NVI 40 Meter Antenna Modeling
The idea of using a low mount dipole, enhanced with reflector wires directly beneath the dipole, on the ground, appears to be a very good approach to creating an NVI specific antenna for local HF operation. Based on other parasitic designs, one would expect the optimum gain to be at a spacing of 0.16 wave. This was based on a report of a similar antenna design used for military short range HF communications. One might think of the idea as a two element Yagi pointing straight up.
In NVI or Near Vertical Incidence propagation, you are shooting a signal straight up and hoping it will bounce off the ionosphere and come more or less straight back down, like shooting a water hose up in the air and making a circular rainfall pattern. It is good for several hundred miles, filling in the area from where VHF line of site and HF groundwave propagation fades out, and where the first skip zone begins.
So what you want from an NVI antenna is a major radiation lobe pointed upwards, with most of the energy above 45 degrees elevation. Exactly opposite of what most people strive for in a DX antenna, with the emphasis on getting the primary energy radiation down around 25 degrees or lower. While 40 meters is a proven NVI band during daytime hours, it is possible for it to occasionally fail on NVI at night.
NVI requires that the critical frequency be above the transmit frequency. The critical frequency is often just 1/3rd of the MUF, maximum usable frequency. During low sunspot numbers and low solar flux, the critical frequency can drop below 7 Mhz after sundown. So 75 meters would become the NVI band of choice, but the antenna properties desired, high angle radiation, that is above 50 degrees elevation or so, stays the same.
Keep in mind that for a good general purpose HF antenna, an excellent starting point is a good old fasion dipole or folded dipole mounted at 3/8 wave above the ground. That is about 52 feet for a 40 meter antenna. At that height you get a great general purpose pattern with little ground loss and some nice low angle radiation. Such a classic is hard to beat for simplicity and performance at 40 meters. Have a look at this pattern. Note that at 3/8 wave, the antenna is not yet quite high enough to start developing the big dip in the center of the pattern, or lobe splitting, but has squashed down and emphasized power in some of the lower takeoff angles. This antenna would best be fed with 75 ohm coax.
By contrast, let's look at the simple case of a 40 meter dipole mounted about where most people manage to mount them, at about 1/4 wave above the ground or 34 feet. We will use high accuracy ground and the usual "good ground" of EZnec of 5 millisiemens per meter. The antenna is 12 gauge wire and 400 inches per side for a total length of 800 inches overall. At this height it feeds MUCH better with 75 ohm coax and the SWR plot assumes 75 ohm feed line. You can still get 75 ohm twin lead and that is a fine option for this antenna as it could be used on multiple bands with low loss that way, and you could use a tuner to force greater simulated band width on 40 meters. You could also resort to a folded dipole with a 4:1 balun in the middle. A folded dipole in general has a bit better bandwidth.
First let's look at the SWR plot of such a typical 40 meter dipole. Notice that the bandwidth is not bad here, remember this is for 75 ohm coax or 75 ohm transmitting twin lead:
Then let's look at the Vertical Radiation Pattern. Notice that there is a LOT of vertical radiation with this installation. This might explain why so many people think 40 meters is a good local band instead of a DX band. Such antennas are better suited for NVI than DX. An 80 meter antenna at 30-50 feet would be even worse in this regard, as it would be even lower mounted in terms of wavelength:
Finall, examine the Horizontal Radiation Slice at 40 degree elevation. Notice that the pattern is NOT like that in the antenna books. This is the real world. Notice how the big null at the wire ends is just is hardly there:
For a really low mount dipole let's look at a 40 meter dipole mounted at just 0.16 wave, or a height of 266 inches for 7.09 Mhz. Included in this version is three reflector wires underneath. It was this idea of added reflector wires that was in the described military version, an attempt to be certain where the RF ground really was relative to the radiator. At that height, the SWR at the feedpoint is a good match to 50 ohm coax, and the bandwidth is only narrow, but livable. For instance, a 1.5 to 1 match to typical 50 ohm coax can be obtained over a range of 7.0 to 7.16 Mhz and more or less centered on 7.09 Mhz where the match is closer to 1.15:1. This is with a simple single wire dipole element.
The above is a diagram of the antenna system with its ground mounted reflector elements. The XY plane of the model is the ground plane. The wires for the reflector are right at the ground level but they could be buried just beneath the ground as well. The element here is a folded dipole since this is the 10 foot mounting height diagram, described below, but a simple dipole is used for heights above 22 feet. In the 22 foot case, the dipole would appear twice as high. Relative to the ground as pictured here.
