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Cophasing any 50 ohm antennas(beating a dead horse again)!!!!

Discussion in 'CB Antennas' started by Grim Reaper, Apr 25, 2013.

  1. Grim Reaper

    Grim Reaper Member

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    Ok i have added a couple of attachements,! a document published 1983,and the other ought to be great for a laugh because of my lame ability with paint,but it shows patterns of single and dual antennas on a semi and then a 4 wheeler,truck or car,and the effect of off-setting dual antennas on the 3 wheeler,I brought this up couple years back and its still over looked or not known by 75 to 80% of folks talking co phasing antennas you get a db gain from dual antennas,But Ill get to that in a few minutes.First some basics,11meters or 27 mhz is actually 36' for a full wave length in the middle of the frequency.,alot of people think its 32' so they figure 1/4 is 8' but if you ever measure out a 102'whip its a little short of a 1/4 wave and that is why a 6" spring was added to help bring the S.W.R. down on them.At 108" your 0' tall and guess what your antenna is tuned,1/2 is 18' ok enough drawing the numbers out.there is this belief that you cant run 2 antennas below a 1/4 in distance apart,Well that isnt true and even at as close as 1/8 wave you get a 3db gain,and for the 4wheel auto world that space has and added bonus the radiation pattern is close to a 360*.See added attaches for single antenna pattern.and duals at 1/8 wave to full wave spacing 1/8 wave at a time.also my lack of art skills aside the radiation pattern on mobile antennas which are effect by the ground surface of the vehicle there mounted on,Ill say that for single Antenna operation on a vehicle centered and as high as possible is the very best for that a single antenna,Back to my comments years ago about taking a dual antenna system off setting it to the rear of a vehicle and them pointing it at a station and out talking a single antenna with equal power coming out of both radios,happens because of previously stated db gains of dual antennas.I was told moving them rear bound costs me power of directing them,well that argument would be perfectly true if not for the extra gain of the second antenna,By off setting Im still talking off the back better than a single in the center because of my gain,I would out talk to the sides with 2 even without gain,Why?Because of the vehicles ground my right would talk left stronger because it has more radiating surface and vice versa,but there's the added advantage of the 3db factor again added in,Now to put the kill on the single pointing at some one,I have more surface forward than he does plus again that magic 3db gain,And for those that dont know or understand DB gain well,Im not talking about gain above a vacuum Im talking real world gain,DB gain can be simply broken down for explanation this way,If you have 5 watts and your wanting to increase your power seen at a receives antenna,you and to effectively dbl your power out 5x5=25 watts to gain 1 db,So now use the math and go for 3db gain.So lets see how that becomes cost effective,If you are using a 10 watt radio and you have a single antenna.your going to need to buy alot bigger radio or add a good sized amp to get a 3db gain and neither is very cheap,But on the other hand if your running a 49.99 antenna,then adding another 49.00 and ruffly 20.00 for coax,The attachments are free to copy and keep,I learned this in my youth from 2 of my uncles,and it took me awhile couple years to find where the paper on this was packed away,No I havent had much schooling on this stuff that was in class rooms,But 1 of my uncles spent 20 years in the Navy as a signal corp specialist,Then Taught at O.U. in Norman,Then when he retired from there he worked at my other uncles Business band shop.sideline CB shop.I started working there in the summer times when I was 13 yrs old,I had learn real world,Military says it is OJT,lol Ill check out the responses soon,Be Blessed By our Lord ,Ken(Grim Reaper):bdh: :oops: Forgot to add the attachments,Something about my disability 's pain levels dont let me sleep alot and I get real space cadet qualifications goin on lmao.And just for the fun of it I threw in the effect on patterns on stacked beams, after studying the resulting new output pattern you see why 2 beams acts as a blinder shutting off incoming signals from the sides and back far better than a singal beam.ooops again still playin cadet hear,you get another little bonus that alot of truckers already know about,that a big improvement in your ears too,that magic db works there too.also for those that cant figure out why you actually get that gain,a little refresher course in antenna mathmatics.you know that anytime you raise the efective resistance load of an antenna system you increase its effiency ,so what better way than to dbl its load to 100 ohms?add a second identicle antenna does it cheap fast and easy,that why when you cophase you need a 75 ohm coax to match at the antennas then a 52 ohm for the odd 1/4 wave of that harness back to your radio,and the 52 ohm can be any prcaticle length,cb shops sell harness only S.W.R. gets close usually 1.4 to 1.5,but add a 52 ohm piece between radio and harness you flat flat with antennas matched.be blessed


