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Marconi
| | Posted on Tuesday, November 20, 2001 - 7:35 am: | 
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Hydro, you said you liked discussions about antennas! This past spring I built a 4 element horizontal yagi using parts from an old original Moonraker kit. The first thing I did was to build the driven element. I attached it to the boom and resonant it for frequency and lowest SWR. I followed the specs and it produced a nice curve centered at 27.205 covering a little less than 2-megs, 28.000-26.500, below 2:1. I made this check at about 12’ above the earth with the beam horizontal. This was my first effort to build and tune near earth rather than at installed height.
I assembled the two director elements according specs. I made another director about 3% smaller than #2 director. At first I assembled the thing on the boom, planning to make it without any reflector using only a driven element and three directors. The thing worked OK and showed good rejection. I installed it on the roof deck mount and the matching changed on me. I tried tuning but it did not seem responsive to tune, no matter what I did to spacing or element length, it resonated best in 10 meters at 28.700. I messed with it for a couple of days and then decided to add a conventional reflector thinking the lack of the reflector was messing with the tune somehow.
I made a reflector out of some old tubing material and hardware that I had from another antenna. Note: this reflector element mounted to the top of boom instead of using the MR’r hubs. I added a 6’ foot section to the boom behind the driven element and placed this reflector at the end. First check found the antenna finally giving a reasonable SWR in 11 meters so I thought.
I decided to make an SWR curve to see how it responded below 3:1 since my original bandwidth curve of 2:1 for the driven element did not cover the apparent resonant frequency I was getting after raising. I did not rechecked the driven element after raising, but it may have also indicated the same resonance at 28.700, as noted above. I was surprise to find, based on SWR’s alone, the antenna still seemed to respond best in 10 meters. I felt something strange was going on, but until I produced this new SWR curve did I really realize that I might somehow have a dual resonant antenna.
There are more details to the story but I will not go into that right now. I recall having seen similar unanticipated results with some large beams in the past but I do not recall seeing this before or if I did, I did not realize it.
I have a very old Wilson 4 element horizontal yagi that is very responsive and out performs this new one by a long shot. It is somewhat narrow banded covering 26.850-27.850 showing true resonance at 27.405. The SWR do not ever dip again as you go away from resonance on either side. What do you think might be going on here?
Marconi |
Hamcber
| | Posted on Tuesday, November 20, 2001 - 1:29 pm: |
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Adding elements in the near-field of a driven element changes the impedance drastically. Since your elements are not exactly 'tuned' with your driven element (you indicated that your lengths were guesses), this makes the problem a little worse.
Assuming the driven element is exactly 1/2 wave on 11 meters and you are feeding it with a gamma match, you have to add some series capacitance to compensate for the inductive reactance at the feedpoint. The gamma simply finds a 50 ohm point somewhere along the radiator and does not tune out the reactance.
Now, let's say you add some elements. This usually lowers the impedance and does not affect the reactance much. You now have to move the 'dog bone' of the gamma further from the boom to raise the impedance back to 50 ohms. This will happen a little more with each element you add to the array.
Of course, each time to move the dog bone, you are going to affect the reactance slightly as well. Etc, etc.
Most commercial gamma matches have a piece of dielectric material between the outer tube and the inner tube to form the series capacitor. This gives you a nice broad-band gamma.
The rest can be found in the ARRL Antenna book... |
Hydro
| | Posted on Wednesday, November 21, 2001 - 3:05 am: |
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Ok Hamcber, you beat me to the punch but I agree with you on the gamma match, that was my first thought before I was gonna dig into the Ant. book. But darn it all I'll be quicker next time. LOL
Marconi, Hamcber has your answer there probably in better words than I coulda put it too, darn him, LOL.
Joe |
Marconi
| | Posted on Wednesday, November 21, 2001 - 11:08 am: |
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HamCBer, I guess in a way you are correct, the element measurements were only guesses. I set the driven element, and directors 1 & 2 according to the specs and of course I made the other modifications mentioned earlier. Before I assembled the other elements I checked the driven element just to see how close the specs got me to resonance. The resonance and SWR appeared to be OK at 27.205, so I felt the measurements for the other elements should also be very close as well. As you note, in order to get at or near a 50-ohm load, I figured I might have to adjust the gamma match a bit.
