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Author Topic: Delta Loop question  (Read 958 times)
KG4OXP
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« on: July 22, 2002, 04:22:40 PM »

I am going to build a delta loop thanks to some advice here and have a couple of questions regarding length.

I will be using it temporarily, during a vacation. The shape will be apex at the top. The bottom of the triangle will be at about 15 feet above ground level and the apex at about 40 feet. I'll probably feed it at one corner or maybe 1/4 wave down from the apex, as many have suggested.

I want to use it for 20m, 17m, 15m - I'm feeding with ladderline directly to a tuner.

1. Should I cut this for 20m (phone portion)..(1005/14.250) = 70' 6"

1a.) In much of what I have read about Delta Loops so often for 20m they are cut to 66 feet versus 70+ feet?? Is there a reason as 4 feet seems to be quite a difference...

2. Is there another non-resonant length that I should use if I want to use it on these 3 particularbands... I'm asking as I read here once that for multiband ops you should not cut the antenna to be resonant on one of the bands to be used..(other than a 40m/15m - harmonic -  example)

Thank you for your advice...

pete
KG4OXP/AG


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AC5E
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« Reply #1 on: July 22, 2002, 05:09:30 PM »

Hi Pete: A very low antenna, or one in close proximity to a large conductive surface such as the wall of a metal building, will resonate somewhat below it's calculated frequency. But four feet is quite a bit of resonance shift.

If you want to work 20 phone, cut it at around 71 feet, and trim it to resonance where you want it after you get it up. It's a lot easier to shorten a loop than lengthen it.

And from my experience I still don't recommend twin lead, ladder line, open wire line, or any such - unless you want to consider the project as an educational exercise from the school of hard knocks.

 For reasonably short transmission line runs, a 10'10" chunk of RG59 makes an excellent quarter wave transformer from the loop to the feedline. Then run however much RG8X or RG58 you need to the rig. After you have your loop tuned you may not need a tuner. In any case you will have a lot fewer hassles and complications than with twin lead.

73  Pete Allen  AC5E

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KG4OXP
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« Reply #2 on: July 22, 2002, 05:24:54 PM »

Thanks Pete,

What is the best way to go from the coax to the loop. I've done this for more permanent installs with baluns or also just soldering the coax center pin to one wire and the braid to the other wire (on a dipole)..but Pls. remember this is for portable use, so soldering on site etc. is out and I'd prefer to be able to separate the coax from the antenna while traveling..

Thanks again..
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WB2WIK
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« Reply #3 on: July 22, 2002, 05:48:26 PM »

I'd use an SO-239 connector soldered to the feedpoint (two wires of the loop coming together) for a quick & easy disconnect that could be used either direct to coax, or direct to a coaxial matching section as Pete describes, or have an optional balun attached; but in any case, an SO-239 makes an ideal termination point for the loop wires and allows you to disconnect whatever feedline you use.

WB2WIK/6
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AC5E
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« Reply #4 on: July 22, 2002, 06:04:56 PM »

Pete, I would do the same thing I'm doing for the loops I have up at the farm. I use "Budwig" style dipole center insulators/UHF female connectors. They are quite compact and quite durable.

Bend the ends of the antenna connectors back to make a hook on each side, and it makes it easy to adjust lengths at the feedpoint. If you have a bit of tension to maintain contact there's no need to solder the loop, but if you want to make it permanant you will probably want to solder it.

I would build the matching section with a PL259 on each end and a double female, "barrel," connector to link the matching section to the feedline proper.

Take a look at www.radioworks.com for the Budwig and barrel connectors.

73  Pete Allen  AC5E
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KA4WJA
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« Reply #5 on: July 22, 2002, 06:17:31 PM »

Pete,
Well first off, we never heard back from you, regarding the "swr troubles" you were having with your "delta loops and 1/2 waves".....but in any case, I'll give you a quick answer...

1) Yes, cut it for 20m....  70' 6" will be fine!!

1a) I've been building/using full wave loops for 20 years and been reading/learning about antennas for years before that....and I've never seen reference to a full wave loop being cut short "on purpose"....
So, the answer is:  use the 70' 6" (or longer) length!
I actually find the 1015/F  is a better formula to use....sepecially if your "measurment accuracy" isn't all that great.....
BUT, in any case do NOT cut it short!!! Especially THAT short!!!

