Loading Coils for LF

This is a discussion on where to put the loading coil and does it contribute to radiation; and in what circumstances should a loading coil be screened - and how.

The radiation resistance of a multiturn loop is :

Rs = 31200 x (n x F / L^2)^2 where Rs = radiation resistance, n = number of turns, F = loop area, L = wavelength.

For my loadingcoil n = 120 and F = 0.04m^2, so at a wavelength of 2200m the radiation resistance will be about 30 nano-Ohm. Estimated radiation resistance of my inverted-L antenna is about 40 milli-Ohm.

Shielding the coil (done properly - avoiding short circuit turns) would make my signal 0.000003dB weaker.

One shouldn't expect too much from the radiation qualities of the loading coil. It makes sense to me that you keep the loading coil away from any objects that might absorb the magnetic field and - if that is not possible - shield the coil from these objects.

Rik ON7YD

After my ATU 'hut' burned down (presumably due to an arc setting fire its plastic structure) I am back on the air now. I have lined the ATU hut with metal, both aluminium sheet and aluminium cooking foil! All this is bonded to earth. Everything is much more docile with less corona from the top of the coil so I thought maybe current was down. I measured it and current (measured on the antenna feeder outside the box) is 10 per-cent better for the same input power. I therefore recommend putting your loading coils in large earthed metal boxes! The reason it's better and not worse (by 0.000003dB) is, I surmise, that the coil is no longer dissipating power into nearby lossy items. Hedge, fence, grass etc. Seems blindingly obvious to me really. I can't put the coil in the middle of the lawn because my wife would get fried when she hung out the washing (sorry about the gender-stereotyping there.) I reckon one could usefully cage the bottom few feet of one's vertical feed wire if it was surrounded by bushes.

Additional it may contain a melt-down better!

Dave G3YXM

Lining the antenna tuning house with screening is about standard in LF/MF transmitter installations. Screening of all six sides serves several tasks:

* to provide short and low impedance return current paths for the matching circuit.

* prevent EM fields couple into the coils and interconnecting wiring (inch class copper tubing).

* to prevent losses from humid building material when exposed to high EM fields associated with the matching circuits

*to provide protection for the operator from effects in high EM fields associated with the matching circuits Some antenna tuning houses are lined with aluminium sheets (DHO38 installation in Ramsloh, 23k4), most others are lined with cooper sheet material. Copper sheets have the benefit that the material is electrical compatible with copper conductors and can be soldered for good electrical continuity. Aluminium sheets are cheaper by themselves but need surface treatment (Alodine treatment) for making good electrical contacts. If it where just the inductors, which need screening, then a minimum spacing from coils to screen equivalent to coil diameter should be provided for both, to keep eddy-currents in the screening under control, and to keep capacity to ground under control. Lining the whole building serves all tasks outlined above at moderate cost and optimum performance.

Soegiono, Gamal

Gamal says it all. It was impossible to keep the Decca coils tuned accurately and maintain phase constancy without the copper screening. Even then there had to be an automatic re-tuning device for when nearby lightning strikes made the coils jump. It was driven by a small electric motor and a belt/pulley arrangement that moved a little subsidiary coil back and forth a few inches. Occasionally there was a direct lightning strike on the antenna usually resulting in the belt jumping off the pulleys and a panic callout for the duty engineer. And for Bob, VE7BS - no, there was nothing in the hut except the loading coils - it was only about 5 x 5 metres.

My now-retired Decca engineer pal who had a lot to do with setting up coil huts and tuning coils, Nigel Fenner, tells me that he occasionally ran Decca transmitters without the copper lining in wooden coil huts but only in very dry climates. In the UK the prevailing dampness made it impossible to maintain tuning or coil "Q" and you would get corona all over the place. He warns against using ordinary soda-lime window glass for insulation - it is very lossy and will shatter, the only glass that will stand up is Pyrex-type. He says an excellent insulator he used many times was the largest Pyrex baking dish he could find with a hole bored in the bottom for the feeder. Also, ordinary plastic ropes (terylene, nylon, polypropylene etc) will melt. Either use steel wires broken up by many insulators or (expensively) sleeved Kevlar rope. And if you're going to run real power you MUST put in voltage limiters! He thinks the fire in Dave's hut was probably caused by the plastic walls breaking down, getting hot and catching fire.

Walter G3JKV.