After installing all the other elements I determined the antenna seemed to be resonant in 10 meters. I decided to try to tune it down into 11 meters by making the driven element length a bit longer. That is when I noticed the non-responsiveness I mentioned earlier.
I am going to go back and re-check the driven element and see if it will respond to changes in frequency by simply adjusting the length in either direction. Originally I did not do this, thinking that it was OK when showing very good resonance values in the middle of 11 meters. I was surprised to see this element give such a predictable result by itself, but I figured Avanti probably knew what they were doing.
I think I get your point that true resonance will only occur when you are able to balance out the reactance. I assume you are suggesting I adjust the element length to where the capacitance and inductances cancel each other out and then tune the gamma match to a point at or near 50 ohms. I realize the other elements presence will have a rather large effect on the matching and to a lesser degree on resonance. I also realize that all this may require a little tweak here and a little tweak there due to compound effects.
I am going to start over and make the antenna to the 4 element specs. I will maintain only the horizontal sided design and retain the conventional reflector. That should work. So, I guess my question to you is. Do you think that the absence of the vertical side material is possibly producing the dual resonance results I seem to be getting when using the factory specs as starting measurements?
Marconi
Webmaster, this should be in the regular antenna section I think, this may not be considered a review subject. Change it if you wish and can. |
Hamcber
| | Posted on Wednesday, November 21, 2001 - 10:56 pm: |
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Hey Marconi.
One polarization's elements have almost no affect on the other. On a cross-polarized yagi, you can retune the horizontal driven element without affecting the tuning of the vertical and vice-versa.
Good luck, and have fun! |
Marconi
| | Posted on Thursday, November 22, 2001 - 8:15 am: |
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Hamcber, in general I cannot disagree with your statement. I guess my question goes more to fact that the measurements I used were factory specs, not necessarily tuned, which originally included the elements for both flat and vertical.
In prior antenna assemblies I have been way off before. However, I don't recall these problems.
1) Non-responsiveness - does not react to tune
2) Producing a dual SWR resonance curve – two close resonant frequencies with low SWR's
BTW, at some point you mentioned tuning the individual director elements. I do not have a procedure for testing this part of an antenna's tune. I have simply used some variation of the math (percentages of the driven element) for these measurements. If you have another procedure that is more precise, could you give me a little description of a device or procedure that I might use specifically? I use a simple field-strength meter to tweak the antenna but I only do this after I think I have control and am real close to resonance and match. I have made length adjustments to the passive elements before, but I rely on these FS results to determine if I made a good move or not, but this is very labor intensive.
I am not talking about spacing adjustments here, just element length. After I get one tuned to a point, I work it for a while then I may mess with spacing a bit. But, I don’t have any way of effectively checking the results in making spacing adjustments. Spacing is a whole different kettle of worms. I have worked with reflector to driven element spacing a little and of course driven element to director #1 spacing but I am never real sure about the results. I think it is kind of like starting all over again when you start adjusting the spacing.
I have two questions.
In this case and as specified by the manufacture, tuning aside, do you think my removing the vertical elements from the “mix” may be throwing me off enough to cause these effects, without all the elements included?
You stated earlier, “Assuming the driven element is exactly 1/2 wave on 11 meters and you are feeding it with a gamma match, you have to add some series capacitance to compensate for the inductive reactance at the feedpoint.” How do you compensate for this condition?
Thanks! Your information is very helpful.
Marconi |
Hamcber
| | Posted on Thursday, November 22, 2001 - 1:11 pm: |
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Ummm... It's time you went to the library and checked out the ARRL antenna book, my friend.
I'm not trying to be evasive at all. This is the point in which it is not practical for me to type what would amount to the entire text of a chapter of that book. My time online is limited and you need some help. |
Marconi
| | Posted on Saturday, November 24, 2001 - 8:09 am: |
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Ah! come on Hamer, surely you could at least give an opinion. I asked twice, in two different ways, the question regarding dual resonance effects and that was no doubt asking for an opinion. I have read something somewhere, regarding tuning or matching, someway or another, for improved broadband results that produces similar results. If this happened in my case it was purely coincidental, that was not my plan. I just thought maybe you have had some experience with this effect.
