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# Loop antenna design for 100. kHz

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#### drkirkby

##### Full Member level 6
I am looking to make a loop antenna to receive one of the 250 kW 100 kHz Loran-C transmitters, which like GPS are designed for navigation but can be used to develop a frequency standard.

I am in South-East England, but the nearest transmitter is at Lessay in northern France about 400. km away, although there's one in Anthorn northern England too.

The output of the loop will feed into a common base amplifier, which will have an input resistance of about 3 Ohms. The amplifier will be powered by 5.6 V which is on the BNC antenna connector on a Stanford Research FS700 frequency standard.

I am interested in finding out the best way to make this. A few things are obvious

1) The capture area increases with the area of the loop. I have made my loop 1.5 x 1.0 m, as that's about the biggest practical size I am willing to tolerate

2) The output voltage from the loop rises as the number of turns is increased. But assuming a constant thickness of wire, increasing the number of turns increases the resistance so increases the thermal noise voltage.

One reference I found stated that the signal to noise ratio depended on the mass of copper used. It stated a single turn using 1 lb of wine has the same S/N as 100 turns if 1 lb of wire is used.

A few things are puzzling me though

1) Is there any point worrying about the thermal noise of a loop antenna when the atmospheric noise at 100 kHz is so high?

2) Should the loop be wound with a piece of wire which has the same DC resistance as the input impedance of the amplifier? I have not measured it but I am led to believe that the input resistance of the common bsse amplifier is 3 Ohms.

I happen to have abiut 100 m of 2.5 mm^2 wire which has a DC resistance of about 0.7 Ohms, whuch us about a fifth of the input resistance of the amplifier. Would I be better to replace the 2.5 mm^2 wire with a 100 m of thinner wire (lets say one fifth the area, so aboutv0.5 mm^2) to better match the amplifier to get maximum power transfer? Doing so would give the same voltage, but sqrt (5) as much thermal noise voltage.

Buying another 400 m of 2.5 mm^2 wire is too expensive, so if I do decide to increase the loop resistance to 3 Ohms it will be by using a thinner wire.

Due to the fact that the Loran-C transmission is around wide (100 +/- 20 kHz), the loop must be untuned.

Any thoughts?

When I designed a VLF antenna , I just used a 2m whip antenna, of course longer the better. Then I used low noise FET front end with 2 stage 5deg X cut Xtal resonator filters and a PLL. I used the US Navy stations from Norway to Cutler Main to Hawaii for navigation. The lowest common denominator was 100Hz for each frequency which was used for the CD4046 mixer for each channel to compare each phase shift. Loran C is a bit different but similar RF wise. The capture range was determined by the thermal drift of the VCXO and now you can get TCXO's under 1ppm for a few \$.

I expect you can get forehand information about the expectable magnetic field strength. This would allow a systematic design instead of guessing about copper pounds.

I would expect to receive the signal with a small ferrite antenna.

You are right about capture area "size is important" definitely applies here.
You can cheat with a flux concentrator such as a large ferrite rod, but that really only applies to something physically quite small. A really big open loop, optimally oriented in the H field will definitely beat it.

One thing you need to realize is that at 100 Khz, skin effect becomes very significant. The ac current only flows on the outside "skin" of the wire, and thick wire is just a waste of copper.
https://chemandy.com/calculators/skin-effect-calculator.htm
The solution is to use many fine strands in parallel, each no thicker than 0.4mm, and all insulated from each other, and you can buy this commercially as "Litz wire". Or you could make it yourself.
This thin wire is not expensive, and you will get much better results.

It will have a much lower source resistance at 100 Khz for any given amount of copper.

At 100Khz thermal noise will not be what limits you, it will be atmospheric noise, especially from lightning discharges even from great distances.

You could use a dual orthogonal loop with tuned resonant coils on each and with separate front ends with LC filters, then AGC and use the strongest signal for processing.

