(Originally written in May 2012)
So this project came out of necessity at my station. I had lost the 4 detector diodes in my MFJ-259B and I was trying to decide if I wanted to replace them myself, which is difficult or send the unit back to MFJ and wait two months. Since the MFJ analyser was a single point of failure in my 630-meter station’s standard operating procedure for system tune up, I needed a quick fix.
The Scopematch has been seen through the years in various forms. The version I will discuss today was developed by Jim Moritz, M0BMU (http://myweb.tiscali.co.uk/wgtaylor/LFTA.pdf) and was originally intended for use at MF and LF but there is really no reason that they could not work at higher frequencies.
The components are fairly straightforward, requiring only a two-channel oscilloscope, a hand-wound toroidal transformer, a resistor and some silver mica transmitting capacitors. Some connectors and an enclosure round out the system. The system works by sampling voltage and current of a signal on a coax and then displaying both simultaneously and in real time on the scope. At resonance, voltage and current are in phase and have equal absolute magnitudes (opposite signs on the magnitudes, in practice, however).
Picture (a) represents the basic simple layout. Many operators literally use this approach when trying to troubleshoot a problem. (b) is another options but the voltage sense transformer on the right side must be wound more carefully as it sees the full voltage on the transmission line. The windings must not breakdown and the transformer must provide enough choking inductance to keep RF from going directly to ground. I don’t like (b), if you can tell. (c) is the best option for a permanent option. The capacitors in the voltage sense are silver mica transmitting types of 250-500v. In my opinion, higher voltage handling on these is better and both Tanners and Mouser stock them.
As I have mentioned in the past, I hate dealing with enclosures for projects. Some guys love the challenge of creating nice looking front-ends but I cringe at the thought of having to do the metal work and for that matter, enclosures are generally the most expensive part of any project if you are using “true”, professionally built enclosures. I was in luck, however. In an unused receive chain, I had an ICE broadcast band filter that I used ahead of a preamp on the main low band position in my station. As I am using band pass filters that are switchable, the broadcast band filter is not being used so I carefully opened the box up and clipped out the circuit, to be returned to an altoids tin at some point, and used this really nice and well-designed box.
The next step is to wind the secondary of the transformer. I had number of cores that were unused and picked one that I thought would offer enough impedance in the region that I was operating. For HF, a Type 43 is probably fine. I am using this for MF and LF and I think this one is a T106-2. The core is wound with 50 turns of enameled wire, each pass through the center of the core equalling a turn.
Next, the primary is prepared from a piece of coax. The coax literally bridges the SO-239 connectors at each end of the box. Remember that we are not modifying the signal; we are simply sampling it so there is very little loss incurred.
The core is placed over the coax and the ends are soldered onto the connectors in the box. The braid of the coax is connected to ground.
The resistive network on the secondary of the transformer causes a voltage drop. At 50 ohms (3-150 ohm 2-watt resistors in parallel), 1 volt generated equals 1 amp of RF current on the coax. I chose to use 6-watts worth of resistors for a lot of safety factor. In reality, there is probably only a fraction of a watt being dissipated in the resistors. Since resistance changes with temperature, I did not want to see variations in the waveform on the scope. The capacitors in the voltage network are similarly installed.
Plugging in to the scope channels, we see both waveforms here. Note that they are not pure sine waves as expected. This indicates a mismatch; in fact, the elbow on the smaller waveform is the result of an impedance mismatch. Adjusting the variometer for best tuning on 630 meters yields perfect waveforms where the current and voltage are in phase and of equal absolute magnitudes. Voltage is displayed on top and current is on bottom.
So why is this better than an SWR meter? SWR meters only show that there is a reflected voltage, it says nothing about impedance other than it is mismatched. The scopematch does not directly indicate an absolute impedance like an analyser would but it gives you a graphical display to tune up with while providing data that can be used in calculations to determine impedance, voltage on a coax, SWR, etc. Furthermore, simply getting both waveforms to look the same and in phase with each other is usually good enough to ensure that a sufficient match has been achieved.
Taking that last thought a little further, the in phase, equal magnitude waveforms indicate a 50 ohm, resistive load. If the waveform were still in phase but voltage were greater than current, it would indicate resonance with a resistance greater than 50 ohms. Looking at the reverse where current is slightly greater in magnitude than voltage, which is the case in my system, the resistance is slightly lower than 50 ohms. In fact, at the time these pics were taken, the SWR Analyser indicates about 49 ohms.
Sometimes the phase is different which indicates a lack of resonance. When current leads voltage, we say that the system is capacitively reactive and when voltage leads current we say that the system is inductively reactive. REMEMBER – “ELI” the “ICE” man! Its very likely that you can have a combination of R and X to deal with when tuning the system.
So here is another tool for the shack if you own a scope and don’t mind analysing data to draw conclusions about you system. My scopematch stays in line all the time and I use it to periodically monitor as wall as during the checkout process. While it has not replaced the analyzer in my station’s SOP, it does enhance the capabilities by allowing me to look at the signal on the coax which, along with information about antenna base current and PA voltage and current, I can draw some reasonable conclusions about the performance of my system at any given time.
By the way, I opted to order parts from MFJ to fix the analyzer. It was four high pitch surface mount schottky diodes that are impossible to get to with out taking the entire unit apart. It only cost me $7 shipped and while it was a headache to do the repair for a number of reasons, it was worth it. If you find that your analyzer stops working for one reason or another and you have not directly transmitted into it, lets talk about how you might fix it. As bad as it is, it’s worth it.