EME234 HF Magnetic Loop Antenna Module

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EME234 HF Active Magnetic Loop Description:

The HF Active Magnetic Loop Antenna module has been designed to be easy to construct and install, and covers a wide frequency range from 1 to 50MHz. It is particularly useful for those that have antenna restrictions or limited space to install a full size receive only antenna, or have very high man made noise levels. The magnetic loop is particularly effective at reducing power line and common mode type noise and being a figure 8 pattern with vertically polarized signals, can be rotated to further notch interference. The amplifier is based on the design by Chavdar Levkov LZ1AQ, and incorporates static and lightning protection, and has a broadband 50ohm output impedance suitable for driving most receivers. The amplifier is designed to be used with a 1 metre single turn loop, and a balanced low input impedance common base amplifier is used to match the input. Low cost RF Transistors are used as the active devices, which produce high output second and third order figures reducing the possibility of inter modulation on received signals. The design incorporates power supply decoupling so that power can be fed to the Active circuit via the coaxial cable.

Test on prototypes indicated that high levels of broadband noise across the Radio spectrum could be nulled completely allowing many weak signals to be heard with very high signal to noise ratios. The same signals on a dipole were either unreadable or the noise made it difficult to copy them. It would be useful to be able to remotely rotate the loop so that it could be more useful at reducing interference. The null is very sharp maybe on a few degrees.

In very strong AM broadcast areas, some lower quality receivers without good filtering may suffer from inter modulation from strong signal overload. A suitable 1.8MHz high pass filter should be fitted at the receiver input to reduce broadcast band signal levels.

Kit Constructors Alert:

1/ There has been a report of a BNC PCB socket that had an intermittent short internally. The issue was traced to a small piece of plating that was loose inside the connectors cavity. Thanks to Michael VK5ZEA

Construction Notes:

The images below show both sides of the board. The construction is quite easy if you start with the SMD components on the bottom side of the board first, and then fit the conventional leaded components after. The only difficult part is the winding of transformer TFMR1 as the windings have to be correctly connected for the correct phasing polarity. An incorrectly phased transformer cancels the signals and produces no gain. Click the pictures below for a more detailed view.

EME234 Module Top View
Top view of the EME234 Active Module
EME234 Module Bottom View
Bottom View of the EME234 Active Module
EME234-TFMR1-Secondary Winding
How 6 Turns is wound for the secondary winding of TFMR1
EME234-TFMR1-Secondary Winding
How 8 Turns bi-fillar is wound for the primary winding of TFMR1
EME234-TFMR2 Wiring
Be careful with the polarity of the wire connections for TFMR2, F goes to G, and H goes to I

EME234 Active Circuit Tests:

Response 0.1 to 3MHz
EME234 Low Frequency response 0.1 to 3MHz using 3.3nF coupling capacitors.
EME234 S22 0.1 to 100MHz Response
EME234 0.1 to 100MHz Frequency Response using 150nH and 82pF capacitors for the low pass filter.
EME234 OpIP2
3MHz OpIP2 -113dBm with 0dBm input two tone 8 and 11MHz
EME234 OpIP3
5MHz OpIP3 with 0dBm input two tone 8 and 11MHz This equates to an OpIP3 of +39dBm
EME234 S22
S22 Output Return Loss 0.1 to 100MHz
EME234 OpIP3

Antenna and Enclosure Mounting:

The recommended Antenna is constructed using off the shelf hardware from Bunnings Hardware stores in Australia. A 3 metre length of 25mm diameter PE100 high pressure pipe is bent into a loop and is used to house a loop wire element, Bunnings Part# 4811263. A 25mm Philmac Poly T piece, Bunnings Part# 4813797 is used for the connection to the ends of the poly pipe, and the T piece is mounted onto a 3 metre long 25mm diameter galvanized steel pole, Bunnings Part# 1130457. Thanks to Tom VK5ZTS and Richard VK5ZLR for the Poly tube loop idea.

1/ A 4 metre long length of stranded 3mm diameter PVC covered wire is run inside the poly pipe for the loop antenna element. You can use a larger diameter if you like but it probably will make no difference with received signal or performance. Bend the 25mm Poly tube into a loop and connect it to the Poly T piece and firmly tighten the gland nuts. On the underside of the loop drill 2x 3.5mm holes 10mm out from the T piece nuts at an angle to suit feeding the 3mm PVC covered wire through the tube. Remove the T piece from the poly pipe and feed the 3mm wire through one hole in the poly tube until it exits the other end of the tube. Feed some thin tinned copper wire through the second 3.5mm hole and out through the poly tube end, and solder it to the end of the 3mm PVC wire. Pull the tinned copper wire and feed the end of the 3mm wire through the second 3.5mm hole to a length of around 40cm out of the hole in the poly tube. At least 35cm of 3mm wire from each end of the poly tube is needed to reach the Active module.

2/ Cut the steel pole to a 2.5 metre length and drive the pole vertically 0.5 metres into the ground. The recommended height of the bottom of the loop is 2 metres above the ground which is at a height that will clear most obstructions, but is still close to the ground avoiding pickup of noise on the outer of the coaxial cable.

3/ Mount the Poly T piece with loop onto the top of the steel pole, and then mount the GME enclosure directly beneath the Poly T piece using the supplied cable tie.

4/ Work out the length required for the ends of the 3mm wires so that it can be routed evenly in length to the Active modules A connections. Use small cable ties to tie the two wires together as shown in the picture below, routing them evenly and away from the galvanized pole to the Active modules A terminals. Drill a hole in the galvanized pole just under the masthead enclosure and connect a crimped eyelet terminal ground wire from the pole to the Active modules AGND connection. This adds some lightning and static protection to the Active circuit.

5/ Run a length of RG6 Quad coaxial cable from where the Active module is located to the receiver. The cable should if possible be run across the ground to avoid noise pickup. Fit suitable male F connectors onto the coaxial cable, and cable tie it to the pole.


Philmac 25mm Poly T Piece
Philmac 25mm Poly T Piece Bunnings Part# 4813797
PE100 25mm High Pressure Pipe
PE100 25mm High Pressure Pipe Bunnings Part # 4811263
Spring Removal
3 Metre long 25mm Galvanized Tube Bunnings Part# 1130457
Completed EME234 Active circuit wired
Picture shows the EME234 Active circuit mounted onto the galvanized mounting pole and wiring connections to the Active module
Active circuit mounting
Picture shows the wiring from the poly tube Loop cable tied neatly and symmetrically routed to the Active module
Completed EME234 Active Magnetic Loop Antenna
Completed EME234 Active Magnetic Loop Antenna hidden from view in the garden

Changes and Modifications:

1/ There has been a report of a BNC PCB socket that had an intermittent short internally. The issue was traced to a small piece of plating that was loose inside the connectors cavity. Thanks to Michael VK5ZEA