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Author Topic: Dipole for 2 meter ssb questions.  (Read 5594 times)
KF4CZV
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Posts: 142




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« on: May 12, 2010, 06:16:50 PM »

I'd like to give 2m ssb a try so I'm planning to build a dipole cut for 144.2 center frequency. My first question is: Will a dipole work better than my copper j pole for sideband? Second question: Since a 1/4 wave dipole for that frequency is only a little over 3 feet in total length, would a 12 foot full wave dipole work better? Thanks!

Terry
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N3OX
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« Reply #1 on: May 12, 2010, 07:54:51 PM »

I'd like to give 2m ssb a try so I'm planning to build a dipole cut for 144.2 center frequency. My first question is: Will a dipole work better than my copper j pole for sideband?

Yes, probably 30dB better.  Cross polarization is a killer.

Quote
Second question: Since a 1/4 wave dipole for that frequency is only a little over 3 feet in total length, would a 12 foot full wave dipole work better? Thanks!

Terry, a slightly-more than three foot dipole is a HALF wavelength.   

A full wavelength antenna at slightly over 6 feet ( 2 meters  Wink ) has a couple more dB gain than a half wavelength dipole.  However, the  problem is that the feedpoint resistance of the full wavelength antenna is very high and a terrible match to coax.  So without a matching section, your SWR would be terrible and the loss in your coax would be very high.

You can build a matching section for a full wavelength dipole in the same way that you feed a J-pole.. Take the U-shaped section that forms the bottom of the "J" in a J-pole and instead of having one leg 3/4 wavelength tall, put elbows and put two half wavelengths out horizontally. 

As far as a 2 wavelength/12 foot antenna, it is NOT the case that "bigger is better" in antennas.  A 12 foot long antenna would also need a matching section and it would have FOUR lobes... so instead of having two directions broadside to the dipole it would have four directions about 45 degrees from broadside.  As you keep making the antenna longer without adding special phasing sections, you just get more and more lobes like flower petals. 

You could add phasing sections, but you start to get a long and unwieldy antenna.  A full wave dipole with a matching section might be a fun thing to try, but I wouldn't go further than that... I would step up to a yagi instead.  There are lots of good plans... look at WA5JVB Cheap Yagis for some practical ideas.  Here's one page.

http://www.fredspinner.com/W0FMS/CheapYagi/vjbcy.html

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

Monkey/silicon cyborg, beeping at rocks since 1995.
KF4CZV
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Posts: 142




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« Reply #2 on: May 12, 2010, 08:15:55 PM »

Thank you very much! That's just what I needed to know.

Terry
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WX7G
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« Reply #3 on: May 13, 2010, 05:55:03 AM »

Using 1/2 inch copper plumbing pipe the length for 146 MHz is 37.5 inches.

A simple horizontally polarized two meter antenna is the 3-D Folded Loop turned on its side. Easier to build then a turnstile antenna (crossed dipoles).
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K5CQB
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« Reply #4 on: May 13, 2010, 02:09:37 PM »

http://www.qsl.net/dk7zb/start1.htm
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N8EKT
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« Reply #5 on: May 13, 2010, 02:33:44 PM »

Your best and easiest solution may be to make a 3 element quad.
It will produce great gain for it's size and is very easy to build.
Virtually all 2m ssb activity will be horizontal so this is a must
But I have routinely checked in to the North Carolina sideband net from my qth with 25 watts
and a 4 element quad.

 
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KF4CZV
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« Reply #6 on: May 13, 2010, 02:37:19 PM »

Thanks everyone! You've told me what I needed to know. Now, if I can just find some 2m ssb activity........!

Terry
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K9KJM
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« Reply #7 on: May 13, 2010, 10:40:12 PM »

You will get lots better results on two meter SSB if you install some type of high gain antenna, As already pointed out, Horizontal.

My favorite low cost, Easy to build and tune VHF/UHF is the "Quagi"

http://commfaculty.fullerton.edu/woverbeck/quagi.htm

Be sure to use low loss coax like Times LMR 400 if running any more than just a few feet, Do keep the feedline as short as possible.

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KI4SDY
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« Reply #8 on: May 18, 2010, 06:23:15 AM »

This would be a good opportunity to build your first beam antenna, since 2 meter beams are small and lightweight. You can make a three element (or more) cheap, out of a variety of junk materials you probably have laying around the house. It will beat the pants off a dipole!  Grin
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KF4CZV
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« Reply #9 on: May 21, 2010, 11:58:32 AM »

I have a friend across town who loves 6 meters as much as I do, but he always tries to use a horizontal loop on 6m-which is essentially a dipole. I have a 3 element beam on a 6 foot boom. Much of the time I can work stuff that he can't even hear.  Shows you what a little gain can do.

73s John AA5JG

I appreciate all this great info! Since a beam is directional, don't you have to be lucky enough to happen to have it pointed in the right direction to initially receive the a station you want to work? Does this mean I'd need a second omni-directional antenna to find the stations first and then switch to work them? How well does a beam receive on it's sides? I've always used verticals so beams are a mystery to me. I was planning to use dual stacked Par OA50s for 6 meter work. Are you saying this won't work well for 6m ssb? That would be really disappointing since I already bought them.

Terry
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AA4PB
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« Reply #10 on: May 21, 2010, 01:22:34 PM »

You are right and some people have both Yagis and horizontal loops for just that reason. Most of the time however you can tell that stations are there even if the Yagi is not pointed in their direction. Either you hear them very weak or you hear other stations working them. That's assuming you don't have a very long boom or a stack of several Yagis.

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KF4CZV
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Posts: 142




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« Reply #11 on: May 21, 2010, 01:43:24 PM »

One other question: Is there even enough 2 meter ssb activity to make it worth the trouble and expense of setting up for it? I certainly never hear anything through my copper j-pole. I realize the conditions aren't great right now for any band but when they are is there much activity?

Terry
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WB2WIK
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« Reply #12 on: May 21, 2010, 02:49:26 PM »

One other question: Is there even enough 2 meter ssb activity to make it worth the trouble and expense of setting up for it? I certainly never hear anything through my copper j-pole. I realize the conditions aren't great right now for any band but when they are is there much activity?

Terry

This is a little bit like saying, "Should I bother getting on 20 meters?  I listen there without an antenna connected and really don't hear anything."

A j-pole (vertical) is likely to hear absolutely nothing on 2m SSB, no matter where you are.  It's a waaaaaay less than "0 dB gain" antenna, which actually has about 20 dB or more loss when used on SSB, because everyone on SSB is horizontally polarized.

I hear lots of activity on 2m SSB (at night) using a 12 element Yagi up 62' on a tower.  If I switch my my "6 dBi" gain omnidirectional vertical collinear, all that goes away and I hear basically nothing.  I keep the vertical for FM, of course, where everything is vertically polarized and my horizontally polarized beam works very poorly.

The tricks to 2m SSB are:

-Horizontal polarization is a must
-A well elevated antenna, high above local obstructions like houses and trees is a must
-A low loss transmission line is a must
-Being able to operate when there's the most activity (typically from late afternoon through late evening) is very helpful -- there's also usually some activity on weekend mornings
-Being able to "aim" your beam at local activity centers, which are usually in and around the most densely populated areas, is extremely helpful
-Run 100W PEP output power if possible, as that's about what most are using
-Know and understand where the activity is: Generally around 144.200, but other frequencies are in common use, depending where you live
-Get on to the VHF weak signal nets and make contacts there to find out more about your local activity

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