Would you like an antenna analyzer and don't have ten thousand to buy it? Do you have handy hands and want to make something useful yourself? On the South Coast Amateur Radio Club Inc. website www.scarc.org.au you will find a description of the antenna analyzer VK5JST. The connection is a combination of RF circuits and a single-chip microprocessor. SCARC offers interested parties a complete kit containing a PCB, components, display module, battery holder, case, buttons, and front panel. The price including postage is 150 Australian dollars, which is about 2,300 crowns (76.34EUR).
The reproducibility of the connection is confirmed by a thousand successfully made analyzers worldwide. You will also find commendations in reviews on www.eham.net/reviews and on the website of the well-known designer PA0FRI http://www.xs4all.nl/~pa0fri.
Features of VK5JST antenna analyzer
The display shows battery voltage, frequency, real component of impedance, imaginary component of impedance, and PSV in the range of approximately 1.3 to 31MHz. With a small tweak, it is also possible to determine the nature of the load (inductive, capacitive). Other source codes are available allowing measurement of capacitors, inductors, or displaying impedance in a different form.
The antenna analyzer interested me so much that I ordered and assembled one kit.
Making
Nine days after placing the order, the shipment from Australia arrived. Inside the package, you will find accurately sorted and packaged components, circuit board, mechanical parts, display module, case, and manual. It is worth reading it before construction and marking important passages. It will significantly facilitate the construction, as there are many components, and orientation on the circuit board is not easy.
Sort the components into groups - resistors separately, capacitors separately, etc. It is advantageous to pre-mark holes on the panel according to the unpopulated circuit board. It is advantageous to solder the interconnections first and then all resistors. I recommend marking the installed components on the diagram with a pencil. Next are capacitors, then transistors and IC sockets. Lastly, we will install the range switch and tuning capacitor. Do not connect the display module, fine-tuning, and integrated circuits yet.
When I saw how the number of components steadily increased, I doubted that such a complex VF circuit could work on the first power-up. Despite my concerns, after connecting the power supply, the ammeter showed a consumption of about 70mA and correct DC voltages at the measuring points. We will disconnect the power supply and install operational amplifiers and a 50-ohm load (e.g., by combining two 100-ohm resistors). After reconnecting, we can check the function of the measuring circuit and perform pre-calibration. This part of the construction took me about 6 hours of pure work.
Now we will drill all the holes for the controls, display, and connectors. The hole for the display deserves the most attention because it must be correctly positioned relative to the circuit board. We will glue or screw the battery holders into the case. The mechanical work took about 2 hours.
The remaining integrated circuits need to be soldered and the circuit board connected to the display module. It requires patience and a steady hand. After the connection, we can connect the power supply, and if everything is okay, the display will first show the voltage value and then the frequency, R, j, and PSV. Fantastic, it works!
The kit is not only excellently designed electrically but also mechanically, and if we follow the recommended procedure, the antenna analyzer will be ready in another 2 hours.
Calibration is a matter of a few minutes and only requires a good 50-ohm load and precision voltmeterMost radio amateurs will want to connect a coaxial cable with a connector to the analyzer, it remains to make a reduction from the analyzer terminals to the desired connector. And we can measure!
Measured results of Diamond W-8010 in the SSB part of the 80m band
3.60MHz 11ohm +/- j6ohm
3.62MHz 13ohm +/- j9ohm
3.64MHz 15ohm +/- j13ohm
3.66MHz 19ohm +/- j18ohm and 2.9PSV
3.68MHz 29ohm +/- j22ohm and 2.2PSV
3.70MHz 37ohm +/- j21ohm and 1.8PSV
3.72MHz 49ohm +/- j0ohm and 1.0PSV
3.74MHz 47ohm +/- j8ohm and 1.2PSV
3.76MHz 32ohm +/- j18ohm and 1.8PSV
3.78MHz 19ohm +/- j14ohm and 2.8PSV
3.80MHz 15ohm +/- j11ohm and 3.5PSV
As can be seen, the antenna is indeed significantly narrowband due to its small length in the 80m band. However, the analyzer confirmed correspondence between the resonant frequency, which is 3.72MHz (zero imaginary component) and the ideal impedance, which is an important assumption for the effective radiation of the antenna.
In addition to antennas, the antenna analyzer can be used to measure matching circuits, tune input and output circuits of amplifiers, and determine the shortening factor of coaxial cable.
Evaluation
The antenna analyzer is an excellent addition for radio amateurs - technicians. For them, the kit is appropriately complex. Certainly, the analyzer should not be completed by a beginner or technician without reading the construction procedure.
The antenna analyzer VK5JST will bring fun to building and using - I recommend it!