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Author Topic: Question on Elevation Pattern  (Read 1925 times)
KD2CJJ
Member

Posts: 369




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« on: November 18, 2012, 07:06:47 AM »

Have two questions being a relatively new ham:

1.  Can you point me to a website or good book (I have the ARRL antenna book and it does not get into any details) how to properly read an Antenna Elevation Graph and an azimuth graph .

2.  Since its very clear that height for a dipole affects the take off angle, does the laws of reciprocity apply?  In other words, does it also affect RX at the same angle?  In other other words, if I were to simulate a dipole at 33 feet and has a peak gain of 6db at 20 degrees TX does this mean RX would have a similar pattern (I understand that there will be no gain)? In other other other words is my RX affected by height for DX as much as TX.

To give some context as to why I am asking: 

I ask I can not hear anything further than New Mexico West, Brazil South, Almost NOTHING North, Lithania Eastern Europe - this is on 20M and of course 10m is much better and I understand why but again is it just because TX is overall better due to gain?... so, I am focusing on 20M for the question.
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73

Mike
KD2CJJ
AC5UP
Member

Posts: 3842




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« Reply #1 on: November 18, 2012, 07:42:30 AM »

You might be assuming this is more difficult than it is........... Take a good look at radiation pattern / elevation plot for an antenna. What you'll see are lobes pointing in some direction on a circle marked in degrees. That's the beam heading with 0 degrees as North and 180 degrees as South.

Let's use this as an example.

Notice the lobe at 30 degrees. Notice how the tip of the lobe crosses the second inner ring. Go to North, then look at the scale markings of the various rings. The outer ring is usually 0 dB gain or the 'nominal' strength of the signal. Could be one watt, twelve, whatever. We're looking at relative levels. If the outer ring is 0 and there are five steps to 10, then each step is worth 2 dB below nominal. Therefore... The lobe at 30 degrees is 4 dB less than the nominal level of the outer ring. Notice the lobes at 90 and 270 degrees. Both reach the outer ring and are 4 dB hotter than the lobe at 30 degrees.

Looking the other way, the null at 60 degrees is 22 dB below the East and West lobes. Notice also how this antenna plot says it's essentially deaf at 125 degrees. Deep null.

Disclaimer:  I'm assuming the scale is in dB. Typically it is, but most plots will have a small block of text in a corner detailing what the scale shows. Frequency, power, etc. Be sure to understand what you're looking at before trying to interpret the plot. But, even a quick glance will tell you where the signal is hot and where it's not and that's the value of a pattern plot.

As for height of a dipole.... Height, soil composition, moisture and surrounding objects all contribute to the elevation angle of the radiated signal. As a general rule a half wavelength above RF ground (which might be a foot or more below physical ground) is a good height for most situations. This will vary by season (rain in the spring, snow in the winter) but the greatest variation will be propagation from one day to the next. On some bands from one hour to the next. As for reciprocity between RX and TX, on complex antennas like a long boom Yagi they do hear best over a specific range of elevation angles. For a dipole, any signal crossing the wire will induce a current and a dipole doesn't care too much about the angle. With a dipole some angles work better, but all angles work.

You will also find you can work some countries only in the winter, spring or fall. Summer DX is generally the worst but your mileage may vary.

Don't sweat the perfection of your antenna as a good band opening can compensate for a multitude of minor issues...  Wink
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KA4POL
Member

Posts: 1967




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« Reply #2 on: November 18, 2012, 07:45:09 AM »

Looking at page 2.6 of the Antenna Book I think it is pretty well explained how those patterns are generated and how they have to be read.
Yes, in is like out.
Just a link: http://en.wikipedia.org/wiki/Radiation_pattern
A search will reveal lots more.
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W5FYI
Member

Posts: 1044




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« Reply #3 on: November 18, 2012, 08:28:17 AM »

You also need to check the propagation charts. Recent solar activity has been playing havoc with HF propagation. In other words, you can't rate your antenna analysis problem on take-off angles alone; you've got to know what's going on in the ionosphere. GL
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KD2CJJ
Member

Posts: 369




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« Reply #4 on: November 18, 2012, 08:41:00 AM »

Perfect.. Your advice of looking at the legend was the key to my understanding - I assumed the scales were the same, but after now paying attention to the legend now it clears everything up!  Duh!  I feel like an idiot!.

