I wrote up last years' Puckeridge tests for Radcom thinking it would be
published shortly afterwards but for some reason it never was. It has now
re-surfaced and is proposed for publication in the July issue. Obviously
much of the reportage on DX records is out-of-date but apart from that I
thought those who took part would like to have a look and let me know if
anything should be changed, since I quote several people
verbatim. Text-only file is attached - please let me have any comments ASAP.
Walter G3JKV.
THIS IS THE STORY of how one particular mast in the 21-station UK Decca
Navigator
chain was used to test some theories relating to LF propagation in the 73 and
136kHz amateur
bands. When the mast at Puckeridge, Hertfordshire, had ceased its primary
function in March
2000 its owners, Racal, allowed a team of radio amateurs access to the site and
mast for three
weeks. The article was written shortly after the event and, although the LF
technology and
expertise have advanced somewhat since then, the story is no less intriguing.
The Puckeridge Experiment
By Walter Blanchard, FRIN, G3JKV *
THE HISTORY
THE DECCA NAVIGATOR was a system for providing a ship or aircraft with its
position by
measuring the difference in time of arrival of radio transmissions from several
transmitters
simultaneously, using their phases. It took its name from the Decca Record
Company, famous
for its "ffrr" (Full Frequency Range Recording) LP records of the 40s and 50s,
and still
perpetuated in re-releases of its landmark recordings.
Transmissions were in the low-frequency band between 70 and 127kHz and could be
heard on
AM as a series of periodically interrupted melodic tones centred around 71, 85,
112 and
127kHz - no doubt many amateurs heard it while they were tuning around. At its
peak in the
60s and 70s it was used by more than 35,000 ships and 10,000 aircraft but, like
many other
radio-navigational systems, it was overtaken by satellite technology in the
form ofthe
American Global Positioning System (GPS).
Latterly, it was operated by the Racal company (who bought Decca in 1981) on
behalf of the
Department of Transport and, in the face of an almost complete turnover by
navigators to
GPS, it was decided to close it down. The closure of the British transmitters
occurred at
midnight on 31 March 2000, followed shortly by the shutdown of the Irish
transmitters. Since
all other European transmitters had already closed, that left only the Japanese
still running
Decca chains, but even these were due to close at the end of 2000.
THE CHAIN
DECCA HAD 21 transmitting sites in the UK using an assortment of antennas. The
original
Decca chain covered the English east and south coasts. The 'English' Chain was
built in 1946
and, as often happens with the first of anything, no expense was spared to
ensure it worked
properly. It used self-supporting vertical masts 325ft (lOOm) high with an
extensive ground
plane of copper radials also lOOm long fanning out every 10 degs around it. It
had an
efficiency of around 10% atthe lowest frequency (71kHz) and a transmitter
output of 1.2kW
radiated 120W, easily enough,it was thought, to cover the whole of the UK with
just one
chain. Unfortunately, it was found that the skywave destroyed accurate phasing
at much
shorter ranges than had been anticipated, and the only answer was to use more
chains with
restricted range. So, from 1 April 2000, there were 21 excellent LF masts and
sites
throughoutthe UK doing nothing for a short period until they were either
demolished or
turned to other uses.
ENTER THE ENTHUSIASTS
AMATEURS ON THE 73kHz and 136kHz bands have always been handicapped by their
inability to erect aerials of a decent size - at 73kHz a half-wave is over 2km
long - so aerial
efficiencies were very low and, even for the permitted 1W ERP, needed kilowatt
amplifiers. In
spite of this, considerable distances were worked on both bands; at the time of
writing, the
record stood at 2200km, OH1TN to IK10DO. But the matter of big versus small
aerials
became a subject of hot debate within the LF Group and many opinions were aired
on whether
they were worthwhile. Did a big antenna have a different radiation pattern from
a small one?
Should it be very high vertically, or would itbe much the same if it were very
long
horizontally? It seemed to be rather a sterile debate until it became known
that the Decca
antennas might be available for a few weeks and thoughts turned to seeing
whether one could
be used for a comparison test. At one time it seemed a forlorn hope because of
legal and
insurance problems but, eventually, these were overcome with the result that
Racal granted
temporary permission to use the 325ft antenna at Puckeridge, Hertfordshire, for
a time slot of
only three weeks! This slot being only two weeks away, the next hurdle was to
find out
whether authorisation could be obtained to radiate a power higher than just 1W.
