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Author Topic: Omnidirectional Antenna  (Read 54481 times)
NW7N
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Posts: 17




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« on: February 05, 2015, 11:44:11 AM »

Do I need an omnidirectional antenna for MARS? I currently have a dipole running north/south and have not been able to hear the nets that I am told are occuring to the south of me.

Thanks!
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WD8DBY
Member

Posts: 97




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« Reply #1 on: February 05, 2015, 01:34:48 PM »

Do I need an omnidirectional antenna for MARS? I currently have a dipole running north/south and have not been able to hear the nets that I am told are occuring to the south of me.

Thanks!

Thanks for your question.  Dipoles typically have best reception perpendicular to the direction the wires are oriented unless you are operating NVIS.  MARS members run every type of antenna...dipoles, verticals, long wires, beams, logs.  You may need to check the times and frequencies of the nets you are attempting to contact; they may have changed.
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AA4PB
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Posts: 14421




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« Reply #2 on: February 05, 2015, 01:41:40 PM »

Unless your dipole is mounted at least 1/2 wavelength above ground it will be pretty much Omni-directional anyway.
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Bob  AA4PB
Garrisonville, VA
K6BBB
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Posts: 0




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« Reply #3 on: February 06, 2015, 12:50:18 PM »

Unless your dipole is mounted at least 1/2 wavelength above ground it will be pretty much Omni-directional anyway.

I'm using a rotatable dipole (Hustler DPK-1 with  60-meter resonators) 13 feet above ground level. I find it to be very directional as I have to rotate my antenna to work a station more than 30 degrees from where it was previously aimed.

Also, I'm able to use this antenna with the tuner on 60, 40, 20, 17, 15, 12 and 10 meters with under 1.5:1 SWR and usually 1.2:1.
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WB6BYU
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Posts: 17353




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« Reply #4 on: February 06, 2015, 04:39:06 PM »

Quote from: K7BOG

... have not been able to hear the nets that I am told are occuring to the south of me.



How far south, and on what frequencies?  What time of day?

For any path there will be times that it works on some frequencies and doesn't work on other frequencies.
That depends on the current state of the ionosphere and the operating frequency.

A couple useful tools for propagation predictions:
http://www.voacap.com/prediction.html
http://www.ips.gov.au/HF_Systems/7/1

Before you blame the problem on your antenna, make sure that propagation supports that frequency for
that path at the time of the net.  Note that the results won't exactly agree, but they can give you some
good clues as to what to expect.


A dipole doesn't show much directivity at relatively high angles (shorter path lengths) but will show more
directivity at low elevation angles (longer path lengths).  However, I've never found the pattern to be
that sharp - I had a rotatable dipole up for 10/15/20m and after trying to turn it to optimize signals I
finally just left it fixed, as it appeared to work about as well.

If you decide that you do need to change your antenna and the path length is a few hundred miles,
you might consider extending just one end of it by half a wavelength instead.  That puts a strong lobe
in the direction of the wire at about 30 to 60 degrees elevation angle.  I've had good results with
such a pattern over distances of 200 to 500 miles.
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NW7N
Member

Posts: 17




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« Reply #5 on: February 07, 2015, 12:12:50 PM »

Quote from: K7BOG

... have not been able to hear the nets that I am told are occuring to the south of me.



How far south, and on what frequencies?  What time of day?

For any path there will be times that it works on some frequencies and doesn't work on other frequencies.
That depends on the current state of the ionosphere and the operating frequency.

A couple useful tools for propagation predictions:
http://www.voacap.com/prediction.html
http://www.ips.gov.au/HF_Systems/7/1

Before you blame the problem on your antenna, make sure that propagation supports that frequency for
that path at the time of the net.  Note that the results won't exactly agree, but they can give you some
good clues as to what to expect.


A dipole doesn't show much directivity at relatively high angles (shorter path lengths) but will show more
directivity at low elevation angles (longer path lengths).  However, I've never found the pattern to be
that sharp - I had a rotatable dipole up for 10/15/20m and after trying to turn it to optimize signals I
finally just left it fixed, as it appeared to work about as well.

If you decide that you do need to change your antenna and the path length is a few hundred miles,
you might consider extending just one end of it by half a wavelength instead.  That puts a strong lobe
in the direction of the wire at about 30 to 60 degrees elevation angle.  I've had good results with
such a pattern over distances of 200 to 500 miles.

About 50-100 miles south of me. The nets occur in the early morning and early evening hours around 5214 mhz. The highest I can get the center insulator off of the ground is 12 feet, which is my roof.  I don't have a lot of real estate for gentle angles on the dipole, either. My roof is 12 feet and the furthest out on the ground is about 50 feet each direction. Right now, my dipole is attached to a gutter, but if I lay it flat on my roof, I can probably go out about twice the length with it resting on my roof as opposed to hanging on a gutter.
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AA4PB
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Posts: 14421




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« Reply #6 on: February 07, 2015, 12:46:36 PM »

Well, a 1/2 wave dipole on 5.214 MHz is 90 feet long, or 45 feet on each side so it should fit in your space. Being only 12 feet above ground is certainly not ideal. Running parallel (and close to) a metal gutter is also not good. You really need to get the center point of the dipole up in the air 30-40 feet on some type of mast. It would be okay for the ends to slope downward closer to the ground as long as you keep the wires away from the metal gutters. Laying it flat on the roof is not good either.
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Bob  AA4PB
Garrisonville, VA
N5EG
Member

Posts: 315


WWW

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« Reply #7 on: February 07, 2015, 02:36:03 PM »

Hi,

You don't say what your QTH is.

