Pages: [1] 2   Go Down

Author Topic: buried in noise  (Read 392 times)

KJ4SKP

  • Member
  • Posts: 106
buried in noise
« on: January 01, 2023, 07:59:37 AM »

My buddy is a recently licensed general class ham in Nashville.  I live just outside Atlanta and have made a few (very noisy) unreadable contacts with him on 20m and 40m this AM. We have attempted contact a few times with similar results.  On both ends the receive audio is very faint/weak.  I am able to receive audio from other stations in various parts of the country/world with strong audio.  My friend is using an Icom-7300 at 100W and I have a Ten Tec Omni VII at 100W.  I'm assuming we are experiencing propagation/bounce issues (receiving other stations clearly).  At the distance we are attempting contact Nashville-Atlanta, is there any recommendation you could make that would increase our readablity? Perhaps a different band, setting etc? At some point an amp is in my future but not quite yet.
Logged

AI5BC

  • Posts: 456
    • HomeURL
Re: buried in noise
« Reply #1 on: January 01, 2023, 08:24:38 AM »

Cell phone gets through every time with crystal clear communications anywhere in the world, or across town. No amps required, operates on less than 1/4-watt.
Logged

NA4M

  • Member
  • Posts: 212
Re: buried in noise
« Reply #2 on: January 01, 2023, 08:28:59 AM »

Nashville-Atlanta is probably too close for 20m and likely 40m with 100w.  You didn't mention what antennas are being used on each end.  A high angle radiation antenna (i.e. low dipole/inverted V) will probably work better than verticals etc. for distances that close on 40/20m. 

Higher power will definitely help if/when available.
Logged

SWMAN

  • Posts: 2117
    • HomeURL
Re: buried in noise
« Reply #3 on: January 01, 2023, 08:29:27 AM »

   BC,   Dah !!
Logged

KJ4SKP

  • Member
  • Posts: 106
Re: buried in noise
« Reply #4 on: January 01, 2023, 08:46:26 AM »

Logged

AC2EU

  • Member
  • Posts: 2793
    • McVey Electronics
Re: buried in noise
« Reply #5 on: January 01, 2023, 09:25:29 AM »

Maybe your friend has his mike gain down too low.
Have him do an ALC check.

NA4M

  • Member
  • Posts: 212
Re: buried in noise
« Reply #6 on: January 01, 2023, 09:54:51 AM »

Maybe your friend can build/install a simple 40m inverted V to try instead of the multi-band vertical.  The vertical has a vertically polarized low angle of radiation which is not helpful for regional HF coverage. 
Logged

WA3SKN

  • Member
  • Posts: 8126
Re: buried in noise
« Reply #7 on: January 01, 2023, 10:11:04 AM »

OK, about 250 miles with hills/mountains in the way.
I would try late afternoon/early evening with NVIS inverted Vees on 40 and/or 80 meters.  Keep the antennas about 1/4 wavelength high for best results.  You could use a horizontal loop at the same heights.
Late AM local time is not the best for that path.

-Mike.
Logged

K6AER

  • Member
  • Posts: 7159
Re: buried in noise
« Reply #8 on: January 01, 2023, 10:28:42 AM »

My buddy is a recently licensed general class ham in Nashville.  I live just outside Atlanta and have made a few (very noisy) unreadable contacts with him on 20m and 40m this AM. We have attempted contact a few times with similar results.  On both ends the receive audio is very faint/weak.  I am able to receive audio from other stations in various parts of the country/world with strong audio.  My friend is using an Icom-7300 at 100W and I have a Ten Tec Omni VII at 100W.  I'm assuming we are experiencing propagation/bounce issues (receiving other stations clearly).  At the distance we are attempting contact Nashville-Atlanta, is there any recommendation you could make that would increase our readablity? Perhaps a different band, setting etc? At some point an amp is in my future but not quite yet.

Your in that no man's land of HF propagation (100-300 miles). At less then 100 miles use verticals and depend on the ground wave for communication.

Placing the dipoles so the main lobe face each other at 10-20 feet will be the cheapest option.
 
A couple of legal limit amplifiers will also help, about 12 dB signal increase. This is a $2500 option times two.