     

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  2. vo1ks

    vo1ks Active Member

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    Directional Antennas Made Simple

    Get the book Directional Antennas Made Simple by Jack Layton. It covers AM broadcast antennas and how to phase two or more to create directional patterns. The same concepts and techniques can be applied to 27MHz or any other frequency.

    Directional Antennas Made Simple - Jack Layton - Google Books

    I have a copy and scanned it as a pdf. Lots of math errors in the first chapters, but going through and making corrections is an even better way to learn and better understand.
     
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  3. Needle Bender

    Needle Bender Well-Known Member

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    A little hard to read, better to use a little paragraphing. [​IMG]

    Appears you have a few ideas I would debate, basically a 1/4 wave matching transformer of 75Ω from each 50Ω antenna is used to raise the coaxial impedance to 100Ω so each of the two antennas can be connected to a T connector and end up paralleled back down to a 50Ω load as far as the radio is concerned, splitting the energy evenly between the two antennas.

    Using a 1/4 wave matching transformer only raises the impedance within the coax, not changing the radiation resistance of the antennas to open air.

    Placement will effect the pattern.
     
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  4. Grim Reaper

    Grim Reaper Member

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    I liked the book I had read it awhile back,and basically your right about splitiing the load evenl,but with rf being a magnetic field the power from the radio effiency is increased, in other words single antennas have a far higher energy loss than duals,this is why the 3db gain is made,think about stacked beams dbl ground planes they all talk better than single ones, hear better too.and i agree this application goes beyond 11meters alone,hince the title any 2 50 ohm antennas.And the military has spent untold millions of dollars threw the years on design engineers researching mobile antenna application,and that as wel as my uncles teachings is whee I got alot of my lesson from,not just one source or 3 but from life lessons,and my obessions with learning electronics ,but because of side effects of meds I cant go to a school daily to get a certification or degree,as far as better photos go,i wasnt trying to be the ultimate teacher,I was simply trying to inspire a few folks to start hunting on here and the web to fill their desire to get more knowledge on 50 ohm antennas of any kind. btw I am not even a high school graduate, its been over 35 years since I have been to school. So my proper writting skills havent been applied or used even a little,but thank very kindly for your response sir,
     
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  5. Grim Reaper

    Grim Reaper Member

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    The 1/4 wave part is correct in your respone but when you use the math formulas on that page to figure coax lengths for any freq, you will see that the coax wave length doesnt come out any where near as long as the antennas length to get 1/4 wave,and you can do the co-phasing of any wave length antennas 5/8 as an example,the coax actually on 27 mhz is only 6'.You also have to use odd multiples of this figure to tune the antenna,like 6' 18' and so on,ok Im taking a break for couple days, My meds finally high enough levels to allow me to sleep then go outside and enjoy spring here.God bless all,
     
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  6. The DB

    The DB Well-Known Member

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    First off, when it comes to your writing, wow. That was entirely to hard to read...

    You are correct, a full wavelength is near 36 feet (give or take depending on the specific frequency). Also 108 inches is near 1/4 wavelength. The thing is the width of an antenna plays a part as well. If a quarter wave whip were a 1/2 inch thick for its full length then its resonant length would be closer to 102 inches. In reality the resonant length happens to be in between these figures as the 1/4 wavelength antenna is rarely 1/2 inch in diamater for any real amount of length. Unfortunately, if you are only using an SWR meter, you have no way of determining where the resonant point is. Also factoring in the losses from the imperfect ground plane that almost every vehicle presents can throw things off even further.

    OK, here we are going to start bumping heads. If you want to run them at that distance go for it, it is a free country after all. That being said, you won't see 3 dB gain over a single antenna. You are trying to say that you have effectively double the output power going front and back, and you have nearly as much gain the other directions as well. Unfortunately the laws of conservation of energy do not allow for this. If you are not increasing the power being fed to the antennas, the only way to achieve a stronger RF field in one or some direction(s) is to sacrifice field strength in other directions. That is physics 101. Adding a second antenna does not increase the power being fed to the antennas and instead divides it.

