75 meter antenna design tradeoffs

[AB2EZ]

Steve

You commented: "If it were the electron gyro effect, typical NVIS antennas would not work as well as they do, since horizontally polarized signals are greatly attenuated by the gyro effect."

If the attenuation is caused by energy being scattered out of the incident wave (i.e., rather than being converted to heat/vibrational energy) then that could be a good thing for NVIS.

Normally, one thinks of the electrons as weakly interacting with the incoming wave... i.e. absorbing energy and reradiating it at the same frequency (with a 90 degree phase shift). The reradiated energy adds coherently to the incoming wave; and the 90 degree phase shift is what results in a reduction of the apparent velocity of the wave (i.e. the index of refraction of the medium)

However, if the electrons are moving, the absorbed and reradiated energy may not add coherently to the incoming wave.

The effect may be strong scattering, with the backscattered energy being what we receive in NVIS.


Stu

[WB4AIO]

I could be wrong, but I see no reason to believe that there is any qualitative difference between "NVIS" and ionospheric refraction at angles slightly less than what is considered NVIS, whatever they may be.

There is no evidence of a zone of transition. There is no signal strength anomaly, no noticeable nulls or peaks (that are not explainable by our current understanding of antennas and propagation) as we move downward in angle from 90 degrees to, say, 60 degrees. There is no noticeable change in selective fading.

The changes that do occur in high-angle paths -- daily, seasonal, and cyclic -- are parallel to the changes that occur in medium-angle paths, and the differences between the angles are adequately explained by long-standing theory.

If there is a special mode of propagation involved in near-zenith refractions/reflections, I suspect it only has a noticeable effect at extremely high angles, very near 90.00 degrees, where the returned wave is probably swamped by the antenna's direct wave anyway.

As to 75 meter antenna design tradeoffs, I agree with K1JJ on dipole heights. The only thing I would append to that is to say that adding a cross dipole fed 90 degrees out of phase makes the antenna fully omnidirectional in the horizontal plane. By doing this, you lose a dB or two in the formerly favored direction, but you gain much more than that in the former nulls.


With all good wishes,


Kevin, WB4AIO.

[k4kyv]

It's not worthwhile to do anything to a 1/2λ horizontal antenna to make it "omni-directional" to distances of less than 500-600 miles or even to 1000 mi.  It is already essentially omnidirectional.  The nulls off the ends occur at very low angles, which may affect signals at great distances, but that is not even a dominant take-off angle for an antenna less than about 3/8λ high. For shorter distances, any null will be imperceptible at the receiving end.

The single-wire OCF dipole aka the Windom, is driven with a single wire feeder.  If the feed wire is attached at the right point, there will be little or no SWR on the wire.  There will be some radiation from the feeder since there isn't a parallel wire carrying currents of opposite polarity to balance it out, but apparently it isn't enough to significantly degrade performance of the antenna. Likewise, single-wire feeders may be used to feed the base of a vertical in the same manner, instead of coax. The old style "open-wire  coax" that uses 5 wires, with 4 outer wires maintained at ground potential while the signal is fed through a 5th central wire is actually a variation of single-wire feed, because the 4 outer wires are not sufficient to shield the inner conductor to any great extent.

The Laport antenna book that Walt, W2DU has offered to distribute on CD has a section that describes "unbalanced open wire lines".  This includes single wire feed, unbalanced two-conductor open wire line, as well as the so-called open wire coax.

[K1JJ]

As to 75 meter antenna design tradeoffs, I agree with K1JJ on dipole heights. The only thing I would append to that is to say that adding a cross dipole fed 90 degrees out of phase makes the antenna fully omnidirectional in the horizontal plane. By doing this, you lose a dB or two in the formerly favored direction, but you gain much more than that in the former nulls.

I've actually tried one of those, called a turnstile antenna. It feeds one dipole at right angles 90 degrees out of phase from the other.  It did exhibit a quasi-omni pattern. However, after a while I realized the 3db loss was going into directions I rarely needed - up to Ontario, Canada and out to the Atlantic Ocean past Rhode Island. I went back to a standard dipole and felt better about the 3db...