Thanks to
considerable assistance from the RSGB, this was cleared in the record time of
only one week,
permission being obtained for the CrawleyARC station, G3WSC, operating at
Puckeridge, to
radiate up to 100W on both 73 and 136kHz.
RESTRICTIONS
RACAL required that use of its site was to be handled through the author only
and, to help
with this, I had the invaluable assistance of Derek, G3GRO, Peter, G3LDO, and
Lech,
G3KAU, all well-known on the LF bands. A request on the LF Reflector brought in
a number
of other amateurs who were interested in transmitting from this mast and a
suitable roster was
devised. G3GRO and G3KAU wanted to get going as soon as possible and so made a
reconnoitring trip up to Puckeridge. No Racal/Decca equipment could be used and
everything
needed had to be brought on site. Puckeridge was a manned station (the others
were
remotely-controlled) and the team had considerable assistance from the resident
engineer,
Dick Caddy. Directly under the mast there was a coil-house that had once housed
the Decca
antenna tuning unit coils (see the photo). This was big enough to hold the
amateur
transmitters, receivers and other gear. Fortunately, it had mains power laid on
and even had
heaters, very welcome in the cold evenings of early April. The electrical
characteristics ofthe
antenna wereobtained from Racal (375OpF in series with 5Q and 12uH) enabling
the re-design
of Derek's ATU once he was back home so that, when he returned, he could just
plug it in and
go.
COMMENTS
Derek, G3GRO, reported on the first weekend (14-16 April2000): "Apart from the
Decca
mast and an RF thermocouple ammeter, none of the original equipment, such as
loading coils
etc, was used. For 136kHz operation, a relatively small variometer (about 500uH
maximum)
and a tapped toroidal autotransformer were connected to ground from the copper
pipe lead-in
to the base of the mast, which itself is supported on four massive ceramic
insulators. The
exploratory visit earlier in the week(11 April) allowed a purpose-built
additional loading coil
for 73kHz to be built by Lech, G3KAU, back at base, in time for the main
exercise at the
weekend. On 136kHz we did not need any additional loading coil: in fact to
start with, we had
to insert in series with the antenna one of the very large 500pF 25kV
capacitors we found
lying outside the hut. Later on during the weekend we discovered that, by
moving the
input/output taps down the auto transformer to reduce the base inductance but
keeping the
same ratio, we avoided the need forthe series capacitor in the antenna lead.
Three transmitters were employed at various times; the G3GRO 300W muchmodified
BKE
linear amplifier was used for the 1W ERP tests on 73/136kHz earlier in the
week, and then
again on Friday and Saturday, for running between 1 and 5W ERP. Additionally,
we used the
well-tried GOMRF 500W set-up and the G3YXM 1kW class-D rig used previously on
many
/P expeditions. The BKE linear was driven from an FT-990 transceiver via a
100:1 digital
divider and BPF from either 136kHz or 7.3MHz. During the overnight sessions on
Friday in
beacon mode, the ERP was 50W and 100W ERP overnight on Saturday. We had a few
problems to start with in getting the variometerto handle the 500W RF, and we
had a few
cracks and sparks resulting in VSWR trips, before we realised that the
capacitive voltage
divider in the forward/reflected power meter in the variometerwas arcing over.
Thiswasthen
by-passed. Fortunately,we had another SWR meter in line. We also found that an
RF sampler
unit brought along by Jim, MOBMU, was very useful in setting up the matching
and tuning in
conjunction with an oscilloscope. We realised on Saturday that the range of our
RF ammeter
was too small for high-power operation and rescued the original very large 30A
RF ammeter
from the pile of redundant scrap units outside in the rain and pressed it into
service. During the
beacon sessions it was reading 14A into the base of the mastwhich looks like 5Q
resistive
(mainly duetothe earth) in series with 375OpF. That represented an RF power of
1 kW into
the antenna - about 100W ERP, allowing for an overall antenna efficiency of 10%
on 136kHz.
One modification we made between the early session and later was to change to
keying the
emitter of the buffer amplifier following the divide-by-100 stage with added
keying shaping to
minimise key clicks which had been reported. That seemed to clear the problem
completely,
although we found out later it was also present on at least one of the class-D
transmitters used
for the higher-power and night-time beacon sessions.
