76 / 100
February 2016
76
A
re you one of those people
working the ‘same old, same
old’ stations on 144MHz,
especially in the UKAC contests?
Well how about taking the step up
to the lowest of the GHz bands,
1296MHz (23cm)?
Making the move from VHF
What’s involved? How will it differ from
144MHz? What’s it going to cost me? These
are all questions that you will be asking
yourself before taking the step. This short
feature tries to answer such questions and
encourage you to have a go.
The UK 23cm band covers 1240 to
1325MHz and we are there as guests
(technically, Secondary Users). We share the
band with a number of L band radars that
are close in frequency in the band 1215MHz
– 1400MHz including civil and air defence
en route primary surveillance radar (PSR). A
map of civil radar sites can be found at
[1]
.
The band is subject to major replanning
[2]
,
but this feature reflects the situation today.
The 23cm narrowband section covers
1296 to 1298MHz. This contains the bulk
of CW/SSB, digital modes and repeaters.
Satellite operation takes place between
1260 and 1270MHz, but in order to prevent
interference to Primary Users, caution must
be exercised prior to using 1250-1290MHz
in the UK. TV repeater outputs are between
1300 and 1325MHz, with inputs between
1242 and 1249MHz.
Equipment
A basic beginners’ setup for 23cm is
similar to 144MHz as far as equipment is
concerned. It consists of a transceiver with
a 1296MHz module, or a transceiver plus
transverter, some low loss coax, and a Yagi
antenna (
Figure 1
). Due to the high losses
in coax at 23cm (RG213 is typically 9dB
per 100m compared to 3dB at 144MHz),
it makes sense to have a remote receive
preamp at the masthead and switch it out
of circuit on transmit. I covered the effect of
masthead preamps on system performance
in the GHz column in August 2015.
Figure 2
shows two ways of implementing
a masthead preamp. The first uses a single
coax feed and two masthead relays (or a
transfer relay) and the second uses two
coaxes and a single masthead relay. If your
transverter or rig has separate Tx and Rx
inputs, this is my preferred way of doing
things, for two reasons. The first is that only
one relay is needed at the masthead, giving
more reliability and you can split the receive
feed in the shack to an SDR panadapter such
as an RTL or FUNcube dongle. The receive
coax can be relatively lossy, thinner cable
such as RG58 (60dB/100m at 23cm), as
the masthead preamp gain will compensate.
1296MHz equipment is not expensive
these days; a basic fully assembled
transverter such as those at
[3]
and
[4]
will
cost you well under £200; a Yagi is in the
order of £90. If you are comfortable with
surface mount construction, a 45W PA kit
can be had for under £100
[5]
and a preamp
for around £50
[6]
. Such a setup will be
very competitive indeed and will enable you
to make use of all the propagation modes
available for the band. You’ll even be able
to work some of the ‘mega
stations’ such as PI9CAM
and HB9Q on JT modes
EME (moonbounce).
Antennas
With the tripling in frequency
from 2m, antennas become
smaller by a factor of three,
so for a given boom length
you can get three times the
performance. The visual
impact of a high gain 1296MHz Yagi is
much less than one for 144MHz, being
smaller than a typical TV antenna.
Photo 1
shows a dual band array with 44 elements
on 23cm and 44 elements on 13cm, with a
domestic chimney for comparison.
Propagation
23cm really comes in to its own when
you consider propagation. It retains much
of the DX characteristics of 144MHz, with
line of sight, hilltop to hilltop performance
and
weather-dependent
tropospheric
enhancement. When this tropo becomes
extreme and ducting takes place, 1296 can
often be better than 144MHz and enhanced
ranges up to 2500km are possible. The UK
terrestrial record, set in August 2008, stands
at 2617km. Due to the high ERPs possible
with relatively small antennas, tropospheric
scatter becomes an important mode when
Step up
to
Technical
1296MHz!
Low-loss
coax
Transverter
Transceiver
Yagi
PHOTO 1:
23cm and 13cm antennas aren’t that big!
FIGURE 1:
Basic 1296MHz setup.