SWR Meters and Radiated Power

As W1SUJ recently demonstrated, a low SWR is not an indication of antenna performance.  Richard got a 1.2:1 SWR on 2 meters with his new antenna, but could just about make the club repeater.  SWR is supposed to tell you if the antenna is matched to the feed line, but the SWR meter is at the transmitter end of the line, so it measures the forward power coming out of the radio, but the reflected power is that that goes from the radio up the transmission line to the antenna, and is reflected back towards the transmitter, and goes down the transmission line again, so if there is loss in the transmission line (and all lines have loss) the reflected power at the SWR meter is way less than the real value.

For example, lets say we have 100′ of RG-8X.  At 144 MHz this has 4 dB loss.  The balun, antenna switches, and connectors may add another 2 dB.  So we have 6 dB loss and therefore only 25% of our power gets to the antenna.  Lets say because the antenna is defective or out of resonance, the SWR, at the antenna, is 10:1.  So the antenna only accepts 18% of the 25%, or 4% of the transmitter power.  The rest of the power is reflected back down the line.  So the line loses 75% of the reflected power.  This means that the SWR meter sees a reflected power of 25% of 82% of 25%, or 4.5% of the original power.  Therefore, the SWR meter sees 100% forward power and 4.5% reflected power and reads 1.1:1.  With this SWR you would expect everything to be great, but using a 25 watt transmitter you would be getting only 1 watt into the antenna.

At the same time, the antenna may receive great, as the SWR (and loss) when receiving is determined by the match between the transmission line and the receiver, and will be entirely different from the transmitting SWR.  So, as Larson E. Rapp (WIOU), would say, “in hock vertis” and build or buy a field strength meter and find out what is really going out…

Chuck Teeters
W4MEW

Bigger is Better

Some of the antenna stuff you hear on the air these days makes you think that 2 plus 2 now equals 5. A few basic facts about radio waves and antennas are therefore needed. If you look around at the radio waves passing by, you will see some strong ones, some weak ones, and some in between. The way you measure a radio wave is in volts, just like a battery, except RF volts. Since the radio wave doesn’t have terminals on the end, we pick some spots on the wave to measure the voltage. The normal measurement is between two points on the wave on meter, about 39 inches, apart. The voltage is not much, so we usually measure millivolts or event microvolts. We have to measure the wave in the plane of polarization, if it’s vertical we measure the vertical meter distance, and if it’s horizontal we measure horizontal. The result is that we get a measurement of so many microvolts or millivolts per meter. For example, if you measured a broadcast signal from a 1000 watt station at one mile, you should get about 150 mV/meter. A TV station at 30 miles should measure about 3 mv/meter. Most ham band signals will measure between 1 and 100 uv/meter, if you are a few miles away.

Now how much signal does your antenna get from that radio wave? It depends upon how long it is. If the signal is 50 uv/meter and your antenna is 10 meters long, in the plane of polarization of the wave, you will get 50 x 10 = 500 microvolts into your receiver. If your antenna is 50 meters long, you will get 2500 uv. So, the longer the antenna, or the stronger the radio wave, the louder the signal. So also, small antennas = small signals and big antennas = big signals, no way around it, so hang up some wire.

Chuck Teeters
W4MEW