2) Full-wave / Multi-wave loops do NOT suffer from the "even harmonic troubles" that 1/2wave (or multiples of 1/2waves) dipoles do!!!!!!
They are "closed loops" NOT open-ended antennas...and
they are "balanced" antennas......

Therefore there is NO need to worry about building an "off-reasonance" loop.....unless you have limited space to install it in.....

Now that I've answered your qusetions.....with facts...
I'd like to offer some "opinions"....
First, a 20m full-wave loop will have a feed impedance of "close to" 120ohms on 20m....and "close to" 250ohms on 17m and 15m......
DEPENDING on its shape, height above ground, and location of surrounding buildings, etc....
There is NO reason to use ladder-line.....a simple coax feed will work just as well.....AND using a 1/4wave (on 20m) 75ohm section of coax at the feedpoint should give you a VERY good match to your rig on 20m....AND will also be "fairly good" on 17m and 15m....
This is better than using just a 50ohm coax feed....AND significantly better than 450ohm ladder line and a tuner.....
LARGE LOOP antennas "work" differently than "1/2 waves"!!!! And most of the articles you'll find on antennas, feedlines, tuners, baluns, etc... are all based on 1/2 wave antennas, whether dipoles, yagis, verticals, etc.......
As I wrote above I've had over 20 years eperience with large loops, and by far the best resources on these antennas are a) ON4UN's Low-Band DX'ing, and b) www.cebik.com
BUT before you can truly understand what they've got to say, you've got to have a basic understanding of antennas, propagation, etc.....AND that is why I recommend The ARRL Handbook and The ARRL Antenna BOOK!!
{I was willing to give you some slack, since you've asked about these antennas and your troubles under  the context of a "vacation set-up".....but that was a while back, and I assummed that you were pressed for time....however now I see that you've got the time....
so, please buy these books, read them, and you'll get a much more complete understanding of all of this than I (or probably anyone else) could ever give you here on Elmers....

Pete, please know that I do wish to help you, and I will continue to do so....but you've got to get a better understanding of these basic, than I can give you here....

Good luck.
73,
John,   KA4WJA





 
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KG4OXP
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« Reply #6 on: July 22, 2002, 07:20:11 PM »

Thanks John...

I have about 3 days before the trip so I have a bit of time..

I have been reading so much about antenna design and it seems just when I seem to understand something, I read conflicting reports from another "expert."

I did buy the book vertical antenna classics...I've studied the charts, formulas etc..I'll be reading a great deal more.

I'm very analytical and it's frustrating when you get conflicting advice on a scientific subject, like this is supposed to be...I think often there is no point of reference...like is 3db a huge difference or not..I suppose it depends on the individual's application.

I'm going to go with your, and Pete's, suggestion as to how to cut and feed the loop...
Thank you.

Pete
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KA4WJA
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« Reply #7 on: July 22, 2002, 07:47:44 PM »

Pete,
You're welcome!!
Taking Steve's and Pete's advice is a good idea!!

As far as books go.....
In my OPINION, there are way too many hams buying the
"compendiums", "anthologies", and "antenna classics" books, in order to "learn" about antennas.....
BUT those books are usually compliations of QST articles, etc.. and as such are primarily written for "active hams", hams who are looking for new antenna ideas, or those interested in reading "real-world" applications of what they already have at least a basic knowledge of!!!!!!
Those books, having dozens of short (2-3 page) "articles" appeal to the "quick solution" route of many in our society today......
BUT, they DO NOT have any of the basic information NEDDED to "understand" the "whys"!!!!!!!!!!!!!!!!!!!!!

AND, that is where  _The ARRL Handbook_  and  _The ARRL Antenna Book_  both excell!!!!  (basic and advanced KNOWLEDGE!!!!!)

I think you've already figured out a VERY important lesson:   Do NOT be mislead into thinking that reading an article written by some "expert", will give you the knowledge you need/desire.....

The ARRL Handbook  and The ARRL Antenna Book both have the advantage of being full of info/knowledge compiled over 75 years, by many hundreds of hams, engineers, scientists, etc....... AND has been "proof-read", "fact-checked", and most has been backed-up with "foot-noted" and fully-researched info, that allows the editors and YOU the READER to check-out the facts, instead of just taking someone's word for it!!!!

Just for clarification, I am NOT an expert on antennas.....I'm good, but not that good, Hi, hi...