For a 120 mile path due south, at 42 degrees latitude of the northern end, the MUF is
below 5 MHz in the morning for many hours according to W6ELProp and an estimated
solar flux of 120 and a K factor of 2.

This is an almost a vertical  incidence path via the ionosphere, so the MUF is very close
to the critical frequency (i.e. it's the most challenging).

So you are going to have a difficult time of it most days no matter what your antenna.

-- Tom, N5EG




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N5EG
Member

Posts: 315


WWW

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« Reply #8 on: February 07, 2015, 07:08:01 PM »

Update --

The MUF on the NVIS path goes above 5 MHz starting about 8:00 AM local time, and goes
back down below 5 MHz about 6:00 PM.

12 feet for the antenna height is going to cause a lot of ground losses based on both
modeling and actual test experience on 7 MHz.

-- Tom, N5EG
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WB6BYU
Member

Posts: 17353




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« Reply #9 on: February 08, 2015, 05:02:44 PM »

12' isn't bad for such a path.  Yes, the ground losses may be higher, but when the band is open
signal strengths should be good.  Having the antenna running parallel to a metal rain gutter
isn't ideal, but I don't expect that would stop you from hearing the stations.

The problem is propagation.  At this time of year at your latitude, the "Critical Frequency" is
probably dropping too low, and the ionosphere won't reflect your signals back to Earth at
such a short distance.  To cover that distance at that time you have to use a lower frequency.
During the day, or during the summer, it may work, and you also should be able to hear
stations at longer distances (because the ionosphere will reflect signals at lower angles,
just not those that go almost straight up.)

Changing your antenna can't fix that.
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NW7N
Member

Posts: 17




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« Reply #10 on: February 09, 2015, 11:48:51 AM »

Well I hope to get a tower up (about 30 feet) sometime this summer, but for now, I need to get from Cheyenne to Denver, essentially.

I guess I should further clarify that the legs of the dipole are not on the gutter. I tied the center insulator to the gutter and the legs are running off of it down to the lawn.

One idea I was curious about is leaving one leg running down to the lawn and then running the other leg up onto the roof. There isn't much metal on my roofs (just nails for the shingles), would that help my propagation at all?
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WB6BYU
Member

Posts: 17353




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« Reply #11 on: February 09, 2015, 12:40:24 PM »

With a wood shingle roof I've just laid the antenna wire on the roof, or tucked it under the shingles.
(Not a good idea if you are running high power, but 100W to a dipole shouldn't be an issue.)
Try to keep the ends up as high off the ground as possible - ideally above the height where someone
might come into contact with them, but you do whatever you can.

When the ionosphere supports the desired path, you should have plenty of signal even with an
improvised antenna.  I've used dipoles running along hedges and fences.  But when the ionosphere
isn't cooperative, nothing you can do with your antenna is going to help, other than designing it
for a lower frequency and using that, or relaying through a station 2000 miles away.


If you look at the link I sent to the Australian Ionospheric Propagation Service web site, their
"LAMP" display (Local Area Mobile Prediction) gives a plot of what frequencies will cover what
distances out to 600 miles (1000km) for each hour over the course of the day.  It has a default
set of values for the amateur bands, but you can also enter your own list of frequencies for
the MARS nets.  You click on the world map to specify your location, or enter it as lat and long.
Be sure to choose the Northern Hemisphere ionospheric data option.  Based on their forecast,
you can see what times of day you are likely to make contact over the desired path, or, for a
given time, what band is mostly likely to be open.
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KC2QYM
Member

Posts: 875




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« Reply #12 on: February 10, 2015, 07:27:21 AM »

Quick...there's a disaster, the internet is down, no commercial power, you have a generator with gas, radio equipment and plenty of wire and trees.  What do you do to get onto the MARS net and save the world?
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WB6BYU
Member

Posts: 17353




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« Reply #13 on: February 10, 2015, 12:35:19 PM »


That's why we're learning about propagation, antennas, and how to determine what frequency you need
to use to cover distances throughout the day now, so the knowledge is ingrained if we do have an emergency.

My motto is, "If you don't use it regularly, it won't work in an emergency."  The best preparation is learning
and using the skills and knowledge you need on a regular basis.
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WV8RS
Member

Posts: 20




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« Reply #14 on: May 08, 2015, 06:54:46 PM »

Dale, what you're saying about Propagation and take-off angles is something every one of us needs to be aware of... I've been watching "conditions" change over the last 3 months and it's amazing to watch a Band "open" literally within a minute!!!  Ron... WV8RS...
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