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Author Topic: 40 meter wire yagi at 30ft....worth it?  (Read 12291 times)
NR5P
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Posts: 131




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« on: June 23, 2009, 04:39:22 PM »

Here is my situation.  I have an uncle that lives in illinois that recently had his license I live in texas and it seems we have the best luck on 40 meters lately for contacts.  I have a 40 meter dipole at about 30ft. and seems to be working great but sometimes the noise is to much to hold a conversation.  I was thinking of adding a director and reflector I have the room but only could do it at 30 ft. because the elements would be in the trees.  Would i notice enough gain on rx and tx to justify this at only 30ft?  Since it is only about a quarterwave high I was thinking it may just continue to act like an omni directional.
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WB2WIK
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« Reply #1 on: June 23, 2009, 05:05:07 PM »

I'd say "try it" and see.  This is really difficult to predict.

If you have enough horizontal space, you might try a "wire beam" made of just a single element, as a colinear array.  There's a good article on exactly this subject in the July 2009 QST magazine.

WB2WIK/6
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N3OX
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« Reply #2 on: June 23, 2009, 07:14:16 PM »

I think it's worth a shot.

Reports of "omnidirectionality" out of low antennas tend to be greatly exaggerated :-)

And when you're talking about beam antennas, you can get reasonable directivity and gain with respect to a dipole mounted at the same height no matter what.  
In no way are they omnidirectional antennas.

You should be able to build a 3el antenna at 30 feet that at will have 6dB gain with respect to a dipole at 30 feet.

A 3 element 40m yagi at 30 feet is still a beam.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
K0OD
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Posts: 2558




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« Reply #3 on: June 23, 2009, 09:40:37 PM »

Consider the "High Gain Single Wire Beam" article by AL7KK on p 38 of the latest QST (July).

That antenna is very simple, produces huge gain, about 11.4 DBi (!) and has high rejection off the sides. It is bidirectional in line with the wire. That would be a perfect antenna for your purposes if you have high trees about 250 feet apart in the right direction. The article recommends a height of about 50 feet for the dimensions given. You might try modeling it to determine the best dimensions when used at 30'.
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NA0AA
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Posts: 1042




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« Reply #4 on: June 23, 2009, 10:28:03 PM »

Yes, you will improve your signal even at 30' - sure it would not be as much as at 60 but so what?  It's not like you are investing a lot in the wire right?

A broadside array like a colinear would also work if you had plenty of room in the proper direction, but it might not be as easy to hang - only you can decide.

If it were ME doing the experiment, I'd hang either the reflector or director first, then see over a few nights how that works, then decide if the 3rd wire is needed.

You are lucky to have the necessary natural supports!

Oh, and if you do this, come back and let us know how it goes will you?  It'd be great to share your experience with real world work.
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K4SAV
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« Reply #5 on: June 24, 2009, 11:42:42 AM »

K0OD wrote: "Consider the "High Gain Single Wire Beam" article by AL7KK on p 38 of the latest QST (July). ...."

That might be a pretty good point-to-point antenna, like maybe fixed on Europe.  With a beamwidth of 30 to 33 degrees on 20 meters it should be adequate for Europe.  

I modeled the 20 meter version, and when I used his dimensions the antenna resonated at 15.2 MHz.  I disagreed with his statement that the feedpoint on 20 meters is reactive and gets canceled with the 5 uH inductor he added across the feedpoint.  My analysis showed that there was no reactance at the low impedance point.  I played with this a long time trying to discover where the error was, and finally decided that he had put the inductor in series with the feedline, not parallel across it as shown in the QST drawing. Expect an "oops" notice in a couple of issues.  When I put the inductor in series, with 3.3 uH I was then able to get an SWR of 1.17 at 14.2 MHz and changing the inductance did move the resonant frequency across the band like he said in the article.  However there is a better way...

His plots show a 2 to 1 SWR bandwidth of 200 KHz.  My model shows 220 kHz, so he is probably not including the stub attenuation in his model.  This is very narrow and doesn't cover all of 20 meters.  This is mostly due to the affects of the series inductor he added.  A better way to implement this antenna is change the wire lengths to 20 and 44 (he used 17.5 and 45).  Then for a matching section use a 13 ft length of RG11 (or equivalent wavelength) attached at the feedpoint (no inductor used).  Use any length of 50 ohm coax to the shack attached at the end of the RG11 section.  This will produce a bandwidth of 400 kHz at 35 ft height and a bandwidth of 500 kHz at 60 ft height.  I did get a gain close to his number when using my dimensions.  I got 10.7 dBi at 35 ft including the attenuation of the stub and matching section.

Using my 20 meter dimensions scaled for 40 meters, I was only able to get 200 kHz bandwidth, however his version for 40 meters showed only 90 kHz bandwidth when using 50 ohm coax and 120 kHz bandwidth when using 75 ohm coax, which had to then be matched at the shack end.

