SWR doesn’t give you the whole story you need an RF current meter.
บทความโดย Paul Danzer, N1II (จากหนังสือ QST ฉบับ September 2009)
In a recent QST article, Eric Nichols, KL7AJ, presented a good idea monitoring the RF current into your antenna system to insure optimum tuning. Actually, it was the second time it was mentioned to me — George Peters, K1EHW, suggested the same thing to me several months before. At the end of Eric’s article, he proposed using a current transformer to do the monitoring.
Making a Current Transformer
This could be as simple as a turn of wire through a ferrite core and several turns of
wire around the core to form a transformer. The output of the transformer would be proportional to the current through the wire.
Making it Happen
This seemed simple enough, and the result of one hour’s work is shown in Figure 1. The core used was a T37-6. T37 designates the size (0.37 inches OD), picked so the insulated center conductor of RG-58 (or RG-59) would fit comfortably through the core center. The 6 designation relates to the frequency application of the ferrite mix, in this case 2 to 50 MHz.
Searching the ARRL suppliers’ data base, it appears that Alstar Magnetics offers this core; an alternate would be a Palomar F37 with mix 61. There is no criticality here — if you want to try it, strip a core from any old source — perhaps from a junked PC power supply or computer cable. It may not be the most efficient RF transformer ever built, but if it works it will do the job.
Wrap 20 turns of 24 gauge enameled wire as the transformer secondary. The secondary is connected to half wave rectifier consisting of a silicon diode (1N914), a 10 kΩ resistor as the load and a 0.1 μF capacitor as a filter (see Figure 2). A high-impedance voltmeter (the $10 variety) is connected to the two pin jacks to serve as an indicator.
The unit shown was tested with a 100 W transmitter on all bands from 80 through 10 meters. Performance across each band was relatively uniform, considering the probable variation of SWR and power though the feed line as the frequency was varied.
Hook up the rest of the circuit as shown in Figure 2 and connect a meter to the terminals.
Figure 1 — View of the transformer and simple circuitry that make up the relative RF current meter.
รูปที่ 1 โครงสร้างประกอบด้วยหม้อแปลงและวงจรแบบง่าย ๆ ลักษณะเดียวกับวงจรวัดกระแส RF
What we Have
The object here was not to get an exact measure of the antenna current. What I wanted
was a relative measure, so I could see if anything was going wrong, or use it as a way to
adjust my antenna tuner for the maximum signal to the antenna. Commercial stations use a current meter, mounted at the connection of the feed line to the antenna, to monitor output. Since they know the antenna impedance and their meters are calibrated, they can determine precise power into the antenna.
If you enjoy low power (QRP, typically 5 W or less) or very low power (QRPP, less than 1 W) operation, more turns may be needed and can easily be added. Similarly, if your meter does not have enough sensitivity, more turns may be called for. If after assembly the core is not firmly in place, held by the friction of the secondary on the primary wire, a drop of glue can be used to secure it all together.
Putting it to Use
The current meter consumes a miniscule fraction of the output power, so, can be left in the line, or removed when not in use. You may even find a meter case and a surplus meter at a hamfest that will work with it to give continuous indication without tying up your bench meter. I suggest writing the relative current indication for each band in your log. Later, if something seems amiss, it is then an easy job to compare your readings to the recorded ones to find out if the problem is in your antenna system.
Figure 2 — Schematic diagram of the relative RF current meter. Nothing about the circuit is critical. See text for parts information.
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