Robert Connolly has sleepless nights, compares two vintage radio direction finding receivers and supplies his list of NDB loggings for a somewhat quieter summer season.

 

 

Following the acquisition and restoration of the 1970’s Seafix handheld marine radio direction finder (RDF), I purchased some months ago, I have now acquired another marine handheld RDF receiver. This time it is a Lokata 7 handheld marine RDF, dating from around 1989/90.

Regular readers will be aware that I have owned one of these for a number of years. However, as I have previously indicated in this column, I had a problem with a small nylon cog in the tuning mechanism that prevented it from changing the frequency; of course, spares are not readily available.

When the opportunity arose to purchase a second-hand Lokata 7 at a sensible price, which came complete with its original headphones and user manual (Fig. 1), I could not resist, especially as the photographs of the item showed it on different frequencies, indicating that it was most probably working.

When it arrived, I found that it was in excellent condition and had not seen a lot of maritime use. At £25 plus postage, it was a little more expensive compared to the Seafix.

However, given its condition and the fact it was complete with working headphones and user manual, still made it a good bargain. I saw another Lokata 7 advertised recently for about £100, seriously overpriced!

The Seafix RDF uses an analogue tuning dial (Fig. 2) requiring some amount of guesswork when tuning; it has a frequency coverage from 200 to (approximately) 425kHz and is powered by 9V DC. It was originally equipped to contain six AA batteries but was later converted to use a 9V DC PP3 type battery.

The Lokata 7 has a digital frequency display (Fig. 3) with tuning in 100Hz steps. Tuning is done with your thumb, using the wheel below the display.

The Lokata 7 also features an internal timer facility, which would have been used in conjunction with the marine NDB beacon chains, operational prior to 1990. The timer is not needed when chasing today’s beacons.

The internal timer switches off the receiver after a certain period of time to conserve battery power. The Lokata user manual states that a set of batteries should last for a season.

Unlike the Seafix, the Lokata does not have a wideband broadcast station selection setting; it is not possible to use it for listening to the shipping forecast. However, the set does have a narrower band setting that may be used to obtain a precise bearing on the signal null.

The headphones are interesting, in that they are using a small speaker in the place where the separation for the earpieces is located. Audio continues from that speaker up through the two air spaced tubes to the earpieces; rather like a doctor’s stethoscope.

 

Comparing Seafix and Lokata

When we are used to high-performance receivers and antennas designed to pull in as much DX as possible, I find that it can always be interesting to back to more ‘basic’ equipment from time to time, as this throws up additional challenges and, sometimes, surprises.

With this in mind, I thought it would be a fascinating exercise to compare the performance of the Seafix and Lokata receivers to see if one or other was more sensitive.

To do this fairly, I decided that both receivers should be used during the same session to reduce any influences caused by propagation variations.

When using a receiver that has a built-in ferrite rod antenna for NDB searching – be it a dedicated RDF or a suitable portable general coverage radio – it is important to remember to ‘swing’ it slowly through 180 degrees on every frequency to eliminate the possibility of missing a beacon that is, possibly, in the receiver’s ‘null’ position. When using this type of equipment, it is always best to operate outdoors and away from as many potential local interference sources as possible.

The comparative listing in Table 1 shows the beacons received by each receiver within their tuning range. As can be seen from the table, the Lokata received several more beacons compared to the Seafix, both during daylight and in darkness. This indicated that it was the more sensitive receiver of the two.

Furthermore, it was easier to use because it has the more accurate digital frequency display. Daylight reception distance was, as expected, very similar on both receivers. Carrying out a similar session during darkness showed the Lokata receiving several additional UK NDBs, not heard on the Seafix unit.

Interestingly the DX signals from 351kHz OV Visby and 421kHz GE Madrid were of similar strength on both receivers. When the darker evenings return, I will wrap up warm and have another play with these receivers.

 

Seasonal Band Conditions

When I ran a daylight-check of the band around Midsummer’s Day, using my main equipment, I not only found the band quite noisy with static and the occasional closer thunder crash, but actual signal strength was also very variable, even for my ‘local’ beacons.

Received signals were quite weak, certainly well below average; however, quite suddenly, conditions picked up and signals became much stronger and clearer. Sadly, this occurred for a minute or so only, before signals were fading out again.

During the hours of darkness, this was no better, and my ears took a severe ‘bashing’ during my session. The handhelds were also affected by this but not to the extent of my main setup.

This left me thinking that I should, maybe, just use the handheld RDF receivers during the summer months.

Interestingly – not being able to sleep one night as I was preparing this column – I decided to have another listen to the band, to see if conditions were any better.

When I started my check around 02.45 UTC, the band was still quite noisy and fading in and out. However just after 03.15 UTC, about an hour before sunrise here, the static and fading suddenly died away, leaving a clear, workable, band. Static is, of course, caused mainly by lightning discharges in the atmosphere and is at a minimum around sunrise.

