Re: Compact Antenna at 100MHz, Can someone help me to build
Hi Ahmad,
There's no such thing as a good short antenna. When an antenna is too
short the impedance rises and the connected cable will mismatch the
antenna so severely that there will be hardly anything left from the
signal. You don't need an amplifier, a suitable unity gain buffer will
do the trick, and I found one. They're generally used for shortwave
reception but this one has a bandwith of 220 MHz and can be used for
your purpose. The design is actually very straight forward and will
not need more explanation, except for a few details. It's a single
sided pcb and all components should be mounted on the component side,
except T1,T2,the bnc connector and the unspecified connector for the
whip antenna. There were holes in the board to fit the transistors
and I cut away a part of the pcb to fit the bnc connector.
L1 and L2 should have a value of 4 mH for reception of the lower part
of the frequency spectrum and 470 µH for the reception of VHF signals.
The dc resistance of these inductors should not exceed 10 ohms.
They also advise you to use a short whip, something between 30 and 50cm
long, they used a spoke from a bicycle wheel with a lenght of 40 cm.
Should the whip pick up any hum from the line voltage, you could cure
this by connecting the whip to the amplifier via a 10 pF capacitor.
The entire amplifier should be mounted in a shielded box.
The voltage at the emitter of T3 should be between 6 and 8 volts,
correct this with P1 and P2. For reception of shortwave signals during
the daytime, when reception is poor, Ug2s should be between 3 and 6
volts. Correct this with P1, the lowest voltage at which a weak
station still can be heard will be the best choice. For reception of
shortwave signals during the night, when the signals are the strongest,
Ug2s should be around 6 volts. Correct this with P1 when receiving a
strong signal and adjust for minimum distortion. They don't say anything
about vhf reception. I translated the most important details of the
article in case you couldn't read Dutch.
About the charts; they were scanned from photocopies that were made
from reprints from "The Electronic Engineer, januari 1968."
on1aag.