CONSTRUCTION ARTICLES UPDATE
10Metre MMIC Pre-Amp
Although MMICs (Monolithic Microwave Integrated Circuits) were never really intended for use in the short wave end of the spectrum, this simple amplifier is cheap and simple to build.
No construction details are given but 'dead bug' style construction on a offcut of single sided printed circuit board is one option. The tapered lead on the MMIC is the input (sometimes also marked with a white dot).
The lead opposite is the Bias/Output and the remaining two connections go to ground...with the shortest leads possible. The coils are 10 turns of wire on a T50-2 toroid. The amplifier can be housed in a simple box made from scrap PCB material. Use decent connectors - BNC or N type - and mount the amplifier as close to the antenna as possible.
A possible switching / bypass
circuit is also shown. An LED - at the
shack end - is useful to show whether or not the amplifier is in use. A two pole relay S1a/b allows the amplifier
to be switched in or out of circuit.
The LA2QAA Special
This simple antenna project is for a dual band (2M & 70Cms) J-Pole antenna system. The antenna sections are matched by adjusting the clamps for the best SWR.
A simple pre-amp for 70Cms is included- (shown on the following page) - mounted in a diecast box on the upright section of the 2 metre J-pole.
The amplifier is powered via the
co-ax cable.
An FO-20/RS-10 Quad antenna
This antenna is intended as a simple transmit antenna for RS-10 in conjunction with an indoor dipole for the downlink.
Use stiff wire and form it into a square of side 525mm.
The matching section is formed from 339mm of 70 ohm co-ax.
LA2QAA used a microphone stand with a wooden 'stand off' - Use your
imagination! The quad loop is supported
by a wooden cross member. For outdoor
use of course, a plastic support would
be more suitable with some attention given to waterproofing the cable - a small plastic box would make a suitable
connection point.
The receive antenna for RS-10 can be a Delta Loop supported 5M off the ground. The length of the loop should be 10.42 metres long (29.4Mhz)
It should be fed by a Q section consisting of an electrical quarter wave of 70 ohm line.
2M Pre-Amp
The 2m preamp uses a cheap low noise dual gate MosFET transistor.
It is not necessary to use an ultra low noise GaAsFET transistor on 2m
for satellite work because the sky noise at 145MHz is approx: 2Db.
The exception to this is EME (Earth-Moon-Earth) communications where an 0.6Db or better preamp is an absolute must due to the extremely weak signals. Only use a preamp if it's necessary. Don't confuse LOUDNESS with a better quality signal. Remember, the preamp will also amplify the NOISE as well as the signal. That's why the preamp should be mounted at the antenna, to compensate for the loss of signal and NOISE in the coax. A run of high quality low noise coax is also cheaper than a commercial preamp. This preamp will allow you to "manage" with RG 213. Cheap RG-058 should only be used for 20m and lower (or to hang your washing on!).
The design shown uses a tuned input circuit adjusted for lowest noise figure and a tuned output adjusted for maximum gain. The 1000pf decoupling capacitors can be feedthrough types allowing one to build the preamp using so called "ugly" construction where the components are soldered together (using shortest possible lead lengths) without needing an etched circuit board. Standard ceramic capacitors can be used if the leads are 1mm or shorter. Use a silver mica or other good quality capacitor at the preamp output. Miniature air type trimmers are recommended for the tuned circuits. Piston trimmers can also be used but only use cheap plastic trimmers as a last resort.
Note that a small RFC (6 turns on a ferrite bead or any 0.4uH RF choke) is included for those wishing to feed the DC via the coaxial cable. If doing this remember to decouple the DC at the rig end as well as using a blocking capacitor (1000pf) between the power supply and the input to the receiver. If the DC is run to the preamp via it's own cable, the RF choke and associated bypass capacitor can be omitted.
The 3SK88 comes in either a plastic "pill" package with four legs or a metal case package. When using the metal case package it is wise to put a miniature ferrite bead on the G2 leg to suppress parasitic oscillations.
This is not necessary with the more
modern plastic package due to the inherent balance of the substrate within the
device. Both devices should be adequately decoupled on the G2, source and drain
leads.
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70cm PREAMP.
The 70cm preamp again uses a cheap low noise device, this time a bipolar transistor. If you cant get hold of a BFR 34A any low noise UHF transistor can be substituted in this circuit with no noticeable difference in performance
Like the 2m preamp, an RF choke and bypass capacitor are included for those wishing to feed the DC through the coax:
At 435MHz it's EVEN MORE important to use the best quality coax you can afford. Again, if the coax is really good quality stuff you won't need a preamp.
A point to note in the 70cm preamp is the input inductor. It's made from copper sheet mounted 5mm over the groundplane. This helps to ensure a low noise figure.
Be aware that LOWEST NOISE FIGURE and MAXIMUM GAIN don't usually coincide. Again, don't aim for LOUDNESS, go for signal quality. The real "measuring instruments" are fastened to the sides of your head. (EARS!).
Adequate decoupling is important (just as in the 2m preamp) together with good quality capacitors. Don't use "caps" desoldered from your old broadcast radio!.
Remember that you'll need a waterproof enclosure for the preamps. A cheap alternative to weatherproof diecast boxes with gaskets is to make the enclosure from PB board soldered along the seams them mount the entire preamp and enclosure at the antenna in a plastic ice cream carton. Remember to bore some ventilation holes (underneath!) to stop condensation. It has been stated by certain prominent people that certain plastics disintegrate in ultra violet light (Ye olde Sun). This is true but at UK latitudes it would take 5 years so it's quite "safe" to use ice cream boxes as rain covers etc.
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1268Mhz 80mw 'linear'.
This simple MMIC based circuit is suitable for raising the output from a typical 23Cms oscillator chain from 20Mw to the 80Mw level.
MMICS are used in pairs with a
simple coaxial power divider / combiner built on a 137 * 50 mm printed circuit
board.