Last night while transmitting BPSK for 15-minute intervals every half-hour well into the night, I was making use of the opportunity to get on with the shack rebuilding. My radio room is an upstairs bedroom and the LF antenna top loading wires pass directly above, attached to the apex of the roof probably 3-4m above the shack ceiling. While transmitting, every time I brushed against a piece of earthed metal I could feel a slight RF burn or shock, not detectable if the metal was grabbed but unpleasant if touched gently with an arm or similar. The weather was extremely dry and the antenna load resistance the lowest it has been for some time, which may be why I have not seen this before with the new 600 W amplifier. The field is probably in the region of 2 - 3 kV/m here. Occasionally the fluorescent tube on the ceiling will light faintly with the RF - this was much more noticeable and spectacular on an older fitting that was not on an earthed metal box but left the tube in free space connected to its starter remotely - on 73kHz that would flash brightly with CW keying to the extent that I had to leave the light on when beaconing at night! So, if you are sitting in the fields generated by 1kW transmitters be prepared for a few minor burns / shocks. And don't ever touch the hot antenna feed, I guess that would give a third degree burn if the arc is maintained for more than a few seconds To once demonstrate the plasma arc, I struck one from a (well insulated) screwdriver and managed to draw it out to 50mm, burning the PTFE wire insulation and marking the screwdriver tip - and that was only on 150W of 73kHz ! Plasm arc welders work like this.

Andy G4JNT

There is a photo in the Low Frequency Handbook, page 13, that shows the Decca loading coils and the walls of the ATU building, with Lech, G3KAU; this gives some idea of the coil and building dimensions. There were some aluminium boxes around the Decca site containing coils. I always felt that the boxes were rather large for the size of the coils inside them - now I know why

Peter, G3LDO

> The radiation resistance of a multiturn loop is : > > Rs = 31200 x (n x F / L^2)^2 > where Rs = radiation resistance, n = number of turns, F = loop area, L = > wavelength. > > For my loadingcoil n = 120 and F = 0.04m^2, so at a wavelength of 2200m the > radiation resistance will be about 30 nano-Ohm. Estimated radiation > resistance of my inverted-L antenna is about 40 milli-Ohm. > > Shielding the coil (done properly - avoiding short circuit turns) would > make my signal 0.000003dB weaker.

I am not sure whether this calculation is applicable to a loading coil. The coil in a mobile antenna for the HF bands is considered to radiate as a linear conductor with a length equal to the length of the coil. I think this could also be the case for a loading coil at LF. So if the coil is 1 metre long (high) with its axis vertical it acts as a 1m long vertical antenna and when at the bottom of a vertical antenna it makes the radiator 1m longer.

Dick, PA0SE

John, G4CNN wrote

"The loading coil was double wrapped in large plastic bags and the matching transformer inside yet another internal one and all cables and wires thoroughly taped at point of entry with duck tape but all in vain. Repairs are underway and should be operational again soon, but I would appreciate advice on better waterproofing. Anyone got any good ideas?"

... I don't really understand why and how my own setup works ... a large loading coil (3.8 mho) about 1/3 height up the mast in wind and rain and snow, without any protection! I have to re-adjust my small variometer in the shack (about 0.9 mH) if it is wet outside, but no problems with flashovers or any other problems ... maybe it is worth a try ...

Geri, DK8KW

Same here, a 2.2mH coil hanging at 14m height and a 0.8mH coil on a plastic box about 0.5m above ground. Both have no protection against wind, rain, hail, snow etc ... and survive for about 3 years now (keep fingers crossed). No problems with flashovers or tuning, thanks to the large loss resistance (about. 120 Ohm in winter, about 140 Ohm in summer). One of the few advantages of all the lossy green capacitors (brand : birch and oak) surrounding my antenna.

Rik ON7YD

I have just read some recent correspondence on this net concerning the pros and cons of placing the antenna-loading coil in the shack or at the base of the vertical aerial wire. The loading coil is not just a matching device, it is also a primary source of vector potential and it is the vector potential, rather than its offspring, the magnetic and electric fields, which is the fundamental source of electromagnetic radiation. It is therefore advantageous to place the coil upright at the base of the antenna. Furthermore, as a large component of the vector potential is locally concentrated in this region, it may be possible to gain a few dBs by placing an aluminium or copper plate, somewhat larger than the diameter of the solenoid, a few centimetres under the solenoid, insulated from the earth but it may be connected directly to the TX ground terminal. (It reflects the vector potential instead of allowing it to be absorbed in the terrain, but DON'T stand on the plate to adjust the solenoid whilst running at full power !!!). I appreciate that some readers may not be au fait with vector potential and perhaps its fundamental role deserves to be lightly aired in the amateur radio journals.

Roger, G2AJV.

I hadn't thought of the loading coil that way (never too old to learn ...). It sounds interesting to me as my loading coil consists of 3 parts : - a 2.5mH coil at the top of the vertical part - a big (7mH) coil that can be tapped, at the moment I am using only 0.5mH of if (so a lot of 'useless copper' around - a smaller coil that can be adjusted between 0.8 and 1.3mH by sliding a ferrite rod (now without the rod in, so 0.8mH) In an attempt to be smart I placed the small coil horizontal (to avoid that the ferrite rod would 'slip away' by gravity) and the big coil vertical (to minimize coupling between the 2 coils). Both coils are on a plastic box about. 0.4m from the ground and about 0.3m separated. I wonder if I would gain anything if I placed both coils vertical ? Also it is tempting to try the plate under the coils, for a first try I could cover the plastic box with aluminium foil (household) or is this a too thin metal layer ? If I find some time over the weekend I will try to do some tests.

Rik ON7YD