My list of experts includes DR. L.B. Cebik, W4RNL  and
Dr. Walt Maxwell, W2DU........there are others, but if you want to take the word of an expert, these guys are a couple of the few that I would not question....

I hope my advice and opinions help....

Good luck.
73,
John,   KA4WJA
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KG4OXP
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« Reply #8 on: July 22, 2002, 08:42:01 PM »

Hi John, Steve & Pete..

Pls. help me understand this section:

http://www.cebik.com/scvhv.html

Most advice I've rec'd thus far suggests side feeding as being much better for low angle vertical work...yet this article seems to contradict this.

He also talks about using balanced line and a tuner for multiband use??

Please understand I'm not trying to be argumentative in asking...I'm just trying to reconcile the differences..
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AC5E
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« Reply #9 on: July 22, 2002, 09:59:23 PM »

Pete, it's Amateur Radio which means you get to try what you want to try. But I have had the best luck with bottom center feed. I have heard a lot of that "top feed, side feed, corner feed" stuff but it doesn't work as well as bottom feed for me.

ANY antenna must obey the same physical laws, and one of the most important of those is that most of the radiation comes from the portion of the antenna nearest the feedpoint.

Most HF antennas are horizontally polarized - and while Faraday rotation has a strong effect on the lower bands it has less effect above 10 mHz - which means a horizontally polarized signal will probably induce a somewhat stronger voltage into recieving antenna. Feeding an antenna at the center of a horizontal section will provide a signal that is primarily horizontally polarized - which seems to work best for me.

73 Pete Allen  AC5E
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RobertKoernerExAE7G
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« Reply #10 on: July 22, 2002, 10:12:12 PM »

7.15 MHz     1.97           16 degrees       49j0
7.15 MHz     5.83           42 degrees      255j75

Same antenna feed at different points.

The first one, side feed, has about 2db gain at a very low angle, and a sharp null towards over head--great for low noise DXing.

The second, great gain but aimed way too high to be of use for DXing, and probably with no null over head.  A great antenna for stateside ONLY, and picking up a lot of stateside QRN.  

To compare the two, you need to know the gain of both at 16 and 20 degrees, for DXing, along with an estimate of how “quiet” each will be for DX signals.

One is easy to feed, about 50 ohms, the second, not to good for coax to a modern rig.

If you download a demo copy of EZENEC.com, you will get graphic representations of where the signal lobes and nulls are; you will be able to see why one antenna is better for "Long Haul" DX, and the other just for stateside.

Plus, you can have one antenna, a delta loop, feed it at one point, and change the frequency it is used on; you can see what happens to the lobes and nulls as you go from one band to another.  You can move the feed point gradually from side to bottom and see what happens to the lobes and nulls.

Enjoy your vacation with an easy to build antenna that works reliably on many bands.

Later on, you need a Hand Book, modeling software, and ON4UN’s 3rd edition, a great antenna primer on a variety of HF antennas.

Bob


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KG4OXP
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« Reply #11 on: July 22, 2002, 10:12:59 PM »

I want to thank you again Pete, and others, for your patience and lessons...now to do some experimenting...

Pete
KG4OXP/AG
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KA4WJA
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« Reply #12 on: July 22, 2002, 10:27:06 PM »

Pete,
Without being rude.....I think you are really just confusing yourself, by trying to read fairly advanced papers on specific antenna designs, and trying to figure them out......WITHOUT the basic knowledge to draw from....

It is beyond the scope of this forum to give you all of that knowledge....

BUT, if you must know....
L.B.  is writing about the use of a:
  Self-Contained-Vertical (SCV) "antenna" that is shaped as a "delta"...BUT is "open" at the bottom, NOT a closed loop.....
  AND also, having a switching circuit that would allow this antenna to be remotely switched into a center-bottom-fed delta loop....
   He does reference delta loops, rect. loops, and half-squares, as a way of introducing this VERY unique antenna design....AND he also compares the relative gains, take-off angles, etc.

IF you are trying to "read more into it" than that, you cannot!!!!
You must understand the basic concepts (that he assumes you do) before you can grasp the importance of what he is writting about.....

