Especially in the technical world, it is extremely important to pay attention to the meanings of words. If you think a certain word or phrase is the one you want to use, maybe you should check the technical definition first. If you use a word in a way that does not suit it’s meaning, then at best you will confuse the topic, and you may lead your readers far astray.

Here is an example of what I am writing about. I recently realized that I needed a wattmeter for my Amateur Radio station. I wanted a fairly inexpensive meter, but I also wanted a dual meter, with two needles on one meter face, so I can read forward power, reflected power, and standing wave ratio (SWR) at the point where the two needles cross.This can be much more convenient than having two separate meter faces to look at, to read forward and reflected power. That type of meter also does not generally have a simple way to read the system SWR. I also wanted a meter that included the ability to read at least 1500 watts.

After searching several equipment dealer’s websites, I found several meters that met most of my criteria. The Daiwa CN-801 HP meter seemed like the best match, at a price of about $160. I searched the ARRL website for Product Reviews, and read a couple of Daiwa wattmeter reviews. I didn’t find a review for the CN-801, but I wanted to see how other Daiwa wattmeters did in the Product Review process. In the February 1991 issue of QST,  I read the review for the Daiwa NS-660PA wattmeter. The reviewer noted “one humorous gaffe in which actual forward power is referred to as “effective radiated power” in the instruction sheet.

Well, that may be a curious gaffe, and we might assume that Amateur Radio operators who have purchased this meter would understand the difference between the forward power measurement and effective radiated power. I don’t believe that makes it okay, however. In fact, some, especially newer Amateur Radio operators, may be completely deceived by this technical error in the language used to describe this meter. Since the FCC Radio Regulations in Part 97, governing the Amateur Radio Service, in several places make references to power limits on certain bands in terms of effective radiated power, it is important to be specific about this terminology!

The effective radiated power (ERP) from a radio station takes into account the actual transmitter power and also the gains and losses of the antenna system (including the feed line between the transmitter and the antenna). To imply that a wattmeter can measure ERP is completely wrong! What the Daiwa Japanese translates probably meant was that the meter will display forward power (from the transmitter to the antenna) as measured by the meter’s directional coupler. Power reflected from the antenna back towards the transmitter will be measured by the directional coupler and displayed on the reflected power meter. Now, where some people get confused is that when that reflected power reaches the transmitter output circuitry, it will be reflected again, this time back toward the antenna. This re-reflected power will now be read by the directional coupler as forward power, and thus it is added to the actual transmitter output power. That means the forward power reading will be higher than the actual transmitter power.

Let’s imagine that our transmitter is putting out 100 watts of power, and that our antenna system is reflecting 10 watts back to the transmitter. When this 10 watts of power is re-reflected from the transmitter and is added to the original 100 watts from the transmitter,  the forward power meter will now indicate not 100 watts, but 110 watts. The reflected power meter will indicate 10 watts. So the forward power reading can be a bit misleading if we don’t take into account that additional reflected power. We can guess that the Japanese translators wanted to indicate that the forward power meter will indicate the total amount of power going through the directional coupler from the transmitter towards the antenna, so they called it “effective radiated power.” That is a very poor choice of words to describe this situation. I have no idea what the Japanese Radio Regulations might use to describe what the US regulations call ERP, but I will guess it is something very similar. A technical definition is a technical definition in any language.

You might be wondering how far this reflection and re-reflection process needs to be taken. Well, it turns out that one reflection and re-reflection is probably enough. Consider that additional 10 watts getting to our antenna. Of that, 10% or 1 watt will be reflected from the antenna again, so instead of 10 watts, we have 11 watts after two reflections. Then instead of 110 watts, we have 111 watts coming through the directional coupler towards the antenna. The next time it will be 11.1 watts and 111.1 watts, and so on. We really don’t need to consider more than one reflection in this process.

Oh, and in case you are wondering, the instruction sheet for the Daiwa CN-801 wattmeter, still has a heading for “Effective Radiated Power & SWR!” I really would have expected someone to correct this error in the ensuing 25 years!