75 meter antenna design tradeoffs

[Steve - WB3HUZ]

Which leads me to question whether the reflector really enhances the NVIS pattern much on transmit.

In the modeling I've done, little or none. I found one "design" on the Web called a super-gain NVIS antenna. It was for 40 meters. It was nothing more than a folded dipole 7 feet off the ground with three approximatley half-wave wires on the ground beneath the antenna. One was directly under the antenna, the other two spaced 6 feet away. All were oriented in-line with the folded dipole.

This so-called super-gain antenna was several dB worse than a dipole at 25 feet in height. Moving the height of the super-gain radiator up to 20-25 feet increased the gain to about 0.5 dB more (at 90 degrees elevation) than the dipole.

For an antenna that has some real gain at high elevation angles, check out the Jamaica antenna. My modeling shows almost 6 dB gain at 90 degrees elevation compared to a dipole.

[K5UJ]

Stu,

How are you searching the literature?  I have been looking in IEEE Xplore.  If your university has that it is a good place to start.  I found this:


The extraordinary wave mode: Neglected in current practical literature for HF NVIS communications

Walden, M. C.;
Ionospheric radio Systems and Techniques, 2009. (IRST 2009). The Institution of Engineering and Technology 11th International Conference on
Publication Year: 2009 , Page(s): 1 - 5

IET Conferences

AbstractPlus  |  Full Text: PDF (8180 KB)
Quick Abstract

Current practical literature for HF NVIS communication places significant emphasis on foF2 as being the maximum frequency for vertical propagation. This, however, fails to consider the extraordinary wave. This paper presents the analysis of 5 MHz beacon data, showing the relevance of the extraordinary wave in the MUF calculation for NVIS propagation. The results are in full agreement with established scientific theory and ionospheric propagation prediction methods, the detail of which the HF NVIS user community may not be aware of.

That seemed interesting.  maybe this:

NVIS Communications During the Solar Minimum

Johnson, Eric E. ;
Klipsch School of Electrical and Computer Engineering, New Mexico State University, Las Cruces, NM


This paper appears in:  Military Communications Conference, 2007. MILCOM 2007. IEEE
Issue Date :  29-31 Oct. 2007
On page(s): 1 - 5
Location: Orlando, FL, USA
Print ISBN: 978-1-4244-1513-7
Digital Object Identifier :  10.1109/MILCOM.2007.4455217
Date of Current Version :   22 February 2008
Abstract

A popular technique for providing non line-of-sight coverage in mountainous or urban terrain is high frequency (HF) radio near-vertical incidence skywave (NVIS) mode. In NVIS operation, HF energy is directed vertically, and is refracted from the ionosphere to return nearly vertically to the area surrounding the transmitter; this nicely avoids nearly all obstacles. Direction finding versus a NVIS transmitter is also made difficult to the extent that the energy is directed vertically and groundwave is suppressed. However, NVIS operation requires the presence of substantial ionization in the ionosphere directly overhead the transmitter, so some concern is warranted about the effectiveness of NVIS operation during a solar minimum. In this paper, we present measurements and analysis of a multi-year NVIS experiment conducted during the current minimum of the sunspot cycle, including the effectiveness of the NVIS mode of operation, and evaluation of the accuracy of standard HF propagation programs in predicting usable operating frequencies.



Latitude, altitude, azimuth and polarization angle dependence of scattering of radio waves in the ionosphere considering anisotropy

De, S.S.;
Radio and Electronic Engineer
Volume: 38 , Issue: 3
Digital Object Identifier: 10.1049/ree.1969.0092
Publication Year: 1969 , Page(s): 165 - 168

IET Journals

This paper describes the latitude, altitude, azimuth and polarization angle dependence of scattering of radio waves in the ionosphere. The anisotropic behaviour which arises due to the presence of the geomagnetic field has been taken into account. The formula obtained for scattering cross-section may be verified experimentally and the importance of the geomagnetic field in the production of anisotropy may then be analysed.


