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.

40 Meter Dipole at 52 feet

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:

SWR of 40 Meter Dipole at 34 feet

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:

Vertical pattern of 40 meter dipole at 34 feet

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:

40 Meter Horizontal Slice

Ficus

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.

Diagram of 40 Meter NVI antenna

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.

Ficus

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.

Elevation Plot of NVI 40 at 10 feet

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.

Azmuth Slice of pattern at 39 degrees

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.

Bandwidth of NVI 40 meter antenna at 10 feet


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|


Return to UH Ham Club Home Page

08/02