However, someone living out in the middle of the country would find it very useful, indeed.

You are correct - the nulls, sometimes as much as -20db off the ends of a dipole can be filled in for a small 3db loss in the other directions using the antenna.  Not a bad compromise, really,

T

[K4TLJ]

I built the antenna Frank Witt described in the April 1989 QST. It indeed does cover the entire band from 3.5 to 4.0 mHz. It is heavy though and requires some support. 65 feet is better but my antenna is only 25 feet. Local rag chews are fine throughout the band. I use RG213 and a current balun at the feed point. No tuner is required.

[Steve - WB3HUZ]

Very interesting Stu. I guess we may never know unless we get an antenna that only radiates straight up. Wasn't that big array in Alaska able to do this?   

[Steve - WB3HUZ]

There are no nulls in the pattern of a dipole 0.25 WL above ground at high elevation angles. The turnstile does little or nothing for the added complexity.

As to 75 meter antenna design tradeoffs, I agree with K1JJ on dipole heights. The only thing I would append to that is to say that adding a cross dipole fed 90 degrees out of phase makes the antenna fully omnidirectional in the horizontal plane. By doing this, you lose a dB or two in the formerly favored direction, but you gain much more than that in the former nulls.

I've actually tried one of those, called a turnstile antenna. It feeds one dipole at right angles 90 degrees out of phase from the other.  It did exhibit a quasi-omni pattern. However, after a while I realized the 3db loss was going into directions I rarely needed - up to Ontario, Canada and out to the Atlantic Ocean past Rhode Island. I went back to a standard dipole and felt better about the 3db...

However, someone living out in the middle of the country would find it very useful, indeed.

You are correct - the nulls, sometimes as much as -20db off the ends of a dipole can be filled in for a small 3db loss in the other directions using the antenna.  Not a bad compromise, really,

T

[KG6UTS]

For 75/80 at home I'm using a 135ft sloped dipole fed with 450 ladderline through a matchbox and DX Engineering 1:1 balun. The high end (south) is at about 62 feet, low(north) at 15. Not the best maybe but what I can put up at home. On 75 the main interest is working AM and BoatAnchor nets on the West Coast and its been an OK performer......as condx allow of course. San Diego to the San Francisco Bay area is about 500 mi. On 20 and 17 it does represent significant wire in the air with barefoot contacts to NZ, JA land, South Africa, Uraguay, Caribean, and Germany. It would be hard to model the thing, the pattern probably looks like a stepped on spider.

At work I can use the range after hours so have an 80m dipole at 90 feet above about 2 acres of bonded hardware cloth, all on a ridge 350 ft over the Pacific. Thats pretty darn ideal but hard to do at home. For a while we had a three element delta loop array on 75 but that was a bother to put up and down when the range was needed for real work. The height is the thing.

Portable I've been using an inverted L with some of those fiberglass poles originally comoflage netting. The wire is 150ft fed against 4 radials, both directly to a tuner and a short coax to the TX. Again, probably not the best but it all rolls up and fits in the Jeep or VW van. This thing is usually set up out in the Borrego Desert for weekend check in to the MRCG 3985 net at night and 20/17 during the day.

[K1JJ]

There are no nulls in the pattern of a dipole 025 WL above ground at high elevation angles. The turnstile does little or nothing for the added complexity.

I see nulls with my older Mini-Nec software. It's probably giving a false impression cuz of it's poor ground modeling reputation. (Yeah I know, I need to get into the 21st centruy with some new software) 

Could you post the horizontal pattern of a dipole at 1/4 wavelength high using your software?

T

[Steve - WB3HUZ]

Here ya go. A dipole at 60 feet over average ground.

The first plot is the azimuth pattern at an elevation angle of 70 degrees. There's about a 1 dB "null."    There will be even less of a null at higher angles.