Dipoles set for under 20 feet will produce the greatest NVIS effect.

I have found that about one hour after sunset I get my best short skip on 40 meters, but that is in New Mexico.

Forget 20 meters for short skip.

You did not mention what your noise floor is at both locations. Possibly a phase null reduction system like the QRM  Eliminator, MFJ-1025 would help lower your noise floor.

Good Luck,

73 k6AER
« Last Edit: January 01, 2023, 10:31:50 AM by K6AER »
Logged

KH6AQ

  • Member
  • Posts: 9292
Re: buried in noise
« Reply #9 on: January 02, 2023, 12:52:20 PM »

VOACAP HF online shows your best paths to be:

80m, 2000 - 1400 Z, 5 pm to 9 am EST, signals peak round 1330 and 2300 Z.
40m, 1400 - 2300 Z, 9 am to 6 pm EST, steady signals the entire time
20m, never

Probably the best antennas for this 215 mile path are dipoles mounted 1/4 wavelength above ground. Lower dipoles or inverted-vees are good also. Vertical antenna may provide the weakest signals.

https://www.voacap.com/hf/
« Last Edit: January 02, 2023, 01:03:11 PM by KH6AQ »
Logged

KA4WJA

  • Posts: 1601
    • HomeURL
Re: buried in noise
« Reply #10 on: January 03, 2023, 01:52:03 PM »

Jon, KJ4SKP,
Good for you for asking questions!
And, kudos to Dave, KH6AQ, who gave you some wonderful info here!

Maybe I can add something even more on-point (and maybe even something that you should commit to memory, and not look to the internet for?):

1)  First the really short answer:
95%+ of the time:
Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!

(some rare times you'll need to be flexible in regards to mother nature....such as in the depths of winter, in low sunspot years, 160m might be needed at nighttime....and, in the heights of summer, especially in high sunspot years, in addition to high-summertime noise levels (lightning static crashes in SE US can be quite high), 80m absorption (D-layer) can be high and you may find your self switching bands sometimes)

Do NOT use vertical antennas here!


2)  Now, the longer answers:
"Regional" radio comms over the "mid-ranges", between 20-30-40 miles and out to 300 - 500 miles, requires (and has since the days of Marconi, hi hi) using some smarts....some understanding of radiowave propagation and antenna system design / antenna patterns (and, if using MF, the ground conductivity becomes are larger part of the calculations....but no need to go there here).


3)  Without rambling on and on about "ground conductivity", "NVIS", "amplifiers", "low dipoles" [sic], etc....I'd like to point out some hard facts (the "Physics") of radio comms:

a)  the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain) is NOT possible to cover via "groundwave" on any MF or HF ham band....not even close!

b)  the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain) is not possible (or VERY unlikely, except for some rare back-scatter or very short sporadic-E skip) to cover via "skywave" on any HF ham band above 10mhz(30m)!

c)  the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain) is most suited for Near Vertical Incidence Skywave (NVIS) communications, and this is done on the lower HF bands 40m, 60m, 80m, and 160m.  (although, assuming high gain antennas and high transmit powers, 6m SSB or 2m SSB, could also be fairly reliable along this path....the costs and complexity involved, not to mention the fairly point-to-point directionality of this vhf-ssb solution, puts it way, way down the list of recommendations, and I might regret even mentioning it)

d)  Near Vertical Incidence Skywave (NVIS) is, just as the name implies, an ionospheric skywave path where the signals travel "nearly vertical", and are below the ionosphere's current "critical frequency" so they are refracted back down to earth at similar / reciprocal angle.

Jon, please make a mental note here of all the above....all four of these points (facts) here, are in the ARRL Handbook, etc....and all four should be known to pass a General Class exam....so, if you commit them to memory and then go and buy an ARRL Handbook, you'll be on your way to learning / understanding a lot!  :)


Now, some things that might not be in the "Handbook" and/or might not be needed to pass a General Class exam.

e)  As I just wrote, Near Vertical Incidence Skywave (NVIS) is an ionospheric skywave path where the signals travel "nearly vertical", and are below the ionosphere's current "critical frequency" so they are refracted back down to earth at similar / reciprocal angle.

