Winnipesaukee Forum - View Single Post - Last VHF question--I promise!

jrc · 05-22-2009, 08:43 AM

Winni,
You pretty much have it right, the antenna must have the right resistance and the right wavelength. But the devil is in the details.

In the radio frequency world, resistance (ohms) is not accurate enough, you need to consider impedance as well (also ohms). You can think of impedance as resistance that changes with frequency. So a 50 ohm antenna at 27 Mhz is likely not 50 ohms at 156 Mhz. As Skip says, your radio is designed to have 50 ohms and with the wrong antenna it won't see 50 ohms.

Second thing is wavelength. The length of radio waves is inversely proportional to frequency. So at 27 Mhz your wavelength should be around 36 feet and at 156 Mhz its closer to 6 feet.

05-22-2009, 08:43 AM	#7
jrc Senior Member Join Date: Apr 2004 Location: NH Posts: 2,689 Thanks: 33 Thanked 439 Times in 249 Posts	Winni, You pretty much have it right, the antenna must have the right resistance and the right wavelength. But the devil is in the details. In the radio frequency world, resistance (ohms) is not accurate enough, you need to consider impedance as well (also ohms). You can think of impedance as resistance that changes with frequency. So a 50 ohm antenna at 27 Mhz is likely not 50 ohms at 156 Mhz. As Skip says, your radio is designed to have 50 ohms and with the wrong antenna it won't see 50 ohms. Second thing is wavelength. The length of radio waves is inversely proportional to frequency. So at 27 Mhz your wavelength should be around 36 feet and at 156 Mhz its closer to 6 feet.