The second plot is the azimuth pattern at an elevation angle of 45 degrees. We're venturing near low angle territory now. There's still is only a 4 dB reduction off the ends of the dipole. At the very low angles, you will see a null.

[K1JJ]

OK, I see.   So at the higher angles that really matter for local work, a dipole at 60' will be within 3-4 db omni-directional. But at the lower angles, which have poor gain anyway, the nulls are more pronounced.

It's been said that a low dipole is pretty much omni, but I think most never considered it THAT omni-directional at 60' high. Interesting.

BTW, my turnstile antenna was at 90', so it appears it was doing some reasonable fill-in at the middle angles and lower..

T

[Steve - WB3HUZ]

Yep, at 90 feet the signal would be down off the ends by about 5 dB at 45 degrees and about 3dB at 70 degrees.

[KM1H]

The turnstile provides circular radiation....straight up.

[WA1GFZ]

Cebik did an article in QEX just before he died where he folded the dipole elements into a loop shape but left a space between the ends. There was no real directivity but it took up less area in the yard. Kind of like the AHE antenna spread out. I had a few emails with him before he signed out. He built a number of 2 meter antennas but thought it would also play on the HF bands. I want to simulate it  for a 160 meter antenna at gfz south. I think the pattern goes crazy on the higher bands.

[Steve - WB3HUZ]

Stu,

Take a look at the attached graph. It's from the ARRL Antenna Handbook, 20th Edition. Zeroing in on the upper left corner (the area of interest for NVIS) it would appear there is very little difference in the one-hop distances for the range of about 70-90 degree take-off angle for E and F1 layer propagation. Even with F2 propagation, the difference is minimal.

[AB2EZ]

Steve

It appears that these curves are based on a very simple trigonometric model... where the propagation path is modeled as a line from the ground up to some equivalent altitude (where a reflection takes place), and by a line returning from that equivalent altitude back to the ground. The angle of incidence equals the angle of reflection... which makes sense in the case of a simple reflection model.

For example, consider a 30 degree takeoff angle. For an isosceles triangle having a 30 degree base angle on either side... the length of the base is given by [2 / tan (30 degrees)] x the height = 3.464 x the height. For the layer heights shown , the "one-hop" distance is exactly 3.46 x the layer height. I.e. if H = 99.5 miles (the equivalent reflection altitude used for the F1 layer), the the one hop distance is 3.46 x 99.5 miles = 345 miles. This is exactly what Figure 27 shows for the one-hop distance corresponding to the F1 layer and a 30 degree take-off angle.

As another example, if the takeoff angle is 10 degrees, the the simple trigonometric formula is one-hop distance = 2 / tan(10 degrees) x H = 11.34 x H. For H= 131 miles (the bottom of the F2 region), the one-hop distance is 11.34 x 131 miles= 1486 miles. Again this is what Figure 27 shows for a takeoff angle of 10 degrees and the bottom of F2.

This simple trigonometric model is fine for refraction (where the equivalent height of the reflection is chosen properly... to account for the gradual bending of the wave as it passes through the ionosphere)

I don't believe that this simple trigonometric model is useful for NVIS. In NVIS, we are not looking at a refraction phenomenon. We are looking at a scattering phenomenon... in which the scattered signal is emitted in all directions from each scattering point in the ionosphere, and where the scattered signals from each scattering point do not add coherently when they arrive at the receiving location on the ground.

The simple trigonometric model serves us well as an intuitive model of a wave being reflected by the ionosphere (even though it is actually a refraction phenomenon, and not a reflection from a discontinuity at a fixed altitude). That simple model does not serve us well for NVIS.

It is often the case that simple models... that work well for predicting some real-world behaviors... will break down (because the approximations and assumptions are no longer valid) when used to predict other real-world behaviors.

Stu

[WB4AIO]

[...]
In NVIS, we are not looking at a refraction phenomenon. We are looking at a scattering phenomenon... in which the scattered signal is emitted in all directions from each scattering point in the ionosphere, and where the scattered signals from each scattering point do not add coherently when they arrive at the receiving location on the ground.
[...]

