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Author Topic: Help creating polar antenna plot? (thx!)  (Read 1356 times)
KC9KEP
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« on: September 15, 2009, 08:20:43 AM »

Hello,

Can someone point me to a source of reference materials or offer suggestions on how I can generate an antenna polar plot?  (Maybe that’s not even the correct name for such a plot?)

I would assume that it would take two people.  One with a field strength meter and another to key a transmitter attached to the antenna to be characterized.

I’d further assume that one would want to record multiple samples of the signal strength at equal distances in a circle around the antenna.  The person who is taking the “field readings” could contact the operator (via cell phone or whatever) and request that the transmitter be keyed in order to take a reading.

I suppose that the various signal strength readings could then be plotted on a polar graph.

I’d also assume that one would want to apply as little transmitter power as would be required as to not clog up the air waves unnecessarily.

For that mater, would one need to identify them self for each transmission?  I would think so because it’s the regulation, no?

Thanks for any information!

KC9KEP
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K0OD
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« Reply #1 on: September 15, 2009, 09:16:02 AM »

What bands and type of antenna are you going to plot? Yagi or say, phased verticals. How are you going to measure the signal? Radio S-meters are almost useless other than to determine whether ***some*** F/B exists. Do you own a good field strength meter?

I do some testing along those lines. I wonder how far away from the antenna a field strength meter should be.

Per recent eham thread, there's little reason to transmit while doing most such tests. You can use your own receiver for most purposes. Use its attenuator to get some idea what each S-Unit means. On 40 I use foreign broadcast or, formerly, Canadian time station CHU which was just above the band. Ham nets are useful.


ID after short tests? LOL! Very Ned Flanders-ish.
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WB6BYU
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« Reply #2 on: September 15, 2009, 10:12:11 AM »

I do this frequently for VHF antenna patterns.  I set
out a remote transmitter at a convenient distance depending
on the sensitivity of my receiver and how far I am willing
to walk.  I'm in the country and can put it up to a mile
away line-of-sight, but when I'm lazy I settle for 50 to
100 feet.  I also have hidden transmitter boxes that are
self-contained (including an ID'er) that are ideal for
this use.

Then I put the antenna under test on a stand where I can
rotate it by hand with a compass rose and record the
signal strength every 5 to 30 degrees around the circle
(depending how detailed I need the plot to be.)  While I
have access to a commercial calibrated field strength
meter, I usually do this with just a step attenuator
to set the signal strength to a convenient reference
mark on the receiver's S meter.  That gives me 1dB
resolution, which is good enough for most purposes.


Trying to do this on HF gets a bit more complicated,
especially if the antenna doesn't rotate.  I've done it
on 40m by lugging around a Ten-Tec Argonaut with a mobile
whip and some radials, and taking measurements in
various directions.  I had a second person operating
the station, but a keyer in beacon mode will work.
Now I'd probably use a short active antenna or loop
to make the antenna more portable.  Ideally you would
take the measurements from several wavelengths away, but
this isn't always practical.  I think in the 40m case
I paced off the distance, but a length of string would
make it more consistent.  If you can't take the measurement
at consistent distances in all directions, you can adjust
the measured intensity for the actual distance based
on the inverse square law.

You do need to be far enough from the antenna that it
can be considered a point source.  This probably isn't
a problem with a dipole or simple vertical, but one is
rarely concerned with measuring the pattern of those!
Basically, if there is a significant difference in the
bearing to different parts of the antenna from where
you are standing, you are too close.  A reasonable
minimum distance might be 10 to 20 times the width of
the antenna you are measuring.  (You also have to
consider the height of the antenna, especially if the
ground is sloping.)

An alternative to carrying a receiver around is to use
a "test oscillator" (as opposed to a transmitter) and
listen on the station receiver.  If you are using a
rotator, you can set the source out in a convenient
place and take bearings as you rotate the antenna in
the shack.  You can also do this with signals on the
air, though you may have to average your measurements
over time to account for propagation changes.

