Loop Skywire Mysteries Explained?

Transcription

1 Introduction: Many antenna books dismiss the horizontal loop or loop skywire as a non performer. However user s of the loop will tell you that it does work; why? I used 80 & 40m loops in the past with good success. So why the differing data? After 2 years of using a 160m loop, and some modeling, I may be able to shed some light on this little problem. Now, I am not ant antenna guru, but the performance of the 160m loop on 40m intrigued me so much, that I decided to model it in Nec Win Pro. The intent of the article is to clarify the use of the loop as an effective gain antenna and explain some of its characteristics and its best characteristics. Description My loop is 160m in circumference. I optimized the length to match operating resonance points on 80 & 40m, with no regard to 160m resonance. This turned out to be part of the secret of the antenna. Loop is about ft high in the horizontal position, strung between the trees. It is fed on a side (not a corner) with a 4:1 balun. Performance Summary The first year I had the antenna up; it was fed in a corner with 450 ohm ladder line. That feed was changed in the middle of the season due to interference complaints. The second year, after some modeling I changed the feed point to a side which indicated there should be more gain. I also observed exceptional performance on 40m. Here is a summery of anecdotal performance. Summertime: Generally, the performance is not very good on 20m and above in the summer. I found that a 20m ¼ wave elevated ground plane could out perform it on some days. Performance below 20m was good, i.e. I could work some DX, and usually the loop outperformed a 40m ground plane and 80m inverted-l. The winter season is the loop s time to shine. Either the snow on the ground helps this antenna (Modeling suggests this), or the fact that DX conditions on the low bands is just better (propagation allows ducting at higher takeoff angles?). Performance on 160m is not very good, although I have not used it extensively on 160m. I just can t seem to get out!! 1

2 On 80, the performance is good, I can usually work DX. In the winter, working Europe on 100W is standard with 59 reports. Generally, 1 full s-unit better than the inverted L. On 40-30m the winter performance is really good, especially during the grayline. Sometimes, I can simultaneously work UK foundation license operators using 10W and VK s during the same opening. Noise: This antenna is very noisy, especially early in the season. I.e. there is so much wire up there that is just picks up atmospheric noise. Later in the season (feb/march), when the noise dies down, it is fine. I now use a K6SE pennant as my main receiving antenna Helps on the static crashes! Detailed Description: Shape: The shape of the loop is not perfectly square. I tried the best I could but my terrain was the limiting factor. Here is the shape of my loop. I Power Line Feed point A power line is just under the loop and I added that to the model. Dimensions: All dimensions in meters. 3D Patterns 2

3 80m side view 80m Top View The shape resembles a donut; take note of the shape of the bottom. 40m side view 40m Top View Notice the bottom of the 40m side view shape. The bottom is flatter indicating more gain at lower angles, and less at higher angles. As well, there is some directivity. 3

4 The feed point position flattens the pattern. Feeding in the corner allows similar gain, but at higher elevation angles. 30m Side view 30m Top View Again, 30m seems flat on the bottom Horizontal Gain Patterns OK so how much gain do we really have? Here are the horizontal gain patterns for 10, 20 and 30 degrees of elevation for 80, 40 & 30m. The thicker line being 30 degrees. On 80m, you can see there is little gain at 10 degrees, 3-4dB at 20 degrees and 4-5dB of gain at 30 degrees! Mmmm, this is looking interesting. 4

5 40M This is the most interesting plot. At 10 degrees there is 6 DB gain in an omni directional pattern! The gain drops off as elevation increases and is less uniform. Interestingly, there is also about 6dB of gain at 5 degrees elevation! The directivity gain occurs at higher elevation angles. 5

6 30M There is about 4-5 db of gain at 30 degrees elevation angle and drops off as you go lower. The perfect 160m Square Loop When compared to a perfect square, here is what the patterns look like for the 160m loop excited at 7Mhz. 6

7 Modeling parameters All examples are modeled using real ground in NEC Win Pro. Gain is over Isotropic or dbi 7

8 Conclusions The loop skywire will perform best for DX under the following conditions: Use a 4 wavelength loop for the band of interest. Feed the loop on a side, this will flatten out the pattern. Great success can be obtained during grayline openings Of course this means having a 320m length loop for 80m; I have not figured that out yet.linear loaded? mmmm But for good performance on 20m, an 80m loop will do! Observations/Questions It is interesting that there is simultaneous gain at both low (5) and higher (20-30) degree take off angles, could this characteristic be encouraging ducting to occur during grayline openings? Have fun with loops, they can perform well, but under certain conditions. Tony, VE1ZA, 11 Nov 06 8