75 meter antenna design tradeoffs

[AB2EZ]

For those of us who like to communicate on 75 meters with other hams within a 500 km radius... what is the best antenna design to use ... and why?

Stu
AB2EZ

[K1JJ]

Stu,

A flat, straight, 123' long, 1/2 wave  dipole at 65' high over AVERAGE ground, (fed with any feedline you wish, in the center) is the best antenna you can put up for local work. If it is significantly lower than 65', then the ground losses and extreme useless highest angles dominate. If it is higher than say, 85', then the lower angles not useful for ~300 miles or less start to dominate.

Many of us have tried and many still resist kicking and screaming, but the simple dipole at 65' (at 1/4 wave high) is still the best antenna going for LOCAL work.  The 1/2 wave long dipole also forms a nice textbook figure eight bi-directional pattern for general coverage. Longer dipoles become too sharp for non-rotational, fixed use.

BTW, zenith beams, reflectors under the dipole, etc generate take-off angle patterns at 80-90 degrees - too high take-off. The dipole at 65' high is more useful for medium distance local work. The optimum angles for local 50 to 400 mile use are usually between 60-75 degrees, much depending on the height of the ionosphere, solar cycle, time of day, etc. This makes the dipole at 65' perfectly positioned.

T

[Jim, W5JO]

I dunno, what is a kM?

[KC2YOI]

500 kilometers = 310.685596 miles

[Ed/KB1HYS]

I know this is just anecdotal evidence but I use a full wave loop on 75m at an average height of 35 feet.  From NH I can hit All of New England, NY, PA and NJ with the Valiant running ~100 watts. I have hit further detroit etc but that is not reliable.

[K5UJ]

I know this is just anecdotal evidence but I use a full wave loop on 75m at an average height of 35 feet.  From NH I can hit All of New England, NY, PA and NJ with the Valiant running ~100 watts. I have hit further detroit etc but that is not reliable.

Ed,

I had a loop like that too about the same height and it seemed to be okay but after a few years I wanted it higher because I was starting to suspect it could be better.  Only problem was putting it up where I wanted it to be here in town was not possible, due to the number of structures needed, where I'd have to put them and their cost.  I took it down and on JJ's advice put up a 130' dipole center fed with ladder line and got it up to 45 -50 feet.  Guys were telling me I was 10dB stronger.  I wish I could have it even higher but on my lot the supports I can put up only go so far.   The loop doesn't really get you much in practice--height is everything until you get to about 80 feet, and that's the whole antenna.  Don't fall into the inverted V trap where the feedpoint is 50 feet and the ends are 15 or 20 feet.  IOW you loose a lot of power in ground proximity and the loop may even be counter productive because it increases your ground coupling because of its size.  I think loops are great and am a fan of them but only if they can be high enough to be decoupled from ground, i.e. around 70 feet or more.  If you can get it that high go for it, otherwise do da dipole

Rob

[WBear2GCR]

...then there are arrays, parallel fed and phased... but then you need realestate...

              _-_-bear

[KM1H]

Im using vertical real estate, crossed inverted vees at 180' soon to be joined with another pair at 60'. A 4 square with elevated radials is also in the works if the weather ever cools down. Most 80/75 activity is chasing DX on CW/SSB.

In the past Ive had excellent results with dipoles or inverted vees at 50-75' for local chatter on 75 and even some high angle grayline DX.

Carl

[W1VD]

BTW, zenith beams, reflectors under the dipole, etc generate take-off angle patterns at 80-90 degrees - too high take-off.

Wonder if offsetting the reflector (from vertical) would steer the pattern? If so, one could have several reflectors and switch them with relays that open the unused reflector wires at the center. Might be interesting to run through simulation. 

[Steve - WB3HUZ]

If you had several (how many ??) this might work. Just one reflector offset will only have an impact over a very small set of angles (maybe only one) and probably be swamped out by the numerous other reflection points off the ground. The first relection point for each angle is a different distance from the antenna.

BTW, zenith beams, reflectors under the dipole, etc generate take-off angle patterns at 80-90 degrees - too high take-off.

Wonder if offsetting the reflector (from vertical) would steer the pattern? If so, one could have several reflectors and switch them with relays that open the unused reflector wires at the center. Might be interesting to run through simulation. 

[flintstone mop]

I dunno, what is a kM?

ohoh someone is not ready for the metric conversion.
ahhh yes antenna time is approaching. I have a reminder to buy some vinyl covered welded fencing for a big ground screen.

