introduction to LF (2200m)

Many countries have or are in the process of allocating LF spectrum space mainly in two areas - 136Khz (2200m) and 160-190Khz (1750m).

In the Asia-Pacific region New Zealand Amateurs have been granted LF privileges for over 10 years in the 1750m band (and recently 2200m) and have a lot of collective experience on the LF bands.

The WIA (Australia's National Amateur representative body) had applied many years ago to the Australian Communications Authority for Amateur access to the LF bands with no success.   However - WARC-07 should clear the path for access privileges to the 2200m (135.7kHz - 137.8kHz) for VK amateurs sometime in 2009.

European countries have had access to the 2200m for a long time now and the US have had low-power privileges for 1750m for some years as well.

The focus here will be on the 2200m band in anticipation of VK amateurs being granted full amateur privileges (expected to be available to Advanced Licencees with up to 1W EIRP).

axso scientific licence (Historical)

In order to gain access to the LF bands an application was made in the second half of 1998 for a Scientific Assigned Licence.  It was necessary to submit a technical application (and then re-submit a more detailed application) outlining the reasons why a licence should be granted.   Some communications back and forth plus several hundred dollars resulted in granting of a Scientific Assigned Licence (callsign AXSO) for the spot frequencies of 177.5kHz and 145.5kHz.  The allowed power was 100W pZ (100W RF power into antenna feeder).

I held the AXSO Scientific Assigned for a number of years until the ACMA withdrew these privileges circa 2005.

LF propagation

The LF bands have some similarity with topband propagation, but with their own unique characteristics such that it has been said that the propagation characteristics of LF are as different from topband propagation as topband propagation is different from 20m band propagation characteristics. 

Courtesy of the author and Amateur Radio Magazine,  an excellent article on "Propagation of Long Radio Waves" by John Adcock, VK3ACA is now available on the Long Wave Club of America web page.

lf receivers

The first task to tackle is the LF receiver.  Many Amateur operators already have LF receive capability as most of the fairly modern HF transceivers will tune down to, usually, 100KHz on receive.  The problem you are going to find is that the sensitivity at those frequencies is likely to be poor.  You will have to build a preamplifier or LF/HF converter to remedy this.  If you have a general receiver (such as an FRG-100 receiver) you are more likely to find the barefoot sensitivity more suitable.

My FRG-100 is sensitive enough without a preamplifier converter, so I don't use any at the moment.  The local noise is high enough (S2-S3) that it easily overcomes the receiver noise.  However, if you live in a quiet location (in the sticks) you will find these useful.

lf antennas

Antennas for LF are not necessarily reciprocal.  That is, you may not want to use the same antenna for receive as you might for transmit. Any Amateur antenna is likely to be small compared to the typical LF wavelength (> 1.5kms) and so will be very inefficient compared to the usual Amateur installation.

receive antennas

Short Whips. The easiest LF antenna to get going on receive is a short whip with a suitable preamplifier.  The limitation is that the short whip is untuned and so is susceptible to overload from local BC or NDB stations. I haven't used short whips so I can't add any information here. However, many LF operators recommend installing a remote whip somewhere in the backyard as far as possible from any interference sources

Tuned Long Wire (Inverted L for example). Any length of wire substantially shorter than 1/4wave (approx 400m) can be tuned by inserting a series inductance to tune out the capacitance reactance of such antennas. 

Earth Antennas.  A somewhat unusual LF configuration is to simply bury two earth stakes into the ground separated as far as possible (10 - 100's of meters) and run a wire from each to a coupling transformer, the other side of which connects to the receiver as shown below.

Earth Probe - impedance between probes approx. 100ohms (varies with local geography and frequency)

Experience with these suggests that they are as good or better than long wires especially as regards to rejecting local electric field interference.    Directional characteristics are uncertain at the moment.

Loops.  Loops are magnetic antennas, as opposed to the first two (which are essentially E-probes), and have some interesting and useful characteristics.   Basically you can view loops as resonant circuits with the coil (the loop) tuned to resonance with a capacitance.  The difference with the magnetic loop antenna is that the coil is deliberately made large as possible so that it has a useable (but tiny) radiation resistance at LF to make it behave as an antenna.

Magnetic loop antennas are useful for receive as they only respond to magnetic fields and so can be useful for rejecting electrical field interference emanating from nearby sources (closer than about 200m).  Further than about 200-400m away, any electric field emanating from a source will start to be accompanied by the companion magnetic component.

