Issue link: https://iconnect007.uberflip.com/i/1507822
44 PCB007 MAGAZINE I SEPTEMBER 2023 ple, it's not a coincidence that router anten- nas are only about 4-5" long. at is a full wave at 2.4 GHz (936/f = length of full wave in feet, where f is frequency). At 2.4 GHz, it's actually 4.68". As the frequency increases, the actual antenna length requirement shortens. As the 2.4 GHz band is 2.401–2.484 GHz, the manu- facturers will target a mean resonance at the middle of the band, 2.445 GHz. at is roughly 4.58". You can double check by measuring the length of your router antenna(s). In our router example, these typically trans- mit between 10-25 milliwatts. Matching the transmission line to the antenna is critical so as to radiate as much of the signal as possible. As SWR increases due to mismatch, the ERP of that 10–25 milli- watts is degraded. We start hav- ing loss on the circuit. When we talk about signal gain and loss, we use decibels (dB). When we start reflecting power back, we are losing ERP. Just for refer- ence, a 3 dB loss is half the transmitted power, so, if we have a 3 dB loss when transmitting 10 milliwatts, we are now only effectively radiat- ing 5 milliwatts. at's not good when we are using such low power to begin with. But, you say, my router has 5 GHz capability, too, and your calculation was only for 2.4 GHz. Why use the same antenna? e beauty of radio waves is they can work harmonically, or more specifically, using multiples of themselves. e Wi-Fi 5 GHz band is not just by accident. e 5 GHz band is a close harmonic of the 2.4 GHz band. So, using the same antenna at 1 full wave for 2.4 GHz, they can obtain matches on the 5 GHz band. A second antenna is not needed. e matching will be a little fuzzy regarding SWR but anything less than 1.5:1 is considered optimal while 2:1 is considered acceptable. I'm sure you have noticed those huge tow- ers on the hills rising hundreds of feet in the air. e actual transmission antennas are very small by comparison. Using our antenna calcu- lator, I know of an FM station transmitting here in Oregon on 92.3 MHz on a tower over 1,000 feet in elevation. However, the driven element for a full wave is only 10 feet, 3 inches. When you look at these giant towers, remember the giant footprint is just to elevate the antennas as high above the ground as possible to propagate the transmitted wave as far as possible. Okay, we now understand the significance of matching transmission lines to loads (anten- nae). In the finished circuit board, we have many other variables: ICs, capacitors, induc- tors, and resistors. Again, the transmission lines (traces) running around on the board need to be constant regarding cal- culating responses to sig- nals. Mismatching can cause changes in waveforms that may not be desirable. Here, inductive and capacitive reactance can manipulate fre- quency waveforms, which may be desirable if designed. How- ever, a mismatched transmission line can skew those waveforms, resulting in unde- sirable results. If the circuit becomes more inductive, the voltage waveform will lead the current waveform by 90 degrees. Conversely, when the circuit becomes more capacitive, the current waveform will lead the voltage by 90 degrees. Using these electrical properties, cir- cuits can provide either gain or attenuation. However, this is only successfully achieved by stable transmission lines as a constant. So, now we know why TDR can be very important in circuit design. Matched trans- mission lines provide optimal circuit perfor- mance, while mismatched lines can provide high reflected waves, signal loss, and even transmitting element failure. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns, click here. As the frequency increases, the actual antenna length requirement shortens.