Polarisation of v.l.f. radiowaves reflected from the ionosphere

Jones, T.B.; Spracklen, C.T.;
Electrical Engineers, Proceedings of the Institution of
Volume: 123 , Issue: 2
Digital Object Identifier: 10.1049/piee.1976.0022
Publication Year: 1976 , Page(s): 115 - 122

IET Journals

The polarisation of v.l.f. radiowaves reflected from the ionosphere at steep incidence has been determined from simultaneous measurements of the reflection and conversion coefficients. Changes due to the diurnal and seasonal variations in electron density are reported and the marked anomalies observed during sudden ionospheric disturbances produced by solar-flare activity are discussed. The propagation parameters have been calculated using the fullwave analysis for a number of idealised models of the electron density/height distribution. The calculated values are in close agreement with those obtained experimentally. It is suggested that these models could be used to determine the propagation parameters as an aid in the development of any new v.l.f. system.


Those reflectors on the ground do nothing unless they are scaled to what you see with a TV antenna bow-tie reflector for UHF.  On 75 m. the reflector screen would be huge.   As Frank says, anything on the ground is probably a benefit only if it reduces ground loss.

[AB2EZ]

Rob

Thanks! I guess it's a matter of using the right search phrases in the right search engine.

I've seen a few of these... but several of them are new to me ... and quite useful (they also appear to contain additional useful references).

I am still going through them, but I found the first one particularly helpful.

I think that an important clue is in the mention of the complementary roles of the "ordinary" and the "extraordinary" wave... as components of the overall ionospheric propagation process. I'm going to have to wade through the older references that contain the detailed analysis that is based on modelling ionospheric propagation in terms of those components.

Stu

[K5UJ]

That's great Stu; glad I could help.  Part of my day job is finding things for people but I would not know an extraordinary wave if one washed over me at high tide  

I can't remember what I searched with.  It may have been some combination of "NVIS ionospheric physics"   I also tried scholar.google.com but did not get anything that looked useful but it was also nearing the end of my lunch time so I had to quit.  IEEE and scholar.google.com were the only two databases i tried.   Just for the heck of it now, I queried the NASA ADS database for NVIS and came up with this:  http://adsabs.harvard.edu/abs/1991RaSc...26..971R

if I get any more productive ideas I'll let you know.

Rob

p.s. trying the "find similar abstracts" send query button at the bottom of the page might produce something

[WB4AIO]

Which leads me to question whether the reflector really enhances the NVIS pattern much on transmit.

In the modeling I've done, little or none.
[...]

For an antenna that has some real gain at high elevation angles, check out the Jamaica antenna. My modeling shows almost 6 dB gain at 90 degrees elevation compared to a dipole.

Never heard of the Jamaica antenna -- I'll have to look that up.

I have spent a few hours modeling "cloudwarmers" -- dipoles or the like with single reflectors underneath. I used the Eznec demo with real wire losses included and used "real, poor quality" ground to simulate what I'm usually stuck with.

By tweaking the spacing and size of the reflector, it is apparently possible to significantly reduce ground losses of medium-height 75 meter antennas with such techniques. *But* I always found that the loss reduction only occurred for a narrow range of frequencies: A reflector that got my losses down to a nice 1 dB or so on 3885 kHz made things much worse on 3733 kHz -- 6 dB worse or more!


With all good wishes,



Kevin, WB4AIO.

[Steve - WB3HUZ]

If you aren't using Somerfield ground modeling, your results are likely erroneous.

[WB4AIO]

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.

Interesting on Cebik. If you make your antenna a horizontally-oriented square 40 feet on a side, fed in the center of one side, and open directly opposite the feed point, you will achieve a beautiful omni pattern and it will be efficient on 75. I built one once but didn't have the four supports needed to get it high enough to reach its potential.


With all good wishes,


Kevin, WB4AIO.

[ke7trp]

Here is my antenna on 40 and 75 meters.  It works really well.  There is some interaction going on with my 160 L which is off to one side. If I ground that antenna out I can change direction of this antenna to some degree.  Several people have come over to measure my antenna as it puts out a good signal.

This antenna is copper weld wire fed with 600 ohm open wire line to a KW matchbox.  I have worked TONS of DX on 20 with this thing as well.

C

[Steve - WB3HUZ]

Looks pretty similar to a dipole for each band at the same height, except for 20 meters. There you are getting more lobes than you would with a 0.5 WL dipole and what appears to be a smaller low-angle lobe.

[Steve - WB3HUZ]

Here is the Jamaica versus a dipole comparison for 80 meters. Both antennas are at a height of 50 feet over average ground.

For an antenna that has some real gain at high elevation angles, check out the Jamaica antenna. My modeling shows almost 6 dB gain at 90 degrees elevation compared to a dipole.

Never heard of the Jamaica antenna -- I'll have to look that up.