by Peter Parker VK3YE - first appeared in Amateur Radio February 1997
INTRODUCTION
One of the most enjoyable facets of amateur
radio is operating portable. You may live in an area plagued with electrical
interference, antenna space restrictions or dwell in an 'RF hole'. In such
cases, portable operation allows you to obtain contacts not possible from home.
Or you may be planning a holiday or
excursion. While not a substitute for proper RFDS or Marine equipment, amateur
radio can add to the enjoyment of the trip, whether you take a VHF handheld,
homebrew QRP rig or full HF station.
Those involved in amateur radio publicity,
WICEN or community service activities will find a properly functioning portable
amateur station an asset. Having such a capability also allows you to respond
quickly in emergencies.
So there are many reasons to go portable.
With the warmer weather upon us, now is a good time to make your amateur
station ready for portable operation. An additional incentive is next month's
John Moyle Field Day Contest, an event which specifically encourages portable
operation (see this month's AR, for the rules).
This article deals with both VHF/UHF and HF
portable operation. It also gives a few tips on equipment, power supplies,
antennas and operating sites.
EQUIPMENT
With today's compact transistorised
transceivers, almost any hand-held, mobile or home station transceiver can be
used portable.
Many operate portable with hand-held VHF or
UHF transceivers. While these are fine for limited local operation, one soon
finds that short battery life and poor antenna efficiency place a limit on what
can be achieved. In addition, many hand-held two metre transceivers have poor
receive front-ends, which make them susceptible to overload from pager
transmitters in some areas. Fortunately, as you will read later, many of these
disadvantages can be overcome.
Mobile VHF or UHF transceivers are also
suitable for portable use, particularly where they have several output power
settings to conserve power. Another benefit is that their immunity to pager
interference is often superior to that of hand-held equipment. While many older
ex-commercial crystal-controlled mobile transceivers exhibit even better
immunity, having just a few frequencies available is a severe limitation for
hilltop operation. The author has found the 2.5 watt Yaesu FT290R a good
performer for portable operation. This particular transceiver also features
SSB/CW facilities, permitting extended-range terrestrial and satellite contacts
to be had as well as the possibility of later adding a transverter for the HF
bands.
There is not a lot of difference between
most modern HF rigs for portable operation. However, the standard 100 watt
power output is likely to limit transmitting to short periods with anything
less than a generator or substantial battery. An alternative is to modify these
rigs to low power (QRP)
operation. Such modifications are not hard - often you do not even need to
remove the lid (Reference One is an example).
Another possibility is to purchase the older
lower power solid state HF transceivers designed specifically for novice
licensees. Examples of such transceivers include the Yaesu FT7 or Kenwood
TS120V. An advertised price of $300 to $400 is typical for these sets.
If your scale of operation is such that a
full HF station is not warranted, operation on one or two bands can be contemplated.
Eighty and forty metres is a good combination for South-Eastern Australia,
allowing contacts at most times. Home construction of suitable equipment is
quite feasible for those with previous practical experience.
At this time of the year ten metres comes
into its own, with many quality interstate sporadic-E contacts able to be had.
A converted CB will yield good results during such times. However, at this
phase of the sunspot cycle, 28 MHz is unpredictable, and one can go for days or
weeks without hearing anything. For this reason, portable operators should
consider taking equipment for other bands until conditions improve.
POWER
The power source for a portable station can
range from a nicad battery pack on a handheld transceiver to a generator capable
of running a full-power HF station. Others use car batteries, sealed lead acid
batteries and/or solar panels to power their station.
The power source eventually adopted depends
on:-
* The period of portable operation
* Transceiver current drain
* Size/weight considerations
Portable operators find that there is a
trade off made between operating period/transmit power output on the one hand,
and battery capacity/size/weight on the other. Your need for transmit power
depends on the results desired. In a comparatively densely populated area, a
power output of five watts from a good site should yield results on VHF/UHF,
and allow several hours of operation from a relatively compact battery. On 80
metres, a power output of between 2 and 20 watts on SSB should suffice for most
short and medium distance work. Such powers are also practical on 28 MHz when
the band opens.
