The 'tropical' bands

[Panrock]

It was always a rare treat to find the 60, 75, 90 and 120 metre bands on a vintage radio, at least in the UK.

In more tropical zones, where these bands were a mainstay, how reliable and free from fading were they? Did listening to them seem like a coming-and-going 'short wave' or a more steady 'medium wave' experience.

How did their range compare? Was their range intermediate with true short waves and therefore appropriate for serving large, sparsely populated territories with just one station?

Then there was also the 'Trawler Band', which - I think - was of longer wavelength (160 metres?) but still shorter than the medium waves.

Are these bands still in use today?

Steve

[Synchrodyne]

As far as I know, a common transmission mode in the tropical bands is near vertical incidence, or NVIS. Basically, the radiation is concentrated around the vertical axis, then reflected back to cover an area of maybe 1000 km across, give or take. Reception is skywave not groundwave, and I’d guess more like MF skywave than long-distance HF skywave. So, there would be some selective fading, but perhaps not as bad as with say a long transpolar HF path.

The actual tropical bands are 60, 90 and 120 metres. 75 metres is a regular HF broadcasting band, intended for the shorter hops.

I don’t think that the “trawler band” (sometimes referred to as the “marine band” and occasionally as “MSW”) ever was an official band as such, but that the name was used by receiver makers and others as an informal designation to cover some or all of the range between the top of the MF band and the 49 metre HF broadcast band. Thus, it covered the lower marine HF bands, used by coastal shipping and fishing vessels. Outside of the tropical zones, communications in these bands were probably the signals most likely to be heard by anyone tuning through the band.


Cheers,

[Synchrodyne]

Here is a tabulation of the MF and HF marine bands from Wireless World 1956 August:




So one might take the "trawler band" as running at least from from 1.6 to 3.8 MHz, and possibly extended upwards to encompass the 4.063 to 4.238 MHz ("4 MHz") marine band. With a 3:1 tuning range, spans such as 1.6 to 4.8 MHz would be reasonably possible.

Not all of the 1.6 to 3.8 MHz was allocated to marine, as there were other uses, such as the 120 metre broadcast band, in that range.


Cheers,

[turretslug]

Interpolating from the 75m band, which was a good place to find European stalwarts such as DW and SRI as well as the BBC with strong and steady reception, they would have been good regional reception bands but with longer range reception sketchy and noisy. Even the 49m band got used as a similar-task supplement to MW in Germany in particular, hence the presence of 49m on many Continental car radios and as a "privileged" bandspread on domestic sets. There is (or was) a niche DXing scene for the tropical bands, I seem to recall that one endeavour was to try and log the Falkland Islands 2470(?) kHz station with its (1970s) 500W in the UK!

I remember a few early transistor radios that had a 4-10 or 4-12ish MHz SW band, thus encompassing 75m and 60m- whether this reflected their Japanese manufacture for a wide international market or simply that early transistors might have struggled with the accustomed 18MHz typical reach I'm not sure.... My received wisdom was that LF/MF broadcasting was near-untenable in the tropics because of high lightning incidence, hence the adoption of the "tropical" bands in Africa and Latin America in particular.

[paulsherwin]

The 49M band bandspread is often marked '49M Euro Band' on 60s and 70s dials, presumably under German influence. I always found the 41M band more useful for European broadcast reception, but maybe it was different on the continent itself.

'Trawler Band' seems to have been British terminology, with 'Marine Band' used elsewhere, particularly the US.

[Station X]

Originally Posted by paulsherwin

'Trawler Band' seems to have been British terminology, with 'Marine Band' used elsewhere, particularly the US.

Known to UK amateurs as "Fish Fone"

[G6Tanuki]

Originally Posted by Station X

Known to UK amateurs as "Fish Fone"

Often to be found on certain 'spot' allocations within Top Band.

And renowned for the very fruity language used by many of the North Sea trawler-fleets!

[sometimes every second word was an expletive]

I had many happy hours listening to fish fone on a regen. radio I made in the early 70's an OC44/OC71 job.

[Panrock]

This waveband was also once used by analogue cordless phones. Sometimes you could hear one side of a telephone conversation. Not very private! I believe the other side used 49 MHz.

Steve

[AC/HL]

I've always associated them with broadcasting to the tropical regions, rather than within them. BBC World Service for instance (other broadcasters are/were available). All to do with skip distances and ionospheric layers, not that my understanding goes much deeper than that. If you can get hold of a copy of "London Calling" it lists frequencies versus time of day for target regions from memory.

['LIVEWIRE?']