Fred

[John Holotko]

Years ago I had pretty good results with a balanced line fed inverted vee doublet cut for 80 meters up about 65 - 70'.  It gave me pretty good coverage, with a respectable signal at 100 watts, throughout New York, New England and into the South into South Jersey, PA, Virginia,  and further depending on band conditions. and out west into Ohio, Michigan, etc. It also tuned up nicely via my Johnson Matchbox, on 40, 20 and even 10. It was a good compromise considering I don;t have the room to stretch a full length flatop dipole. .  Unfortunately the Nor easter that swept through here this spring put the kabosh on that antenna. I hope to get something new up there this fall.

[WD5JKO]

I use a single wire feed (insulated tower) OCF Windom antenna with the vertical feed at 38', and the ends droop down inv V style. The dual polarization seems to give me an advantage over similar folks using just a low Inv V dipole. A few years ago I partnered with Lynn K5LYN, and took over as net control of the 'Boat Anchor' net and I was running a modified CE 20A barefoot (80m LSB). I had no issues being heard well over a 400 mile radius at about 8 pm. When I use my amplifier, I'm often one of the the top signals on that net. As crude as this antenna is, it works. Just yesterday on 3725 at 11 am, a station about 60 miles away said I sounded like I was in his driveway.

I keep meaning to do some modeling. It seems to me that the old fashioned Windom with single wire feed is seldom used anymore. Instead some commercial products use dual wire (coax) feed and make attempts to minimize feedline radiation. In my case I want the feedline to be part of the radiator, and then have a tuner at the ground attached to the ground rod and radial system.

This IS NOT a low noise receive antenna however.

Slightly off topic:
While repairing my lawn sprinkler system after my recent lightning hit, it occurred to me that all this underground wiring going to 16 zones on 1/3 acre may have use as a counterpoise for my 80m Windom antenna. Has anybody done this? I'm thinking adding multiple ferrite beads over the sprinkler power cord (to a large wallwort), and then take a 0.01 cap to each zone wire, and common return wire (17 in all). The sprinkler controller is about 10' away from my antenna feedpoint. I see this idea as a significant boost in my 80-160m ground radial system. Then I fear what the controller electronics might do when I key up. 

So as it is now, with the sprinkler system floating, do those wires have any effect on my transmitted signal?

Jim
WD5JKO

[W2VW]

A simple current fed resonant half wave at 50 to 70 feet would play best for local regional work.

IIRC the antenna there was a parallel set of dipoles with 75 and later 160.

Stu, with your skills I'd just love to see you go through all the analysis of a balanced line fed antenna there.

The 160 dipole could be left up all by itself and used as a pair of half waves in phase on 75.

Have fun!

[K3ZS]

I use a single wire feed (insulated tower) OCF Windom antenna with the vertical feed at 38', and the ends droop down inv V style. The dual polarization seems to give me an advantage over similar folks using just a low Inv V dipole.
Jim
WD5JKO

I could never understand the operation of the old OCF single wire fed windom.    It seems to me that it is just an end fed antenna of some random length.    That is not to say that it can't work well as Jim as shown.

[W9GT]

I'm sure that Stu could analyze these options from the technical standpoint much more proficiently than most of us, but I think that many of the antenna options referenced here work well.  Walt should also chime in on this discussion!

My experience has revealed that a three-wire dipole (three parallel wires fed with open wire feeders)  at 65 ft. AGL works better than any other 75 meter antenna that I have tried.  This antenna is actually what amounts to a double folded dipole with a feedpoint impedance of something close to 600 ohms.  It is fed with open wire and a remote antenna tuner with coax back to the shack.  I fully recognize that , in theory, the only real advantage of such an antenna is bandwidth capability.  Otherwise, it should work as any other dipole antenna.  I seem to receive, however, consistent unsolicited great signal reports when using this antenna system.

What has not been really discussed, however, is the environment in which an antenna is installed.  Various objects in the near field of an antenna, as well as the effective height of that antenna above average terrain and ground conditions have much to do with performance.  There are lots of variables to consider and there may not be a definitive answer as to what works best, at least for all locations.  It seems, however, that some form of 1/2 wave (or greater, if a non-resonant system) dipole fed with open wire might be the best overall antenna.

73,  Jack, W9GT

[AB2EZ]

Hi!

Thanks to all for the comments that you have posted so far.