Your response to my initial post was very good. But your follow up to my question, though correctly stated, was not exactly on point. "...One polarization's elements have almost no affect on the other. On a cross-polarized yagi, you can retune the horizontal driven element without affecting the tuning of the vertical and vice-versa." You further stated, “Adding elements in the near-field of a driven element changes the impedance drastically.” Again, I agree. I know you meant this in a cross polarization sense, but that is not what I was talking about. My dilemma has to do with the presence or non-presence of reflective material in the radiated field regarding tuning results. I am not asking about making simple tuning adjustments to a reflective element that happens to be present in the antenna's radiated field.
I may not agree, but I can understand your reluctance to be more specific in response to my second question about the tuning procedure, for passive elements, that you alluded to. It has to be that you just don’t want to give away any secrets. My thoughts on that subject are, “that is why I am on these forums.” I will share what I think I know when interested. My statements may be fact or maybe not, but I am always truthful and honest. Of course I have changed my opinion on many topics over the years and therein the degree of truthfulness of a statement that I may make. I am not reluctant to share however. There is always the very good likelihood that few will understand completely what I have said if the subject seems to be technical in nature. But, maybe they will glean something and go study, as you suggested. I figure what I give up is of very little loss to me personally and you know it doesn’t seem to hurt a bit.
You are correct. I need to get back to the drawing board. I have to may variables involved for anyone to tell what is going on with this thing.
I am still curious about your idea of tuning passive elements though.
Marconi |
Hamcber
| | Posted on Saturday, November 24, 2001 - 12:41 pm: |
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Sorry if I offended you, that was not my intention.
I was referring to tuning elements in the same polarization when referring to the impedance change on the driven element. Adding parasitic elements in the same plane effects the driven element impedance by lowering it by a lot. Parasitic elements in the opposite polarity have little, if any effect.
On the subject of 'dual resonance', it is not really dual resonance. An antenna element will resonate on one frequency and multiples of that frequency (harmonics). This is not the same as gain, tuning, or pattern characteristics, I'm just speaking of resonance.
What you are likely seeing is two points that make a simple SWR meter happy. At one point is where the reactance is at or near zero, at the other 'resonant' point, you are probably seeing the 50 ohm point. On a Smith chart, you can plot the reactance and resistance curves. The object is to get them to intersect (50 ohms, j0) right on the frequency you intend to use the antenna on. This will give you a wide bandwidth as well. If they do not intersect or if the curve is sharp at design fundamental, the bandwidth will be narrow.
Get the book. My fingers are tired! Like I said, my typing info on this forum is not going to explain it as well as the illustrations in the book can. Also, I get PAID to do this normally. If my clients knew I was willing to give it away for free, I'd be in trouble.
Happy thanksgiving too! |
Marconi
| | Posted on Saturday, November 24, 2001 - 4:07 pm: |
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No offense taken.
Best of the Holidays for you and your's!
Marconi |
Taz
| | Posted on Monday, November 26, 2001 - 7:49 pm: |
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I have a question here. Who is to say that the arrl is correct? thats all theroy and math on paper, isnt it? I always thought that the peformance in the feild and how it really works is what makes the difference? and not what the math says.
Taz |
Hamcber
| | Posted on Monday, November 26, 2001 - 10:17 pm: |
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However, without the 'math', you have no antenna. You can't get very good efficiency out of a random wire antenna, and you don't get good swr from a 'guess'. To go one step further, you don't get a predictable pattern out of a 'that looks good to me' antenna.
Don't be fooled, the scientists know a lot more than Joe's CB shop down the corner, good buddy. |
bruce
| | Posted on Monday, November 26, 2001 - 11:19 pm: |
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Taz ill bet you beleve that a beam has audio gain too trust us hams the ARRL knows what its doing the books they put out are some of the best low priced books on radio ever writen |
Marconi
| | Posted on Tuesday, November 27, 2001 - 8:21 am: |
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Taz writes: "I have a question here. Who is to say that the arrl is correct? thats all theroy and math on paper, isnt it? I always thought that the peformance in the feild and how it really works is what makes the difference? and not what the math says."