Actually the tropical bands were, and maybe still are, used by stations in tropical countries, I still have QSL cards from YVMQ (Radio Barquismeo) Venezuela on 4990kHZ, Ecos del Torbes, San Cristobal, Venezuela, and a couple of other S. American Stations, plus one in Ghana, which I heard in the early hours of the morning, UK Time, on a Radio I built to a PW design, back in 1971/72. These were received in my ground floor flat, using only an indoor long wire aerial. I doubt whether such reception would be possible now, because of all the QRM (Man-made Noise), though I haven't tried to listen to the Tropical bands for a good many years. I still have the radio, though it would need an overhaul as it uses four OC170 (AF117) transistors in it's RF & IF stages, plus a faulty Denco 1.6Mhz IF transformer.

[Synchrodyne]

We may obtain a precise definition of the purposes for which the tropical bands were established from the Annex to the 1947 Atlantic City ITU meeting. Quoting therefrom:

“In these Regulations, the expression "broadcasting in the Tropical Zone" Indicates a type at broadcasting, for internal national use in countries in the zone defined in 252, where it may be shown that because of difficulty of high atmos¬pheric noise level and propagation it is not possible to furnish economically a more satisfactory service through the use of low, medium or very high frequencies.”

Copies of the pertinent pages from that ITU document are attached.



Essentially the tropical zone is that between the Tropics of Cancer and Capricorn, with some specific northward and southward extensions, notably to 35 degrees south in Regions 1 and 3. Here is the map from the ITU Annex.



As far as I know, atmospheric noise level would preclude the use of LF broadcasting in the tropical zone. The LF broadcasting band was authorized for use in Region 1 only. There was significant use in the European Area, a sub-region within Region 1 that did not extend further south than 30 degrees latitude. The eastward extension of Region 1 also included Russia and Outer Mongolia, both well above 30 degrees latitude, and LF was/is used in Russia. So, one might say that LF was suitable for use only above 30 degrees latitude, and perhaps an even higher number, 35 degrees of more, would be indicated for good reception. Applying the 35-degree number one may see that LF would not have been of major utility in the southern hemisphere, where there are not too many countries entirely or at least majorly below this parallel and with dimensions that might benefit from typical LF transmitter coverage.

MF is certainly used, and by inference usable in the tropical zone, but I suspect mostly for relatively local coverage where highish signal strengths offset the atmospheric noise. For example, there are MF transmitters in the cities and major towns in Australia that lie above the Tropic of Capricorn, examples being Cairns, Darwin and Broome. But HF in the tropical bands had the advantage that much larger areas could be covered. Until recently, the ABC in Australia operated I think it was three HF transmitting sites in the Northern Territory to provide wide-ranging coverage. These all used frequencies in the 120-, 90- and 60-metre bands, changing seasonally and diurnally as dictated by reception conditions. As far as I know all were more-or-less centred in their respective service areas and used NVIS propagation. Going back further, the ABC also operated some coastal HF transmitters to provide service to inland areas. I’d guess that these were beamed, more like the case for international HF broadcasting. These used various frequencies in the regular HF bands, and I think some in the tropical bands. The transmitters were located in Brisbane and Perth, both below 35 degrees latitude and so in the Region 3 tropical zone.

I have seen it said that in jungle areas – usually in the tropics - the dense foliage causes rapid attenuation of MF ground waves, so in such situations HF could be preferable. Thus, one imagines for example that any MF transmitters in Manaus, Brazil would have service areas that did not reach very far into the surrounding jungle. (The anacondas would have to slither into the city to hear the annual opera festival in reasonable fidelity.)


Cheers,

[Synchrodyne]

Originally Posted by paulsherwin

'Trawler Band' seems to have been British terminology, with 'Marine Band' used elsewhere, particularly the US.

Both designations were used in this part of the world. Thus, one found some receivers marked "TB" and others marked "MB".


Cheers,

[Valvepower]

Hi,

Ah, Trawler Band….

I remember excitedly getting a PYE P76 radio with a Trawler Band from the local Council tip in the early 70’s, which wasn’t working - one EBC41 later - bought from Bentley Acoustic, it was working. I remember this being a rather nice radio picking up some interesting broadcasts. The nearest I’d got to Trawler band at the point was a two-transistor radio built using a Denco Blue range 3 Transistor coil.

About the same time I was given a Perdio Town a Country radio also with a Trawler Band, but sadly I couldn’t get it going – maybe it was a whisker in one of the AF117!, but whatever I couldn’t get it going. This model was popular especially in coastal areas.

Terry