Some further context with a question, If I am not hearing Japan then its because of band conditions, which is pretty much what your saying..  I have my dipole up about 30 feet.  And was wondering why I just dont hear at all from Asia, Africa, very little Canada, or even lesser on West Coast of US... I hear a slew of Europe and generally can work the station if I hear them as far as Cyprus, even hit Italy a few times with no issues.  I have my dipole setup as a sloppy flat top (hot side is perfect, the sloppy side is pulled into a tree).  The antenna is angled generally NE / SW.  You think I am hitting NULLS in the pattern due to the flatop?  From my simulations, it seems well formed with single lobes, but as you said I should at least hear them, maybe not work them - I dont hear them.   Do you think a rotatable dipole would be better due to the fact I could rotate it?  Unfortunately the way my property is I am stuck in the current position and the XYL doesnt want a beam (im protesting at least for a Hex but a rotatable dipole is easier to digest, Verticals are a NO NO here in Long Island NY, me being spitting distance from 20KV power lines)

Thanks for the advice and clearing up a few points.

You might be assuming this is more difficult than it is........... Take a good look at radiation pattern / elevation plot for an antenna. What you'll see are lobes pointing in some direction on a circle marked in degrees. That's the beam heading with 0 degrees as North and 180 degrees as South.

Let's use this as an example.

Notice the lobe at 30 degrees. Notice how the tip of the lobe crosses the second inner ring. Go to North, then look at the scale markings of the various rings. The outer ring is usually 0 dB gain or the 'nominal' strength of the signal. Could be one watt, twelve, whatever. We're looking at relative levels. If the outer ring is 0 and there are five steps to 10, then each step is worth 2 dB below nominal. Therefore... The lobe at 30 degrees is 4 dB less than the nominal level of the outer ring. Notice the lobes at 90 and 270 degrees. Both reach the outer ring and are 4 dB hotter than the lobe at 30 degrees.

Looking the other way, the null at 60 degrees is 22 dB below the East and West lobes. Notice also how this antenna plot says it's essentially deaf at 125 degrees. Deep null.

Disclaimer:  I'm assuming the scale is in dB. Typically it is, but most plots will have a small block of text in a corner detailing what the scale shows. Frequency, power, etc. Be sure to understand what you're looking at before trying to interpret the plot. But, even a quick glance will tell you where the signal is hot and where it's not and that's the value of a pattern plot.

As for height of a dipole.... Height, soil composition, moisture and surrounding objects all contribute to the elevation angle of the radiated signal. As a general rule a half wavelength above RF ground (which might be a foot or more below physical ground) is a good height for most situations. This will vary by season (rain in the spring, snow in the winter) but the greatest variation will be propagation from one day to the next. On some bands from one hour to the next. As for reciprocity between RX and TX, on complex antennas like a long boom Yagi they do hear best over a specific range of elevation angles. For a dipole, any signal crossing the wire will induce a current and a dipole doesn't care too much about the angle. With a dipole some angles work better, but all angles work.

You will also find you can work some countries only in the winter, spring or fall. Summer DX is generally the worst but your mileage may vary.

Don't sweat the perfection of your antenna as a good band opening can compensate for a multitude of minor issues...  Wink
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73

Mike
KD2CJJ
KD2CJJ
Member

Posts: 369




Ignore
« Reply #5 on: November 18, 2012, 08:43:08 AM »

Yes I watch them every day...  Im still learning but from my analysis it seems sun spot activity is at the highest, which from what I am reading is a contradiction as to why the bands are all over the place.  Im still learning how to properly analyse and interpret band conditions though and may not be reading them properly....

You also need to check the propagation charts. Recent solar activity has been playing havoc with HF propagation. In other words, you can't rate your antenna analysis problem on take-off angles alone; you've got to know what's going on in the ionosphere. GL
Logged

73

Mike
KD2CJJ
KA4POL
Member

Posts: 1967




Ignore
« Reply #6 on: November 18, 2012, 10:19:50 AM »

We are somewhere in the maximum of this sunspot cycle. However, it is a weak maximum. And if push comes to shove the next cycle will be even lower. This is what the solar currents indicate.
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N3OX
Member

Posts: 8854


WWW

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« Reply #7 on: November 18, 2012, 12:13:40 PM »

If your antenna is an issue at all it could be because it's close to your house, provided the photo on your QRZ page is accurate.  But in any case it's very unlikely that your antenna is limiting your ability to get to the West Coast, etc. 

Antenna differences, even nulls on a basic dipole, will almost never explain "missing" whole regions of the world unless you're using a very horribly broken antenna or one that's really and truly blocked by a huge terrain feature or a very big wall (house  doesn't really count: even a house that's perfectly opaque in the radio spectrum will have some diffraction around it)

We could look at nulls...  if you had super deep nulls of 30-40 dB or more which might start to reduce your ability to hear and work even strong stations, you wouldn't be able to null the whole west coast or anything.  There's 10 degrees heading difference just between San Francisco and Los Angeles from here in Maryland, for example.  32 degrees between Vancouver, BC  and San Diego, CA.   Too broad to null.  As you get further away, the differences get smaller for a given region, but

Plus you won't get very deep nulls like that out of a dipole over real ground.  More likely would be 15-20dB at housetop height and that's not enough to exclude strong stations from stateside stuff. A null problem would be enough to exclude a WEAK Lithuanian even during good propagation, maybe enough to make it hard to hear Asian stuff over the pole, but certainly not enough to remove the West Coast or limit contacts with Canada or cut out those monster stations from Hungary that get on the air from time to time.