 

DGPS Correction Stations

Differential Global Positioning System (DGPS) correction stations are enhancements to the US Global Positioning System (GPS) and Russian GLONASS GPS network.

These provide improved location accuracy, from the 15m (nominal) GPS accuracy to about 10cm, in case of the best implementations. Accurate navigational position information is vital to shipping, especially in crowded sea areas close to ports in poor visibility.

The closure of the Decca navigation system in 2000 resulted in a move to GPS maritime navigation and entailed the requirement for greater position accuracy.

Therefore, it is no accident that many DGPS stations in Europe are located at lighthouses that were former marine radio NDBs, in the frequency band of 283 - 325kHz.

This is the main reason why I include these in this column from time to time. In the UK, the only exceptions to this are the stations located at Stirling and Wormleighton, both of which were former Decca navigation transmitter stations in a previous life.

Each DGPS signal employs a network of fixed, ground-based, reference stations to broadcast the difference between the positions indicated by the GPS satellite system and known fixed positions. Stations broadcast the difference between the measured satellite pseudo ranges and actual (internally-computed) pseudo-ranges. Receiving stations may correct their pseudo ranges by the same amount.

The digital correction signal is broadcast locally over ground-based transmitters, Fig. 2 shows a received example. To decode the signal, you need to have your receiver and PC sound card connected, preferably using the input socket of your sound card, although as some laptop sound cards only have the microphone input socket, this will suffice.

 

Decoding Software

You will require software to decode the audio signal. I use COAA’s DSC Decoder, downloadable from this website:

www.coaa.co.uk/software.htm

There is a 28-day free trial before this software needs to be registered or stops working. The other piece of software I use is MultiPSK, downloaded from this URL:

http://f6cte.free.fr

This excellent piece of software has its own DGPS decoder within the professional modes section. An unregistered version will run for about ten minutes of decoding DGPS signals. After this, you have to restart the programme if you wish to continue or register the software to achieve no such restriction.

DGPS transmissions use several different signal types, as follows:

Type 1 (or Russian Type 31) signals provide corrections for a full set of satellites.

Type 9 (or Russian Type 34) signals transmit sub-set differential corrections.

Type 7 (or Russian Type 35) messages contain Radio Beacon Almanacs.

Integrity monitoring within the header of Type 1 or Type 9 messages indicates the health status of the reference station; healthy, unhealthy or unmonitored. Reference stations and transmitters have assigned numbers for broadcast stations and reference stations.

Number blocks are allocated to all countries. In the case of the UK, station reference numbers (Station ID in the table) run from 440 to 449, and station transmitter numbers (BC in the table) from 680 to 699.

The transmitted signal always includes the reference station number.

The example in Fig. 3 is from Point Lynas, with a transmitter reference number of 682.

The purpose of the transmitter reference number is to indicate which transmitter the station is using, in those cases where stations have two transmitters available.

It is not known for certain whether Russian DGPS stations use the station ID or transmitter reference number or, perhaps, utilise the same number for both elements, as shown in the table.

There are two DGPS transmission speeds available. The most common one is 100bps; this is used by most European stations. A speed of 200bps is used in Norway, Portugal and the United States.

Software like MultiPSK will scan between the two speeds automatically if that option is selected, and manual selection is also available. You will often find a clue in the software waterfall display as the 200bps station transmission display looks wider compared to the 100bps.

DGPS signals are not confined to coastal areas. The system also covers some inland waterways such as the Danube. Table 2 has details of DGPS stations I received recently.

As in the case of NDB DXing, there are reception variations depending on propagation conditions, although many close European stations are received regularly during darkness, with some also present during daylight.

 

Summer Logging

While the summer months are not a great time for NDB chasing, they provide an opportunity to carry out routine maintenance on your antennas, feed lines, etc. This could save you potential damage caused by winter storms that may require you to get ladders out and climb up to roof level to prevent possible damage to your property. Trust me, when it is windy on the ground, the higher up you go, the windier it becomes, making the job dangerous.

Tony Stickells, who normally supplies an extensive log for this column, contacted me to advise that, since March, he has been plagued by local interference on a 24/7 basis that was totally wiping out the NDB band at his location.

When Andy Thomsett submitted his logs, he mentioned just how un-spectacular conditions were, concurring with my opinion regarding contemporary reception conditions.

However, the summer can throw up the occasional surprise, if conditions are suitable. For instance, I did find that some stations from Morocco made their existence known.

On a brighter note (as it were) by the time you read this the daylight hours will be shortening, and the winter season will be just around the corner. We can only hope that conditions will improve.

Table 3 details our loggings for the summer period. Until then, good hunting.

 

DXers for Table 3:

NDB reception this period during daylight (*).

A: Robert Connolly, Co. Down, Northern Ireland. NRD 525, Datong AD370 active antenna + Timewave DSP+9.

C: Andy Thomsett, Southwest England, ELAD FDM-DUOr and PA0RDT Mini-Whip.

 

Table 1 Seafix versus Lokata RDF receiver reception comparison