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Author Topic: Asking advise on a 20 meter dipole.  (Read 13744 times)
K4RLK
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Posts: 29




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« on: March 06, 2011, 04:52:30 PM »

I'm putting up a 20 meter dipole at 35 feet and have 2 questions. 1) Is a flat top better or inverted V. I can do either. 2) I live in Florida, central part close to the west coast. Would a north/south or east/west direction be best? Thanks in advance for all your advise.
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WB6BYU
Member

Posts: 17279




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« Reply #1 on: March 06, 2011, 05:11:20 PM »

Quote from: K4RLK

1) Is a flat top better or inverted V.


A dipole would be slightly better in terms of gain, but the difference is small as long as the
slope of the wires isn't too steep.


Quote
2) I live in Florida, central part close to the west coast. Would a north/south or east/west direction be best?


It depends - who do you want to talk to?  Running it East/West would be best to work
the New York, while North/South would be better for Los Angeles.  Find a globe and look
at what parts of the world are in various directions.  (Don't rely on a flat map for this unless
it is an azimuth projection centered on your QTH - the bearings won't translate correctly.)

The truth is that a dipole really isn't that directional.  I had a rotatable dipole up for several
years and rarely bothered to turn it.  On Field Day from Oregon I could work the whole
country with it running North/South, including Southern California and British Columbia
(which were off the ends),  Alaska and Hawaii.  Try to avoid pointing one end at an area
of interest, especially a more distant one, but a single antenna should cover at least half
the world, and in my experience about 80%.  (Perhaps not good enough to break pileups, but
sufficient for enjoyable QSOs.)
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K0ZN
Member

Posts: 1782




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« Reply #2 on: March 06, 2011, 07:31:09 PM »

Hi,

That antenna will show some deterioration in performance off the ends....significantly at times....especially long haul contacts.

If you want good USA coverage, I would suggest having it broadside NW to SE. As a rule, a dipole gives very good results with a
broadside "beam width" of about 90 degrees or so (45 deg. either side of the main lobe maxima). You can and will make contacts
off the ends, but you won't get or give really good reports.

If there is any easy way you could move it 90 degrees, it would be the best option.

Pick up a copy of the ARRL Antenna Book.  Check the various patterns out. Respectfully, spending some time studying the Antenna
Book would be the best investment you could make in your station at this point. You can pick up an old copy off of Ebay cheap and
the basic data in it is still totally current. The more you know about antennas the better your signal will be and the more fun you will
have.

73,  K0ZN
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K7KBN
Member

Posts: 3491




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« Reply #3 on: March 06, 2011, 08:09:53 PM »

Try both and see which works better for YOU.

Experiment.

Learn.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
K2OWK
Member

Posts: 1277




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« Reply #4 on: March 06, 2011, 08:54:45 PM »

Hello, The inverted "V" configuration has the advantage of being able to operate both Vertically and Horizontally. The horizontal dipole works only horizontally.

Regards
73s
Barry K2OWK
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N4JTE
Member

Posts: 1169




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« Reply #5 on: March 06, 2011, 09:07:54 PM »

Hi Ron if you have the option, a horizontal flat top dipole will out perform an inv Vee about 90% of the time, same with a vertical. There will be the occasional signal that might have a little more strength on a Vee. The Vee is a compromise antenna for folks having only one support, the myth about the vertical polarization, DX capabilities, only applies when the Vee is up about 2 wl and it will still have a problem competing with any dipole at the same height most of the time.
Regards,
Bob
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N4JTE
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Posts: 1169




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« Reply #6 on: March 06, 2011, 09:29:50 PM »

Forgot second question; In Fl., if you are interested in stateside contacts a NW/ SE broadside heading will be useful. However if you want Europe then a NE/SW broadside will serve you well, by broadside I mean the ends of your antenna are 90 degrees to the broadside intended directions.
A dipole at your height will not be very directional most of the time, as you move it higher it will show more directivity.
Regards,
Bob
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N4JTE
Member

Posts: 1169




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« Reply #7 on: March 06, 2011, 09:59:44 PM »

One more thing, verticals will work dx if phased with one raised radial, over a dipole, just worked Moscow on 7.187 at 1 am eastern.
Worked better than 185 EDZ at 65 ft.
5/9 both ways on 2 ele vertical, 3/3 on Dipole.
I'm done,
Bob
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K0ZN
Member

Posts: 1782




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« Reply #8 on: March 07, 2011, 07:46:14 PM »


Follow up, Re:  Horizontal dipole vs. "Inverted V" configuration.

Based on another comment, there seems to be some misunderstanding.  First, a DIPOLE is a DIPOLE.  Period.  You can bend it and orient it
in different configurations:  To wit:  straight horizontal, "inverted V", "regular V', a circle (the Halo antenna), a "Z", vertically, an "L" or sloping... in any
of these cases, it is still just a dipole. (the basic definition of a dipole, basically being a half wave antenna fed in the center). (Comment to other
"experts":  I am trying to keep this definition simple. This is not an IEEE paper!)