---  Theoretically NVIS comms can occur at freqs from just below 1mhz and on up thru 12mhz (and military does use NVIS beyond the freq ranges that hams do)....but in our world of ham radio, NVIS is a propagation path that is most useful on 80m, 60m, and 40m....as well as less-so on 160m (both due to the critical freq rarely falling below 3.5mhz, and due to antenna sizes and noise levels)

---  The terms "near", "near vertical", "nearly vertical" do not have "carved-in-stone" / scientific definitions, but rather we have an "acceptance" among most RF / Antenna engineers and propagation scientists, that these generally refer to angles of ~ 50 - 60 degrees up to 90 degrees....(which, depending on ionosphere height, represent ranges from 10 miles out to ~ 250 - 300 miles....although, many hams [including myself] use "NVIS" to describe ranges out to ~ 350 - 400 miles, where the propagation angle would be ~ 45 degrees)

---  The term "critical frequency" is a scientific term --- the frequency below which, the ionosphere currently supports reflection/refraction at a vertical angle / straight-up-and-down....and, this "critical freq" is usually 1/3 of the current calculated MUF at the same location (although, this last point, being 1/3 of the MUF is rather moot, as you will almost never know the MUF at your location and along this short path, not to mention the D-layer absorption can be quite high on the lower HF bands when the MUF is high, etc.)   


4)  Some specific answers to your specific query, for your desired comms to/from your house and Nashville, TN....although, not knowing the space each of you have / what trees or other supports you each may have to string up an antenna on, etc...it makes it difficult to be precise in recommendations, but here goes:

As I wrote above....for 95%+ of the time:

--- Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
--- Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!
--- Do NOT try to use a vertical antenna for this path!  It is the worst choice here!  (especially a very poor / inefficient 15' long trap vertical)


---- And, while many will say that at these low heights, dipoles are omni-directional so their orientation is moot, this is actually not the case...

You see, if looking at just 2-dimensions of the azimuthal pattern you may not see a deep null off the ends...but, if you look at the elevation pattern (of say a 40m dipole at ~ 35' high), you will quickly see that while the "broadside" pattern is quite wide and allows you to even have a signal less than 1-db down at angles as low as 45 degrees, the "end-wise" pattern is already 2-db down at ~ 60 degrees and ~ 5db down at 45 degrees....and, since your path to Nashville is going to be a NVIS path at ~ 60 degree elevation (at a true bearing of ~ 318 degrees).

So, orienting your dipoles in a NE-SW directions, placing your broadside pattern in the NW-SE directions, could give you 4db of gain versus having the dipoles strung up the opposite way...


----  Also, while there are some hams that regurgitate the BS / myths that NVIS comms are better when you use "lower" antennas....this is a ridiculous myth, that would apply only when "lower" means lower than 0.4 wavelengths, but higher than 0.15 wavelengths!

This BS comes from the ground losses of very low dipoles reducing the noise levels AND the signals as well (just like their radio's receive attentuator and RF Gain controls do!)   

I could add more graphs and images here to prove all of this, but if you simply accept this as fact, and understand that as dipole height gets below 0.2 to 0.175 wavelengths high, losses go up....and at really low heights of 0.02 wavelengths (about 3' on 40m, ~6' on 80m) loss is ~ -12db versus a dipole at 0.2 wavelengths!   


----  In your area (and your friend's area), having below average ground conductivity, the "optimal" heights for maximum zenith gain (straight-up) is about 27' to 28' high for 40m (and 46' - 49' for 80m)....and, while maybe only adding one db or so, to improve your dipole pattern at angles of approx 60 degrees, the optimal height for your locations, for a 40m dipole, would be about 31' to 34' high (and 56' - 62' for 80m)....and, only losing one-tenth to two-tenths of a db max (.1 to .2db) "straight-up", compared to using the zenith optimal height.


----  So, you'd each need horizontal antennas for 80m and 40m (a fan-dipole is cheap and easy to make, and a 40m/80m fan-dipole is easy-peasy to construct and tune)....40m dipole wires up 27' - 35' high (32' - 33' is best here) and 80m dipole wires up 50 - 60' high, would be great!  Simple coax feed (with a good 1:1 current balun at the feedpoint), efficient, good swr bandwidth, etc...

