Homebrew Antenna analyseR Project
It’s been a long time since I’ve posted anything new here. Development work is always difficult to do in between a day job. There have been several projects that I have been working on for a long time and this is one which I started off eons ago and never got round to testing and publishing it.
First a few details; there are of course several commercial antenna analysers and all doing a pretty good job of it as we can see from the price tag. There are also several good experimental designs and choosing one over the other depends on several factors. For me, one of the key factors is the simplicity and availability of components as also the simplicity of the code engine that drives the calculations.
Having already familiarised myself with Microchip devices and having access to good technical inputs always helps. The next is the core frequency generator and here too ready code and design from an early project came in handy (See D-SPiRiT from an earlier post). I decided to split the Digital and Analogue sections into the following areas: Motherboard comprising of the digital sections (PIC18F26K22) and the DDS module [Picture 1]. 2 daughter boards comprising the RF amplifier [Picture 2] and the Detector [Picture 3] section.
DDS: The module does seem to run warm(hot) and maybe it is best to run the module from a 3v3 supply and add 1K resistors on the 3 signal lines between the PIC and the module. I have not done this in the current design, but maybe I will shortly and report back (don’t hold you breath)!!
RF Amplifier: Built straight out of the circuit, with an introduction of a preset(not shown) after Q3 to vary the signal level (I’m not sure about what it does to the impedance/waveform but I thought I would give it a try.
Detector/Instrumentation Amplifier: I chose to use LM358 instead of theLM324, since I needed 6 opamps and the LM324 would have given 4 or 8 and added a wiring complexity that I did not need.
Note: Transformer T1 is ferrite ring 7mm in diameter and permeability 1000-2000, bifilar – 8 turns
I wrote a small test program to set the DDS at 14MHz, connected a dummy load and calibrated the analyser by setting the Vin to 1011, V50 to 505, and VL to505. The SWR at this point is 1.0. I then decided to test it further by varying the DDS from 13MHz to 15MHz and displaying the raw values of Vin, V50 and VL on Line 4 of the display and the results on line 1 and 2. You can see the changes in the video.
I’m not sure if the parameters (SWR, R and X of the dummy load should change so widely across the range from 13 – 15 MHz, maybe it does, a resistance test of the dummy load showed it to be 83 ohms.
Check out the video here: HARP
It is best to use good quality trimpots to calibrate the meter at the band of interest. Now to make a PCB and test it further.