That's fascinating. Are you certain of this, or is this an hypothesis?

Does refraction cease to happen at angles that are high enough, and scattering take over? Is there an overlap zone where both phenomena take place? Would not scattering produce lower received signal levels than refraction? -- if so, why do we not notice that for very short paths on 75?


With all good wishes,


Kevin, WB4AIO.

[AB2EZ]

Kevin

Depending upon the frequency and the ionization state of the ionosphere... there is a maximum incident angle (relative to horizontal)... above which refraction is not sufficient to result in enough bending to cause the incoming wave to return to the ground (i.e. thinking of this as equivalent, for our purposes, to a reflection... no reflection will occur for incident waves above that angle.)

In the technical literature, this maximum incident angle is often referred to as the "critical angle".

Below the critical angle, the effect we call refraction results when electrons in the ionosphere weakly interact with the incoming wave. They absorb (very briefly) and re-radiate it (with a 90 degree phase shift) in all directions. This is similar to the way a parasitic antenna element absorbs and re-radiates power emitted by the driven element (and other parasitic elements). In the case of refraction, the re-radiated/phase shifted power adds coherently to the incident wave... and the net effect is that the incident wave appears to be traveling slower (i.e. the speed of light in the ionosphere will be slightly less than the speed of light in free space), and the direction of propagation of the incident wave will gradually change. Again, this is very similar to the way in which parasitic elements of an antenna change the direction of portions of the power emitted by the driven element. Power that is re-radiated in other directions (other than the direction of the incident wave) tends to add up in a way that causes it to cancel out. Thus, there is very little power "scattered" in other directions.

However, there is some power scattered in the backward direction... whose power level depends upon various types of inhomogeneities in the ionosphere. This phenomenon (back-scattering) is similar to (but not exactly the same as) the phenomenon that makes the sky appear to be blue. I.e. blue light is scattered more strongly (in all directions) by the atoms in the atmosphere than green and red light. Thus blue light is removed from the incoming sun light... and the sun appears to be yellow (white minus blue = yellow).

In the case of NVIS (I believe, but I have not found a good technical reference): the refraction effect is not present (because the angle of the wave it too high). What is left if the scattering effect.

Since the allowable loss between the transmitter is so large (e.g. 100 Watts = 123dB over S9), provided the background noise levels are not too high, the back-scattered NVIS power is (apparently) sufficient for good communication along some short distance paths.

For example, if I radiate 100 watts into an area whose diameter is 375km, and if the equivalent area of a full length (half wave dipole) 75 meter receiving antenna is a circle whose diameter is 37.5 meters, then the propagation loss associated with the transmitted power spreading out is: -20 log (37.5/375,000) = 20 log (10,000) = 80 dB. Therefore, I still would have 43 dB of path loss margin left for an S9 received signal. That means that only 1/20,000th of the transmitted signal needs to be back-scattered into the 375km diameter receiving zone.

Stu

[Steve - WB3HUZ]

In the case of NVIS (I believe, but I have not found a good technical reference): the refraction effect is not present (because the angle of the wave it too high). What is left if the scattering effect.

NVIS is always performed below the critical frequency.

[WB4AIO]

Kevin

Depending upon the frequency and the ionization state of the ionosphere... there is a maximum incident angle (relative to horizontal)... above which refraction is not sufficient to result in enough bending to cause the incoming wave to return to the ground (i.e. thinking of this as equivalent, for our purposes, to a reflection... no reflection will occur for incident waves above that angle.)

In the technical literature, this maximum incident angle is often referred to as the "critical angle".