If you (and/or your friend) don't have the room for an 80m antenna, you can make one fit in smaller space by bending it / zig-zaging it, etc....and even letting the 10' - 15' at each end hang down, etc. is just fine....(or if you wish to accept some loss and narrow-swr bandwidth, some will just add coils / loading to the dipole)

Of course, if all you want is daytime comms between the two of you....your present 40m dipole should be good (get it up ~ 30' high), just get your friend to build a simple 40m dipole and string it up ~ 30' high, and you're good-to-go!  :)


5)  So...you see....the short answer is a really good answer!
And, while the long answer is more informative, it doesn't change the basic answer....which is:

95%+ of the time:
Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!
Do NOT use vertical antennas here!

(some rare times you'll need to be flexible in regards to mother nature....such as in the depths of winter, in low sunspot years, 160m might be needed at nighttime....and, in the heights of summer, especially in high sunspot years, in addition to high-summertime noise levels (lightning static crashes in SE US can be quite high), 80m absorption (D-layer) can be high and you may find your self switching bands sometimes)


6)  BTW, your question is about propagation (and antennas), band choice, etc....but you also generically asked about "readability"....and, here you hit upon the very basics of HF comms....it's all about S/N (signal to noise ratio)!
Anything you can do to improve your S/N will help....and, while we've all been discussing way to improve each of your signals, we've ignored the "noise" part of that ratio.... :(

Not to worry....the ARRL Handbook (and "antennas", "elmers", and "RFI" forums here on eham) will help you here!  :)

But, in a nutshell:
a)  rid your house of any/all RFI producing devices!  (sometimes even the power supply running your rig can be a problem)
b)  get your antennas as far away from any RFI producing devices as possible (whether in your house, or your neighbors)
c)  do NOT use a "pre-amp" on the HF bands, at least not below 21mhz!  (make sure both of you have  your radios' pre-amps OFF!)
d)  do NOT use the "noise blanker"! (except in cases where you actually do have some impulse noise or powerline noise)

e)  turn down the RF Gain (and/or use the attenuator) to improve readability
f)  try not to use "noise reduction"...as most "noise reduction" adds some "watery" DSP artifacts and can actually reduce "readability"
g)  use headphones!!!  (good communications headphones, no need for expensive "noise cancelling" headphones)

{remember that the human brain, and human ear-brain-connection, is one of the best "signal processing" devices on-the-planet!  and, the more you use it, the better it gets....it is one of the advantages of actual human intelligence!  Don't try to use a computer (like the DSP in a radio) to do what the human brain can do better!}

h)  use common-mode chokes / current baluns on your feedlines (and not using EFHW, etc. antennas)
i)  use multiple turns thru some Mix 31 torroids on any wiring coming from RFI producing devices that you cannot throw away.

There are other things to do....but, think this should keep you going for now.  :)

So, in addition to the advice about what bands, what antennas, what books to buy, etc....I hope these additional pieces of advice will help you out?




I hope this helps.

73,
John,  KA4WJA


{P.S.  "Groudwave" and "NVIS" are unfortunately misunderstood by many hams (personally I blame the "internet", hi hi )....so, just a very quick, fyi...although use of vertical antennas (with good ground systems, and across regions with good ground conductivity) on 160m can yield good "goundwave" comms out to 80 - 100 miles....if coverage area has good-to-excellent ground conductivity, and noise levels are low (and can be 200-300 miles over a seawater path...and, I've done this myself, ~ 250 miles comms on 2mhz maritime over seawater, using vertical antennas...)

but, while the "books" say 80m "groundwave" range using the same good-ground systems and over regions with "good" to "very-good" ground conductivity, can be 50 - 60 miles, in actual real-world practice 80m "groundwave" comms is much less, typically only 30 - 40 miles, as the NVIS signal (even from the null off a vertical antenna) can overtake the weaker groundwave signal, and as noise levels are usually high for most hams these days, but if you're in areas with very low noise levels (and very good ground conductivity), you may be able to obtain the "book" ranges....(fyi, years ago I personally did some experimenting, using a random-length vertical sloper, with only one elevated radial....and my friend, just 16 miles away, using his GAP Voyager vertical....we personally found NVIS comms using our full-wave horizontal loop antennas at 35' - 50' high, produced higher signals than the groundwave signals from the verticals....real-world, for a range of just 16 miles!...not scientific, but just an fyi...)

and on 40m, this is even a shorter range (figure only ~ 20 miles for S-9 signals, but twice that if you have low noise levels and can maintain comms with low signal levels of ~ S-3 or S-4).....

