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Author Topic: Magnetic or True North for antenna positioning?  (Read 8129 times)
WV4L
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« Reply #15 on: July 25, 2012, 04:59:26 PM »

I down loaded a compass App for my phone that takes into account your GPS position and adjusts for variation to get true North.
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K7KBN
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« Reply #16 on: July 25, 2012, 05:36:47 PM »

I determined where true north is whenever I've moved, using Polaris.  I use trees, houses (or whatever's available and not going to be moving) to determine direction from the shack.  Never a problem.
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73
Pat K7KBN
CWO4 USNR Ret.
W1EL
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« Reply #17 on: July 25, 2012, 05:41:34 PM »

Another "natural" way to get a ballpark idea of where true north is, is to look at where the Sun is at 12 o'clock (noon).
It will be in the direction of true north, give or take a bit.
The reason it will not be exactly at true north is that we now use time "zones" which means it is 12 noon over a large area of land.

When the sun is at its greatest altitude above the horizon, it is pointing at true north.
In the era before time zones, each town had its own 12 o'clock when the Sun was at its highest in the sky.
This caused mayhem when trains became commonplace - since their speed meant you would now be changing time rapidly.
Standard time zones were introduced to overcome this inconvenience.

Also, the ancient greeks proved that the world was round a few thousand years ago by noting that on a certain day of the year, when the Sun was directly overhead, a well had no shadow inside.
By noting the shadow cast in another town further away at the same time, they were able to work out the curvature and so the size of the Earth.

With all our technology these days, it is a shame if we would lose the simplicity of finding our way, that our ancestors used routinely.

73 - Rob


Respectfully disagree. From NA, the sun will be due south at noon. Stand with the sun at your back and face south.
Or get a compass.

GL de W1EL
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K7KBN
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« Reply #18 on: July 25, 2012, 05:47:39 PM »

StayVertical -

For those of us who live north of the Tropic of Cancer, the sun will never "...be in the direction of true north...".  Now, the shadow of a reasonably straight stick, poked into the ground so that it's truly vertical, will be pointing true north at about local noon.
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73
Pat K7KBN
CWO4 USNR Ret.
STAYVERTICAL
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« Reply #19 on: July 26, 2012, 02:01:35 AM »

Whoops!
I am currently living in the southern hemisphere, so habit had taken hold.
Of course for you northerners, the Sun will be be true south when it is on the meridian.

For anyone who has confused north with south I apologize.

73 - Rob
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K7KBN
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« Reply #20 on: July 26, 2012, 07:02:40 PM »

Hey Rob -

That thought flitted through my mind when I posted:  "He could be in the Southern Hemisphere", but I decided that would be too easy an explanation.

Enjoy your winter!

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
STAYVERTICAL
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« Reply #21 on: July 27, 2012, 12:02:44 AM »

Thanks Pat,

Yes, I once heard that each Tibetan rug has a flaw intentionally added so that the gods won't take it away.
It could be entirely bunkum, but it is a good excuse to give for my calling south north (hi).
I was waiting for someone to bring up living within the two tropics but thankfully I did not have to eat humble pie for that one.
Hope you are not in drought there, and it doesn't get too hot.

73 and regards - Rob
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K3GM
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« Reply #22 on: July 27, 2012, 05:17:17 AM »

I can't vouch for its accuracy, but my Android based Motorola Razr Maxx will display either magnetic or true north.  For the true north setting, the Compass app first determines current location based on GPS, then goes out to a lookup table to get declination data which it uses to adjust the compass bearing.  Unfortunately, I can find a Motorola specification that states how accurate its three-axis magnetometer is when properly calibrated. Outside in my small field, away from metallic objects, the magnetic north reading on the phone exactly matches my handheld hiking compass.  Switching the phone's compass to true north results in a 15 degree jog off the traditional compass bearing which matches the declination for my location.
« Last Edit: July 27, 2012, 05:22:13 AM by K3GM » Logged
ZL1BBW
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« Reply #23 on: July 27, 2012, 10:56:24 PM »

So, I just created a map from the NS6T free site,   is that magnetic or a True North map? and when I look a Google Earthn to plan out the longwires, is that Mag or True.

Thanks
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.
STAYVERTICAL
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Posts: 875




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« Reply #24 on: July 28, 2012, 12:24:56 AM »

Most maps use true north in their grids.
In radio use you would use a map which allows great circle determination of heading.
A great circle is a line which also passes through the centre of the Earth.
The azimuthal map basically gives you great circle headings from your location to anywhere else you wish on the globe.

So to answer your question, the headings on the map would be in true format, and you would use these to determine your target heading.
You would use magnetic only to figure out where true north is on your land, so you can then aim your antenna from there.
If you already know where true north is, then you don't have to bother with magnetic at all.

