OFF-CENTER FED ANTENNAS AND/OR MULTIBAND ANTENNAS G5RV vs. Windom

This was the start of what became radio propagation. What is NVIS?   The many purposes for NVIS propagation includes military communications and emergency communi NVIS stands for Near out to about 200 to 300 miles. - - Vertical Incidence Skywave A Little Bit Of History In 1946 by Louis Varney, whose call sign was G5RV9, was looking for a better version of the standard dipole antenna for 20 meters. His experiments became the G5RV antenna. This was the start of what became the OCF or off-center fed antenna  cations (EMCOMM). NVIS is used for short range communications, that is

After some experimenting, Varney had a dipole that worked okay on 20 meters but the bandwidth was not as wide as expected, this was the standard half-wave dipole. With some additional research he became aware of the Velocity Factor! Keep in mind that he was not the first person to find this out.

A REVIEW OF THE VELOCITY FACTOR If we divide 492 by the frequency in MHz we get the length of the standard half-wave dipole antenna. Check your Technician, General and Extra Class license manuals. However, that length is correct if your antenna is in the (almost) perfect vacuum of deep space. Down here on earth, because of the atmosphere, the ionosphere, the Van Allan belt, radiation from the Sun and a slew of other factors, the actual length is somewhat shorter and we call it the effect of the Velocity Factor.

All of those obstructions slow down the speed of our radio waves by 5 percent (or parts there of). So if we get 5% of 492 and subtract that from 492, we come up with 468 as the magic number here on Earth. It’s a little easier if you multiply 492 by 95% to get the same answer. Back to Mr. Varney. He proved (to himself) that without a doubt 468 divided by frequency in MHz was better for actual TOTAL antenna length.

A PROBLEM ARISES! Mr. Varney determined (again, for himself) that the dipole antenna had an impedance of about 73 ohms. Sure enough his 20 meter antenna worked good at a specific frequency but without much bandwidth. This was because of electrical currents at the antenna and feed line junction points. The only practical feed line available in 1946 was “build your own” ladder line. The ladder line is used to get a matching impedance at the load point of the dipole antenna. But just an arbitrary length of ladder line creates a high SWR. BACK TO THE DRAWING BOARD!

PROBLEM RESOLUTION The center of the 20 meter band is 14.175 MHz. Using 492 / 14.175 we have a length of 34.71 feet If we make our ladder line a half-wave in length and use 97% instead of 95% for the Velocity Factor (because we are dealing with ladder line) we get 33.67 feet and the SWR problem goes away – unless we change our transmitted frequency. It turns out that if we use an Antenna Tuner we resolve the bandwidth and the SWR problem at the antenna feed point.

A DISCOVERY Using an Antenna Tuner it was discovered that the antenna could be “loaded” up on other ham bands. However, for many Hams, it can be somewhat difficult to get an acceptable SWR. If the antenna is designed for 20 meters, the antenna works okay on 80 thru 30. It is excellent on 20 meters and okay thru 10 meters. Keep in mind – the G5RV was not designed to be an all band antenna. The antenna is a CENTER FED dipole. Add coax and a 1:1 current balun

SO………………………….. 1.We find that the G5RV is NOT designed to be an all band antenna. 2.We find that the G5RV is not a true multi-band antenna. 3. It is however, a true power-house on 20 meters without an antenna tuner. 4. We can probably refer to it as a cousin of the standard dipole.

ONWARD AND UPWARD

Now we end up with The short end at 45 feet & 7 inches with 6 inches stripped off each end The long end at 90 feet & 2 inches with 6 inches stripped off each end Here is our almost final configuration 45 feet & 7 inches 90 feet & 2 inches All we need now is a 4:1 balun BECAUSE at the junction of the two wires the antenna impedance is very close to 200 ohms. A 4:1 balun is easily feed with 50 ohm coax.

TIME FOR THE FINAL COMPARISON Overall, the G5RV and the Windom Off-Center Fed Antenna both function as Multi-band antenna’s. The G5RV works very well at 20 meters but, it is not too happy at other frequency’s even with an antenna tuner even though it had a narrow band width. Remember, Louis Varney was trying to design a really good 20 meter antenna. It just so happened that it could be used on a few other bands. The OCF dipole was designed specifically for multi-band operation. It operates quite well on 180 thru 6 meters, however, it doesn’t like 30 and 60 meters very well. It has a wider band width than the G5RV on most bands. It really shines with a good antenna tuner.