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EME157 B2 ( 2M KIT ) CONSTRUCTION NOTES
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, & 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.
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This Image shows the completed
2M board before fitting the Sub PC board. It is recommended that the Relays are fitted after everything else has been completed. |
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These
Pictures show how the Sub PC board is aligned to the main
board & tack soldered into place. After you are sure that
you have it aligned correctly, then solder it on both sides
to the main board. |
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GME MASTHEAD BOX MOUNTING
The GME Masthead box will accept
cables up to around 8mm in diameter. Therefore larger cables like
RG8/213 cannot be used with this box. Larger cables would also
place too much strain on the BNC sockets on the Preamplifier board. To
connect the Antenna to the Pre amp, & Pre amp to the main feed line,
we recommend the use of LMR240 cable with crimped BNC connectors.
LMR240 is double shielded & has losses similar to RG213 cable,
but has a much smaller 6mm diameter. Refer to our catalogue section3.pdf for LMR240 cable & connectors.
We
don't expect that many will have any problems with the standard
plastic Mast Head box compared to a shielded one. It may help
only to keep some strong out of band Ham signals
from entering the preamplifier & possibly overloading the
Radios Receiver.
CONNECTING TO YOUR TRANSCEIVER
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 & 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 & preamplifiers so that the relays etc have enough time to switch RX to TX mode befor 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, & are able to directly power the pre amp through the coaxial cable & require no additional circuitry. Popular transceivers like the TS2000 & IC706 series do not have internal bias tee circuitry to send +12vdc out via the rear antenna to power external pre amps. In most cases additional circuitry will be required to interface the transmit switching from the transceiver to power the pre amp in RX mode.
3/ The new 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, & 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. Future products will include a new EME166 Sequencer Kit that can control the sequencing of preamplifiers & RF power 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, & 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.
EME157 KIT PROTOTYPE TESTS
Above chart shows the measured
results of a prototype on a HP8970B Noise Figure Meter.
6M
Pre amp in TX mode through path Return Loss Measured
on a Network Analyzer
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2M
Pre amp in TX mode through path Return Loss Measured
on a Network Analyzer
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KIT
CHANGES & MODIFICATIONS EME157B2 KIT
1/
Kit change after April 2011. The BF988A has been replaced with the Mini-Kits BF998R-M.
2/ There can be various faults with this Kit which are mainly to
do with construction. On one Kit we could not get the RX/TX switching
circuit working. We found a 62ohm leakage from the gate of the
2N7000 fet to ground. We removed all components on that junction
except the 680kohm resistor & still measured 62ohms to ground.
We then removed the 680k & the 62ohm leakage disappeared.
We can only conclude that it could have been due to something
conductive on the board so we cleaned the board thoroughly &
reinstalled the components. There
appears to be a problem with the last batch of PC boards that
were made. The board should be checked thoroughly for any short
circuits to ground on the 12 volt input to the board before construction.
Some problems where a pad has been shorted to ground has been
found under the 680k resistor, BC547 transistor, & also the
plated through hole near the 2N7000 Fet. If you have a short then
use a sharp scalpel to carefully cut the short.
3/ There is a small 2 second delay of the
RF sensing circuit engaging the relays when the pre amp is first powered up. This is normal operation & not a design fault.
4/
On later kits we have been adding a 6dB attenuator on the output
of the preamplifier before the relay to reduce the gain &
add more protection to the MesFET & 1N5711 diodes. Some have
complained of blowing the diodes & Fet when using up to 100
Watts on FM. The RF sensing circuit should not be used alone for
high power, & a separate control voltage to the pre amp should
be used.
5/
Inter mod problems don't seem to be an issue with this Kit &
most problems can be directed to the receivers design. I recently
have been using the pre amp on 2M SSB with local & DX signals
up to 60dB over S9 with sporadic E openings only some 10kHz away
without any problems. I use an IC275H which are a rather reasonable
receiver. Don't expect your TS2000, IC706 or 7000 etc to perform
as good as the older receivers. I recommend turning the radios
pre amp switch off on late model Radios if you are going to use
the EME157 2M pre amp. On the IC820/821 the attenuator switch can
be turned on when using the preamplifier at the Antenna.
6/
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 a sure way to blowing the
pre amps diodes & BF988 / BF998R. You will have to hard switch the pre amp
as the RF sensing will not be able to cope with this high level
spike. If you have problems with the pre amp being damaged then
check on the Internet for any problems that may be associated
with your Radios model.
KIT
FAULTS & MODIFICATIONS EME157B6 KIT
1/
Kit change after April 2011. The BF988A has been replaced with the Mini-Kits BF998R-M. Also L4 has been changed from a BN43-202 core to a T50-2 with 19 turns. This was done to reduce the losses when used with higher RF power.
2/
Also refer to the above notes for the 2M Kit version.
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Thursday
5th January 2012