    Now there are some cases where this may come close or even achieve a 3 dB gain at 1/8 wavelength separation, but this will not happen with vertical omnidirectional antennas right next to each other. In reality you will get a 0.3 dB gain over a single antenna, at best. Now if you put one antenna directly above the other (a colinear setup), or phase two yagi antennas 1/8 wavelength apart you will see noticable gains that can approach 3 dB over a single antenna, but not when they are omnidirectional antennas side by side.

    Ah yes, the attachments. Do you have source information for the first one? It has incorrect information to start with. It claims that the circles on the graphs shown are 3 dB over a single antenna. That is not how those types of graphs work. The outer circle shows the maximum gain point of said antennas, not some specific gain figure. I can also tell you that the total gain from these varies wildly.

    I happen to have a book here that shows very similar graphs and I am happy to give the gain information presented with it. It is the ARRL Antenna Book 22'nd Edition.

    All of these are maximum gain over a single antenna, and assumes a full 1/4 wavelength antenna.
    1/8λ apart - 0.3 dB gain
    1/4λ apart - 1.1 dB gain
    3/8λ apart - 2.4 dB gain
    1/2λ apart - 3.8 dB gain
    5/8λ apart - 4.8 dB gain
    3/4λ apart - 4.6 dB gain
    7/8λ apart - 3.7 dB gain
    1λ apart - 2.8 dB gain

    Unfortunately it is a hard back book or I would be able to scan those pages in and show you.

    The next few lines you typed talked about forward directionality resulting from moving antennas towards the back of a car, yes, that is true, but at the cost of efficiency. And with cophased antennas this cost is even greater.

    "magic 3db gain"? For one there is no magic to gain, it is both predictable and measurable.

    Interesting, you are claiming that doubling your power is 1 dB gain? This is simply not true. The use of any of any decibel calculator available online clearly demonstrates that for power, 3 dB is double the power. Make sure you are using one that gives you the chance to choose between power and voltage. If you are looking at voltage differences it takes 6 dB to effectively double the voltage. I think you have the right idea for how decibels work, but your numbers are way off.

    For the record I started at age 14. I spent 7 years working at a CB shop in my free time doing nothing but tuning antennas to help pay my way through college. This was half a lifetime ago for me. I thought I knew a lot then, but I didn't. More recently I started reading and learning, and began to see just how much I didn't know, and how much I was clearly mistaken about. I learned how things work in the real world. I learned the math behind it including the truth about things like SWR, resonance, return loss, and other factors.

    And one other thing on this section of text. To my knowledge the military does *NOT* make the claims anywhere near you are making. I have several publications printed for military use and *none* of them talk about cophasing at all. Should you have access to a military document that refers to "cophasing" or supports these claims I would *love* to see it.

    Two things on this chunk of text. For one, yes, if you have any real gain while transmitting you will have the same amount of gain for your receive. The reciprocity theorem clearly states this.

    And the other part, your electronics knowledge is way off. To start with you want maximum current flow at the current nodes, so so far so good. The problem with how you said it is to increase current flow you want less resistance, not more. This is electronics 101. It doesn't matter if its AC power, DC power, RF power (which is actually AC power), the more resistance the less current will flow.

    Electronics 101 also disagrees with parallel antennas "resistances" adding. In parallel, the resistances divide, not add. That being said, with the AC RF signal we are referring to, the presented "resistance" is being converted back to a theoretical 50 ohms. Long story short, the current stays the same, and is thus *divided* between the two antennas making each antenna *half as effective* as a single antenna would be as each antenna is radiating half as much as a single antenna would. This is before potential additional losses are factored in.

    Going on to your next post here...

    Huh? First off, RF is electromagnetic radiation. If you have only a magnetic field you have nothing.

    Also, you have your efficiencies backwards. A single antenna is more efficient than two antennas. I have not once read anywhere that the opposite was the case until your post here.

    Stacked beams are a different beast from cophased omni's. Similar principles, different antennas, and different results.

