Tech Net Notes January 29, 2010

Loop Antennas

nb5b, Dave, (I believe it was) mentioned that he was thinking about putting up a loop antenna and suggested Loop Antennas as a topic several weeks ago. Having been intrigued with the loop antenna concept for some time, I was very interested in pursuing that topic when I had a chance. That chance came with the January 29th, 2010 Tech Net.

I received the following information from: http://www.bloomington.in.us/~wh2t/loop.html

From WH2T - Dr. Ace's Full Wave Loop Antenna Info

"I have personally used a 160 meter band Horizontal Full Wave Loop antenna with very good success. I worked all 50 states and several countries with 100 Watts using the antenna on the 160 meter band. With a good antenna tuner, the antenna will work 6 thru 160 meters. I currently use a 75 meter Full Wave Delta Loop on the 6 thru 75 meter bands with a tuner. I am very pleased with it."

He goes on to mention that on horizontal polarization he gets about 2.1 dB gain.
He claims the loop is:
Much quieter than a dipole or vertical
Has a broader bandwidth
Usually outperforms a dipole antenna

He feeds his with 75 ohm coax but says 50 ohm can be used.

Dimensions: The formula is 1005/Freq in Mhz = Length in feet
160 Meters at 1.9 Mhz = 529 ft.
75 Meters at 3.85 = 261 ft.

Trimming may be necessary to obtain a low VSWR.

The impedance of a Full Wave Loop is theoretically in the vicinity of 100 ohms.

Connect the center conductor of the coax to one end of the wire and the shield to the other end. Be certain to seal the shield end to protect against water. \

Any shape will work - square, octagon, pentagon, triangle (delta loop), or circle. "The larger the area or aperture inside the loop, the better. A circle has the largest area but is impractical. If you use a triangle shape try to make each leg an equal length as this gives the largest inside aperture or area."

"Note 1 - Don't use a Balun on this Antenna! On a horizontally oriented loop you can feed a corner, center of a side or anywhere. It is unimportant."

"Note 2 - If you know you will be using a Loop, Dipole, Zepp, etc. on Multiple Bands and you want the most efficient performance of the antenna system you will always get less feedline loss if you use Open wire 450-600 Ohm window/ladder line."

"If you feed the Full wave loop antenna direct with a single piece of coax you can only adjust add/prune the antenna til the VSWR gets down to about 1.7:1 at resonance, so you will probably want to use a tuner if you want to cover an entire HF band.. And yes the tuner will work fine as long as you are not using a linear amplifier."

In the article he also includes information on how to determine the length of the feedline using a combination of 75 ohm and 50 ohm coax.

With that we began the net.

kd5avk, Harold, on his mobile from Greenwood, Arkansas, asked if a turner was needed with a loop. After looking back at the information from WH2T it was determined that yes, a loop is needed if using the antenna multiband. Otherwise the antenna will work best only for the frequency for which it is cut.

ae5mn, Hank, asked if the loop antenna can be strung aroung gutters, downspouts and etc. Having installed a myster antenna around the eves of my house with good results, I relayed that I had kept the elements of the dipole in the middle of the eve which was at a minimum the recommended 5 1/2 inches away from most metal objects (including gutters, downspouts, cable TV wiring, and AC service wiring.

kc5zqm, Doug, mentioned that he had used 2 Meter loops for fox hunts and tracking balloon beacons with kc5trb, Harry. In those instances he's used them mostly in the vertical orientation. He mentioned that in SSb mode the horizontal orientation would more likely be used.

k5bbm, Brian mentioned that the loop antenna that kc5zqm referred to may be able to be found at arrowantenna.com.

kf5if, Wade, mentioned a 40 meter Delta Loop that he and the late ae5ft, Gene built at a field day that had a 40 ft peak. The broadsides were to the North and South and that is where they made most of their contacts. It was bottom fed, near the center. No turner was used and SWR was below 2:1 across the band. He mentioned that he was a little disappointed in the amount of noise he had on it and that the high noise may have been due to the vertical orientation and it being in the heat of summer. He did not use an analyzer during the construction, but did use an analyzer afterward to confirm it's characteristics.

End of Net