# Estimating near-field electric field strength at LF (137 kHz)

I'm experimenting with low frequencies, but due to the inherent nature of LF, I strive for more transmit power and bigger antennas.

Now of course I want to comply with regulations, which allow (in my country) an average exposure of 80 V/m at this frequency.

If I'd have a T-antenna that was 15m high and 20m wide. How do I calculate/estimate at which distance I would expect this field strength? Let's say 100W PEP, and an estimated radiated power of 10 mW EIRP.

The formula that I've found applies only to the far-field, I think: https://ham.stackexchange.com/a/10078/16455

Thanks.

• More information needed, please: 1. At what distance from the antenna system does its radiation need to meet or not exceed the 80 V/m limit? 2. Is that limit applicable to physical locations not available for human occupancy? 3. What is the configuration of the r-f ground system (buried radial, length/number), or ? 4. What is Earth conductivity at the antenna site, in mS/m, and permittivity? 5. Frequency of interest? Commented Feb 19, 2020 at 20:38
• 1) About 6 meters from any part of the antenna. The area is very uneven, with lots of trees and other properties which will discharge the E-field to ground. 2) The limit is only applicable for human occupancy. Below 6W EIRP there are no other regulations. 3) Burried radials with a total length of 800 meters in varying length. The impedance of the antenna is conveniently 50 ohms, which consists of ground losses (40 Ohms?), loading coil/wire losses (10 Ohms?) and radiation resistance (Close to zero) 4) Very inhomogenous. 5) 137 kHz Commented Feb 19, 2020 at 21:40
• A friend with a similar setup measured 1.5 V/m at 2m distance from the loading coil at the base of the T. But I don't know how reliable his measurements were. Commented Feb 19, 2020 at 21:41

## 2 Answers

Below is a rough estimate of the near fields calculated by NEC4.2 for an elevation of 3 meters above level Earth from a transmit antenna and system as generally described in the OP, and the followup comments of the OP.

"Use with due diligence."

• Thank you very much, that is very helpful! The coil inductance is only 3 mH I should have mentioned. I assume that is due to stray capacitance of the antenna to the environment. So the antenna voltage will be smaller. Commented Feb 21, 2020 at 0:39
• NEC4.2 includes the capacitance of the radiating wires to the propagation environment within ~1/2WL around the antenna when calculating the L or C needed for resonance of the wire model it is evaluating. In this case it tells us that a ~6,720 _micro_henry inductor is needed at the base of the vertical wire of the T antenna for system resonance (j=0 ohms), at 137 kHz. Commented Feb 21, 2020 at 11:04
• The simulation matches the suggested 6 pF capacity per meter antenna wire from this page strobbe.eu/on7yd/136ant But I have 448 pF antenna capacity, thus only 3010 uH are needed for resonance, resulting in an antenna voltage of about 3 kV. Commented Feb 21, 2020 at 22:16

Estimating the near field is best done with modeling software or empirical measurement. The details of the antenna construction and environment (the ground and any nearby conductive objects in particular) can significantly impact the near field.

That said, 100W is not much, and often regulations specify that no particular evaluation need be done if the power is below some threshold. Furthermore, low frequencies are generally safer, as sensitive parts of the body (such as the cornea) don't readily absorb the radiation. For example in the US, no evaluation is necessary below 500W PEP on the 160 meter band.

• Similar here, no evaluation necessary below 6W EIRP, and I'm at an estimate of 10 mW EIRP. But the 80 V/m field strength limit still needs to be met, independently from that, as it is an "Environmental law" - sorry I don't know the English term. Commented Feb 19, 2020 at 21:59