    We are in agreement here, phasing does work the same in bands other than 11 meters. All you have to do is design your layout for the appropriate frequencies you are using.

    If they spent this money on studies there will be published reports. If it provided the benefits you claim the information would be readily available in their antenna literature. Where are these reports. Where is the literature that has this information. The (limited obviously) information I have available to me from the military does *not* include any information on phasing whatsoever. I would *love* to see the military reports from the studies you are referring to.

    I have news for you, very few antennas are at their best when tuned to 50 ohms. As an example, a 1/4 wave whip with 1/4 wave horizontal groundplane radials will present something closer to 35 ohms to the feedline at resonance. That translates to about 1.4:1 SWR at resonance. The further off of this you go the more losses will be presented.

    In your third post, the shorter length of coax is from velocity factor. This is because the RF signal does not travel at the speed of light through the feedline, and because of this the feedline calculation for wavelengths needs to be adjusted for this slower speed. Most coax will have a velocity factor in the writing on them, and it will always be a number less than one. You get the feedline equivalent by taking the actual length as calculated using the speed of light calculations and multiply this by the velocity factor of the feedline.

    Wow, another long post... I need to start breaking some of these up.


    The DB
     
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  7. Needle Bender

    Needle Bender Well-Known Member

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    Agreed, and an example of using the velocity factor to calc the 1/4 wave matching transformers for, say, a 75% VF coax;

    VF X 1/4WL = .75 X 108" = 81" long.
     
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  8. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    First I just want to say that it is 100% possible to achieve good gain, up to about 4 dB when using 1/8 wave spacing but the pattern is NOT omnidirectional.

    Most everywhere I read on CB forums people say it is impossible to achieve gain at less than 1/4 wavelength spacing and trying to do so is fruitless. They are so wrong and the reason they are wrong is they do not understand phase angles. They assume that both antennas are fed in phase.

    Let's take two vertical antennas that are identical and mount them 1/8 wavelength apart on a vehicle. Lets assume they are on either side of the vehicle. With both antennas fed in phase the gain is a miniscule 0.3 dB and the pattern is almost completely omnidirectional with a slight flattening toward the sides and a slight bulge out the front and rear. Now if we do something radical like feed the antennas with unequal lengths of coax we end up changing the phase angle between the antennas and this is where we start to see big differences in patterns and gains. If we feed one antenna with an extra half wavelength of cable then the antennas are 180 degrees out of phase and the pattern becomes like a figure eight with the main lobes out the sides and the nulls towards the front and rear. The gain is near 3.8 dB. If we use an extra 3/8 wavelength of cable the gain is 4.2 dB with the pattern unidirectional towards one side, the side that has the extra cable. This can all be seen and explained better on page 8-6 of the Radio Amateurs Antenna Book 17'th edition if anyone has one. Omnidirectional gain can only be achieved by vertical stacking of antennas HOWEVER gain can be developed with as little as 1/8 wave spacing if one can live with the resulting pattern.

    I just wanted to say this because I am tired of hearing that YOU CANNOT GET GAIN FROM LESS THAN 1/4 WAVE SPACING. You can. You just need to know what you are doing and be aware of what the end result will actually be.
     
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  9. vo1ks

    vo1ks Active Member

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    And the resulting pattern can be further refined by changing power to each antenna. It doesn't have to be split 50/50...
     
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  10. The DB

    The DB Well-Known Member

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    This is absolutely correct, but not what "Grim Reaper" was talking about. He was referring to antennas that are 1/8 wavelength apart with 0 degree phasing between them. Unless I misread, which is entirely possible.

    Pages for other editions of the antenna book with the same information:
    14'th Edition of the ARRL Antenna Book page 6-5 (*see note below*)
    16'th Edition of the ARRL Antenna Book page 8-6
    22'nd Edition of the ARRL Antenna Book page 6-8

    From what I can tell this information is not in any version before the 14'th edition, as it is not in the 13'th edition.

    *Note*, the 14'th Edition of the ARRL Antenna Book has different values for gain on most of their graphs. Only three have more than a 0.2 dB difference, although one of those three shows a full 1.0 dB difference. The graph for the antenna Captain Kilowatt was referring to is one of the three, it reads 4.9 dB gain where the other books read 4.2 dB gain.


