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Author Topic: Monoband Yagi's Rule!  (Read 10090 times)
KY6R
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« Reply #15 on: August 07, 2012, 10:44:47 AM »


It would be very easy to build a 30/17M nested Moxon, and 4 dBd on 30M would be killer. The width of it would be about the width of an A3S, so its a great sized antenna. If I were to build one - I would make it look like a yagi - using aluminum for the largest part of the director and reflector - and then use wire for the sides.

Does anybody know of a book devoted to Moxons? I have a book by Moxon but it covers all kinds of antennas. My knowledge of the antenna is seriously lacking. I've read individual articles with plans from time to time but I'd like a book solely devoted to the topic.

30 meters just doesn't have the activity 40 does so a monobander doesn't make sense for me. I like to participate in contests for the sport and to pickup fills so I'd opt for a 40 meter beam. Plus for my small lot a 30 meter beam would be a real waste of space.

73,

Chris/NU1O

The best Moxon reference is here and free:

http://cebik.com/

And here is a wonderful and free Moxon calculator:

http://www.ac6la.com/moxgen.html


I toyed with a 40M Moxon as well - but since I could not get one up at least a half wave, at one time I had a pair of phased verticals and used the Christman method plus a DXE antenna switch to switch broadside and end fire. The easier way to do this - at the same cost is the PVS-2 by DXE.

You could also put up two 40M half squares at right angles for 3 dBd gain in 4 directions . . . . . that's the simplest - no radials . . .

Finally - if you have a 50' tower, you could hang 4 or 5 half wave sloping dipoles and build a K1WA or K8UR Array - which would be like a 40M four square - but also doesn't need radials . . . 4 dBd in each direction. See the ON4UN Lowband DX-ing book for details - that's the best lowband antenna book, period.

« Last Edit: August 07, 2012, 11:17:45 AM by KY6R » Logged
K0RS
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« Reply #16 on: August 07, 2012, 12:16:42 PM »

OK, here's a little heresy for you.  Four elements on a 26 (or 31) foot boom on 20m won't give you any more gain than three.  You can change the pattern slightly with careful placement of the 2nd director at such short boom lengths, but the difference in gain is insignificant.  Four, or even five, elements won't do any good until the boom length approaches three quarters of a wavelength, and at that length the extra elements make a big difference.  But that's nearly a 50 foot boom on 20m.  In practice, you will start to see advantages in gain from those extra elements with around a 44 or 45 foot boom.  

This is of course true on any band, but higher frequencies have correspondingly shorter boom lengths.  On 10m for example, you only need a 25 foot boom to achieve 3/4 wavelength.  I built a five element 10m Yagi on a 27' boom and it was a killer antenna.  Some time with your modeling program will confirm this.  The advantages of a three element over a four at shorter boom lengths are obvious; lighter weight, less wind resistance, lower cost, accessability to the balun or matching system from the tower.  Four elements normally won't hurt you, but it won't help you either.  The fourth element does add another variable when designing the antenna, making it more difficult to get everything right.

One option for a four element that is valuable to to move the first director very close to the driven element (OWA design).  This will broadband the SWR response if both CW and SSB are important to you with a low reflected power.  Still will have the gain of a three element.  The first director serves only to broaden the SWR curve.
« Last Edit: August 07, 2012, 12:29:19 PM by K0RS » Logged
NU1O
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« Reply #17 on: August 07, 2012, 12:57:03 PM »

K0RS has a valid point, Rich.  All the big guns running 20 meter monobanders (W6CCP, Seymour, et al) are running antennas like he describes.  If you can't fit a 45 or 50 foot boom it's probably back to the drawing boards.


73,

Chris/NU1O
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KY6R
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« Reply #18 on: August 07, 2012, 01:09:45 PM »

OK, here's a little heresy for you.  Four elements on a 26 (or 31) foot boom on 20m won't give you any more gain than three.

Yes - YW confirms this. Now, a 24' boom 3 element 20M yagi should be quite a bit better than my A3S . . . . not sure how much because I don't believe the Cushcraft specs at all . . . . . I'll spend some time and see what would be an optimal boom length for a 3 element 20M yagi on a 2" boom.

« Last Edit: August 07, 2012, 01:27:14 PM by KY6R » Logged
N2RJ
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« Reply #19 on: August 07, 2012, 01:26:50 PM »

I use a SteppIR which is "a monoband on every frequency."

I don't think I will ever use a trapped antenna again.

