I used the IFR 1600 and the return loss bridge to run a sweep of the 6 Meter beams I recently built, and here are the results. On a 10 MHz wide sweep, the peak resonant frequency shows up at 51.2739 MHz. At that frequency the analyzer is seeing -83.7 dBm coming back through the RLB. Since our reference with no load on the RLB was -40 dBm, our actual return loss is 43.7 dB, which equates to an SWR of 1.015:1. Nice. Now let’s look at the sweep at 1 MHz bandwidth.
At 1 MHz bandwidth (100 KHz/div) we can narrow down the point of peak resonance even more closely. At 51.2603 we’re getting -85 dBm, which is 45 dB of return loss compared to our reference of -40 dBm. This is just a tiny bit better than 1.015:1, being closer to 1.01:1 (which would be a 46.064 dB return loss, to be exact, so we have to interpolate a bit.)
So what is the actual useful bandwidth for this antenna. If we’re going for 1.5:1 SWR or better without requiring any sort of tuner we need to look for a return loss of 13.979 dB or more. So, let’s round that to 14 db. 14 dB down from our 40 dBm reference is -54 dBm. Looking back at our 10 MHz sweep (figure 1), each horizontal division is 1 MHz. According to the graph of the sweep, our return loss drops to that level or better at about 48.5 MHz, and remains below that level up to about 51.8 MHz, yielding a usable bandwidth of 3.3 MHz. If we were looking for a 2:1 SWR or better (9.542 dBreturn loss or more), the sweep in figure 1 tells us that we can expect that from about 47.7 MHz to 52.7, a usable range of 5 MHz. That more than covers the lower end of the 6 Meter Amateur Radio band up to almost the upper edge of the FM Simplex area of 6 Meters, which is agreed to be at 53 MHz.
In summary, this homebrewed set of 6 Meter, 3 Element beams is performing as desired. I can work the entire lower end of the 6 Meter band (the SSB portion) without an antenna tuner and have an acceptable SWR of 1.5 or better. The recent QSO with a station in Ecuador using the Icom IC-7200 straight to the antenna (with the antenna only up at about seven feet on a tripod out in the yard) confirmed that pretty convincingly in my estimation. 🙂
~ 73 from KK4ICE