Quote
And was wondering why I just dont hear at all from Asia, Africa, very little Canada, or even lesser on West Coast of US

It's probably different reasons for each, even within a region. A problem with a lot of East Asia from the East Coast U.S. really is propagation.  Any paths that cross near the pole are very hard.  Conditions are very variable when the sun is active because solar flares and so forth tend to really perturb the ionosphere around the poles.  I run a few hundred watts to simple small beam antennas up around 30 feet or so and I have VERY rarely heard and worked similarly equipped stations from East Asia.  Usually when I work someone in say, Thailand or Vietnam or S. Korea or Indonesia, I look them up and they have a big beam at 100 feet and a kilowatt.  For hard polar paths, a couple of 100W and dipole stations lead to a very marginal link on 20m.  You will tend to hear the countries that have more big gun stations. 

You probably have really stellar propagation to a lot of Africa, but many countries have fairly low day-to-day activity or smaller stations.  Canada and the West Coast should be just timing or luck.



Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KD2CJJ
Member

Posts: 369




Ignore
« Reply #8 on: November 18, 2012, 01:30:31 PM »

You make some interesting points... I took the time to look at my antenna and its truely NE / SW... I suspect I could be missing most if not all California with its position..  Im probably about 20 degrees from pure East - West... I suspect the weak signals I would not hear... but hear strong ones with good propagation.  I hear Texas likes its no tomorrow and anything in between.  The Photos are not representative anymore as I have made it a flat top dipole and its up about 10 more feet to roughly 30 feet.

I hear Hungary you are right they are monster signals...

Next year I might spring for a rotatable dipole (or build one)... thanks for the feedback!

If your antenna is an issue at all it could be because it's close to your house, provided the photo on your QRZ page is accurate.  But in any case it's very unlikely that your antenna is limiting your ability to get to the West Coast, etc. 

Antenna differences, even nulls on a basic dipole, will almost never explain "missing" whole regions of the world unless you're using a very horribly broken antenna or one that's really and truly blocked by a huge terrain feature or a very big wall (house  doesn't really count: even a house that's perfectly opaque in the radio spectrum will have some diffraction around it)

We could look at nulls...  if you had super deep nulls of 30-40 dB or more which might start to reduce your ability to hear and work even strong stations, you wouldn't be able to null the whole west coast or anything.  There's 10 degrees heading difference just between San Francisco and Los Angeles from here in Maryland, for example.  32 degrees between Vancouver, BC  and San Diego, CA.   Too broad to null.  As you get further away, the differences get smaller for a given region, but

Plus you won't get very deep nulls like that out of a dipole over real ground.  More likely would be 15-20dB at housetop height and that's not enough to exclude strong stations from stateside stuff. A null problem would be enough to exclude a WEAK Lithuanian even during good propagation, maybe enough to make it hard to hear Asian stuff over the pole, but certainly not enough to remove the West Coast or limit contacts with Canada or cut out those monster stations from Hungary that get on the air from time to time.

Quote
And was wondering why I just dont hear at all from Asia, Africa, very little Canada, or even lesser on West Coast of US

It's probably different reasons for each, even within a region. A problem with a lot of East Asia from the East Coast U.S. really is propagation.  Any paths that cross near the pole are very hard.  Conditions are very variable when the sun is active because solar flares and so forth tend to really perturb the ionosphere around the poles.  I run a few hundred watts to simple small beam antennas up around 30 feet or so and I have VERY rarely heard and worked similarly equipped stations from East Asia.  Usually when I work someone in say, Thailand or Vietnam or S. Korea or Indonesia, I look them up and they have a big beam at 100 feet and a kilowatt.  For hard polar paths, a couple of 100W and dipole stations lead to a very marginal link on 20m.  You will tend to hear the countries that have more big gun stations. 

You probably have really stellar propagation to a lot of Africa, but many countries have fairly low day-to-day activity or smaller stations.  Canada and the West Coast should be just timing or luck.




