100ohm power resistors for use in Wilkinson power splitters are often available at low cost. A pair of these resistors will make an excellent dummy load. I recently acquired a power film resistor which has four 50ohm resistors mounted on a large copper heatsink flange. As all of the resistors are connected to the mounting flange at one end, it was not possible to connect the resistors in a series/parallel combination to make 50ohm. The solution was to connect the four resistors in parallel to make a 12.5ohm load and use a 4:1 transmission line transformer to provide a 50ohm match. The transformer consists of two turns of RG58 on a Maplin N90AB ferrite core. This gives a perfect 1:1 SWR from 1.8 - 30MHz. The finished circuit is in Figure 6. Note that the DC voltage from the diode is only half the peak-input voltage because of the 2:1 voltage ratio of the transformer. I haven't been able to make much of an impression on this load during testing. Several minutes of SSB speech at 500W PEP will bring it from room temperature to lukewarm. I'm sure my transmitter would catch fire long before this load gets hot!
Sunday, June 29, 2014
Saturday, June 28, 2014
5W 14MHz (20m) AM Transmitter
This is a simple audio transmitter for the 14MHz (20m) band. Its principle is seen from the diagram, so I will describe very briefly: The first stage is a simplified oscillator, the other three stages are amplifiers. The coils are wound on the 4mm diameter core with screw ferrite core to fine tune to the highest brightness of the light bulb. As a modulation choke, a secondary of small transformers (eg 220V/9V) is used. The tuning capacitor is also tuned for the highest brightness of the bulb. Then you can replace the bulb by a short. When you connect other antennas However, it is necessary to re-tune the output. Never operate without antenna!! Current consumption of the transmitter from the 12V supply during the normal operation of about 800mA. The coils have 30, 25 and 12 turns.
5W 14MHz (20m) AM Transmitter schematic
1W 7MHz (40m) AM Transmitter
This is a simple transmitter for amplitude modulation (AM) in the short wave band. It contains three transistors. The first stage as a quartz oscillator and can operate from about 4.5 MHz up to 12MHz. The second stage of the amplifier. For simplicity there is the resistor connected to collector and not coil. The third transistor is traditionally connected with the coil and forms the output stage. Using a DC-blocking capacitor it is connected to an antenna. For a simple start-up and high-quality sound it is modulated to the last stage. Sensitivity is therefore small, and the audio amplifier is required at the audio input. The AM modulation is built using the modulation inductor, which must have enough inductance. You can also use a secondary of small transformer. I used the secondary of the 220V/9V 1.6 VA transformer. Double choke in the oscillator has 2x 6 turns of wire 0.5 mm on the ferrite rod, 5mm diameter. Choke of output stage is without core and has 24 turns coil wire diameter of 0.5 mm wound on 10 mm diameter.
The circuit is powered by a well-filtered 12V voltage, current drawn is about 300mA. If you need to destroy the output power transistor, simplest way is to switch the transmitter on without the antenna connected :).