As you see, "groudwave" on the ham bands is not all that useful for most....of course, the "exceptions" here are if the region covered has good-very-good ground conductivity, and the stations in question both have very-low noise levels and sustain comms even with low receive signal levels, then the ranges quoted in the "books" might be usable...and, 160m groundwave comms is done by some hams everyday, and some even use 80m groundwave, if there is poor 2m FM coverage, etc...}
« Last Edit: January 03, 2023, 01:58:56 PM by KA4WJA »
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: buried in noise
« Reply #11 on: January 04, 2023, 09:00:13 PM »

So let's put some numbers on it...

For your signal to go up and bounce off the ionosphere,
then come down about 200 miles away, the vertical angle
above the horizon is about 70 degrees.  (Assuming a typical
ionosphere height of 250 miles.)

So you don't quite need to shoot straight up, but close to
it.

The problem is that the ionosphere doesn't reflect signals
at such a high angle unless the frequency is low enough.

How low?  That varies with conditions.  Sometimes 40m will
get through with good signal strengths.  Sometimes I have
to use 160m, especially at night at the bottom of the sunspot
cycle.  (I'm at 45 degrees north latitude, and 40m isn't open
as often over that distance as it is along the southern tier of
states.)


Here is an introduction to NVIS, which includes a link to a
coverage map that you can run to see the optimum frequency
vs. time of day for distances out to about 600 miles from
your location.

How well does a vertical radiate at 70 degrees above
the horizon?  That article includes comparative plots
of the vertical and a dipole.  The answer is about
10 to 15 dB down.  Not the end of the world, as signals
are often strong when the band is open, and QRP
contacts are common.  So it is still possible, but there
isn't as much margin.

Don't have room for an 80m dipole?  Elsewhere on the
site is a design for an 80m dipole that fits in the same
space as a 40m dipole, and operates 20m as well.
Other small antennas are possible as well.  Even a
mediocre antenna on 80m can outperform a good
antenna on 20m when the critical frequency is down
to 5 MHz, and signals above that frequency just pass
off into space rather than reflecting back down to Earth.


Are there other possible modes?  Yes, E-skip and
backscatter can both cover that distance occasionally,
but they may require more power for a reliable
connection, and they aren't as predictable.

Are there other possible reasons for the weak signals?
Sure.  Could be lossy coax, a problem with one of the
antennas, or insufficient audio so the radio isn't putting
out full power.  But from the symptoms and the distance,
I'd first make sure you are using the optimum band for
that path, which the LAMP forecast can provide.

KA4WJA

  • Posts: 1601
    • HomeURL
Re: buried in noise
« Reply #12 on: January 05, 2023, 12:18:44 PM »

Dale, et al,
1)  Just a curious mind here....regarding the NVIS "angles" at various distances.  (to be clear and blunt, I'm NOT arguing your data...just sharing mine and asking what you and others observe)

With many variables here ---- sunspots / solar activity....height and density of the ionosphere, both E and F layers....as well as D-layer absorption....also the variables of station locations, particularly their latitude....and, of course the biggie: differences in angles depending on the frequency/band being used ----  I know there is no "exact" answer...and, I know this post might be getting way too deep-into-the-weeds of something that is most probably a moot point for 99% of hams...but...

Now, certainly when the coverage range desired is very local (within 100 miles), maximizing zenith gain is the only concern....but when farther "NVIS" ranges are desired, such as covering the whole state of Florida, or California, etc., where even a centrally located station needs to cover 300+ mile radius, we might want to use somewhat higher than optimal antenna heights (higher than optimal for max zenith gain), such as 0.25-wavelength up to 0.3-wavelengths?