The author considers that the rechargeable
sealed lead acid battery (or 'gel cell') is the most reliable and
cost-effective power source for several hours of operation with low to medium
transmit powers. As remarked earlier, small nicad battery packs give only a
short 'talk time'. As well, they seem to run out at the most inopportune
moment. Much more use from a hand-held transceiver can be obtained by running
it from a 12 volt sealed
lead acid battery. However, caution should
be exercised here. Apart from checking the polarity of connections, one should
verify that the handheld is able to take the full voltage put out by a
well-charged battery. If not, a voltage regulator should be constructed to
bring the voltage down to that required. Alternatively, batteries complete with
in-built regulators are commercially available (Figure 1).
When using larger pieces of equipment, it
makes sense to draw up a power budget so that you know approximately how long
your battery will last. Transmitting demands more from the battery than does
receiving, with the difference increasing with transmit power. Different
activities have differing transmit:receive time ratios. For instance,
one-to-one operating is more power intensive than a large net with many on
frequency.
To draw up a power budget, you need to know
the following:-
* Transmit current consumption
* Receive current consumption
* Transmit:receive ratio
* Battery capacity (in amp hours)
The first two can be obtained from your
transceiver's instruction manual. The battery's capacity should be printed on
the battery itself, while you can take a guess for the transmit:receive ratio.
We will use an example here of a Kenwood
TS120V, a popular transceiver for HF portable operation. Its transmit power
consumption is 4 amps, while on receive, it draws 0.7A (at 13.8 volts). We
assume that we will be
listening twice as long as we're
transmitting (ie transmit:receive ratio of 1:2). Thus in an hour one would be
transmitting 20 minutes (0.33 hr) and receiving 40 minutes (0.67 hr). The
object of this exercise is to find the mean power consumption of the station in
a given hour of activity. Armed with this information, one could then determine
how long a particular battery is likely to last, or, conversely, how long one
can operate with a battery of a given capacity.
A calculation for the TS120V would be as
follows:-
Receive contribution:- 0.7A x 0.67hr = 0.47
A.H
Transmit contribution:- 4.0A x 0.33hr = 1.32
A.H
Total (for 1 hour):- = 1.79 A.H
Thus if we were to operate in this manner
for one hour, we would need a total battery capacity of around 2Ah. With a 6 Ah
battery, it should be possible to operate for at least three hours. Empirical
tests with a TS120V and a 6.5 Ah battery showed that ample capacity was
available for this period of operating.
The figures above are conservative. This is
because the transmit:receive ratio assumed is more typical of active contesting
than casual operating, which tends to involve more listening. As well, the SSB
and CW modes have lower duty cycles than FM or digital modes, where the
transmitter is radiating full power (and thus drawing maximum current) during
the entire time the operator is transmitting.
Performing a few calculations like this will
reveal why much portable operation takes place with lower transmit powers than
is usual from home. Doubling output power will not necessarily improve the
quality of contacts achieved, but will have a large effect on power
requirements and/or the operating time permitted per battery charge.
CHOOSING A SITE
On not every occasion can one choose the
operating site. However, when a choice is possible it should be made with the
following factors in mind:-
* General site amenity (access, shade,
freedom from high winds, etc)
* Distance from power lines (for both safety
and interference reasons)
* Distance from other man-made interference
sources
* Availability of trees (for supporting HF
wire antennas)
* Altitude (particularly for VHF/UHF
operating)
* Clear paths in desired directions
(particularly for VHF/UHF operating)
* Distance from pager transmitters
(especially for two metre activity)
The relative importance of each of these
factors will vary according to the bands used, and the type and distance of
contacts desired. Depending on equipment quality, some sites will be unusable
for two metre operation because of there being strong signals from pagers and
other transmitters. However, with better equipment, a cavity filter or even
horizontal antenna polarisation, the site could prove useable. As well, other
factors can come into play. For instance, if one were experimenting with
vertical antennas which rely on a good ground system, an area surrounded by
salt water or wetlands might be a good choice.