However, additionally, the magnetic loop has a figure-8 antenna pattern as shown below.

Pattern looking down on a vertical mounted loop from above

That is, there is a null at right angles to the loop with maximums in the plane of the loop (off the ends).  The nulls can be used to advantage if the interference source is in a different direction (90 degrees is best!) from the desired signal.

Loops have the further useful characteristic of being larger unaffected by their environment (except of course magnetic materials, unintentional shorted conduction path closeby and sources of magnetic interference such as TV oscillator circuits). My first receive loop was an 8 turn loop 2m high by 4m long (the lower leg about 0.8m off the ground) run around the wooden frame of a screened room.   This worked but had the disadvantage of not being able to be rotated and unfortunately placing the nulls in the direction most needed :-(

Top-loaded Verticals. This configuration is probably popular as a receiving antenna as most operators use this configuration for transmitting and so it is convenient to use it for receive as well.  Not only are they likely to be the most efficient in the Amateur arena for transmitting they are also the most efficient for receiving.  Unfortunately, they are also very efficient in picking up local electric field noise as well.  As a result many operators use the top-loaded vertical for transmission, but use a magnetic loop for reception.

Interestingly, many operators have indicated that it is necessary to de-tune their top-loaded vertical when receiving on a separate magnetic loop as the vertical intercepts and then re-radiates local interference into the quieter magnetic loop quite nicely!!

transmit antennas

Although I realise that authorisation to transmit on LF hasn't been granted yet I will describe some of the problems and characteristics of LF transmitting antennas in anticipation of that event.

Radiation Efficiency. LF transmit antennas, like LF receive antennas are likely to be small in terms of wavelength for the average Amateur installation.  That means that the radiation resistance will be very low (in the order of 0.1 ohms or lower) resulting in a radiating efficiency in the order of less than 1% (more likely closer to 0.1% or lower!!!).   Most of the RF power going into the antenna will be dissipated in non-radiating losses (coils, trees, ground, etc) while a tiny fraction will be radiated.  Typical impedance of a LF vertical can range from over 200ohms to less than 10ohms. Remember this impedance is almost entirely non-radiating losses.  My own antenna has an estimated efficiency of about 0.015% or about 30dB down on a full-size antenna with impedance of about 80 ohms and an estimated radiation resistance of about 15milliohms!  Other operators (ZL Amateurs) have installations with efficiencies probably over 10dB better than this.  Most of the effort in developing efficient LF antennas revolves around designing low loss (high Q) tuning coils and keeping the antenna away from trees and buildings and other objects.  Additionally, provision of a good earthing system can result in substantial improvements in efficiency, although the effectiveness of more than a simple stake in the ground appears to depend on other factors such as local ground conductivity and height of the radiator itself.

Polarisation. Authoritative literature states that it is impossible to launch a horizontally polarised signal with an LF antenna on the ground because the electric field is parallel to the ground and so is shorted out by the conducting earth beneath.    Consequently almost all Amateur antennas (and commercial ones for that matter) are vertically polarised.  I remain unconvinced of that this is true for DX contacts and intend to experiment using horizontal polarisation in the future.

Resonating Short Vertical Antennas.  For most antennas except the magnetic loop, the task is to resonate the electrically short antenna which exhibits an apparent capacitance reactance with the same value of inductive reactance.  This involves placing a coil in series with the antenna and adjusting its value to achieve resonance.

A straight vertical antenna could therefore be constructed and an inductor inserted in series with the antenna at the base.  Depending on the height of the antenna the required inductance at around 136KHz would be about 5-10mH.

Improving Radiation Efficiency.  The two main factors in antenna efficiency (not only at LF) is to maximise the radiation resistance and maximise the antenna current.

Maximising the radiation resistance for the vertically polarised component of the radiation involves making the vertical portion of the antenna as long as possible.    Double the height and you quadruple the radiation resistance (Rr = height * height).  If all things remain equal this will, as a rough rule of thumb, double your range up to a couple of hundred kilometres (where diffraction and other losses start).

Maximising antenna current involves getting the losses as low as possible.  The losses come from the loading coil itself and losses associated with nearby objects and the ground itself.