The ground reflector used is three wires parallel to the dipole. They are 840 inches long from end to end. They are four feet apart side to side. One is directly below the dipole and there is another reflector 4 feet on each side. All of the ends of these reflector wires are connected together. The center of the middle reflector wire is grounded with a ground stake.
The gain is 7.17dbi straight up. The beam width is 112 degrees and the half power point is at an elevation of 34 degrees. As you can see below in the discussion of takeoff angles, one does not really want all the power going straight up, even for 100 mile paths. The pattern is an elongated circle with maximum radiation broadside to the dipole, but with the end radiation minimums normally associated with a dipole greatly reduced.
Such an antenna would have reasonable omni directional coverage as well as some distance capability since it does have considerable radiation at angles below 45 degrees.
For a permanent installation this could be a very workable antenna. The primary problem is use of such an antenna over a wider frequency range. The reflector greatly decreases bandwidth along with the low mounting of 266 inches, just over 22 feet. There is a significant problem getting this antenna to cover frequencies like 7.29 and 7.09 simultaneously. One solution would be to feed the antenna with open wire or very good windowed twin lead, or 75 ohm transmitter twin lead and use a tuner to give the band coverage needed. In that case it could be shortened a bit to move resonance closer to band center. At 80 meters this is agrivated even further, and even in the best case of 3/8 wave mounting which is quite high at 80 meters, covering the entire 80 meter band with a single wire dipole is not easy.
For portable use this antenna can be redesigned. Based on the 10 foot poles available at Radio Shack, we can lower the height of the antenna to 120 inches. These poles are a convenient size, light and easy to transport. They can be cut in the center and because they have necked down one one end to fit together, the two halves can be easily rejoined. This makes a package with the longest part only five feet long. But most of all, it does not require trees or other supports, it is self contained. It only needs some space to set it up.
Several things happen when an antenna is mounted this low, and placed this close to a reflector element. None of it is a surprise, the same things happen in Yagi designs.
Most dramatic change is the feed point impedance, which plunges to about 12 ohms. But wait, 12 * 4 is 48 ohms. The easiest way to fix this is with a folded dipole. Usually people think of folded dipoles in free space and associate feed point impedances of 300 ohms with folded dipoles. But the folding works as an impedance transformer. It works for any feed point impedance.
Using a folded dipole made from 450 ohm window line, raises the feed point to a near perfect match of 1.05:1 for 50 ohm coax in this case. A simple, once folded dipole will have 4 times the feedpoint impedance vs the same single wire dipole.
But the bandwidth has become like a notch filter! The 1.5:1 range is now 7.06 Mhz to 7.12 Mhz. Fine for 7.088, but hopeless at 7.295 or so. It has 8.9 dbi gain straight up and a beam width of 101 degrees. It has its 3db point at an elevation of 39 degrees. Also this low mount means one needs to be a bit concerned about RFI exposure underneath the antenna. Also the antenna will respond heavily to anything between it and ground, like a parked car, bushes, or small trees.
This particular model had the reflector wires 36 inches apart. Also the radiator had to be shortened partially due to the use of a folded dipole driven element and partially due to the proximity effects of such low mounting. Such ground reflector wires do not have a lot of induced current and can be thin hookup wire.
In the Diagram Below, notice that indeed the maximum lobe is straight up and radiation below 39 degrees is seriously restricted.
In this case the dipole and the reflectors were 800 inches long. The "reflectors" are more of a ground enhancer in this application. So the length is not really important as long as they cover the same area as the antenna, especially near the feed point of the antenna were the currents are maximum. They serve to establish the exact ground level as well as reduce ground loses.
In the above diagram notice how the horizontal field and the vertical field are at right angles. But notice mostly how the pattern is not the one you would expect from a dipole, be very much like one you might expect end on from a two element Yagi, looking down the boom from the front.
Changing to 48 inch spacing on the reflector wires instead of 36 inches and making them 420 inches long had virtually zero effect on the antenna performance. In each case ordinary good ground was assumed.
Changing to very good ground raises the above case to 9.17dbi and 96 degrees beam width, so there might be something to say for the wider spacing of the ground reflector elements and the slight lengthen of them.
Unfortunately, the ground effect does not improve the main problem which is very narrow band width. For this to be practical either it would have to be feed with parallel line and a tuner, or hams in Hawaii need to decide which 40 meter frequency they are going to use, our unique 7.088 frequency or the more common 7.290 area frequencies.