PORTABLE ANTENNAS
Like the operating site, the antenna chosen
depends on the contacts desired and the type of activity. Again the popular VHF
and/or UHF hand-held transceivers with their short helical antennas are not
ideal. Fortunately, better antennas can be made at home. Perhaps the next step
above the standard antenna is a quarter wave whip plugged directly into the
rig's antenna socket. Or, a 1/4 or 5/8 wave ground plane can be fabricated from
easily available materials - designs frequently appear in the popular amateur
handbooks. Still others use J-pole antennas, made either from metal tubing or a
length 300 ohm feedline small enough to be rolled up and carried in a pocket.
Transmit and receive coverage can be further
enhanced through the use of a small beam antenna. Gains of 8-10dB over a dipole
are possible with portable yagi or quad antennas. However, above this level, the
antenna's boom needs to be lengthened considerably to achieve even a small
amount of additional gain. Figure Two shows a two element yagi suitable for two
metre portable use. Built on a wooden boom, it makes use of elements from a
discarded television antenna. No balun or special matching section is required
if the antenna is fed directly with 50 ohm coaxial cable (RG58 suggested for
short runs). A wooden mast or tripod can be used to mount the beam. The gain of
this antenna would be approximately 4-5 dB above a dipole. Empirical tests have
confirmed the yagi's gain.
Amateurs transmitting from good sites in
densely populated areas may find that two or more repeaters are accessible on
the one frequency. By concentrating the transmitted signal in one direction, a
yagi or quad allows operation through one repeater without disturbing those
listening on other repeaters who would otherwise be hearing one side of the
conversation taking place. In such cases, a beam is desirable, not so much
because of its gain, but its directivity.
Note that on VHF/UHF, vertical antenna
polarisation is favoured for FM and packet radio operation, while horizontal is
used by SSB operators. Failure to observe this convention will result in many
lost contacts; cross-polarisation losses can be very high. On HF antenna
polarisation is much less significant.
On the lower HF bands, most portable
operators use simple dipole or inverted-vee antennas. The latter are
particularly convenient, as only one central high support is required; the ends
can be tied off at only a metre or two above ground. Those using several bands
have the choice of multiple dipoles, trapped dipoles, or tuned feeder antennas.
Generally the antenna should be mounted as
high as possible. In most areas, it should be possible to find a suitable tree
to support an antenna, though there are cases when it would be wise to bring a
collapsible mast.
Further details on antennas and their
construction can be found in the standard handbooks or back issues of AR.
ACCESSORIES
We have looked at equipment, power supplies
and antennas required for portable operation. However, there are a number of
other accessories that help make portable operating more enjoyable.
These include:-
* External speaker-microphone (if using a
hand-held transceiver)
* RF field strength meter/SWR bridge
* Multimeter
* Back up batteries
* Logbook/pens
* Clock/watch
* VK Callbook (particularly for lists of
repeaters, bandplans, addresses)
* Personal needs (food, water, insect
repellent, shelter, etc)
It may be a good idea to make a checklist of
the above items so that none are left behind, particularly if operating some
distance from home. Some go further and mount their equipment in cases large
enough to carry accessories as well as the transceiver itself. Such an
arrangement makes it easy to go portable at short notice.
CONCLUSION
This article has provided some pointers on
amateur portable operation on the HF/VHF/UHF bands. A good thing about it is
that you will probably have most of the equipment required. As well, a portable
station can start off very simple and be gradually added to as time and
inclination allows. Don't forget the John Moyle Field Day a little over a month
away on March 15/16; the rules are in this month's AR, page 30.
REFERENCE
1. Buxton, S Use of IC735 HF Transceiver for
QRP Operation
Amateur Radio, August 1996, page 24
ILLUSTRATIONS/DIAGRAMS
Figure 1: A commercially-available
rechargeable battery pack and regulator
Figure 2: A simple two element yagi for two
metre portable use
(not included on Novice Notes Online - please refer to Amateur Radio, February 1997)
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This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.