คำถามจาก Mark เมื่อต่อระบบดังรูปทำไมเวลาปรับ ATU แล้วอ่านค่า SWR จากภายในตัววิทยุ และ SWR ภายนอก ได้ค่าไม่เท่ากัน
Mark, KG4UDL, asks: I run my 5 W HF transceiver through an external antenna tuner and a separate SWR and power meter as shown in Figure 3. The transceiver also has an internal SWR meter. Why is it that I can adjust the tuner for minimum SWR on the radio’s internal meter and read an SWR of 3:1 or more on the external meter? Also, when I obtain this reading I am showing a full 5 W output. I thought that high SWR and high power to the antenna were mutually exclusive — that high SWR indicates low antenna efficiency. I have had great success with the system, by the way, including multiple European contacts during the recent ARRL International DX Contest.
ตอบ To answer your more general question — SWR is just a reflection (pun intended) of the impedance match at the interfaces between the two systems where your measuring device is located. It really has nothing to do with antenna efficiency, and an antenna with a high SWR can radiate very well as long as you can couple power into it and you have low loss in the transmission line.
Many transceivers fold back or reduce power if the SWR is higher than a certain level, often 2:1. This is done to prevent excessive voltages or currents from damaging transmitter components. This is often a major issue — so even if the antenna could work fine, if the transmitter reduces power, your signal will be weaker.
The concept of reflected power is a mathematical way of considering the standing waves on a transmission line. If you have a 1:1 SWR and put 5 W into the line, you will measure 5 W forward power and 0 W reflected power. If you have a 3:1 SWR (in the usual 50 Ω system, that could be a 16.6 or 150 Ω resistive load, or an infinite combination of reactive loads, for example), you will have a reflected power of 25% of your forward power. If your transmitter could put out a full output into a 3:1 SWR, you would see a forward power of 6.67 W and a reflected power (at 25%) of 1.67 W. The net actual power going to your antenna would be the difference between the forward and reflected, 6.67 – 1.67 = 5 W.
Your wattmeter is in the right spot to read the same as the radio’s SWR meter — unless the coax between the two is not of 50 Ω (75 Ω RG-6 or RG-59 are two examples). I have found that many patch cables purchased at hamfests, for example, are not really 50 Ω. If other than 50 Ω, there will be an impedance transformation in them so that the impedance at each end (and thus the SWR) will be different.
Note that this really doesn’t cause any harm or significant loss — it just confuses the measurement process. The difference will be a function of electrical length and would be most significant with a cable that is a quarter wave long, around 6 feet on 10 meters, for example. If so if the load at one end is 50 Ω, the other end would see 100 Ω and vice versa. If this were the situation between your external meter and your radio, a 1:1 SWR at the external meter would result in a 2:1 reading at your radio.
One way to find out what’s happening is to put a good 50 Ω dummy load right at the transmitter and see what SWR it reads. Then move it to the end of the patch cord and repeat. If the cable is 50 Ω, the reading should be the same.
Another possibility is that the SWR/wattmeter is providing an erroneous reading. This is not uncommon, although most show 1:1 when matched. The readings away from being matched are not always as accurate. See any product review on wattmeters in QST. There was one in March 2009, for example.
If in doubt, tune with the meter in the radio. The SWR, as measured by that meter, is what determines the amount that the transmitter will “fold back” and reduce power. Thus, that’s the important place — if the transmitter ain’t happy, ain’t nobody happy! to borrow a phrase.
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