EME182 HF Active Antenna

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Mini-Kits Active Antenna Description:

The HF Active Antenna has been designed to be easy to install, and covers a wide frequency range from 100kHz to 80MHz. It is particularly useful for those that have antenna restrictions or limited space to install a full size antenna, and can be used in antenna arrays for noise cancellation or beam forming. The design incorporates static and lightning protection, and has a broadband 50ohm output impedance suitable for driving most receivers. A J310 FET is used as the active device, and along with a complementary push pull output buffer produces high output second and third order figures reducing the possibility of inter modulation on the received signals. The design incorporates power supply decoupling so that power can be fed to the Active circuit via the coaxial cable.

The recommended Antenna is a 1.9 metre long telescopic stainless steel side mount truck antenna. It is physically short as an antenna for the low frequency HF bands, and will appear as a small 22pF capacitor to ground on the active input circuit. Antennas longer than around 2.5 metres, may increase the possibility of strong signals overloading the active circuit causing inter modulation. The antenna is ideal for mounting directly onto a 2 metre long 1 inch 25.4mm diameter square or round aluminium tube. The active circuit can then be easily mounted onto the same tube, and then short connection wires are connected from the antenna to the active circuit. At least 3 ground radials 3 metres long each are laid along the ground for the best performance.

Testing indicated that receive signals below 10MHz may require an 10dB gain amplifier on the receiver input to increase signals to a more comfortable level due to the short antenna length. Some modern receivers incorporate preamplifiers that can simply be switched on, on the lower bands.

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 problem with not being able to adjust the 50k trim pot for the correct current for the J310 Fet. This equated to around 30mA which is around 1.17 volt across the 39ohm source resistor on prototypes. There may be some variation with the J310 FETs that cause the current setting for best IP3 to be lower than 30mA. So the source resistor may need changing to a higher value around 100ohms to set the current for some FETs. Aim for around 25 to 30mA by measuring the voltage across the source resistor for best IP3.

Construction Notes:

The images below show both sides of the board. There are a number of modifications to the board including wire links and the two ceramic capacitors that have been fitted to the first PC board design.

EME182 Module Top View
Top view of the EME182 Active Module
EME182 Module Bottom View
Bottom View of the EME182 Active Module
12pF Ceramic Capacitor Mounting
Mounting of the 12pF ceramic capacitors
EME182 Module Bottom View
Power and RF de-coupling inductors
Transistor Mounting
2N2222A 2N2907A Transistor orientation
EME182 Module Bottom View
Mounting of the J310 FET

EME182 Active Circuit Tests:

Response 0.1 to 30MHz
Frequency response 0.1 to 3MHz, 10dB attenuator fitted
Response 0.1 to 100MHz
Frequency response 0.1 to 100MHz no attenuator
Two Tone 8 and 11MHz
OpIP2 -96.7dBm with 0dBm input two tone 8 and 11MHz ( add 10dB to measured result above ).
S22 Return Loss
S22 Output Return Loss 0 to 100MHz

Antenna and Enclosure Mounting:

The PC board has been designed to use the GME masthead enclosure. These are fully waterproof and last for many years in harsh environments. The recommended Antenna is a Aerpro AP90 Universal Truck side mount antenna, that can easily be mounted onto 2 Metre long x 25mm diameter aluminium pole. The pole is driven into the ground by around 50cm leaving a length of 1.5 metres above ground. Two 3mm holes are carefully drilled through the aluminium tube at a spacing of 135mm between the holes. The holes on one side of the tube are then drilled out using larger drill bits, and finally a 12.5mm side drill bit to suit the antennas black plastic insulator diameter. Some filing may be required to fit the insulators through the holes. The bottom insulator has a wing nut and threaded bolt that needs to be removed, as inside the insulator is a spring that is not required so has to be removed so that it does not short the antenna to the aluminium mounting pole. Next fit the top insulator onto the pole using the standard screw supplied. Next fit the bottom insulator, and then slide the antenna through the two insulators and attach the antenna to the bottom insulator with the screw. Attach the GME enclosure to the pole between the two insulators using the supplied cable tie. Fit the two supplied cables for the antenna and ground connections as shown in the pictures.

Two Tone 8 and 11MHz
Front view of the board mounted inside the GME Masthead Enclosure
Drilling the holes
Drilling of the pole to suit the AP90 Antenna
Spring Removal
Discard the spring inside the bottom plastic insulator
Bottom Insulator Mounting
Bottom Insulator Mounting
Completed Antenna Mounting
Antenna mounted to the pole
Active circuit mounting
Mounting the Active Circuit to the pole
Ground wire connection
Connecting the ground cable wire
Antenna wire connection
Connecting the Antenna Cable wire

Changes and Modifications:

1/ There has been a problem with not being able to adjust the 50k trim pot for the correct current for the J310 Fet. This equated to around 30mA which is around 1.17 volt across the 39ohm source resistor on prototypes. There may be some variation with the J310 FETs that cause the current setting for best IP3 to be lower than 30mA. So the source resistor may need changing to a higher value around 100ohms to set the current for some FETs. Aim for around 25 to 30mA by measuring the voltage across the source resistor for best IP3.

2/ Low 12vdc to the Active circuit when fed with a bias tee through coaxial cable. Check the bias tee to make sure that it is capable of supplying at least 100mA @ 12vdc. This can be done by placing a load resistor on the 12vdc output of the bias tee. If the test fails then replace the PTC inside the bias tee.