When choosing a remove measurement site, make sure it is
clear of metal objects that will affect the reading,
especially power lines.  These will pick up the signal
and reradiate it, causing very quirky results.


Without a lot of attention to detail, these methods only
give an approximate pattern, and aren't really adequate
by themselves for absolute gain measurements.  But they
are good enough for relative patterns or comparative
gain measurements (comparing the gain of one antenna to
another in the same location.)
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N3OX
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« Reply #3 on: September 15, 2009, 10:15:16 AM »

If your antenna is rotatable, G4HFQ's "Polar Plot" program is what I use, along with various types of signal sources... local ham keydowns, AM radio stations (for my low band receiving flag), TTL can oscillators used as small battery power signal sources which I think probably fall under Part 15 rules because emissions are so low.

The program is available here.  You need to have a sound card and turn your radio's AGC off and the RF gain to the point where the audio doesn't distort.

http://www.g4hfq.co.uk/

This is a real pattern taken on my rotatable flag antenna using a 1500kHz AM radio station for the signal source:

http://n3ox.net/projects/flag/flagpattern_lg.JPG

Here are comparison patterns using a 1843.2kHz TTL can switched on and off by a 555 timer (switched on and off too slowly, so you can see the "pings"

http://n3ox.net/projects/flag/160pattern_lg.jpg

That one reveals some kind of weird problem which I think I've fixed and also compares two termination resistors.

If the antenna is not rotatable, you have to walk around a circle with a field strength meter or a signal source, like you say.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
N1LO
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« Reply #4 on: September 15, 2009, 10:27:19 AM »

Hi Tom,

Sounds like you're on the right track. I've done this with mobile stations.

You don't need to id each transmission. Choose a band that is closed or find a clear frequency. In a phone band, I like to ID (ask if in use), then switch to rtty or cw for consistent test transmissions, returning at least every few minutes to ID as 'testing'.

I tie a string around the vertical to maintain the same distance and use the minimum power that gives a good reading on the field strength meter.

Receiving tests aren't practical for generating a plot quickly; you can't control the transmission strength or direction. Also, RX plotting won't tell you anything about your TX efficiency.

If you record the data then modify your antenna, you'll be able to directly compare your results.

For consistency:
Use a radial string to set the distance.
Put your meter on a stand/tripod.
Step away just a bit, and always stand in the same position relative to your stand and antenna.

GL,

Mark
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AB7E
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Posts: 117




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« Reply #5 on: September 15, 2009, 10:56:19 AM »

Try the very nice (and free) program from G4HFQ.  It's called Polar Plot and it's available from his web site at

http://www.g4hfq.co.uk/

The program requires you to connect the audio output of your rig to the sound card of your computer, and to turn off the AGC in your rig to make the audio chain as linear as possible.  Any decent sound card will be quite linear (that's what keeps the distortion low) if you don't overdrive it.

You tune to a steady signal and click on the start button in the program, and it will automatically generate a polar plot for you as you rotate your antenna.  It's very slick, and the help file to learn how to use it is pretty thorough.

A few things to consider:

1.  A signal from a local station will be steady, but the resulting plot will be for a pretty low takeoff angle.  The signal from a signal far enough away to propagate via skip would arrive at a more relevant angle for DX, but it REALLY tough to find a signal steady enough to make an accurate plot.  My recommendation is just to use the closest local ground wave signal you can.

2.  Make a test plot to make sure that you don't flat-top your sound card at the peak response of the antenna.  If necessary, ask the guy providing the signal to reduce power until you get a smooth lobe.

3.  The program expects you to know approximately how long it takes to rotate your antenna 360 degrees and you enter that time in seconds in the dialog box, but you don't need that to be super accurate.  Set the time a little on the high side so the plot will overlap itself, and then the program allows you to trim the extra plot until the ends of the curve join smoothly.  

As I said ... it's pretty slick.  Lot's of folks use it, so if you get stuck just come back here to this forum and ask for help.