You must use litz wire the more cores the better. The "turns" must be spaces away from each other say by using comb like spacers. Use as much inductance as you can to get a high impedance, high Q circuit. Use a capacitive tap, say 1000 PF silver mica from earth to the output then your resonance capacitor to the live end of the coil.
I see no reason to use a common base transistor for your input device. No linearity problem, no problem with instability at this frequency, you are just throwing gain away.
Picking up noise will be your problem, It could be worth while to have a second circuit to act as a noise blanker, it all depends on how your PLL reacts to the noise pulse.
Have you considered using Droitwhich on 195 KHZ, its is could be subject to short periods of over modulation.
Frank

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I made a 60x60 cm loop antenna with 0.5 mm dia.wire, tuned with 0.1 mm ceramic cap to 23.8 kHz transmitter in West Germany, to detect SIDs. It works just fine, pointing is sensitive to 5 deg.approx..
The main problem I detected when starting is the strong noise from many switching-mode power supplies. In my case, the worst was my own laptop AC-DC converter running at 19.5 kHz. I can assume harmonic extend to above 100 kHz.
If you observe similar interference, use a shielded loop.
Thick wire and a good tuning capacitor (air if possible) is the best way to success. At resonance, natural noise should not be a problem as the noise bandwidth of the receiver is defined by loop Q. It can be of the order of thousands.
Good luck!

- - - Updated - - -

For the receiver I used a gyrator circuit with two TL082 opamps, very nice, linear and easy.

This is a picture I found on the internet, but this guy has the right idea.

I built one of these years ago. It was very similar to the one in the photograph. That isn't me though, despite the World being in monochrome when I was younger.

1. Forget noise figures. Background interference from TV, SMPS, CFL lighting etc will be MUCH higher than natural atmospheric or circuit generated noise.
2. The type of wire does make a difference but I wouldn't go overboard with exotic wire sizes or types. I used standard 7/0.2 equipment wire and it worked as adequately.
3. Do consider tuning it, making it resonant will hugely increase the sensitivity at the frequency you want and more importantly, help to reject the frequencies you want to avoid. You can reduce the 'Q' if necessary to broaden the bandwidth by adding a parallel resistance.
4. Don't make one, make two. The second one doesn't have to be identical, a much smaller design will work. Make it rotatable in horizontal and vertical planes and subtract it's signal from the main antenna. You can use it to null some of the unwanted signals, believe me there will be many of them.
5. Beware of high voltages from it and take measures to protect circuitry. The loop will generate several tens of volts if there is a nearby lightning flash.

I never followed up initial experiments but if I had, I think making it center tapped ground and using the ends differentially might have been next to try. The size makes it pick up by capacitive coupling and most of the 'noise' was 50Hz from the mains supply. Getting rid of that while preserving the LF response might be a challenge. The 50HZ pick up was magnitudes higher than the signals I was looking for. That was in a rural area near Bristol in the of SW England in the early 1970s.

Brian.

If its going to be pretty big, and located outside, one way to do it is by using something like 25 pair telephone cable in one huge loop suspended between bamboo poles.

All fifty wires are then connected in series giving 50 turns, and the joining point potted in epoxy. The outer aluminium foil will act as a Faraday screen if its left open at the joining point, and one end only grounded.
It makes for a pretty simple robust weatherproof loop antenna.

Hi,

I´m not familiar with building antennas.

But DCF77 (for sending time signal in Germany. It uses 77.5 kHz wich is not far away from 100 kHz) uses ferrite antennas in the receivers.
There are solid copper wires. Ferrite is about 5cm in length and 1cm in diameter.

Klaus

Yes, the reference to DCF77 made be guess a ferrite antenna could be sufficient. DCF77 is said to achieve 2000 km range with 50 kW transmitter power.

There's however a difference in modulation methods and occupied bandwidth between DCF and Loran-C.

Someone just brought me (in the UK) a 'faulty' radio controlled clock they just bought. The fault was it was always wrong by one hour. Label on the back says "DCF77". :lol:

Brian.

Hi,

since fortunately time is leading, the solution is simple:
Just buy 720 million km of RG-58 as a 1 hour delay line. ;-)

Klaus

I'll tell them you suggested it. :wink:

As it's an analog clock, I think bending the hour finger might be more practical, if not as aesthetically pleasing! This if off topic though (slap wrists).

Brian.

