# How many turns of wire inside 60 cm shielded loop antenna (9kHz - 30 MHz). How can I find this number without destroying the antenna?

Lots of information about shielded loop antennas is available on the internet. However, antenna manufacturers do not indicate the number of turns. How can I find the number of turns of a manufactured 60 cm shielded loop antenna rated for 9 kHz – 30 MHz without destroying the antenna?

• Hello and welcome to this site! Can you please add a photo and/or the make and model? Oct 12 '20 at 17:22
• May I ask, why are you interested in the number of turns? Do you want to know the inductance? Oct 13 '20 at 20:52

## 2 Answers

Measure the inductance of the loop (without anything else connected). One winding: 2.5 uH, two windings: 10 uH (quadratic relation).

Most shortwave wideband (non-tuned) loop antennas are just a single turn. Inductance will be around 2 to 3 uH. To have constant conversion from field strength to unloaded output voltage, the short-circuited output current of the loop must be converted into an output voltage. The active part of the antenna is a transimpedance with input impedance of 100 milli Ohm and noise matching to the source impedance/inductance.

The low load impedance is seldom realised: the input impedance of the transimpedance amplifier (for example a transistor or FET circuit in common base /common gate) with or without an input transformer is in the order of one to ten Ohm. Result: frequency response for lower frequencies is reduced. Example: Wellbrook ALA 1530 conversion (Vout/Field strength) is reduced below 1 MHz. That is not a problem for the reception sensitivity of weaker signals, since the external noise is higher for lower frequencies.

For measurement purposes only, the "load impedance of the loop" must be low.

There are some examples of mult-turn wideband loop antennas with a shield: these antennas mostly have resonances in their frequency response. I have never seen a good multi-turn shielded active loop antenna without resonances in the 15 to 30 MHz region.

PA0FSB Brightnoise

If you can find out the diameter or AWG of the wire used, then you can find a resistance-per-foot (cm) chart in a Google search

If the wire is connected directly to a connector or terminals, then measure the total resistance of the coil of wire, assuming that 60 cm is the distance measured from center-to-center rather than the OD or ID.

With some simple math, you have your answer. 60 cm average diameter * 3.1416 = 188.5 cm/turn or 6.21 feet of wire per turn.

• I'd think that not many hams have good meters for measuring milliohms. I don't. An alternative could be to use a good inductance meter and measure the inductance. Somewhere I have a kit-built microcontroller-based inductance meter I got at a hamfest... Oct 13 '20 at 18:07
• @rclocher3 Since the loop covers 9 kHz, it's almost a given that it contains enough turns of wire to be quite a few ohms. Edit: I moved that comment to the OQ, where it should be. Oct 13 '20 at 20:42
• Roger, that's good to know. Oct 13 '20 at 20:45