The EME157 PC board was designed to suit the construction of low cost masthead amplifiers to suit both the 6 and 2 Metre bands. Both Kits use the low cost BF998R MesFET from NXP which have a low noise figure and were designed for VHF/UHF television tuners. The output of the BF998R is matched to 50ohms using a tapped inductor ensuring stable operation. Onboard relays provide bypassing of the preamplifier in transmit mode, and Schottky diodes provide further protection of the MesFET from high RF levels. The design has onboard circuitry to allow powering of the preamplifier in receive mode via the coaxial cable, and is idea for use with the Mini-Kits VHF Bias Tee. The PC board is designed to suit the GME mast head enclosure which provides high protection of the circuitry from weather.
1/ Two 1N5711 diodes have been added to the input of the preamplifiers for protection. This was required due to intermittent damage of the BF998R MesFET when using high RF power through the preamplifier. Please see the diode modification picture. This problem was only apparent when the trimmer capacitors were tuned to some positions, and was not an issue when correctly tuned for the frequency in use.
The 6M preamplifier has been changed in January 2015 to make construction easier. Both inductors L1 and L2 have been changed to make them more mechanically stable with vibration. L1 has now changed to 24 turns of 0.4mm ECW on a T37-10 Toroid, and L2 has now changed to 21.5 turns of 0.8mm ECW close wound on a 5mm drill bit.
The Images below show how the coils are wound for the 2M 144MHz version of the Kit. Take note of the winding direction of L1, L2, and L4 especially so that they fit the board correctly. On the 2M version the end on L1 is soldered to turn 3 on L2. However this was found to not be all that critical in tests on a noise figure meter.
Kits later than April 2011 are supplied with the Mini-Kits BF998R-M device. BUY THIS PRODUCT
The Images below show how the coils are wound for the 6M 50MHz version of the Kit. Take note of the winding direction of L1, L2, and L4 especially so that they fit the board correctly. On the 6M version the end on L1 is soldered to turn 3.5 on L2. However this was found to not be all that critical in tests on a noise figure meter, but does affect gain.
The GME Masthead enclosure will accept cables up to around 8mm in diameter. Therefore larger cables like RG8/213 cannot be used. Larger cables would also place too much strain on the BNC sockets on the Preamplifier board. To connect the Antenna feed line to the Preamplifier, and Preamplifier output to the main feed line, we recommend the use of LMR240 cable with crimped BNC connectors. LMR240 is double shielded and has losses similar to RG213 cable, but has a much smaller 6mm diameter. Refer to the website for LMR240 cable and connectors.
EME157B2 Preamplifier Gain and Bandwidth Test
EME157B2 Preamplifier input return loss
EME157B6 Preamplifier Gain and Bandwidth Test without onboard attenuator fitted.
EME157B6 Preamplifier input return loss.
EME157B6 Preamplifier Gain and Bandwidth Test showing the two distinct peaks caused by the two input tuned circuits when tuned for minimum noise figure at 50.200MHz.
1/ Mini-Kits cannot be expected to take responsibility to make this product work on your Transceiver, we can only give guidance on how best to use this product. Using this product with high power FM or SSB modes does have the risk of damaging the BF998R FET if the transceiver switches from RX to TX modes faster than the preamplifiers relays. Some protection is built into the preamplifier circuit including a RF sensing circuit and Schottky diode protection, but high RF levels into the preamplifier before it has fully switched from RX to TX mode can damage the pre amp. Some Transceivers are prone to producing a spike on the RF output when going into transmit mode. This can be due to the ALC in the Radio not acting fast enough. This is where a sequencer circuit is useful to sequence the switching of RF amplifiers and preamplifiers so that the relays etc have enough time to switch RX to TX mode before RF power is applied. Mini-Kits has tested the EME157 series Kits up to 100 Watts using the RF sensing circuit with very low failure rates. It is however recommended that a maximum of 50W FM, or 100W SSB be used.
2/ Powering the Preamplifier with +12vdc can be either by running a DC power cable direct to the preamplifier board, or by feeding it up the coaxial cable. Transceivers like the ICOM IC275 / IC820 have a pre amp switch with delay sequencing, and are able to directly power the pre amp through the coaxial cable and require no additional circuitry. Popular transceivers like the TS2000 and IC706 series do not have internal bias tee circuitry to send +12vdc out via the rear antenna to power external pre amps. additional circuitry will be required to interface the transmit switching from the transceiver to power the pre amp in RX mode.
3/ The Mini-Kits EME168 VHF Bias Tee Kit is now available that allows the preamplifier power to be easily switched via the coaxial cable. Many Transceivers have access to a +12 volt TX connection on the accessories socket on the rear. This can be used to control the bias tee switching the +12vdc to the pre amp in RX mode, and disconnecting it in TX mode. At this time the RF sensing circuit should still be used on the preamplifier as a fail safe, as there is no delay sequencing built into the EME168 VHF bias tee. The EME166 Sequencer Kit can be used to control the sequencing of preamplifiers and high power RF amplifiers with a transceiver.
4/ To power the preamplifier board directly, fit a 1nF or larger feed through capacitor through the hole that is drilled next to the RF output socket on the sub PC board. Solder the end of L4 that was connected to the RF output socket on the PC board to the feed through capacitor. Connect +12vdc to the feed through capacitor, and 0 volts GND to the circuit board ground. This Mod also stops any +12vdc from being fed back down the coaxial cable to the Transceiver.
A Bias Tee Kit is available that is suitable for powering the 6m and 2M preamplifiers via a coaxial cable. The SWR return losses have been tested to be better than -20dB and losses less than 0.2dB. The Kit has only been designed for use up to at least 100 Watts. BUY THIS PRODUCT
1/ When using the optional masthead enclosure with nickel spray, there is a possibility of a connection on the bottom of the board shorting out on the nickel spray disabling the relays. The point that shorts to the nickel is the 1nF ceramic capacitor on the gate of the 2N7000 Fet. Refer to the enclosure modification. Cut the high plastic protrusion from the enclosure. Make sure that the nickel paint has been fully removed where the plastic protrusion existed.
2/ Kit change after April 2011. The BF988A has been replaced with the Mini-Kits BF998R-M.
3/ Kit change after March 2013. Two 1N5711 Schottky diode have been added to further protect the BF998R from damage caused when using high RF power through the preamplifier. Refer to the diode modification here.
1/ The 6M preamplifier has been changed in January 2015 to make construction easier. Both inductors L1 and L2 have been changed to make them more mechanically stable with vibration. L1 has now changed to 24 turns of 0.4mm ECW on a T37-10 Toroid, and L2 has now changed to 21.5 turns of 0.8mm ECW close wound on a 5mm drill bit. Please see the picture that shows the Input Matching Circuit This change does not improve the performance so is not required to be done on previous Kits.