November 3, 2012 By Ford-NØFP
4 tower, 28′ high, 284′ long loop
4 supports, 72’ apart, 28’ high, 284’ of wire. Looking west.
More than one antenna is an operating feature. When the two antennas have different performance, noticeable differences can be observed when doing an A-B switch. I currently have an all band vertical. It’s an elevated feed (8’ off the ground) aluminum tower about 68’ high, making the radiator about 60’ vertical. An SG-230 tuner matches the system on any frequency from 1.8MHz to 30MHz. I have many friends located 60 to 100 miles from my QTH. HF propagation is tough, especially during the day. The vertical has a very low launch angle and my friends fall inside the skip zone.
Horizontally polarized antennas are, in many ways, superior performers on HF. While the vertical omni-directional antenna may provide superior performance in all random directions, and occupy a small footprint, they fail to take advantage of the additive effects of the reflection of the wave front in the area near the radiator. The so-called “ground effect” of the Fresnel zone reflection can account for 6dB of additional gain!
Horizontal Dipole (1/2 wL high) vs Vertical Performance
Here we see a horizontal dipole at 66’ (just less than ½ wL) in black and compared to a ground mounted vertical in blue.
The physics behind the phenomena is simple. The wave front emitted directly by a horizontal dipole combines with the wave front reflected off ‘ground’ beneath and in front of the dipole. A vertical has no parallel reflective surface so it can only launch a direct wave front. A horizontal antenna effectively combines the apparent radiation from two points, thus providing about 6dB of apparent gain.
So far, this discussion has only considered the peak radiation at approximately 30° above the horizon. The launch angle is important. Skywave propagation reflects signals off the ionized upper atmosphere to extend signals well beyond the visible horizon. Multiple skips permit Trans global communications. Suppose you don’t want to talk to the next continent, but the next county? Or two counties over? Ground wave communications, which is polarization sensitive, can be limited to a few dozen miles. Skywave propagation becomes impossible when the target is inside or just beyond the “skip” zone.
The classic propagation pattern of a vertical or ½ wavelength high dipole is perfect for long distance skywave. By building a horizontal antenna low, you can introduce skywave propagation that virtually eliminates the problem of skipping over your target. Compare the 28’ high 284’ square loop to a full sized 80M flat top dipole at 130’.
Low Loop (black) compared to 1/2 wL high dipole (blue)
Here we see the 28’ high square loop (black plot) to a full sized 130’ high dipole (blue plot)
Clearly the high dipole does a better job of launching the wavefront to the horizon. The NVIS antenna shows a 15dB improvement at zenith (straight up). Even at 30° it is roughly equivalent to a omni-directional vertical. And it is a true omni by nature.
A 130’ dipole mounted horizontal at 130’ in the air is nothing short of darn near impossible. What if the dipole were also mounted at 28’? How would it compare to the loop? Very similarly:
Low Loop (Black) versus Low Dipole (Blue)
The loop (black) plays only a couple dB better at zenith than the dipole (blue).
At these low altitudes, they are both omni directional, similar launch response, but the loop is far more complex. The loop takes 284’ of wire, and 4 supports. The dipole requires only 135’ of wire and two supports.
The dipole is clearly easier to build. When mounted low, it becomes difficult to match, narrow bandwidth, and virtually impossible to match on the upper ham bands. Here we see an easy match at 3.5, 11, 18, 25.5, and only 3.5 is within a hamband.
Dipole Frequency Response 3.3 MHz to 30 MHz
135’ long dipole mounted at 28’.
What happens with the loop? Using a 50 ohm line, the match is fairly easy on most ham bands.
Loop Frequency Response 3.3 MHz to 30 MHz
When matching to 50 ohms, the natural dips fall within ham bands. 80M and 40M are fairly easy match, although quite narrow. What happens if you use a 4:1 transformer and match it to 200 ohms instead of 50 ohms? Additionally, suppose I add a 255pF capacitor and tune out some of the inductive reactance of the loop—effectively raising its resonant frequency just a bit.
Loop Frequency Response with 4:1 transformer
Here we see an easy match on 80M and 40M with a very easy match on all the upper bands, including WARC bands.
Here is a graph of the 80, 40, and Upper Bands. As can be seen, this is an easy match for any auto-tuner in most radios.
Measured SWR Curve w/4:1 (200 ohm) feed
Performance on Upper Bands:
Building the NVIS:
Each tower has two 10’ sections of 2” PVC (schedule 40) coupled with a pipe coupler. The bottom section is 8’ section of 2’ PVC but schedule 80. A special coupler was fabricated using the 2” schedule 80, ripped in two on a table saw, and drilled to accept ½” ready-rod. A ¾” strap was laminated between the bolts to increase rigidity and strength.
Schedule 80 to Schedule 40 Connection
The “dead man” anchors are 16” pieces of the 2” schedule 80 pipe. A 36” hank of stainless cable threaded through a 2” piece of ¼” copper tubing, flattened, and bent in a loop. A cable clamp secures the cable. The cable and tube is buried at right angles inside a trench approximately 12” deep. 5 gallons of water was used to cement the clay around the tube.
“Dead Man” Anchor
Here we see an anchor before it was buried. You can barely see the outline of where the sod was lifted to expose the loam and clay beneath. A 24” trench, approximately 12” deep, secures the anchor.
The top guy is at the 27’ level. The pulley is just short of 28’ high. The plexi-glass guy ring was fabricated from ¼” stock. A table router was used to round down the edges to prevent it from cutting the ropes.
Feed Point Pulley
RG213 was attached to a SO-239 chassis mount. A 4:1 balun was fabricated from a 1.5” diameter type 61 ferrite toroid. 8 turns of RG58 was used to make the transformer. The entire system was weather sealed using a hot glue gun and coating the entire connection point in hot glue. The high impedance end of the transformer was connected to two 510 pF Silver Mica (500v) capacitors in series to form a 255 pF 1000v capacitor. This brings the resonant point of the loop up about 125 KHz on 80M, about 60 KHz on 40M, and is virtually invisible on the upper bands.
Sky Hook Pulley and Insulator
The other 3 towers have a simple insulator fabricated from 0.4” this HDPE stock.
The rope was 130 lb test baling twine purchased from the farm supply store. This orange rope is about 1/8” diameter and does not stretch. It is also UV protected for durable operation. They make two varieties of rope: Biodegradable and non-biodegradable. I don’t want anything ‘degrading’ so I’m using the rugged variety. 9,000 feet of twine is $26, which is $0.003/ft (1/3 of a penny!)
Guy Ropes and Tensioner
The rope tensioner was made of ¾” strips of oak trim with a 1/8” diameter hole drilled in each end.
The guys are secured through a chain repair link looped through the cable anchor.
Lifting Rope Cleat
A rope cleat is used at about the 5’ level. A 2” strap clamp secures it to the tower. The cleat is handy to secure the lifting ropes. Winding the feed point tower a few turns secures the RG213 to the tower and prevents it from blowing in the wind.
Next summer, the system will be lowered and move to the wall of my shed. The dead man anchors will not be in the way of the mowing operations that happen every week. If pushed in an emergency, this system could be transported on very short notice and reassembled using fence stakes as anchors. The performance is excellent out several hundred miles during the day. The military uses this style antenna for its large area operations. You often saw a multi-turn version of this mounted to the Hum-Vees of military troops entering Iraq a few years ago. They used it because it works!