One of the things that is on my mind is the issue of how strongly the radiated signal is returned to locations that are anywhere from 10km to a few hundred km from my location.

I've been searching the web for published papers that go into the details of the physics of how NVIS (near vertical incidence skywave) works. For some reason, I can find many papers that describe NVIS systems and their applications... but very little regarding the underlying physics of NVIS.

What I believe is happening is that the incident radio wave is "reflected" as a result of physical interactions (with electrons in the ionosphere) that involve "Faraday rotation". Electrons in the ionosphere interact with the Earth's magnetic field ... and end up precessing (spiralling around the magnetic field lines)... analogous to the precession of a gyroscope on a table interacting with the Earth's gravity. [Ref: electron gyro frequency.] When the incident wave passes through the volume of space that contains these precessing electrons, some of the incident power is reflected (but I can't find a useful reference with the details of why/how this happens).

Anyway... one of the questions on my mind is whether the direction of propagation of the transmitted signal (straight up v. small angles relative the vertical) and/or the polarization state of the transmitted signal has a significant effect on the strength of the signal that is returned to the receiving locations when NVIS is the propagation mechanism.

I also believe that the role played by Faraday rotation in NVIS is the reason why "reciprocity" does not always appear in North-South NVIS communication.

Stu

[KF1Z]

Yes Stu,
Many people can talk or write many pages about how to do something, without ever actually knowing how it works, and why.

Isn't it amazing.....

 

[K5UJ]

I do not have a high regard for the NVIS concept as most hams seem to implement it.

First, I want to get past the million dollar term Near-Vertical Incidence Sky-wave, or NVIS, which is nothing more than a cloud burner, usually a low dipole.   What happens (based on my reading of articles on-line and in QST) is that hams freely borrow from the American military use of cloud burner antennas such as dipoles, noting that in their field manuals and such, they (the military) recommend a height of 0.1 to 0.2 wavelength above ground.   For 75 meters this is around 25 to 50 feet.  Most hams take this to mean they have license to put their 75 or 80 meter dipole up 25 feet and everything will be FB because the U.S. Army said so, and they ought to know.   Don't they have all kinds of DARPA Ph.D.s studying this stuff?  But wait--the goals of the military operating in the field are not always the same as the goal for hams.   First, the military want an antenna that is quick and easy to put up that will give cloud burner propagation on a wide range of frequencies.   They want to be heard within around 100 miles on 2, 5 or maybe 20 MHz.  Secondly, the military can run whatever power it takes to do the job.  Hams can't run 20 or 30 kw; the Army can and they are not all that concerned with ground loss and efficiency.  Hams are (or should be).  Heck, the military even use folded dipoles with resistors to get a wide frequency load and if there are any problems they simply crank up the watts. 

So, many hams doing the "NVIS" thing get into trouble thinking anything the Army does is antenna gospel.  It is, if you are tactical communications back to Hq. in combat, but not if you are trying to work the HUZman in Va.  on 3880.    

So, you can have your cloud burner NVIS thing, but at a height that gives both a 90 degree angle and much less ground loss.    Guess what--now we're back to the 65 foot high dipole 

[Steve - WB3HUZ]

There is definitely some scattering going on. The ionosphere is not a smooth "mirror" or homogenous, even though this is often how the basic theory is taught. Tilting also happens at different times of the day.

Hi!

Thanks to all for the comments that you have posted so far.

One of the things that is on my mind is the issue of how strongly the radiated signal is returned to locations that are anywhere from 10km to a few hundred km from my location.

I've been searching the web for published papers that go into the details of the physics of how NVIS (near vertical incidence skywave) works. For some reason, I can find many papers that describe NVIS systems and their applications... but very little regarding the underlying physics of NVIS.

What I believe is happening is that the incident radio wave is "reflected" as a result of physical interactions (with electrons in the ionosphere) that involve "Faraday rotation". Electrons in the ionosphere interact with the Earth's magnetic field ... and end up precessing (spiralling around the magnetic field lines)... analogous to the precession of a gyroscope on a table interacting with the Earth's gravity. [Ref: electron gyro frequency.] When the incident wave passes through the volume of space that contains these precessing electrons, some of the incident power is reflected (but I can't find a useful reference with the details of why/how this happens).

Anyway... one of the questions on my mind is whether the direction of propagation of the transmitted signal (straight up v. small angles relative the vertical) and/or the polarization state of the transmitted signal has a significant effect on the strength of the signal that is returned to the receiving locations when NVIS is the propagation mechanism.