You can get the Force 12 antennas which are multi monoband too, they basically put a bunch of monobanders on the same boom and optimize them.
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KY6R
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« Reply #20 on: August 07, 2012, 01:35:16 PM »

I use a SteppIR which is "a monoband on every frequency."

I don't think I will ever use a trapped antenna again.

You can get the Force 12 antennas which are multi monoband too, they basically put a bunch of monobanders on the same boom and optimize them.

I will only design and build my own antennas from now on - I enjoy it so much more than buying. I also won't have traps any more - the A3S was a quick fix for just this year and maybe next, and was inexpensive and can be turned with a cheap rotator on my AB-952 mast. It will be easy to resell as well.

Having used YW, I don't believe that the SteppIR's perform like mono banders on every band. The only way that could happen is if they also changed the element spacing as well as the element length. They are also heavy antennas and require bigger rotators and a stronger mast than my AB-952. I had a 2 element SteppIR - and one of the stepper motors gave up the ghost.

The Force-12 antennas are expensive these days. I built my 17M 3 element yagi for < $50 - since I was able to buy an old Wilson 5 element 20M 40' boom yagi (with old rusted boom to mast clamps) for $150 and turned that bugger into three antennas. I am going back up to my "source" for more old antennas soon.

I won't put motors or traps up in the air from now on.
« Last Edit: August 07, 2012, 01:42:50 PM by KY6R » Logged
K0YHV
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« Reply #21 on: August 07, 2012, 01:39:36 PM »

I use a SteppIR which is "a monoband on every frequency."

Not exactly, as they are still a compromise on element spacing.  The only way to get optimal spacing is with a monobander.

John AF5CC
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K0RS
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« Reply #22 on: August 07, 2012, 02:44:38 PM »

Now, a 24' boom 3 element 20M yagi should be quite a bit better than my A3S . . . .

Troof be dat.

In fact I built such an antenna.  First configuration was for Field Day.  I optimized forward gain but reduced the F/B.  Pointed the sucker east and never moved it during the contest.  Worked great from CO, east coast, west coast, boom.  We set a 3A record that year.  Later I took the same antenna, changed element length and spacing and made it into a more conventional DX antenna, with F/B as well as forward gain.  A friend still has it installed at his home QTH.

Here's another Yagi truism.  The three main performance characteristics of a Yagi are: Forward Gain, F/B ratio and SWR bandwidth.  You can optimize for any factor, but the others will suffer.  Any Yagi design is a compromise between these three parameters.  Pick one (or maybe two).  I've had Yagis that were broadband but were wet noodles in a pileup.  Others (think KT-34XA) that had forward gain, but poor F/B.  The 'XA was a special case for SWR bandwidth because it had dual driven elements.

Another friend, W0UA, a contester, made a profound statement to me one day.  I had to learn a lot more about antennas before I realized that I was hearing words from a master.  Sometimes knowledge is imparted before one is ready to absorb it.  He said, "Front to back is nice but forward gain makes Qs."  If that sounds too simple, you need to be in some more pileups.  Zen is where you find it...
« Last Edit: August 07, 2012, 03:07:04 PM by K0RS » Logged
KY6R
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« Reply #23 on: August 07, 2012, 03:46:33 PM »

Now, a 24' boom 3 element 20M yagi should be quite a bit better than my A3S . . . .

Troof be dat.

In fact I built such an antenna.  First configuration was for Field Day.  I optimized forward gain but reduced the F/B.  Pointed the sucker east and never moved it during the contest.  Worked great from CO, east coast, west coast, boom.  We set a 3A record that year.  Later I took the same antenna, changed element length and spacing and made it into a more conventional DX antenna, with F/B as well as forward gain.  A friend still has it installed at his home QTH.

Here's another Yagi truism.  The three main performance characteristics of a Yagi are: Forward Gain, F/B ratio and SWR bandwidth.  You can optimize for any factor, but the others will suffer.  Any Yagi design is a compromise between these three parameters.  Pick one (or maybe two).  I've had Yagis that were broadband but were wet noodles in a pileup.  Others (think KT-34XA) that had forward gain, but poor F/B.  The 'XA was a special case for SWR bandwidth because it had dual driven elements.

Another friend, W0UA, a contester, made a profound statement to me one day.  I had to learn a lot more about antennas before I realized that I was hearing words from a master.  Sometimes knowledge is imparted before one is ready to absorb it.  He said, "Front to back is nice but forward gain makes Qs."  If that sounds too simple, you need to be in some more pileups.  Zen is where you find it...

