use of an ATU to ensure the optimum =
antenna=20
operation: In order to ensure that the antenna =
impedance=20
matches that of the feeder it is often necessary to employ a =
matching or=20
tuning unit between the feeder and the antenna itself. These =
antenna=20
tuning units (ATUs) are used to tune the radiating element so =
that its=20
impedance matches that of the feeder so that the maximum power =
transfer=20
is obtained and the level of standing waves is minimised. To be =
able to=20
tune the antenna properly the ATU should be located at the=20
antenna.
- Use an ATU in the shack to reduce the level of VSWR =
seen by=20
the radio transmitter output stage: High levels =
of=20
standing waves can cause damage to the semiconductor output =
stages of=20
radio transmitters. To protect these stages from damage, many=20
transmitters detect the level of VSWR (voltage standing wave =
ratio) and=20
reduce the output power when levels start to rise. Therefore to =
ensure=20
that the transmitter can deliver its maximum power output a low =
level of=20
VSWR must be present. When it is not possible to have an ATU =
right at=20
the feed point of the antenna, one can be placed near the =
transmitter to=20
tune the antenna feeder combination so that the VSWR seen by the =
transmitter is minimised. While not totally ideal it can help by =
allowing the transmitter to see a low level of =
VSWR.
Antenna feeder considerations
The feeder is an important part of any radio = antenna. Its=20 purpose is to ensure that the maximum amount of power reaches its=20 destination, either radio transmitter power reaching the antenna, = or=20 incoming signals from the radio antenna reaching the receiver. Any = power=20 lost will reduce the efficiency of the whole antenna system. The = feeder=20 cost and performance considerations may have an effect on any = decision=20 made regarding the antenna, and it is therefore important.
- Choose the optimum feeder for any situation: =
=20
The cable type should be chosen to provide an acceptable loss at =
the=20
frequencies to be used. For frequencies below 30 MHz feeder loss =
is=20
normally quite low and subject to power limitations thinner coax =
may=20
often be used unless long runs are required. As frequencies =
rise, so do=20
the levels of loss, and thicker, lower loss varieties are =
needed.=20
Although the cost can be high, an investment in a low loss cable =
can=20
ensure that the overall antenna system operates to its full=20
capability.
- Consider the installation of the feeder when planning =
the=20
radio antenna installation: Any feeder should be=20
correctly installed. For example if it is coax or coaxial cable, =
it=20
should not be bent too tightly beyond its minimum bend radius. =
If this=20
is done damage may occur. Open wire feeder should not be run =
through a=20
house as nearby objects will de-tune it and losses will rise. =
Any run=20
inside a building must be kept to an absolute minimum
- Suitably weatherproof any feeder if it is to be used=20
externally: In particular, great care should be =
taken=20
when using coaxial cable outside. The end of the coax should be =
sealed=20
to prevent any moisture ingress. Moisture will itself cause loss =
as it=20
will absorb power, and for the longer term it will give rise to=20
corrosion which will degrade the performance of the =
coax.
Good earth
One good solution for many ham radio HF antenna = requirements is a ground mounted vertical. These and a number of = other=20 radio antenna systems require the use of an efficient earth or = ground=20 system for it to operate satisfactorily. As the ground system is = key to=20 the operation of the radio antenna it is necessary to ensure that = the=20 ground system is satisfactory.
- Ensure the ground system has a low electrical=20
resistance: One of the major requirements for an =
earth or=20
grounding system for an antenna is that it should have a low =
electrical=20
resistance. This can be achieved by ensuring that there is the =
maximum=20
surface area of metal in contact with the earth itself. =
Electrical=20
grounding rods as well as old copper pipes can be used.
- Ensure the ground system has a low RF =
resistance:=20
Although good electrical conductivity for DC is required, =
the RF=20
performance can be further improved by laying radials, typically =
a=20
quarter wavelength long radiating out from the central ground =
point. If=20
compromises have to be made, they should be laid out in the =
direction=20
where optimum performance is required.
- Ensure the main point for the ground system is as =
close to the=20
feed point of the ground mounted antenna as possible: =
=20
Antenna systems using a ground system do not operate well if the =
lead to=20
the ground system is long. Accordingly the main connection for =
the=20
ground system should be as close to the base of the antenna as=20
possible.
- Consider the ground conductivity: The=20
conductivity of the ground itself should be considered when =
choosing a=20
place for the ground system. Dry sandy soil gives a poor =
connection=20
whereas damp fertile soil gives a much better result. If =
possible the=20
earth connection should be made in a place where the earth =
connection=20
will be better. This may alter the choice for the position for =
some=20
vertical antennas, or it may mean that ground mounted systems =
may not be=20
viable.
Antenna safety aspects
When any radio antenna system is fitted up, = whether for=20 ham radio or any other purpose, safety must be one of the major=20 considerations. Sometimes antennas are temporarily fitted up, only = to fall=20 down when the wind rises. Home-built antennas are particularly at = risk,=20 but even commercially made ones can suffer if they are not = installed=20 correctly. Care must be taken to ensure that there is no chance of = the=20 antenna falling and injuring someone.
Particular care must be taken if there are any = power=20 lines in the vicinity, even the relatively low voltage ones used = for=20 powering just one or two houses. The antenna should be sited so = that even=20 if the antenna did fall there would no possibility whatsoever of = it=20 touching the line. In the past people have been killed when this = has=20 happened.
Above all an overriding sense of the safety = aspects of=20 radio antenna design and installation must be employed when = installing=20 antennas. With the weather being what it is, any risks taken are = likely to=20 result in failure before long. It is not worth taking any risks as = falling=20 antennas can cause harm to people and to property, and in any case = an=20 antenna on the ground cannot be used.
Summary
Experimenting with radio antennas can be a = fascinating=20 area or ham radio. Many ham radio enthusiasts find it a = particularly=20 rewarding element of the hobby. Also, experience gained from = setting up=20 and testing various types of radio antenna can be very useful, = adding=20 vastly to some of the basic guidelines summarised here. It can = also be=20 very rewarding when a new antenna performs particularly well, and = much=20 better than previous installations. The performance of the ham = radio=20 station can be greatly improved and this can often be seen in the = results=20 with many distant contacts being made.
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