The receive system was a home-brew up-converter with an input bandpass filter
about 3kHz
wide on both 73 and 136kHz, followed by a Mini-Circuits MAR6 preamp and MC 1496
mixer
IC to a 10MHz IF feeding FT-990 and IC-756 transceivers for most of the time.
The FT990
and converter stood up remarkably well to the very large antenna input with no
sign of
cross-modulation.
We had a switched attenuator at the input to the converter but, for most of the
time, it was
switched out. We had two operator positions side by side, one handling the 136
and 73kHz
traffic with the second op also monitoring 136/73kHz in parallel, but also
handling the HF
crossband input mainly from 7MHz.
I think one of the nice things about the operation was that, with such a big
signal, we could
easily be heard by stations with a very simple antenna not specifically tuned
to LF and give
quite a few crossband stations their first QSO with an LF station. I think the
other memory
was of having to make several journeys humping all the gear a couple of hundred
metres
through the pouring rain and climb over a low fence with the gear and through
all the grazing
sheep in the compound."
G3XDV had a few memories too: "The continuous rain that soaked through my coat
and
made it weigh a ton, then went on to soak the rest of my clothes. I remember
thinking that I
had spent my school days avoiding sporting activities involving getting cold
and wet, but this
was by no means the first radio expedition that had resulted in just that.
I also had an agonising hip problem that started to get better from that day on
- must be the
healing powers of low-frequency RF (RF gets a bad press these days!). On the
arrival of
reinforcements, YXM, XTZ, MRF, BMU and myself wentto the local pub for a
warm-up and
refreshments only to be told that there was a wedding reception on and there
was no food.
They eventually took pity on us dripping into our crisps and offered 'something
and chips',
which went down very well. Also, at one point, it occurred to us that there
wasn't much point
in going on the 73kHz band, because 90% of the active licensees were in the
same room at
Puckeridge...."
G3GRO summarised the results of the first weekend: "There were about 65 QSOs in
total,
including those during the initial setting up period on Tuesday 11 April.
Two-way contacts
were made on both bands and crossband contacts froml36kHz to 73kHz and to 7MHz.
There
has not been a reception report from across the pond from VE or W (which was
always going
to be a long shot), but the longest contact was crossband to 7MHz with Alex,
UB5WF, in
KN58JQ - about 200km north of Odessa on the Black Sea - who gave us RST429 with
normal CW at a distance of 2225km. This was over a daylight path at 1232 UTC on
Sunday
16th. It is not known yet what receiving antenna Alex had for 136kHz, but it is
highly unlikely
that it was a dedicated LF antenna since there is as yet no LF activity in
Russia as far as is
known. Other long distance QSOs were to Valerio, IK5ZPV, 2-way on 136kHz, who
gave us
RST589; IK7HSS, crossband to 7MHz, and Neils, OZ8NJ, (2-way on 136kHz) who
relayed
to us that IK5ZPV was hearing us and would call us shortly. We also got an RST
599 on
136kHz from SM6PXJ, OZ5N and HB2ASB. Two QSOs were also made on both 136kHz
and 73kHz to Finbar, EIOCF, and Ray, GI3PDN. Reports on 73kHz were abouttwo
S-points
down with Ray and Finbar by comparison with 136kHz. We also worked GJ4CBQ and
GU3SQX crossband 136kHz/7MHz, which was pleasing since, due to Loran QRM from
Lessay, they normally have difficulty in hearing stations on 136kHz.
Perhaps one of the most unusual QSOs was with Graham, G3XTZ/M, operating mobile
on
136kHz CW whilst driving to the site to have a spell of operating! We also had
a report via
e-mail from Marc, F5MAF, in Toulouse, JNO3PP, who was hearing us at 599+ on a 2m
diameter loop at a distance of 900km, and bemoaning the fact that there is no
LF activity in his
neck of the woods. Later, e-mail reports on the 71.8kHz signals were received
from Walter,
DJ2LF (569 in JN59NO),and Geri, DK8KW (579 in JO52BH - 697km).
Thanks to all those who took part despite the very wet and freezing cold
weather and also to
those who took the trouble to give us reports which have yet to be analysed. At
one point on
Saturday, as the shifts changed over, there were 12 people in the ATU shack at
the base of the
mast which represented a large slice of the active UK LF operators! They were:
G3KAU,
G3XDV, MOBMU, GOMRF, G3XYM, G3XTZ, G3YSX, G4GVC, Kate, G4JAI (G4GVC's
XYL), G4TSH, G3LHZ, G3GRO, and not forgetting Peter, G3LDO, holding the fort
back
home, and Walter, G3JKV. whose efforts made it all possible."