{As a side note:  I have two separate full size 80m loops....one is a fairly square, horz. polarized loop and the other is a rectangular vert. polarzied loop...
They both work well....and they are both VERY different animals!!!!! AND they both perform very differently!!!! Even though they are both almost exactly the same length and at almost the same height........It is beyond the scope of this forum, to discuss the specifics here with you.....BUT I do have and use them both......Again, not trying to be rude, but I do not know how to explain these specifics until you have the basic knowledge.....}

Reading the articles that L.B. writes is GREAT!!!!
And his website is an excellent resource....I print things out from there and hand them out at some of my antenna seminars.....
BUT, quite with quite a bit of it, you must have a good working knowledge of antennas, etc. in order to grasp it all.....

That is why I recommend the books (ARRL Handbook and ARRL Antenna Book) I do!!!!!

73,
John,   KA4WJA

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KA4WJA
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« Reply #13 on: July 22, 2002, 10:37:46 PM »

Pete (KG4OXP),
Please forgive my rudeness...
But, after reading the other recent answers (that were posted while I was still typing) and your response to them..... I feel compelled to give some unsolicted advice....

You're leaving for vacation in 2 days....
Do NOT go out now and try to experiment based on my (or others) advice here!!!!!!
PLEASE JUST CUT A WIRE  70' 6" LONG.....ATTACH SOME COAX TO IT....AND HAVE FUN!!!!!!!!!!!!!!!!!!!!!!!!!!!

When you get back home, then BUY THE BOOKS!!  READ THEM!!! and ASK some more questions as necessary.....
AND then try out some experimenting!!!!!!

DO NOT WASTE YOUR VACATION!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Good luck.
73,
John,  KA4WJA

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W5RH
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« Reply #14 on: July 23, 2002, 10:47:02 AM »

Hello, Pete,

I've seen you've had a few good responses to your posted question so I'll throw in my 3 cents too.

For your vacation delta I would concur with you and the others on the following:
Apex up, corner fed, cut for 20 meters (1005/14.x), hung at 40 feet apex and 15 feet base.  Allow the corner insulators to slide so that you can 'fit' it to the area at your vacation site.  In this fashion the 20 meter performance will be vertically polarized and will offer you a low angle of radiation (  I assume here that you are after DX ).  Feed Z will be 'nominal' 110 to 120 ohms.  Feed as you stated or as the others stated with coax and series section xfmr.  If the coax is kept to minimal length, using a tuner will be OK there too ( I do it all the time).

This loop is optimized for 20 meter low angle performance.  So the other 2 bands (17, 15) you will have to just accept the performance as they will encounter an antenna that is greater than 1 wavelength and the standing wave current distirbution will not be symetrical and in phase as when on 20....but it will radiate...and sometimes you get lucky and it performs just brilliantly on the other bands.

A couple of tricks I use are as follows.  On a one wavelength loop, if you feed it at a corner for logistical convenience, you can force the standing waves to be completely symetrical around the apex ( this would be equivalent of feeding it 1/4 wl down from the apex -- the best situation ).  Simply open the loop with an insulator at the mid-point of the baseline.  This will give a current distribution of 1/2 wl from the apex to the baseline midpoint on each side of the triangle.  The corner feed is then just like an off center fed dipole situation.  Feed Z might be a bit higher but it is still resonant.  

This opening of the baseline provides another benefit.  It makes the 20 meter full wave loop a 1/2 waveloop on 40 meters (a 40 meter dipole folded into a triangle).  And the baseline opening forces the 40 meter current distribution to have it's current maximimum at the apex (40 feet) so that you will get a semi-low angle/high angle performance on 40.  

Just a comment on the books.....Low Band DX-ing contains the best loop information, bar none....except for L.B. Cebik who does a fine job on his web site with his loop series.

FYI, I have been using delta loops for many years for DX work.  Apex up, vertically polarized, fed at the corner.  I even took a full size 40 meter lop and shortened it with inductors in the non-radiating/horizontally polarized portion of the loop and kept the performance very close to that of a full sized loop....I call it the "W5RH"  :-)).   This allowed me to fit a 40 meter loop between a yagi at 50 feet and my house roof at 15 feet.  Also by opening the base line I was able to use it on 80 meters for some local net work.

Have fun on your vacation and in your delta loop playing.  I have always stated that "Delta loops are the most flexible antennas on earth"  They will do what ever you need in polarization or angle of radiation or physical space limitations....study them for awhile and you'll see what I mean.  They are easily phased and even operated parasitaically.

GL and 73.....Rick...W5RH...Houston      
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