If you want to find true north on your land, get your compass and rotate yourself until you read 360 (or 000) degrees.
Then, you subtract the magnetic variation for your location (whangarei), which is 19 degrees East.
So, your true north would actually be at a magnetic heading of 341 degrees on your compass.

From that point you rotate clockwise to get the various headings 090,180,270 etc degrees.

Hope this helps,

73 - Rob

« Last Edit: July 28, 2012, 12:28:57 AM by STAYVERTICAL » Logged
AA5WG
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« Reply #25 on: July 28, 2012, 07:26:26 AM »

Here is a navigational memory aid that applies to your antenna needs, "EAST IS LEAST and WEST IS BEST".

After finding your magnetic variation by Google or from a VFR aeronautical navigation chart, call a sectional, apply the above memory aid.  i.e. for my 8 land area of Traverse City, Michigan the variation is 6 degrees West. Thus "WEST IS BEST" is applied by adding 6 degrees to my magnetic north compass indication to arrive at true north.  True north is 6 degrees west of what the compass is saying in the Traverse City, Michigan area.  

(The variation line on the aeronautical chart is depicted by a continuous dashed line with the amount of
variation listed every couple of feet or so depending of the chart.)

If the variation was 6 degrees east then, "EAST IS LEAST" would have been applied by subtracting 6 degrees from the magnetic north indication of the compass.

73,
Chuck - AA5WG
« Last Edit: July 28, 2012, 08:03:02 AM by AA5WG » Logged
K7KBN
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« Reply #26 on: July 28, 2012, 09:49:48 AM »

Just a word about Great Circle (GC) maps.  They can be made for any location.  At the very center, the shapes of the continents are pretty much recognizable.  When you get closer to the perimeter, however, the shapes distort wildly.

Think of yourself precisely at the geographical North Pole.  OK?  Now:  which way is South?  It doesn't matter WHICH way you point, you're right.  There is no "east" or "west" if you're exactly at either pole.  Looking at a GC map with your location at the center, the exact opposite side of the world (the "antipodal point", to use the correct term), is at every point along the circumference of the circle.  Short path = long path.  Fun to consider if you don't have something (anything!) else to do  Grin
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73
Pat K7KBN
CWO4 USNR Ret.
AA5WG
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Posts: 498




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« Reply #27 on: July 28, 2012, 10:40:47 AM »

Update to my above post:

I should of said:  True north is 6 degrees greater than what the compass is saying in the Traverse City, Michigan area.

C
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STAYVERTICAL
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Posts: 875




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« Reply #28 on: July 28, 2012, 02:37:35 PM »

Update to my above post:

I should of said:  True north is 6 degrees greater than what the compass is saying in the Traverse City, Michigan area.

C

Yes, it's easy to make a verbal typo in this area.
In a nutshell - the magnetic variation is how many degrees east or west of true north the compass would point.
So if true north is straight up and down, and the compass reads north to the right of this line, the variation is east.
If the compass points left of true north, the variation is west.

Looked at in this way it is obvious that if the compass points to the left of true north(west), you need to add something (variation) to get to straight up and down.
If the compass points to the right of true north(east) you need to move back to the left, and so subtract the error amount (variation).
If you get a mental image or draw it on paper it is pretty easy.

By the way, you may come across compass deviation as well - this is a purely local correction and is applied due to nearby metal etc.
On ships compasses it is adjusted out and tabulated, but for our purposes it can be ignored if you are clear of large metal or powerlines etc.

I don't want to insult anyone if this information is too simple, but perhaps it may save someone confusion down the line.

73 - Rob
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W8JI
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« Reply #29 on: July 29, 2012, 04:54:13 AM »

The real answer depends on the antenna and location.

A very large HF beam antenna, one with a full wave boom, has about 30 degrees either side 3 dB beamwidth. I have fairly wide aiming with a 20 meter Yagi on a 55-foot boom!! I can't tell if I bump  any of my large HF antennas 15 degrees, and sometimes even 30-40 degrees. This is because long ionospheric paths are often full of scattering, like pointing a light through fog.

Also on HF, signals often skew or bend from the great circle path. What anyone SHOULD be doing on HF for long distance paths is turning the antenna and looking for a peak. There are times here when Japanese signals, great circle NW, arrive west. On some occasions they arrive SW. It isn't wise operating to point to a compass heading when working long distances on a polar path, or sunrise/sunset path.

A large VHF antenna can be 15 degrees or less 3 dB beamwidth either side of peak.

Most of the lower 48 states in the USA are within 15 degrees, and toward our middle longitudes or south are near zero. Where I live in Georgia compass error is near zero, just a few degrees.

When everything is considered, the real answer is it all depends on your antenna, where you intend to work, and how you operate. An experienced HF DX operator generally looks for the peak, and uses the heading only as a general guide. For groundwave or shorter distances, especially with narrow beamwidth VHF antennas, aiming by compass can be valuable.
 
73 Tom
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