I also believe that the role played by Faraday rotation in NVIS is the reason why "reciprocity" does not always appear in North-South NVIS communication.

Stu

[K5UJ]

I did some reading about this a few months ago because I was curious about the ionosphere also.  It takes a physicist to explain it adequately and I am not one, but I loosely grasp that the ionosphere really doesn't reflect if we define "reflect" as something bouncing off a surface.  It gets excited by photons and reradiates.   But, I can't explain arrival and departure angles.   All so-called passive media that are what I would call photon-responsive in the RF range at least, have free electrons.   Your copper antenna wire has unbound electrons drifting in its lattice (a metal lattice is sort of like a crystal structure).  Elements with lots of atoms make better conductors because they have more loosely bound electrons in the higher orbits which are more likely to drift when there is no potential (silver).  When gaseous elements get ionized by sunlight in the ultraviolet range there are unbound electrons which are also subject to influence.  What frequencies affect them may depend on the density of the ionized layer.  Like with electrons that are free on the surface of a metal parabolic dish, they are energized and give off photons just like your electrons do in the lattice of your antenna wire when you deliver RF current to it.   I'm not very good at explaining this partly because it has been a long day and I'm tired--sorry about that.  Stu you probably have a better understanding of this.  Make corrections.

[AB2EZ]

Rob

You have summarized a number of the key issues.

[As an aside, even the world's greatest physicists admit that they do not really understand this stuff... when you get down to the deepest details. For example, no one knows why some fundamental particles exhibit the property of having a charge (-1 for electrons)... or why charged particles interact via the electromagnetic force. However, by using abstract models, like Maxwell's equations, one can predict the outcomes of experiments; and one can design things, like antennas, that work in predictable ways. After a while, we begin to believe we understand the physical world, because we can predict some of its behaviors with great accuracy.]

There is an important difference between ordinary ionospheric "reflections" and NVIS.

In the case of ordinary ionospheric "reflections", one can use Maxwell's equations (or equivalent methods) to show that a radio wave that is propagating through a medium, whose density of free electrons varies with position, will travel in a curved path. If a radio wave is launched toward the ionosphere with a shallow enough launch angle, then it will gradually curve around as it travels from lower to higher altitudes (i.e. higher-to-lower density of free electrons)... and end up traveling back toward the surface. This is (for our intents and purposes) equivalent to a reflection.

In particular, if one defines an appropriate equivalent reflection altitude, then the angle of incidence equals the angle of reflection. The largest angle that will result in enough bending to result in a "reflection" depends upon: the details of the density of electrons v. altitude in the ionosphere; and the wavelength (frequency) of the radio wave.

NVIS is a different phenomenon. It also behaves like a reflection in the sense that a radio wave launched toward the ionosphere, from the ground, is partially "reflected" back toward the ground.

However, this does not result from the gradual bending of the direction of the wave as it passes through the ionosphere at a shallow angle. It is caused (I believe, but I can't find a good reference) by the fact that the free electrons in the ionosphere are spiraling around the Earth's magnetic field lines as these electrons move toward the North or South magnetic pole. The mechanical frequency at which each electron spirals around (1/the time required to make one complete rotation) depends upon its location, but is (very roughly) around 1MHz. This is called the electron gyro frequency. Somehow (I am trying to find a reference with the details), as a radio wave passes through a medium (the ionosphere) containing these spiraling electrons, a reflection results... even at vertical or near vertical incidence. One thing that is important (to some extent) for communication purposes is whether or not the strength of the NVIS reflection process is dependent upon the polarization state of the incoming (launched) radio wave. Another thing that is important for NVIS communication is whether the received signal has a polarization state that is different from the transmitted signal.

As I have mentioned before, since the physical process of NVIS probably doesn't satisfy the "if" part of the law of reciprocity ("If A,B,... are true... then the channel obeys reciprocity"), it is, perhaps, less surprising that South-North NVIS paths sometimes exhibit a "diode effect".

Stu

[K5UJ]

Stu,

Yes, I had wondered about ionosphere arrival and departure angles and wondered if a wave front is split in the ionosphere medium similar to looking at an object submersed in a pool of water and thought that some portion of the signal arriving at the ionosphere (let's say for example a 20 meter signal arriving at a low angle) passes through it and only a portion is bent and re-energizing the ionosphere. 