Taz, Hamcber is right, without the science and the math you are at the mercy of chance. Folks often make light of the Isotropic Principal but in order to evaluate any scientific response you must first try to establish a base line. The math only fails us when we do not understand.
The ARRL Antenna Book is full of a mix of science, math, and anecdotal experience. It is only meant to present a series to understanding. Trial and error is always a vital part of learning, but it too is not all there is to the process.
BTW, hams do not have a corner on the subject of understanding and folks that make those claims show no humility. I never let my understanding be clouded by either experience or science. Always try to stay humble and open minded or this cloud will consume you.
Good luck. |
Marconi
| | Posted on Tuesday, November 27, 2001 - 8:27 am: |
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Hey Hydro, you sure are quite out there! Come on guy, let us continue the discussion. Therein lies understanding. HamCber will get back in if we get off track.
Marconi |
Taz
| | Posted on Tuesday, November 27, 2001 - 8:52 pm: |
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ok, I just wanted your opinion. But hamcber I never said anything about an antenna but i realize that I am under the antenna section here. But what i meant is all radio gear in general. and bruce i think jo gunns claims are alot of bull. and bruce be carful what you bet on.
Taz |
Hydro
| | Posted on Tuesday, November 27, 2001 - 10:03 pm: |
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Sorry guy's, I was taken and stuffed in a suitcase by 2 kids that will be beaten correctly when I can walk again. ( who ever took a suitcase hiking, DAMN them) LOL.
Back to the subject though, lookin in the book is the best way to go at this point, like Hamcber said, it's a whole chapter. There's plenty of experience after the math.
Antenna's are gonna react differently, surrondings, ex. trees, shrubs, bldg.'s, and the type of soil, dirt or clay that grounds it.
If it's easily accessible tune it at ground level through the gamma match( Antenna fully grounded) then put it back in the air and tune it again, measurements are pretty crucial but they change due to everything around the antenna.
I'm gonna stop before I get babbling, but most gain is compared to a dipole (mathamaticly correct susspended 20' in the air with no surrondings).
By the way, Hamcber, where the Joe's cb shop, I want to find that guy and @#$%^&^&&^%$#.....
LOL
Joe
P.S. Just incase anyone thinks of it, I don't have a cb shop. |
bruce
| | Posted on Wednesday, November 28, 2001 - 7:21 am: |
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jogunns clams are bull show me how you get audio gain and ill retract this the NBS standards for antenna gain are well known no jogunn product has ever been within these limits. Show me how you calculate audio gain because a number of letters asking them for the formular and the e and h plane plots i sent them only resulted in a catalog being mailed to me. The other antenna makers MFJ MOSLEY ect will send you the data in fact they even publish it you must understand these people ( jogunn ) are dealing with people who for a large part are not well schooled in elecronics. Jogunn ADDS the gain vert/horz (ie) a 8 db gain beam vert + 8 db gain beam horz now is a 16 db gain antenna sorry it dont work that way.
You want a good antenna go get a cushcraft or some ham antenna go look in QST or any mag that is geared to the ham user not playing games with numbers to justify outragus prices. Show me the measured gain equal to what they clame and the measured AUDIO gain on NBS calabrated equiptment under range standards.
bruce
PS nbs standards for a 3 element beam are about 6 db
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Marconi
| | Posted on Wednesday, November 28, 2001 - 1:42 pm: |
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Bruce, JoGunn probably coined the term to support their big loud radio idea with their antennas. Of course they are like most, out to lunch.
Since their antennas are mostly fixed as to resonance and spacing, you can't even play around with them unless you do heavy modifications.
They are sturdy and well built, and they are better than most in that regard.
I have several good operators around me that run mostly the stargunn type, I think it is called. I have no idea of how well tuned they are, but they seem to work OK except they do not appear to have much rejection when pointed away from me. That may be as a result of a good gain characteristic out the front however. From my personal observations working with each, beam to beam, I am not impressed with their response.
