NVIS and the UK/RT. 321 – Does what it says on the tin – on the Battlefield or in the Backyard

While working from the relative comfort of my radio shack (Located in Gravesend), I often leave my UK/RT. 321 switched on tuned to one of the 5Mhz frequencies allocated to amateurs for experimenting in the 60 meter band.  It gives me comfort for some strange reason.

Stations Heard

This week at around 4:30pm on Thursday I heard quite clearly through the ambient noise provided by the set,  Robin, G0GNE in Elstead, Surrey talking to Ian M0MYK – nearby (approx 30km away) in Horsham. Both stations were between 75 and 80km away from me.  Signals from both of them were readability five and strength nine, and they were discussing and testing various configurations of loop antennas they were developing.  Both Robin and Ian were using the UK/RT. 321 and for some battered old military radio sets past their life expectancy, they weren’t doing too badly.  Neither was mine.

What do you use yours for?

Whenever I have aired my 321 it has been for short range work, “Inter G” (around UK or Europe). I regularly work between here, Yorkshire, the South West and Scotland as a radio ham, and Prior to buying into the surplus Clansman market,  I had used the HF sets professionally.  Mostly during the late seventies and eighties.  Working short(er) ranges with HF radio communication appealed to me as well as working DX.

Design Spec

In accordance with the manuals for the 320 and 321 and various signals training pamphlets, the set and their accessories were designed to provide HF Groundwave communications up to 50Km and Skywave communications. This fitted in with the radios war role – to back up VHF communications up to 50Km range on the battlefield, and also to provide long range communications.  During the cold war I and colleagues had many opportunities to test the set in both modes of communication and I had managed to work the Clansman HF sets between bases in UK, Germany and the Falkland Islands and various other locations, bare bones – we did this whenever a “DXpedition” came our way ;- (and even stretched to data communications – but that’s another story)..

Choosing the Mode

According to the military manuals two modes were common – Groundwave for short range – up to 50Km, and Skywave for long distance work – over 1000 km.  During my experience and through experimenting I had also learnt how to use a “third way” called near vertical incidence skywave, to achieve the middle distances.

Near Vertical Incidence Skywave (NVIS)

This involves Near Vertical Incidence Skywave (NVIS) – a common technique developed in the thirties and forties and still in use today.

HF Techniques

NVIS is a technique, comprising the transmission of a signal at the appropriate angle and frequency causing it to reflect back to earth, as close to the transmitter as possible – in order to achieve a pre determined short range beyond 50Km and less than 900Km.   This differs from the techniques used to; gain long distance (DX) working via Skywave, and that chosen to achieve very short ranges using Groundwave up to 50Km.

All HF operating techniques involve the selection of variables such as the frequency, reflecting layers in the ionosphere, timings, antenna, and the variation and choice of antenna support heights and angles etc – to achieve a pre-determined range. Such techniques are beyond the scope of this article – but a good starting point for research exists in Wikipedia and the appropriate pages have been book marked.

Planning an NVIS Link

After the 50km groundwave limit, communications tends to be a bit marginal and fading can become a problem along with noise and other common problems affecting weak signals.  Antenna and frequency selection become critical to planning.  The same techniques apply in the backyard of course.  The selection of an antenna may be a little less easy to achieve due to various factors including space limitations, budget and aesthetics.  But a range of common antenna are available for all types of propagation.  To specifically radiate NVIS a low dipole in the inverted vee mode (^) is ideal for example.

In service the sets are provided with elementary antenna kit for all three modes of propagation.  The handbooks describe a number of antenna configurations which are chosen to suit the terrain and local site.

NVIS occurs between 10 Mhz in the early part of the day and 2 Mhz at the end of the day and into the evening.  NVIS propagation is highly dependent on the state of the F2 layer in the ionosphere or radio conditions.  This makes 5 Mhz a very attractive place to practice NVIS during the day.  As an aide to propagation prediction, Ionosondes (or radio probes) are used to sound the F2 layer in real time. This provides a prediction of the likelihood of the layer being able to support the mode in a particular area and the best frequency, time of day and range available.  (In a modern system of course this presents the opportunity for an ionosonde to be operated on a network and for the data it generates to be used to automatically select the appropriate frequency or range of frequencies to operate from.)  Back to Clansman in the backyard – particularly useful are the prediction charts or Hourly LAMP Charts which are published by the Australian Government Bureau of Meteorology.

Select your Range, Frequency and Operating Time from the Table

Military Communications – Signals Exercises

NVIS, is a technique that improves the chance of communicating beyond groundwave range.  I recalled experimenting with this in mountainous terrain during a regimental exercise where I would have been forced to deploy numbers of VHF rebroadcast stations (manned repeaters) to communicate with sites within a 150Km radius in order to get over the terrain.  This was an unacceptable waste of manpower and Instead I chose to use HF for the job instead.  The exercise yard was located in the mountains of bonny Scotland.  To achieve the distance I consulted the appropriate service antenna handbook and found the ideal solution – the Shirley Antenna.  Calculating the antenna was simple from the table provided, and measuring it out in the yard of the TA Centre I soon realised the area needed to erect it was about the size of a football pitch.

In memory of Shirley…

Undeterred we pre-built Shirley, as she effectionately became known, and installed it when we arrived.  The only parts needed were a lot of D10 cable and spacers, which provided the antenna and the feeder.  See below.  Fortunately space is never an issue for the forces and from our HQ – Shirley gave us precisely what we wanted – Communications over the mountains and into the valleys between – at strength five in military terms or S9 in yours, – good signals all week – job done.

Locked on…

Of course what makes the Clansman sets so special is that they all lock on to precisely the same channel by virtue of a unique (for its time) design.  The set has a synthesised tuner which is super accurate and never has to be retuned.

Using this feature, I realise now in conjunction with the application of NVIS, Clansman HF Radios would have provided the UK with a highly efficient, nationwide, voice emergency network – had the baloon gone up at any time during the cold war for example.  And here it is now, performing the job it was ultimately designed for, out of our back yards 34 years after they first entered service.

73 de Stu – G4IYK