It would be interesting to try NVIS operation near one of the poles and see if the propagation is the same as what we experience here much closer to the equator.

Rob

[Steve - WB3HUZ]

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. This is why vertical antennas are superior on 160 meters. Unless the absorption doesn't occur at very high incident angles.

Rob

You have summarized a number of the key issues.

[As an aside, even the world's greatest physicists admit that they do not really understand this stuff... when you get down to the deepest details. For example, no one knows why some fundamental particles exhibit the property of having a charge (-1 for electrons)... or why charged particles interact via the electromagnetic force. However, by using abstract models, like Maxwell's equations, one can predict the outcomes of experiments; and one can design things, like antennas, that work in predictable ways. After a while, we begin to believe we understand the physical world, because we can predict some of its behaviors with great accuracy.]

There is an important difference between ordinary ionospheric "reflections" and NVIS.

In the case of ordinary ionospheric "reflections", one can use Maxwell's equations (or equivalent methods) to show that a radio wave that is propagating through a medium, whose density of free electrons varies with position, will travel in a curved path. If a radio wave is launched toward the ionosphere with a shallow enough launch angle, then it will gradually curve around as it travels from lower to higher altitudes (i.e. higher-to-lower density of free electrons)... and end up traveling back toward the surface. This is (for our intents and purposes) equivalent to a reflection.

In particular, if one defines an appropriate equivalent reflection altitude, then the angle of incidence equals the angle of reflection. The largest angle that will result in enough bending to result in a "reflection" depends upon: the details of the density of electrons v. altitude in the ionosphere; and the wavelength (frequency) of the radio wave.

NVIS is a different phenomenon. It also behaves like a reflection in the sense that a radio wave launched toward the ionosphere, from the ground, is partially "reflected" back toward the ground.

However, this does not result from the gradual bending of the direction of the wave as it passes through the ionosphere at a shallow angle. It is caused (I believe, but I can't find a good reference) by the fact that the free electrons in the ionosphere are spiraling around the Earth's magnetic field lines as these electrons move toward the North or South magnetic pole. The mechanical frequency at which each electron spirals around (1/the time required to make one complete rotation) depends upon its location, but is (very roughly) around 1MHz. This is called the electron gyro frequency. Somehow (I am trying to find a reference with the details), as a radio wave passes through a medium (the ionosphere) containing these spiraling electrons, a reflection results... even at vertical or near vertical incidence. One thing that is important (to some extent) for communication purposes is whether or not the strength of the NVIS reflection process is dependent upon the polarization state of the incoming (launched) radio wave. Another thing that is important for NVIS communication is whether the received signal has a polarization state that is different from the transmitted signal.

As I have mentioned before, since the physical process of NVIS probably doesn't satisfy the "if" part of the law of reciprocity ("If A,B,... are true... then the channel obeys reciprocity"), it is, perhaps, less surprising that South-North NVIS paths sometimes exhibit a "diode effect".

Stu

[K5UJ]

This is only tangentially interesting as far as the topic is concerned but I'm including it below because it discusses what range of radiation frequencies have the most effect on the ionosphere, something I had not known exactly before.  This is excerpted from this week's propagation forecast from NW7US which is distributed every Friday from an ARRL mail list:

<<<The late Robert Brown, PhD, NM7M, proposed that the hard X-ray
energy present in the wavelengths from 1 to 8 Angstroms provide the
most effective ionizing energy throughout all of the ionospheric
layers in our atmosphere.  The GEOS satellites measure these
wavelengths and the resulting measurements are reported as the
background X-ray level throughout the day.  A daily average is
reported, as well.

Dr. Brown recorded the daily background X-ray levels for several
sunspot cycles, and discovered that during solar cycle minimum
periods, the background X-ray levels remained at the A class level.
During the rise and fall of a solar cycle, the background X-ray
energy levels remained mostly in the B range.  During peak solar
cycle periods, the background energy reached the C and sometimes
even M levels.

Armed with this information, can we discover any clues as to the
current status of Sunspot Cycle 24?  The Space Weather and Radio
Propagation page maintained by NW7US at http://prop.hfradio.org/
includes a graph showing the daily and monthly averaged hard X-ray
flux since the end of Sunspot Cycle 22.  The plot reveals a
noticeable rise in Cycle 24 activity.  We're seeing the energy rise
to the B level more often as 2010 progresses, supporting the view
that Cycle 24 is alive and moving along toward an eventual sunspot
cycle peak in several years.>>>