Capacitive Top-loading.  To assist in reducing the coil loss (and obtain a better current distribution) we can increase the apparent capacitance of the antenna by utilising top-loading (as used in HF mobile antennas) by stringing horizontal wires connected to the top of the vertical portion of the antenna.  More capacitance means less series inductance required for resonance and so less wire in the coil and therefore less coil loss. It also means that the current in the vertical part of the antenna is closer to being constant instead of decreasing with height, resulting in a higher radiation resistance.

So for the best antenna we need as much capacitive loading as possible and as much height as possible.   This means an antenna with large amounts of wire at a great height.  Not an easy task.

However, it turns out in some practical cases that increasing the height of your antenna doesn't necessarily net you the squared increase in radiation resistance you expect.    For one thing, increased height decreases the capacitance of the top-loading wires for a given size horizontal top-loading section requiring more inductance to tune to resonance - hence increased losses.  Secondly,  if increasing the height brings the top-loading horizontal section closer to tree branches, there will be an increase in losses.

Size Does Matter.  Having said that, in the near ideal situation (open ground with no nearby objects, good ground conductivity, availability of high support structures) the situation to aim for is the make the vertical portion of the antenna as high as possible, top-load it with as much horizontal top-loading as possible to reduce the size of the loading coil and wind the coil with as high a Q as possible.

As far as earthing is concerned there is still a lot of uncertainty in some minds from the results of practical experiments, while in other minds there is none.  Some say standard long as possible radials (unlikely to reach 1/4 wave with most peoples resources) are the way to go, while others say that radials approximately the length of the height of the antenna earth-staked at their ends is the best compromise.

Inverted L. Basically we can view the operation of the inverted L antenna as utilising the apparent capacitance of the horizontal section of the L to assist in resonating the antenna with the aim of getting as much antenna current into the vertical section as possible.

Earth Antennas.  A somewhat unusual LF configuration is to simply bury two earth stakes into the ground separated as far as possible (10 - 100's of meters) and run a wire from each to a transmitter.   One UK Amateur has conducted extensive experiments and has been successful in making contacts over reasonable distances (> 100Km) with high powers.  This is a very old form of communication (was used in military applications in earlier times) which originally used voice frequency transmissions.  It certainly is a way of using a low-profile antenna!!

Loop Antennas. Loops have the advantage of not being affected by their environment and as long as the bottom horizontal leg is 1m to 2m above the ground the ground losses are likely to be less than 5 ohms.  Loop currents in the order of tens of amps can be achieved with the use of low-loss wire for the loop itself.  The problem with loops in that their radiation resistance is extremely low (usually in the order of less than a milliohm) so although the antenna current can be high, the radiation efficiency is not outstanding.

Loops of sufficient size to radiate with reasonable efficiency are not likely to be able to be rotated creating nulls in the transmitting coverage.  While several OS operators use loops they are not common for transmission.

Top-loaded Verticals. This configuration is the most popular at the moment as a transmitting antenna for several reasons.  Firstly, the radiation resistance is an order of magnitude higher than magnetic loops and secondly, many of the LF operators are also topband enthusiasts and have large antennas for that band and so simply connect the balanced vertical feeder wires together as the vertical radiator and the horizontal dipole section becomes the top-loading wires.

Ideally, the top-loading wires should be symmetrical to largely cancel out any horizontal radiation but this is not always possible.

band plans

These details are presented here as informational purposes only.    Even in Europe they are voluntary.     Their use here is restricted to educational purposes only.     Apart from the obvious advantage of usage by a large number of Amateur LF users, no comment is made whether a similar plan should be adopted for VK if, and when, access to the 2200m LF is granted.

Un-official band plan from DK8KW for the European LF Band (virtually identical to IARU and UK band plan following)

 

IARU Region 1 Conference (2005) and UK Band Plans

FREQUENCY IARU BAND PLAN	Max Bandwidth	UK NOTES
136kHz		There is no rigid band plan for this band, but amateurs are asked to work within the following guidelines, giving long distance communications and experiments priority.
135.7 - 136.0		Station tests & transatlantic reception window 135.900 - 135.980 preferred transatlantic window for Europe / North American transmissions of very slow CW (QRSS)
136.0 - 137.4		CW 135.980 - 136.050 preferred transatlantic window for Europe / North American contacts
137.4 - 137.6		Non CW digital modes (Hellschreiber, Wolf, PSK etc)
137.6 - 137.8		Very slow CW (QRSS centred on 137.7kHz) 137.700 - 137.800 preferred transatlantic window for North America / Europe transmissions