The configuration of a dipole will change the feed point impedance some and it will change the radiation pattern plot.....but it is still a dipole!

In the real world there is little difference between an inverted V and a straight dipole if the angle of the V is not too narrow.  There seems to be
some slight, marginal data that indicates a straight horizontal dipole in its favored direction *can* have a tiny bit more power, but this is splitting fine hairs.
If the dipole is to be fed with balanced line and used on harmonics as an all band antenna, the horizontal configuration does start to show some
advantages....but you are looking at a mono band antenna.

If you can get the center of an inverted V up measurably higher than a horizontal dipole,
that may have a minor advantage; higher is better.

Keep in mind, that an inverted V configuration starts to show some cancellation of the radiation as the V angle narrows
and approaches 90 deg.  A narrow Vee angle of less than 90 deg. starts to cancel radiation significantly and is to be avoided.

Again, pick up a copy of the ARRL Antenna Book so you get the facts and a correct understanding of this antenna and its properties and radiation patterns.

73,  K0ZN
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K4RLK
Member

Posts: 29




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« Reply #9 on: March 13, 2011, 05:24:21 PM »

Well, here's what I did.  I made the dipole 16'4.5" long on each leg, fed with 50ohm coax. The center of the dipole is at about 33ft, and the coax runs 90* from the center connector. Right now it''s an inverted V with the legs at about 45*. I hooked it to my MFJ-9420, and the swr meter shows the swr to be below 1.2 across the phone portion of 20 meters. I guess I got lucky with the length of the dipole legs.
The antenna is oriented nw to se.
I fired up the MFJ, and found HI3FVA calling CQ. I got him with a 59 signal report! Not bad for my first QRP contact. Thanks for all the advice. I may leave it inverted for a while, then try it as a "T" and see what happens!
« Last Edit: March 13, 2011, 05:38:44 PM by K4RLK » Logged
K0ZN
Member

Posts: 1782




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« Reply #10 on: March 14, 2011, 06:51:55 PM »


Hi, 
Thanks for the follow up feedback.....lots of guys don't do that.  Sounds like you went to some trouble to do it right and are getting
the expected good results. I suspect that will be a good antenna for you. I think you will find you can work a lot of stuff on that antenna.

EXPERIMENT with it !!!  That is what Amateur Radio is all about!

Don't be surprised if the SWR goes up a tiny bit if you convert it to a straight horizontal dipole. In the Inverted Vee configuration, the feed point impedance
is just a little lower (probably a little better match for 50 ohm coax, to be candid), but the antenna will work just as well.  There is also the old saying, which has
some truth!  "If it ain't broke, DON'T fix it!!" 

73,   K0ZN

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K4RLK
Member

Posts: 29




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« Reply #11 on: April 02, 2011, 12:58:08 PM »

Update:
I moved the dipole from an inverted "V" to a flat top. It seemed to make it hear better. I saw no change in SWR, but using the MFJ-9420, I dropped from 11 watts out to 10 watts.

Today I added a 10 meter dipole to the 20 meter. I added 2 8' legs going east.west. The SWR is under 2.0 below 29.000. At 28.500 it's about 1.2

Total cost for the 2 band dipole using parts in the garage is about $20.00. I'm very pleased with my first home made antenna project.
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WB6BYU
Member

Posts: 17279




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« Reply #12 on: April 02, 2011, 03:11:43 PM »

Thanks for the update!  It's always good to hear back to see if we gave the correct
suggestions.


Quote from: K4RLK

I moved the dipole from an inverted "V" to a flat top. It seemed to make it hear better. I saw no change in SWR, but using the MFJ-9420, I dropped from 11 watts out to 10 watts.


Not at all unusual.  Most radios, tuners, etc. care about the actual impedance they see more
than the SWR (which just tells you how bad it is, without telling you exactly what the value is.)
It may have changed from 42 ohms to 60 ohms (or vice versa), both of which would
give an SWR of 1.2 : 1, but the rig may be happier with one impedance than the other.
If that is important, adding a quarter wavelength of coax will shift it back, but a 10% change
in output power doesn't make a lot of difference.

Quote
Today I added a 10 meter dipole to the 20 meter. I added 2 8' legs going east.west. The SWR is under 2.0 below 29.000. At 28.500 it's about 1.2

Total cost for the 2 band dipole using parts in the garage is about $20.00. I'm very pleased with my first home made antenna project.


Great!  One of the things I want people to learn is that homebrew antennas really aren't
difficult to build, and they work at least as well as similar commercial antennas for lower
cost.  This is an important learning experience.

Good luck with your antennas!
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