Below the critical angle, the effect we call refraction results when electrons in the ionosphere weakly interact with the incoming wave. They absorb (very briefly) and re-radiate it (with a 90 degree phase shift) in all directions. This is similar to the way a parasitic antenna element absorbs and re-radiates power emitted by the driven element (and other parasitic elements). In the case of refraction, the re-radiated/phase shifted power adds coherently to the incident wave... and the net effect is that the incident wave appears to be traveling slower (i.e. the speed of light in the ionosphere will be slightly less than the speed of light in free space), and the direction of propagation of the incident wave will gradually change. Again, this is very similar to the way in which parasitic elements of an antenna change the direction of portions of the power emitted by the driven element. Power that is re-radiated in other directions (other than the direction of the incident wave) tends to add up in a way that causes it to cancel out. Thus, there is very little power "scattered" in other directions.

However, there is some power scattered in the backward direction... whose power level depends upon various types of inhomogeneities in the ionosphere. This phenomenon (back-scattering) is similar to (but not exactly the same as) the phenomenon that makes the sky appear to be blue. I.e. blue light is scattered more strongly (in all directions) by the atoms in the atmosphere than green and red light. Thus blue light is removed from the incoming sun light... and the sun appears to be yellow (white minus blue = yellow).

In the case of NVIS (I believe, but I have not found a good technical reference): the refraction effect is not present (because the angle of the wave it too high). What is left if the scattering effect.

Since the allowable loss between the transmitter is so large (e.g. 100 Watts = 123dB over S9), provided the background noise levels are not too high, the back-scattered NVIS power is (apparently) sufficient for good communication along some short distance paths.

For example, if I radiate 100 watts into an area whose diameter is 375km, and if the equivalent area of a full length (half wave dipole) 75 meter receiving antenna is a circle whose diameter is 37.5 meters, then the propagation loss associated with the transmitted power spreading out is: -20 log (37.5/375,000) = 20 log (10,000) = 80 dB. Therefore, I still would have 43 dB of path loss margin left for an S9 received signal. That means that only 1/20,000th of the transmitted signal needs to be back-scattered into the 375km diameter receiving zone.

Stu

Stu --

Thanks for taking the time to explain it all so elegantly and completely. And thank you, Steve, for reminding us of a crucial fact.

I think -- and Steve's reminder that "NVIS" is a below-critical-frequency mode reinforces my opinion -- that what has been recently called NVIS (and everyone always used to call just plain "short skip") on 75 is not a scattering phenomenon. It's the fairly reliable high-angle refraction-type propagation that we always get when 75 is below the critical frequency -- and that's most of the time, even right up to 90 degrees.

When the critical frequency goes lower for the short path you're using on 75, the nearer stations fade way down rapidly and then almost disappear. If the farther interfering signals and background noise allow, sometimes you can still hear a faint wavering trace of the person you were talking to, even after the fadeout. Such faint residual signals may well be due to scattering.

As to the "diode effect" that we occasionally see -- what seems to be one-way propagation -- I think it's explainable if we consider that the ionosphere is not uniformly ionized. (I am writing an article on the magic of radio, and I'm titling it "The Clouds You Cannot See." Like a bank of clouds, the ionized layers are lumpy and uneven and of slightly differing properties, which are constantly changing.)

Since our HF radio waves are not truly reflected but are instead refracted, they must pass through a considerable depth of the "reflecting" layer before exiting it again and returning to Earth.

A thought experiment will illustrate the implications: If the inside-the-layer part of the N-S path from, say, WA1HLR to W3DUQ is more highly ionized on its northern half than on its southern, there may be sufficient bending from just that north half to allow Tim's southward-bound wave to return to Earth and be heard at W3DUQ's Pennsylvania location. But Bill's northward-bound wave, encountering less refraction, may not even bend enough to reach the more highly-ionized region to the north -- it may pass right over it and leave the bounds of Earth.

Hence, one-way propagation. It's fairly rare, but I think most long-term HF operators have encountered it. There may be other explanations, too, and mine might be wrong some or all of the time. But it makes sense to me.


73 and, as WA4GGL used to say, good luck in the contest,


Kevin, WB4AIO.