Marconi |
Taz
| | Posted on Wednesday, November 28, 2001 - 8:16 pm: |
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uhhh bruce i dont understand unless u had a typo.
you said:jogunns clams are bull show me how you get audio gain and ill retract this the NBS standards for antenna gain..
I just said that I "think" joe gunn claims are bull. who's to say i am wrong? "different folks, different strokes"... Its just an opinion. I think alot of people think that there claims are a little outrageios and a little off. but who cares?
Taz |
bruce
| | Posted on Wednesday, November 28, 2001 - 8:28 pm: |
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see my problem with jogunn is they are ripping off people with " most AUDIO " gain but when challenged by people like my self they go to the SGT sholtz defence ( I KNOW NOTHING ! ) Yes they are well built but laws apply to gain as do they to most every thing. The NBS did a well documented antenna gain experiment about 20 years ago and nothing ive seen has proven it wrong in short guys if i tell you my 3 element beam has 16 db of gain and 8 db of audio gain but all other 3 element beams have 7 db of gain ( nothing about audio gain ) something is wrong so when in doubt go look up a ham beam for 2 meters the gain will be the same as a CB beam element for element if it is 6 db on 2 meters bet your d-104 it wll be the same on 11 meters.
bruce |
Hydro
| | Posted on Wednesday, November 28, 2001 - 9:31 pm: |
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My definition of AUDIO gain, Turning the volume knob clockwise, other than that I have to agree with Bruce.
If they can get 16db gain from 3 ele. I would find a way to put one on my balcony in my condo.
I use lots of wire for my antenna's, even have the rail loaded as a counterpoise.
Joe |
Hamcber
| | Posted on Thursday, November 29, 2001 - 12:47 pm: |
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Bruce:
Although you are obviously correct that an antenna cannot possible make audio gain since it is an RF component only I have to tell you why I am always gasping for air after reading one of your posts you see that if there are no periods to break up the sentence I don't know when it is time to breath while reading it therefore I get all out of breath every time I read one of your posts sorry for the tone of this but I am all light headed from reading your posts above but anyway the antenna thing is blown out of proportion by more antenna makers than just Joe Gunn I mean heck how do you get 9.9 Db gain out of an Antron 99 antenna or 5 dB gain out of a center loaded mobile antenna they are all a bunch of crooks if you want to get right down to it so don't let it stress you out anymore go ahead and but one of the antennas you like and let everyone else buy what they like everyone knows the dB claims are a bunch of 'bull' anyway
Oh, here you go: . |
bruce
| | Posted on Thursday, November 29, 2001 - 10:31 pm: |
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english is not my major. sadly my english teacher went to her grave with me on her mind and tied in a white jacket. Dont even get me started on the antron 99 i have stated that a 9 db gain 360 deg vert had to have a vertical beam width of about 9 degrees. Common antenna people cant we have a common standard like DBD and use it corectly. CBers as a rule have only what they read in catalogs or hear on the radio in which to form their ideas of what things like antennas do. There is no QST,CQ or 73 for the CB user sadly S9 went belly up 30 years ago a very well done mag for the cb user. well i calm down and get back to colecting christmas seals
bruce |
Marconi
| | Posted on Friday, November 30, 2001 - 7:16 am: |
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Bruce, I don't think anyone here will disagree with your assessment of the ad schemes, or should I say scams, used by many advertisers today. The world is constantly bombarded with falsehoods, and the majority seems complacent in that truth. We have just gone thru eight years of solidifying that fact.
From what little I read concerning Ham activities, I am not sure about the veracity of that group either.
Hamcber is right on the mark when he says:
“thatyourwrittenwordisabitunuasual.iamsureyourteacheristurningoverinhergrave.hopetheytiedherdownrealgood.iguessshegaveupwhenitcametopunctuationforsure.
If you use a mirror when reading the coments above, it will all seem very clear.
We get your point!
Marconi |
Hamcber
| | Posted on Friday, November 30, 2001 - 12:24 pm: |
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Yea, but now I need to scroll my screen left and right to ready this thread. THANKS, MARCONI.....
Actually...... this gives me an idea........ |
Marconi
| | Posted on Friday, November 30, 2001 - 1:53 pm: |
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OH BOY!
Marconi |
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