Hi,

OT: How can a DCF77 clock be analog? ;-) Ok, this was my last OT for today

Klaus

There are lots of self setting analog clocks around. They use pulse ratchet mechanisms to pull the cog wheels around in the same way quartz analog clocks do. The difference is they initialize to a fixed finger position when the battery is inserted then send the calculated number of pulses to move the fingers to match the received DCF77 time.

Brian.

Back in days before GPS when we did portable seismic recording, onto cassette tape, the B channel had the signals from WWVB at 60kHz using a small OEM card in the recorder box to long wire antenna for coordinated universal time with correction codes to 1ms.

Then all the tape signals with seismic on A channel using FM audio and WWVB were uploaded to server to synchronize all the data from a 16 or so portable recorders used in the field in a linear string towards the man-made blast for research. THe distances involved were several minutes to the blast ( 500km) and far far away from Colorado Tx for WWVB.

I may be wrong but I thought Loran (in the original post) was for direction finding rather than carrying data. I should be able to pick them up from here but the frequencies around 100KHz are S9+20dB of complete wideband noise here from man made sources. It's amazing when there is a power blackout just how much can be picked up on LF. I use an Icom 756 pro II which is hooked up to a standby battery. It isn't the best receiver at low frequencies but when that fog of interference is lifted, I can hear low power local radio stations hundreds of miles away.

Brian.

Just a few comments, based on the many interesting replies. There are too many to make it practical to comment on each individually, so here is a summary.

1) Yes I had considered using Droitwich @ 198 kHz, and might well do that, although according to Wikipedia, it use a rubidium frequency reference, which makes it pretty useless. I don't know if that information is correct though - I would have expected a cesium source.

2) I had not considered using DCF77, but I will do so.

3) I measured the impedance of my loop (1.5 x 1.0 m, 19 turns) using an HP 4284A precision LCR meter. At 20 Hz, it has a magnitude of 0.7 Ohms at a phase angle of 17 degrees (0.65 +j 0.17). This is pretty close to what I would expect based on the DC resistance of the wire. But at 100 kHz, it is very different (14.2 + j 1000 Ohms), which is 1.6 mH in series with 14.2 Ohms. (All these readings were read from the meter, with no averaging, as I changed the display format, so don't expect them to convert exactly.)

The loop has a self-resonate frequency somewhere between 300 and 400 kHz, but I don't know exactly where as my LCR meter only has discrete frequencies it works at - it is not fully tuneable.

4) The common base amplifier was a circuit posted by someone on the time-nuts mailing list, who wrote

"I use a home-made untuned loop antenna with 4 windings of 2.5 mm2 insulated wire on a 80 x 80 cm wooden frame, and with a grounded base pre-amplifier mounted on the antenna frame. A schematic is enclosed for you to copy. The pre-amplifier is powered through the cable, and loads the FS700 input as required. I live about 290 km from the island of Sylt, and get nice noise margin figures from the FS700, normally about 40 dB, often up to 46 dB."

The circuit is attached

If someone has something that works, it is tempting to follow it. I think the use of common based is because of the low input impedance of the amp (estimated to be 3 Ohms), although my loop impedance seems far greater than 3 Ohms.

I changed the transistor for a MAT12, as the original is impossible to obtain from reliable sources. There are plenty on eBay from China, but I suspect they are all fakes.

5) The bandwidth of Loran-C is quite wide, and therefore I am tempted to not tune the loop. Again I go back to the comment from someone who is getting very good S/N ratios with an untuned loop and the common base amplifier.

6) I'd not considered that voltages of 10's of volts could be inducted by lightning. For that reason I will probably put some high-speed back-to-back diodes, to keep the voltages in sensible limits.

7) Someone else mentioned the use of Litz wire, and given how far the real part of the input impedance of the loop (around 14 Ohms) is different from the DC resistance (estimated about 0.7 Ohms), clearly the skin effect is having more effect than I had given it credit for.

8) Yes, I had considered a whip and FET amplifier. In fact the FS700 was supplied with an active whip antenna, but I don't know what the amplifier was.

9) I believe a loop will give better S/N than a ferrite rod antenna, and expect this will lead to better stability.

10) I have a GPS disciplined crystal oscillator (HP 58503A), which should be more accurate short-term than the long-wave systems, but I want to compare them. Long term they should all be the same.

Dave

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