    The DB
     
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  11. jazzsinger

    jazzsinger Bullshit Buster

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    a point many seem to be missing is any gain,comes from loss in another direction, therefore if you've achieved 3d gain to the front and you've lost 3b behind and your talking to a station behind you,what have you achieved, thats not even counting the shit you may redirect in useless directions.

    just because a station is physically in front of you,doesn't necessarily mean his strongest signal will come straight at you, it could arrive via reflection,refraction,multipath,backscatter etc etc

    as far as i can see bottom line is unless your using a good omni for listening or some online program telling you where skip is coming from, and unless your cophasing high gain antennas in perfect conditions, or are wullie wan mate, there is very little to be gained from any cophasing at all, even then its probably a very expensive pointless excercise a 10db gain amp wouldnt blow away easily.

    on a vehicle its more about ego than any practical useability,

    give me one highest point centrally mounted decent antenna anyday.
     
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  12. jazzsinger

    jazzsinger Bullshit Buster

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    whoever taught you about decibels wasn't very bright, how does doubling your power equate to 5x5=25, doubling power is 2 x 5w = 10w, now that isn't 1db gain either, its 3db gain, 3db = 2 x power. you used the maths and got it miles wrong,1db gain on 5w is about 6.something watts,i cant even be arsed figuring it out its so pointless.

    to move a 6db per s unit meter from S6 at 5w to S7 would take 5w x2 x2 = 20w, to go to S8 would require another 6db increase,therefore 20w x2 x2 = 80w. 0r you could just do a 12db increase ( x16) from 5w to get the same 2 Spoint increase, 16 x 5 = 80w, its not rocket science.






    this should be a laugh



    FUCK ME !!!! i wasn't disappointed, firstly you need the 1/4 wave or odd multiples thereoff to be 75ohm,not 50ohm, the reason you raise the antenna impedance to 100 ohms is not for efficiency, its so that when you parallel the two 100 ohm loads you get a 50 ohm load allowing maximum power transfer.

    now on the point off maximum power transfer, that available 5w power is split two ways, therefore creating 3db loss to each antenna, so each antenna gets 2.5w, you don't suddenly turn 5w into 10w,what do you think this is? harry potter :headbang

    that power is then focused in a single or dual usually direction at the cost of deep nulls in others.so it appears as a more powerfull signal compared to an omni, but it covers a hell of a lot less area in certain directions for a bit more in others.

    incidentally the director and reflector of a beam work the same way,they APPEAR MORE POWERFULL BECAUSE THE SIGNAL IS FOCUSED IN A VERY NARROW BEAMWIDTH,at huge cost to behind and the sides in varying degrees depending on mostly element spacing an boom length, hey don't increase power at all,they just waste less,you can use that to your advantage though if your only working a narrow direction.


    you can't create something from nothing,there's never been an antenna made that increases power in all directions, even a linear amp doesn't create power,it transfers mains power to rf power with considerable loss (mostly to heat) with high signal purity or smaller loss at lesser signal purity, but they never ever gain power, for that you'd need an amplifier with over 100% efficiency.

    think off it this way, if you put 100 dollars worth off mains power into an amplifier, a right good amp might give you 70-80 dollars of dirty rf power,55-70 thats a touch distorted, or 45-55 dollars worth of very clean rf, either way you loose something.

    it depends on design and biasing off the amp and the designers attitude to spectral purity,

    be it make it nice and clean,lower powered or make it filthy and show morons on power meters readings that impress them despite the fact they haven't got a fucking clue what it means,not only to their audio,their neighbours,possibly the emergency services and countless other spectrum users.


    i suggest you reread or find new sources as you either don't understand the ones you have or they were written by a complete idiot.
     
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  13. jazzsinger

    jazzsinger Bullshit Buster

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    p.s. you spelled mathematics wrong too ;)
     
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  14. HomerBB

    HomerBB Well-Known Member

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    [​IMG]
     
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  15. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Rarely do i agree with Jazz when he goes off on a tirade but this time I have to agree with everything he has said.

    Oh Lord Jesus it hurt to say that. :cry:
     
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