Jerry, K4SAV
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N4JTE
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Posts: 1157




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« Reply #6 on: June 24, 2009, 01:18:44 PM »

Definetly worth the effort, consider possibly phasing 2 elements along the line of the W8JK variety, or carefully cut outer parasitics in the 3 ele configuration. I have had great success at around your height limitation with 3 ele inv vees and reversable quads, never had the room to flat top 2 or 3 ele on 40, you will be astounded. Check www.n4jte.blogspot.com for some tips.
Regards,
Bob
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K0OD
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Posts: 2558




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« Reply #7 on: June 24, 2009, 01:23:12 PM »

Hey, Jerry, thanks for posting all of that.

I'm thinking of throwing up a HGSW for the CQWW this fall. My trees are perfect for hitting Europe. Too bad the opposite direction is almost worthless (ZL and not much else). The other direction would mostly just pick up noise and QRM during EU openings.

I had noticed that the antenna as constructed in the article was quite narrow banded. What sort of side rejection did your model show. The articles plot isn't calibrated.

You could have lots of fun monoband contesting with just two of these: One on Europe (and ZL), the other on Japan and the Caribbean and most of South America. Use a dipole or vertical to fill in the holes.
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NR5P
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Posts: 131




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« Reply #8 on: June 24, 2009, 02:59:03 PM »

Ok I'm going to try it.  It will probably be about 3 weeks before I get to it.  I'll definately let you know how it goes.  Something I'd love to do is use my existing 40m inverted vee as the driven element.  My 80 meter is going east west.  at an angle.  I'm afraid that the elements may interact with the 40 meter antenna.  I'm going to put up the reflector first and see how it benefits. My uncles call is kc9qan btw
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W6TGE
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Posts: 154




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« Reply #9 on: June 24, 2009, 03:06:20 PM »

I was thinking of an HGSW for 10M. It should be a heck of an antenna and not too long. It also does not need to be very high, either. Now I need to check what was said about the Inductor. I like the RG11 idea and eliminate the inductor. How long would the RG11 sectiom be on 10M? Is it just 1/4 wave?
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W6TGE
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Posts: 154




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« Reply #10 on: June 24, 2009, 03:25:35 PM »

So, the RG11 is attached at the feedpoint. Still, do we use the same data as was in QST for the 2 phasing lines?

Would you change the data for wire length on 10M as you suggest on 20M?
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K0OD
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Posts: 2558




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« Reply #11 on: June 24, 2009, 04:06:19 PM »

"What sort of side rejection did your model show. The article's plot isn't calibrated."

Doh!  Helps to put on the eyeglasses (strong ones) to read the tiny print. Front to side rejection at 90 degrees is about 25 dB. The four side lobes are down about 15 dB. Overall, the sides are down about 19 dB.

F/B is, of course, zero!
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K4SAV
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Posts: 1847




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« Reply #12 on: June 24, 2009, 05:50:46 PM »

W6TGE wrote: "So, the RG11 is attached at the feedpoint. Still, do we use the same data as was in QST for the 2 phasing lines?"

I used the same length stubs that he shows.  The resonant frequency is fairly sensitive to the length of those stubs.  A change of 3 inches moves the resonant frequency by 140 kHz on 20 meters.

W6TGE also wrote: "How long would the RG11 sectiom be on 10M? Is it just 1/4 wave?"

I just put in a section that lowered the SWR close to 50 ohms without checking the wavelength I was using.  You should use a 1/4 wave section.  For 20 meters that's 11.5 ft of RG11.  Its not very sensitive to that exact length.  Before adding that section the impedance was 116 ohms (SWR 2.33) at 14.14 MHz.   With a 1/4 wave section it moved the impedance to 49 ohms (SWR 1.04) at 14.14 MHz.

My wire dimensions were different from his because he made the antenna too short, then tuned it lower with the inductor.  I made the antenna resonant on the frequency of operation then added the matching section to lower the SWR.  

A 10 meter version: For the low end of 10 meters (center freq of 28.4 MHz) my wire length are 10.2 and 21.4 ft.  Using his stub lengths (6.47) and a 1/4 wave transmission line matching section.  Gain is 11 dBi at 20 ft high, 10.2 dBi at 30 ft height.  Bandwidth is 1.1 MHz.  Choosing lengths for the higher part of the band centered at 29 MHz, lengths are 10.0 and 21.0 ft, stub length 6.3 ft.  Bandwidth 1.1 MHz, gain 11.2 dBi at 20 ft, and 10.3 dBi at 30 ft.  No I didn't get those gain numbers backwards.  At higher heights there are some high angle lobes that develop and this robs the low angle lobes of a small amount of gain.

Jerry, K4SAV
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K4SAV
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Posts: 1847




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« Reply #13 on: June 24, 2009, 05:53:10 PM »

I should have added, that at the higher heights the take off angle goes lower which makes up for that small gain decrease.
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WX7G
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Posts: 6128




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« Reply #14 on: June 25, 2009, 01:11:24 AM »

An EZNEC models with a 67' reflector spaced 30' gives a gain of 4 dB over the dipole at the radiation take off angles you will be using. The wide spacing gives an input impedance of 45 ohms.

You will have 4 dB of gain on TX and 4 dB of gain on RX. With improved forward gain and better front-to-side and front-to-back ratio there should be a substantial improvement in the S/N ratio when working your uncle.
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