Logged

73

Mike
KD2CJJ
M6GOM
Member

Posts: 888




Ignore
« Reply #9 on: November 19, 2012, 02:48:38 AM »


2.  Since its very clear that height for a dipole affects the take off angle, does the laws of reciprocity apply?  In other words, does it also affect RX at the same angle?  In other other words, if I were to simulate a dipole at 33 feet and has a peak gain of 6db at 20 degrees TX does this mean RX would have a similar pattern (I understand that there will be no gain)? In other other other words is my RX affected by height for DX as much as TX.

To give some context as to why I am asking:  

I ask I can not hear anything further than New Mexico West, Brazil South, Almost NOTHING North, Lithania Eastern Europe - this is on 20M and of course 10m is much better and I understand why but again is it just because TX is overall better due to gain?... so, I am focusing on 20M for the question.

Seeing as nobody has answered this:

Yes, the laws of reciprocity apply. Gains for TX are also gains for RX. If your dipole is half wavelength or lower, you'll hear stations up to 3000 miles well but anything further than that will be quite poor compared to getting the dipole a wavelength high.

As for what you can hear.....

Go to this website and generate an azimuthal map of the world - it'll look very different to a normal map but it actually shows the shortest paths to places and is what you use to aim a beam antenna:

http://ns6t.net/azimuth/azimuth.html

Now print it out and draw a line on it where your dipole is pointing. Now draw an oval the shape of a peanut centered on the middle of the map and at right angles to that line. Make it so the ends of the long side of the peanut go to about halfway out from the center.

You'll end up with something like this:

http://tinyurl.com/btju5gv


Anything inside that peanut shape is roughly the area you'll cover well with your 20m dipole at 30ft. It will cover outside that but it'll be hard work and mostly strong stations you hear.
« Last Edit: November 19, 2012, 03:00:07 AM by M6GOM » Logged
PA5MW
Member

Posts: 14




Ignore
« Reply #10 on: November 19, 2012, 04:44:37 AM »

Mike,

You might want to take a look at: http://www.ncdxf.org/pages/beacons.html
This allows you to check HF propagation paths 24/7

And perhaps compare to some of the real-time monitoring stations at: http://www.ncdxf.org/beacon/monitors.html

Remember: these HF beacons use max. 100W to a small multi-band vertical antenna, only

73 Mark, PA5MW
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KD2CJJ
Member

Posts: 369




Ignore
« Reply #11 on: November 19, 2012, 02:02:23 PM »

Very very good info.... I see why many of my fellow hams who have Verticals hear relatively more than my dipole at 30 feet, since reciprocity applies they are not behold-ant to height for angle of reception.  I suspected this and why I ended up asking the question.  The net net is I need to get my dipole higher or get an antenna that can introduce gain at the equivalent height or lastly use a vertical but then deal with the other issues that come with a vertical - am I off base?

After looking at the Azimuth map I see why I have relatively no issues hitting Europe and no issues hitting Texas but hear almost nothing North West or South East....  Currently my antennais placed NE / SW which from my QTH based on this map directly hits Europe and TX.. very interesting.  Funny I was thinking of making it true east / west and would help my Cali contacts but then suffer with European contacts.

Next question:  How did you determine the size of the Peanut over the Azimuth map?


2.  Since its very clear that height for a dipole affects the take off angle, does the laws of reciprocity apply?  In other words, does it also affect RX at the same angle?  In other other words, if I were to simulate a dipole at 33 feet and has a peak gain of 6db at 20 degrees TX does this mean RX would have a similar pattern (I understand that there will be no gain)? In other other other words is my RX affected by height for DX as much as TX.

To give some context as to why I am asking:  

I ask I can not hear anything further than New Mexico West, Brazil South, Almost NOTHING North, Lithania Eastern Europe - this is on 20M and of course 10m is much better and I understand why but again is it just because TX is overall better due to gain?... so, I am focusing on 20M for the question.

Seeing as nobody has answered this:

Yes, the laws of reciprocity apply. Gains for TX are also gains for RX. If your dipole is half wavelength or lower, you'll hear stations up to 3000 miles well but anything further than that will be quite poor compared to getting the dipole a wavelength high.

As for what you can hear.....

Go to this website and generate an azimuthal map of the world - it'll look very different to a normal map but it actually shows the shortest paths to places and is what you use to aim a beam antenna:

http://ns6t.net/azimuth/azimuth.html

Now print it out and draw a line on it where your dipole is pointing. Now draw an oval the shape of a peanut centered on the middle of the map and at right angles to that line. Make it so the ends of the long side of the peanut go to about halfway out from the center.

You'll end up with something like this:

http://tinyurl.com/btju5gv


Anything inside that peanut shape is roughly the area you'll cover well with your 20m dipole at 30ft. It will cover outside that but it'll be hard work and mostly strong stations you hear.
Logged

73

Mike
KD2CJJ
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