[AB2EZ]

Kevin

The law of reciprocity is a theorem (if A,B,C ... then reciprocity) that holds true for any path if A,B, and C, are satisfied.

The condition (one of the assumptions: A,B,C... that are in the "if" part of the theorem) that is not satisfied for some channels is: time invariance of the channel. The most common type of time variation in a channel, that leads to a non-reciprocal channel (e.g. in an isolator or a circulator), is Faraday rotation.

[The other conditions (the other assumptions: A,B,C) have to do with how the measurement of reciprocity is performed, and are not related to any details of the propagation path/channel itself]

Thus, to the extent that non-reciprocity ("diode effect") actually occurs, the cause must be a time varying channel (e.g. Faraday rotation).

If the channel is not time varying, then it will satisfy reciprocity.

To find references on-line, you should enter "principle of reciprocity". If you enter "law of reciprocity" you will find mostly references to sociology-related matters.

Stu

[AB2EZ]

Steve
Kevin


I (respectfully) do not agree  that NVIS is a refraction-based phenomenon or a reflection-based phenomenon.

Naturally... the fact that one person says "it is" and another person says "it isn't" doesn't resolve the difference of opinion. That's why I am looking for an actual physical derivation (based on Maxwell's equations, etc.) of the NVIS phenomenon that can provide insights into how to best employ it for communication purposes.

Stu

[Steve - WB3HUZ]

I don't know if it's refraction based or not. I do know that NVIS always occurs below the critical frequency, so by definition, refraction is possible (and to me, very likely).

If the channel is not time varying, then it will satisfy reciprocity.

Most of the time, in HF comms, especially at the lower end of the HF spectrum, the channel is always time-varying.

[KA2QFX]

I had a communications professor whom I thought did a wonderful job of explaining propagation thusly:

   Starting with the re-radiated energy from say a parasitic array element one can readily add and subtract the combined energy of both incident and parasitically radiated energy to see how a directional pattern is formed for any point on a surrounding sphere several wavelengths in radius.  (Assuming of course we know the phase relationships of all element currents, etc. )
   If we then replace the parasitic element with the “image” of the incident antenna formed in say, the earth’s crust, we have a similar situation. The highly conductive and finely dimensioned parasitic element is now lossy and “fuzzy” (that is to say physically larger in volume).  Consequently the ultimate pattern is still defined by the geometric sums of various points of radiation, however those point’s locations have become less well defined and more numerous.
   Such near field considerations are not too difficult to imagine.  But projecting this further, to the ionosphere, requires the additional consideration of tropospheric bending and other affects.  But the idea of multiple points of re-radiated and incident energy still holds to produce a resultant field.  The more clearly defined the layer of ionization, the more distinct will be the resultant wavefronts, and vice versa.  
   The critical angle, which varies most significantly with frequency, was explained principally as a matter of wavelength, not time.  Since the shorter wavelength has the opportunity to produce many more “images” in the ionized layer, the resultant reconstruction of wavefronts back towards earth largely cancel. The radiation scattered away from earth tends to maintain a similar phase relationship and thereby penetrate the ionosphere. Since it does so the predominate phase shift that Stu mentioned we have Faraday Rotation.  (I was not aware that Faraday Rotation occurred on signals returned to earth.)
   
   So, that’s how it was all explained to me and it seems to make sense and predict what we perceive. I therefore do not think in terms of “reflection” or “refraction” as much as “reconstruction”.  

BTW: In answer to the original question my vote is for the ½ wave dipole at a1/4 above ground, or thereabouts.  

Jay: FYI,  I modeled moving the reflector around under the driven radiator and got only slight pattern skewing until ¾ or more above ground. Apparently the ground effect is tough to overcome. Which leads me to question whether the reflector really enhances the NVIS pattern much on transmit.
   
   

Great discussion.

[WA1GFZ]

Maybe reconstruction is best when excited at 90 degrees to get a nvis return.
Any other angle sends the signal away. So the trick is to reduce the reflection losses of the earth below the antenna.