While 80 meters is probably a better NVI band at night, remember that all antenna dimensions are doubled. Using only the 10 foot poles for instance would place an 80 meter antenna tremendously close to the ground. One practical approach to NVI on 80 meters is to use a commercial mounting that places two mobile whips back to back, like a self supporting dipole. This can easily be mounted on a small post. Efficiency would be low and the bandwidth would be terribly narrow, but if carefully tuned and fed, it would have plenty of useful NVI radiation and be easy to install and transport vs a 132 foot wide dipole and a pair of 30 foot poles.
I have created such an antenna using pole sold for paint roller extension handles and a commercial bracket slightly modified. These poles are sold in various lengths and I chose the 12 foot one since it collapes to 6 feet. There is a 16 foot version but it is 8 feet long collapsed and that is a bit hard to stuff in the average car, etc. The antenna tested rather well, but it did show a LOT of interaction with everything underneath it. This meant each deployment was unique and the feedpoint impedance and resonance could not be counted on.
I have for some time insisted that a decent antenna tuner needs to be a part of any emergency antenna design. You simply cannot count on the resonance and feedpoint impedance of any temporarily installed antenna. The tuner is your ace in the hole, your ability to strongarm the feedpoint situation and ensure that you can load what you string up and actually get on the air.
Recent field testing of a vertical has shown very good local coverage with such an antenna. This prompted calculation of just what takeoff angles are involved in the NVI type of propagation mode. Keep in mind that for distances less than 30 miles, ground wave is a likely propagation path. The exception would be a path which was less than say 50 miles, but heavily obstructed by terrain such as a high mountain or ridge line between the two stations. A vertical should be quite good for ground wave output, but it puny radiation straight up makes one wonder why it worked so well for typical NVI paths.
A table of slant path distance and takeoff angle for various high angle skips from the E, F, F1 and F2 layers over various distances from five to 300 miles baseline. This is the basic so called NVI zone, including areas blocked from ground wave less than 50 miles distant, out to reasonable first skip zone distances workable with antennas like beams on towers.
Distances 5-30 miles represent contacts between stations on the same island which do not have ground wave contact, that is blocked by exceptional mountain ridges.Typical effective ionospheric layer heights assumed were 70 miles for E layer, 175 miles for F layer, 140 miles for F1 and 200 miles for F2 layer. Angles are in degrees. Path length is the slant height up and back down. Remember at night the F1 and F2 layers tend to merge into the F layer.
The D layer which is strong during the day is not of any use for skip, but it does cause a very large amount of attenuation both going up and coming back down. It is mercifully a daytime phenomenon, being quickly destroyed by ion recombination and needing constant rebuilding from solar radiation. The D layer dies promptly at sundown and rebuilds promptly at sunrise and is a major reason for timing of the opening and closing of 40 meters for DX around here. The D layer attenuation is worse on lower frequencies than on higher ones.
During yacht race communications on marine frequencies like 4 Mhz., the effect of the rising sun was huge and dramatic with signals from even as close as 100 miles going from armchair to ESP levels nearly lost in the noise. The direct result of the return of the D layer.