Further, if needing to cover a state end-to-end, this can even be 400 - 500+ miles (from the National Hurricane Center in Miami to our state capital, Tallahassee, is ~ 400 miles....but from Miami all-the-way to Pensacola, FL is ~ 525 miles), and we may want to sacrifice our zenith gains by 4 - 6db, and use dipoles up 0.3 to 0.35-wave high?  (or if someone needs to cover somewhat longer ranges, of 600 - 700 miles, we may want a 40m dipole up 0.4-wave high?)


2)  But, many many moons ago, somewhere in an old ITU or IEEE paper, (or maybe something from US DOD?), I read that at a distance of approx. 300 miles, figure about 60 degree "wave angle"....{which is also what you show in your chart}

Then (also many years ago) I saw a chart showing radiowaves with a 60 degree "take off angle", having an "earth radius distance" of approx 400 miles....{which seemed too far in my mind}

Then (about 25 years ago), I read some other stuff showing much shorter distances being covered by these angles....I think it was 200 miles at 60 degree wave angles. 
And, around this time also read some papers from L.B. Cebik, where he also was showing ~ 200 - 225 mile radius covered at angles of 65 degrees (and ~  150 miles with angles of ~ 75 degrees) ....with my interpolation of ~ 60 degrees for a 250 mile path distance.... {both of these pieces of data seemed a bit shorter than what I had always assumed}


Then in 2005, I read Dean Straw, N6BV's article about NVIS (see his chart of average wave angles, from a latitude of ~ 38 degrees / San Fransisco, CA)....where his data seemed to disprove the old adages of "300 - 400 mile coverage for wave angles of ~ 60 degrees" and not only reaffirmed the much smaller coverage, but actually showed a 185 mile distance having an average wave elevation angle of 60 degrees!




And finally, a couple years ago (2016), I read the Dutch IEEE paper (that I think you have some data from, as well?), showing remarkably shorter distances (of approx. ~ 200 - 225km/ 120 - 140mi for the 80m band, and only slightly longer for 40m) for wave angles of approx 60 degrees....meaning much lower angles needed to cover ranges up to 300 miles!





Of course, the differences in wave angles being refracted with the different sunspot activity, has been understood for ~ 100 years....as well as different angles for the different frequency bands, which I've been aware of for many decades...but..

But, something that I was not aware of until reading this article 6 - 7 years ago, was the irony that it is the lower freq bands that have the lower wave angles covering the same NVIS distances!  :(
In my mind, this was anomalistic, compared to the historical / statistical angles of signals traveling much longer distances (multi-hop F2), where somewhat higher wave angles for the lower HF bands, versus the wave angles of the higher HF bands, covering the same paths....
Just have a gander at HFTA, etc., and look at the statistical wave angles for US to EU, or US to JA, etc. for 80m, 40m, and 20m, etc., and you'll see what I mean....and then look at the NVIS data from the Dutch IEEE paper....and, you'll see why I learned something new from those guys in the Netherlands!  :)
Now, I know better....thanks to the Dutch!  :)



3)  So, I guess my query here is more of an observation....
Now-a-days I'm using Dean, W6BV's, and the Dutch IEEE paper's, data for evaluating NVIS ranges and antenna heights.

Which means the ~ 195 - 200 mile (~ 320km) range for Jon, KJ4SKP, to cover from his house to/from "Nashville", with our current position on the solar-cycle, on 40m would have wave angles of ~ 50 - 65 degrees....and, a good rule-of-thumb is to assume "approx 60 degrees for approx 200 miles".

{This (60 degree angle) is a lower angle than I used to (~ 20 years ago) assume / calculate things for....where, > 20 years ago, I used to think that ~ 60 degrees was the average wave angle for ranges of ~ 300 miles, and many, many moons ago, I thought it was even the angle for ranges out-to 350-400 miles...now-a-days, just a few years ago, for a path here in Florida....I think I calculated ~ 62 degrees for a range of ~ 175miles, along Central Florida...}