73,
Dave   AB7E
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WA3SKN
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« Reply #6 on: September 16, 2009, 07:03:51 AM »

On HF, this is not as easy as it sounds... BUT it is a lot of fun to try!
That is beacuse that any metal within 100 WAVELENGTHS will affect readings somewhat.  This is the reason that the "Antenna Contests" used VHF/UHF antenna models for testing... to keep the antenna range within reason!  But you can get some idea of how well your antenna is working or it's directionality with some fairly simple test eqpt.
Can you rotate the antenna, or must you move the signal source?  What types of antennas do you plan to check?

-Mike.
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WA3SKN
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« Reply #7 on: September 16, 2009, 07:04:05 AM »

On HF, this is not as easy as it sounds... BUT it is a lot of fun to try!
That is beacuse that any metal within 100 WAVELENGTHS will affect readings somewhat.  This is the reason that the "Antenna Contests" used VHF/UHF antenna models for testing... to keep the antenna range within reason!  But you can get some idea of how well your antenna is working or it's directionality with some fairly simple test eqpt.
Can you rotate the antenna, or must you move the signal source?  What types of antennas do you plan to check?

-Mike.
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WA3SKN
Member

Posts: 5494




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« Reply #8 on: September 16, 2009, 07:04:55 AM »

On HF, this is not as easy as it sounds... BUT it is a lot of fun to try!
That is beacuse that any metal within 100 WAVELENGTHS will affect readings somewhat.  This is the reason that the "Antenna Contests" used VHF/UHF antenna models for testing... to keep the antenna range within reason!  But you can get some idea of how well your antenna is working or it's directionality with some fairly simple test eqpt.
Can you rotate the antenna, or must you move the signal source?  What types of antennas do you plan to check?

-Mike.
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WA3SKN
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Posts: 5494




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« Reply #9 on: September 16, 2009, 07:05:23 AM »

On HF, this is not as easy as it sounds... BUT it is a lot of fun to try!
That is beacuse that any metal within 100 WAVELENGTHS will affect readings somewhat.  This is the reason that the "Antenna Contests" used VHF/UHF antenna models for testing... to keep the antenna range within reason!  But you can get some idea of how well your antenna is working or it's directionality with some fairly simple test eqpt.
Can you rotate the antenna, or must you move the signal source?  What types of antennas do you plan to check?

-Mike.
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WA3SKN
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« Reply #10 on: September 16, 2009, 07:08:11 AM »

Got an "SQL ERROR" while trying to post... SORRY!

-Mike.
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KC9KEP
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« Reply #11 on: September 16, 2009, 11:06:35 AM »

Thank you everyone for all the information!

It's all good learning for me!

In the near future, I'd like to build a Moxon antenna
with rotator, so I'll be interested to confirm its
performance if possible.

But, what had piqued my interest was an old QST
article in which they were plotting a graph
of their wire antenna.  They just turned on their
transmitter and plotted signal strength over a
large radius.  (and, located some unexpected lobes.)

And, that's what I have right now, an end-feed wire.

Sorry, I do not have the dimensions handy.  I'd guess
that it's up about 25 feet and is around 80 feet
long ..

But, as much as anything, I'm curious how a "field plot"
is accomplished :-)

Thanks again all!

73
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WB6BYU
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« Reply #12 on: September 16, 2009, 01:28:58 PM »

With large commercial installations they will fly a
chopper around it with the appropriate measuring equipment
on board.  That allows them to do both horizontal and
vertical patterns.

But most hams can't afford that approach!
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N3OX
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« Reply #13 on: September 16, 2009, 05:40:52 PM »

"But most hams can't afford that approach! "

You might be able to get something done with a moderate sized helium or hot air balloon with a distance marked string on a very calm day.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
G3TXQ
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Posts: 1523




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« Reply #14 on: September 17, 2009, 02:49:12 AM »

>> WA3SKN wrote: On HF, this is not as easy as it sounds... BUT it is a lot of fun to try!
That is beacuse that any metal within 100 WAVELENGTHS will affect readings somewhat.<<

Do you really mean that? Would a piece of metal 2km (1.2 miles) from my 20m antenna actually affect readings?

That's one impressive metal-detector Wink

Steve G3TXQ
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