I just modeled a 24' boom 20M 3 element with 6 dBd gain and just under 20 dB F/B. That will give me at least 1 dBd better than the A3S, and it would then let me get the 17M 3 element yagi up another 10' - from 35' to 45'. I'd put the 20M yagi up at 55'.

I'll make sure to check the stacking in EZNec (to make sure there isn't going to be a nasty interaction), but I can always keep these on separate masts if that's the case. If I stack them - both antennas can go 10' higher - and that matters quite a bit where I live.

HFTA shows that both bands paths given my terrain open up significantly - by an order of 15% more openings at lower angles.

YW + EZnec + HFTA = good stuff . . . .
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W6GX
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« Reply #24 on: August 07, 2012, 04:54:01 PM »

Rich,

Rather than build several monobanders I highly recommend you consider the interlaced monobander design.  I have one (Bencher Skyhawk) and the performance is fantastic.  It's like having three mono-banders on one mast, without any destructive interactions that are often associated with christmas tree stacks.  The SWR is flat on all three bands.  An added bonus on the Skyhawk is the ability to work the WARC bands with a tuner (12m, 17m, and 30m).  Whether you could design your own or buy one it's totally up to you.  The Skyhawk is a very reasonably priced alternative and a serious contender among other medium sized antennas.

73,
Jonathan W6GX
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KY6R
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« Reply #25 on: August 07, 2012, 06:07:30 PM »

Rich,

Rather than build several monobanders I highly recommend you consider the interlaced monobander design.  I have one (Bencher Skyhawk) and the performance is fantastic.  It's like having three mono-banders on one mast, without any destructive interactions that are often associated with christmas tree stacks.  The SWR is flat on all three bands.  An added bonus on the Skyhawk is the ability to work the WARC bands with a tuner (12m, 17m, and 30m).  Whether you could design your own or buy one it's totally up to you.  The Skyhawk is a very reasonably priced alternative and a serious contender among other medium sized antennas.

73,
Jonathan W6GX

The antennas I would consider buying if I weren't having so much fun designing and building my own would be the Optibeam, Bencher Skyhawk and Skylark and the Force-12 antennas.

But busting pileups first call with an antenna you have designed is totally addicting . . . I'm having too much fun.
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W2IRT
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« Reply #26 on: August 07, 2012, 06:58:34 PM »

I've gotta say I love my C31XR from Force-12. Best antenna I've ever used. BUT, that said, I have some space on my second tower, above my 6m antenna, that I'd like to fill with a 15m monobander for use in contesting. I don't think I'd trust myself with a homebrew design (I'm just not a great designer, nor do I have an engineering or technical background). I'd like to put up something reasonably lightweight, 3 or 4 elements, that can be turned with a medium-duty Yaesu GX-800 rotator, and up 42' AGL. My thought is to keep this one focused on South America during the late afternoons in contests, while my C31XR stays on Japan, switching between them as necessary and saving wear and tear on my rotor. Any thoughts?
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Night gathers and now my watch begins. It shall not end until I reach Top of the Honor Roll.
KD8MJR
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« Reply #27 on: August 07, 2012, 07:19:41 PM »

I use a SteppIR which is "a monoband on every frequency."

Not exactly, as they are still a compromise on element spacing.  The only way to get optimal spacing is with a monobander.

John AF5CC

Actually Steppir says that because they can use variable length elements that is not true!   Comparing my 3 element to a friends Mono Bander I can detect no difference on 20M.  Even if there is a ever so slight difference, it sure is a whole lot better than a multi band Antenna.  You can argue on the reliability point with a Steppir but forget about telling anybody that they don't work Dam good.
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N5UD
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« Reply #28 on: August 09, 2012, 03:15:58 PM »

I thought we went through this a year ago. Working DX:

Clear paths to the horizon.
Near ocean coastlines help.
Horizontal polarized antennas up at least 1/2 wavelength, more the better
Stacked yagis with switching for best ant/angle
Yagi gain is a function of boom length
More elements are better as long as above is observed, but then pattern gets sharper

Well I am sure I missed something, but that's basics. On 20M I always liked good F/B. Try to null QRM.
20M is the money band, especially during low sunspots. A good start ... 3 el on 24 foot boom ? Obviously a 4 would have to be at least 36.

Then again when conditions are just right....a screwdriver can work 'em.

73 Tony N5UD
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WA2VUY
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« Reply #29 on: August 10, 2012, 09:04:21 AM »

Twenty-five years ago (I think it was) the guy at the top of the Honor Roll in most countries worked was a W1 (anyone remember who that was?). Anyway, I was always struck that he only had one antenna, a 20M monobander. His saying was that "all dx comes to 20M".



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