IN RETROSPECT
THE REPORT of reception by UB5WF at 2225km, in daylight and over a
predominantly land
path, was especially interesting. This is about two-thirds of the way to
Canada; Newfoundland
is 3520km from Puckeridge and has a predominantly sea path, so the signal would
only have
had to travel another 1300km to make it all the way to Canada. Unfortunately,
at these ranges
the 136kHz ground-wave signal is falling off very rapidly and, according to the
CCIR
propagation curves, another 35dB would be needed, even over sea water. Just
possibly, it
could be done at a very low-noise site using one of the FFT programs for
reception. Larry,
VA3LK, is already taking steps to set up an LF station for this purpose,
although it is not
known whether Puckeridge will be available in winter, when skywave might help.
Overnight on 15 April 2000, G3WSC was left on-air running continuously as a
beacon with an
estimated radiated power of 100W. This generated a number of listener reports
and, according
to Vaiski, OH2LX, who has some very accurate measuring receivers, the signal
strength was
not far below those of some ofthe commercial stations near the band.
SMALL(ER) ANTENNAS
APART FROM this DX work, as already mentioned, a long-standing discussion
within the LF
group has been about the differences between large and small antennas. Given
the same
radiated power, is there any difference in signal strength between them? After
all, both are
very small in terms of one wavelength. To try to resolve this Jim, MOMBU,
decided to setup
a 'small' antenna at Puckeridge, fairly near the 'big stick'; he aimed to
radiate the same power
from both alternately, and see what reports he got. Fig 1 shows his layout.
Surprisingly, the
small one got slightly better reports, by about 4dB, and Jim's remarks on this
were as follows.
"COUPLING BETWEEN ANTENNAS
Several people raised concerns about the coupling effects that might exist
between the two
antennas. If sufficient power was being coupled into the big antenna materially
to affect the
overall radiated signal, one would expect significant current to be flowing in
the big antenna
while transmitting from the small antenna - but there was none. Even with the
big antenna
resonant, the current induced in it by the small antenna was too small to
contribute a
significant fraction of the radiated power.
RADIATION RESISTANCE
The radiation resistance (2OmQ) of the small antenna was easy to calculate due
to its simple
geometry. The Decca mast was much more complicated - the exact radiation
resistance
depends on how thick you make the various conductors - however, the variation
is not that
great. I settled on a value of 0.65Q. If we assume that the 4dB difference in
signal strengths
was due entirely to the 0.65Q value being in error, the correct R would have to
be about
O.26Q to make the figures work out. I can't see how it would be as low as this,
however.In
summary, I chose the values of 0.02Q and O.65Q for small and big antennas as
the consensus
of a number of estimates, and would be surprised had the errors in the values
been big enough
to explain the differences observed in field strengths.
POSSIBLE CAUSES
My money is on the increased field strengths being due to fields and currents
under the ground
surface, resulting in greater effective height ofthe small antenna. Obviously,
this needs more
investigation, which I hope to try later.
CONCLUSIONS
The main point about this experiment was that, provided sufficient transmitter
power is
available, a small antenna can be made to radiate a given value of ERP just as
well as a big
one. This was borne out by the comparative signal reports. Of course, the big
antenna will
require much lower transmitter power, and will be much more stable electrically
- a Decca
requirement for complete phase stability. This is not a big surprise, and
confirms the result that
we got at the previous Puckeridge expedition. What is more surprising is that
the small
antenna seemed to be more efficient (less inefficient!) than would be expected
from theory.
What is more directly important is to discover the different factors affecting
the power
radiated by small antennas, so that this information can be used by LF amateurs
to generate
better signals. It is already clear that field strengths (and therefore ERP)
depend on many more
things than are taken into account in the simple antenna theory we use, and so
the ERP
figuresthatwe give out have little relation to reality. I hope soon to do some
more experiments
with different antenna environments in the hope of throwing some light on this."
FINALLY
THIS WAS A MOST interesting experiment and should give encouragement to those
who can
only erect small antennas for the LF bands (most of us). The powerful LF signal
radiated by
G3WSC enabled many who had never heard anything on the LF bands to do so; this,
together
with the straightforward DX aspects, must have been one of the most successful
amateur radio
experiments of recent times.
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