So, there's the scoop out of my brain....not sure I'm completely correct in all of this, but since the antennas we are dealing with have very broad patterns, if we are off by 5 degrees in our wave angle calculations, it will make no noticeable difference!  :)   
{It's when we start to consider larger / higher-gain NVIS antennas, such as horz Lazy-H's / Jamaica Array (or even the bigger extended-Jamaica Array), where we are narrowing the pattern (maximizing the zenith gain)....it's only when doing this and desiring to still cover a 300 mile radius, that we should be concerned about the exact wave angles used!}


Fair winds and 73,
John,  KA4WJA
« Last Edit: January 05, 2023, 12:41:46 PM by KA4WJA »
Logged

KA4WJA

  • Posts: 1601
    • HomeURL
Re: buried in noise
« Reply #13 on: January 05, 2023, 12:46:36 PM »

Sorry about the typo, it's N6BV....I got Dean's call right once, and wrong once...

<snip>
Now-a-days I'm using Dean, W6BV's, and the Dutch IEEE paper's, data for evaluating NVIS ranges and antenna heights.

73,
John,  KA4WJA
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: buried in noise
« Reply #14 on: January 06, 2023, 11:12:18 AM »

Quote from: KA4WJA

1)  Just a curious mind here....regarding the NVIS "angles" at various distances.



Yes, it isn't always simple.

The effective height of the ionosphere varies with frequency.
My numbers are geometric calculations for a nominal height
of 250 miles (400 km).  It does vary a lot with the current
conditions, time of day, etc.

And the Dutch paper's demonstration of E-layer modes is an
important advancement.  That probably helps to explain why
verticals work well on 160m:  not just via ground wave, but
also via the E-layer with lower angles of radiation.


On the other hand, the exact angle of radiation doesn't matter
that much in the greater scheme of things.  I'm in the process
of developing some plots of relative antenna gain plotted on
a map.  The 1 dB beamwidth of a horizontal dipole translates
to something like 200 x 120 miles (300 x 200 km).  Using my
numbers, at 500 miles (800 km) it is down about 5 dB broadside
and 9 dB off the ends.  When the band is open, that shouldn't
be enough difference to prevent contacts, although it could be
marginal with low power or inefficient antennas.

Having a "6" in my callsign, I'm quite familiar with the issues
of covering a long, skinny state (~1000 miles end-to-end). 
And, you are right that the antenna needs to be designed for
the required coverage pattern. 

Even here in Oregon, at shorter distances, I can see the effects
of different antennas.  I had an end-fed half-wave wire for
160m running out to a tree.  Other stations to the north and
south were not particularly strong on the 80m net.  But
afterwards a station 250 miles to the East called me and
asked what sort of amp I was using.  (I was barefoot.)
Turns out the 1-wavelength wire (when not fed in the center)
has a broadside null (including directly overhead) that covered
many of the other stations, but a peak at around 50 degrees
off the ends of the wire pointed in his direction.

That's why I'm working on plotting relative gain of different
antennas as contour lines on a map that can be rotated to
account for the antenna orientation.  Especially over longer
distances, I want folks to see how different antennas can
work better (or worse) to specific locations.  And the first
application of those plots will be for gain NVIS antennas,
where there are the most trade-offs between gain and
coverage area.  (That article is in work, but Real Life has
made progress slow for the last few months.)


I have to say that I wasn't too impressed by Dean's paper
when it came out:  mostly, he discredited the use of
160m.  Here at 45 degrees north, we can go years without
40m opening up for NVIS, and much of the time 80m goes
out about sunset.  I've been advocating the use of 160m
as an alternative for about 3 sunspot cycles now, and those
who did use it were amazed how well it worked, even with
low dipoles and portable operation.  (Yep, we strung a
portable 160m dipole across the lawn in front of the county
EOC.  If folks tell you it can't be done, they just aren't
being very creative.)


So, yes, I agree that there is a lot of variation in
coverage distance at a particular angle.  And any
single number will be wrong much of the time,
because conditions change.

I picked a nominal ionosphere height from one
source, and clearly there are other options that
may apply better at other locations.  That's why I
also use the LAMP plots and VOACAP - they give
different results (VOACAP is sometimes a bit
optimistic over shorter paths).

But probably the important thing is that the
effective height, and therefore the coverage angle
for a given distance (or vice versa), will vary
with time, and we need to take that into account
in our planning and signal strength margins.

Pages: [1] 2   Go Up