1 Watt QRP Power Transmitter

 

The 1 watt 20 meter QRP transmitter with VXO. This is a nice QRP transmitter that can be used in combination of one of the simple receivers. Normally these designs have only two transistors: one is the X-tal oscillator and the second the final amplifier. A good example is my first QRP rig that is also described somewhere on this site. Here the VXO (Variabele X-tal Oscillator) has a tuning range of 16 kHz. This VXO is buffered with an extra driver stage for a better frequency stability and a varicap diode is used instead of a variabele capacitor. An extra transistor is added for keying the transmitter with a low keying current. What you can do with such a simple 1 watt QRP power transmitter. This is a real low power transmitter, so do not expect that you can do everything with it but... When conditions are normal, you can easily make many QSO's during one afternoon with stations with distances upto 2000 km with a simple inverted V wire dipole antenna! From Europe, I did even make QSO's across the Ocean!

SMALL SINGLE TURN MAGNETIC LOOP

 

The small single turn magnetic loop (SSTML) antenna consists of a single winding inductor, about 3 feet (1 meter) in diameter, and a tuning capacitor. A second loop, which is one fifth of the diameter of the large loop, is connected to the feedline and this small loop is positioned in the large loop on the opposite side of the tuning capacitor.

The SSTML has some very interesting properties:

a) It has a small footprint. The loop I describe here looks like a circle in the vertical plane and is just a little over 3 feet (1 meter) in diameter.

b)It is a rather quiet antenna. It doesn’t pick up as much man-made noise from nearby sources as a wire antenna would in the same situation.

c) This antenna is somewhat directional, which can benefit you in two ways. You can either aim (rotate) the antenna for maximum signal strength, or for minimum noise pickup. I prefer to do the latter, and here’s why. This antenna has what is called a deep null on each side of the antenna, the broad sides, meaning that signals coming from that direction will be attenuated quite a bit (30 dB is an often-quoted figure). However, this is mostly true for signals we receive directly, like noise sources, and not so much for signals from broadcast stations coming to us through skywave propagation. I aim the antenna for minimum noise pickup, which results in the best signal to noise ratio. In some situations it is quite possible to fully tune out a noise source such as a TV or computer monitor.

d) Since this antenna is really a tuned circuit, it also acts as a preselector. It only receives well in a narrow bandwidth of a few hundred kilohertz (kHz). The antenna requires retuning if you change the frequency on the radio by a hundred to two hundred kHz. This may sound like a disadvantage, but if you have ever tried a long wire antenna on a rather sensitive receiver, you probably have noticed that your receiver may get overloaded, resulting in hearing multiple stations at once or hearing broadcast stations on frequencies where there really aren’t any. This may make it impossible for you to pull in that DX station you’re really interested in or even make listening to a strong broadcast station rather unpleasant. This antenna will help prevent overloading your receiver.

27MHz TRANSMITTER with Crystal

Fig 1 shows a simple 27MHz transmitter producing a carrier.

(receiver for this circuit HERE)

The 27MHz transmitter PC board

Here is the circuit made by Lucian Papadopol  iz6nnh@gmail.com
He has created capacitor values by paralleling two values.

This means it produces an unmodulated 27MHz signal and when picked up by a receiver, such as shown in fig 2, the result is a clean, noise-free reception. To increase the output of the transmitter, the 390R resistor is replaced by a 220R. This increases the current from 7mA to 12mA. The resistor could be decreased to 150R for more output.

Wideband RF Field Strength Meter

 

Field strength meter is extremely useful when working with RF devices. It can be used to quickly diagnose whether a transmitter circuit is working, and can be used to detect RF signals in the environment. The simplest field strength meter could be built with a tuned LC circuit and a germanium diode, just like the way of a building a crystal radio except replacing the ear piece with a high sensitivity current meter. While this approach fits the needs of most simple applications, it has a pretty narrow frequency range (~100 MHz) and requires tuning the LC circuit to the correct frequency before measurements can be made and the design can become complicated if wider frequency range tuning is desired.

Another option is to use an RF detection chip. Most of such chips (from Linear Technologies, Maxim and Analog Devices) offer a very broad testing range and have much higher sensitivity and accuracy than a simple diode signal detector can offer. Here I will use Linear Technologies’ LT5534 RF detector chip as the field strength meter’s front end. Similar circuits can be build with other RF detection chips as well, depending on the types of the specific application.

LT5534 can detect RF signal from 50MHz all the way up to 3GHz, which covers most of the spectrum one typically uses. If your frequency spectrum is significantly different, you may check out the other RF detection chips the above mentioned companies offer.

The core detector circuit is almost identical to the reference design. The LM324 op-amp forms a differential amplifier with a gain of 2. The main purpose of this differential amplifier is to provide the ability to “zero” the meter reading or adjust the sensitivity of the detector. Since the differential op-amp’s output is proportional to the voltage difference between the output of LT5534 and the wiper voltage of the potential meter, we can adjust the potential meter to set the reference point (i.e. zero reading) for the environment. Also, by raising the wiper’s potential, it would take a higher output from the RF detector for the differential op-amp to register an input voltage and thus effectively lowered the sensitivity of the detector.
The output bandwidth of LT5534 is tens of MHz, since we do not care about the signal details in this particular application, the relatively low bandwidth LM324 has no impact on performance. The above circuit uses a 5V regulated power supply.