Chart of: Distance across the ground E layer path length up and down; takeoff angle F layer path length up and down; and takeoff angle F2 layer path length up and down; and takeoff angle [Note: For two E skips, look up the total half distance and multiply] [the path length by two, but take the takeoff angle from the chart for] [half the distance. Such double skips suffer from four passes through] [the attenuating D layer and are unlikely.] 5 mi.|E: 140 88|F: 350 89|F1: 280 89|F2: 400 89| 10 mi.|E: 140 86|F: 350 88|F1: 280 88|F2: 400 89| 15 mi.|E: 141 84|F: 350 88|F1: 280 87|F2: 400 88| 20 mi.|E: 141 82|F: 351 87|F1: 281 86|F2: 400 87| 25 mi.|E: 142 80|F: 351 86|F1: 281 85|F2: 401 86| 30 mi.|E: 143 78|F: 351 85|F1: 282 84|F2: 401 86| 35 mi.|E: 144 76|F: 352 84|F1: 282 83|F2: 402 85| 40 mi.|E: 146 74|F: 352 83|F1: 283 82|F2: 402 84| 45 mi.|E: 147 72|F: 353 83|F1: 284 81|F2: 403 84| 50 mi.|E: 149 70|F: 354 82|F1: 284 80|F2: 403 83| 55 mi.|E: 150 69|F: 354 81|F1: 285 79|F2: 404 82| 60 mi.|E: 152 67|F: 355 80|F1: 286 78|F2: 404 81| 65 mi.|E: 154 65|F: 356 79|F1: 287 77|F2: 405 81| 70 mi.|E: 157 63|F: 357 79|F1: 289 76|F2: 406 80| 75 mi.|E: 159 62|F: 358 78|F1: 290 75|F2: 407 79| 80 mi.|E: 161 60|F: 359 77|F1: 291 74|F2: 408 79| 85 mi.|E: 164 59|F: 360 76|F1: 293 73|F2: 409 78| 90 mi.|E: 166 57|F: 361 76|F1: 294 72|F2: 410 77| 95 mi.|E: 169 56|F: 363 75|F1: 296 71|F2: 411 77| 100 mi.|E: 172 54|F: 364 74|F1: 297 70|F2: 412 76| [Note that even at 100 miles, the E layer is down to 54 degrees] [And even the F layers are down into the 70's] 105 mi.|E: 175 53|F: 365 73|F1: 299 69|F2: 414 75| 110 mi.|E: 178 52|F: 367 73|F1: 301 69|F2: 415 75| 115 mi.|E: 181 51|F: 368 72|F1: 303 68|F2: 416 74| 120 mi.|E: 184 49|F: 370 71|F1: 305 67|F2: 418 73| 125 mi.|E: 188 48|F: 372 70|F1: 307 66|F2: 419 73| 130 mi.|E: 191 47|F: 373 70|F1: 309 65|F2: 421 72| 135 mi.|E: 194 46|F: 375 69|F1: 311 64|F2: 422 71| 140 mi.|E: 198 45|F: 377 68|F1: 313 63|F2: 424 71| 145 mi.|E: 202 44|F: 379 67|F1: 315 63|F2: 425 70| 150 mi.|E: 205 43|F: 381 67|F1: 318 62|F2: 427 69| 155 mi.|E: 209 42|F: 383 66|F1: 320 61|F2: 429 69| 160 mi.|E: 213 41|F: 385 65|F1: 322 60|F2: 431 68| 165 mi.|E: 216 40|F: 387 65|F1: 325 59|F2: 433 68| 170 mi.|E: 220 39|F: 389 64|F1: 328 59|F2: 435 67| 175 mi.|E: 224 39|F: 391 63|F1: 330 58|F2: 437 66| 180 mi.|E: 228 38|F: 394 63|F1: 333 57|F2: 439 66| 185 mi.|E: 232 37|F: 396 62|F1: 336 57|F2: 441 65| 190 mi.|E: 236 36|F: 398 62|F1: 338 56|F2: 443 65| 195 mi.|E: 240 36|F: 401 61|F1: 341 55|F2: 445 64| 200 mi.|E: 244 35|F: 403 60|F1: 344 54|F2: 447 63| [Note that typical Oahu to Big Island distance show takeoff] [angles now dropping into the 50's for even the F layer.] 205 mi.|E: 248 34|F: 406 60|F1: 347 54|F2: 449 63| 210 mi.|E: 252 34|F: 408 59|F1: 350 53|F2: 452 62| 215 mi.|E: 257 33|F: 411 58|F1: 353 52|F2: 454 62| 220 mi.|E: 261 32|F: 413 58|F1: 356 52|F2: 457 61| 225 mi.|E: 265 32|F: 416 57|F1: 359 51|F2: 459 61| 230 mi.|E: 269 31|F: 419 57|F1: 362 51|F2: 461 60| 235 mi.|E: 274 31|F: 422 56|F1: 366 50|F2: 464 60| 240 mi.|E: 278 30|F: 424 56|F1: 369 49|F2: 466 59| 245 mi.|E: 282 30|F: 427 55|F1: 372 49|F2: 469 59| 250 mi.|E: 287 29|F: 430 54|F1: 375 48|F2: 472 58| 255 mi.|E: 291 29|F: 433 54|F1: 379 48|F2: 474 57| 260 mi.|E: 295 28|F: 436 53|F1: 382 47|F2: 477 57| 265 mi.|E: 300 28|F: 439 53|F1: 386 47|F2: 480 56| 270 mi.|E: 304 27|F: 442 52|F1: 389 46|F2: 483 56| 275 mi.|E: 309 27|F: 445 52|F1: 392 46|F2: 485 55| 280 mi.|E: 313 27|F: 448 51|F1: 396 45|F2: 488 55| 285 mi.|E: 318 26|F: 451 51|F1: 400 44|F2: 491 55| 290 mi.|E: 322 26|F: 455 50|F1: 403 44|F2: 494 54| 295 mi.|E: 327 25|F: 458 50|F1: 407 44|F2: 497 54| 300 mi.|E: 331 25|F: 461 49|F1: 410 43|F2: 500 53|