Co-Phasing two mobile antennas

[hossless]

I am going to co-phase two 10k's on my pickup. I read on fgbestcommunications website a few days ago regarding 75ohm coax used in conjunction with 50ohm coax in co-phasing.



>Figure 8 shows the all important arrangement of coax you must use to feed your phased antennas right. A odd multiple of 1/4 Wavelength 75 Ohm coax must be used. Check out the "Coax Basics" section for information on cutting coax to certain lengths.







Figure 8 - The most important step in getting your co-phased antennas to work right. 75 Ohm coax is used as an impedance transformer to match placing two 50 Ohm loads in parallel back to 50 Ohms. Note this is only for co-phasing two 50 Ohm antennas. I am not going to cover harnesses for more than two antennas, if you are going to co-phase more than 2 antennas at a time, you better be reading more than just my web page for co-phasing! Also....each side of the harness must be the same length.<



Let me ask, is it crucial to use the 75ohm coax with the standard 50ohm? Or can I just stick to the 50ohm coax? All of the co-phasing harnesses I have ever seen have been made up of only standard 50ohm coax. Check out fgbestcommunications website for info on co-phasing. Their website seems very impressive, they have alot of great info on their website.



Thanks, Hossless




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[Moleculo]

YES!!!! You must use 75 ohm coax at odd quarter wave multiples if you want them fed in phase. You can alter the lengths to get different signal patterns...let us know what you're looking for on that topic.



BTW, I did see at a cophase "box" somewhere that would allow you to use regular 50 ohm coax instead of 75ohm. Seems that would make things simple. I can try and find it for you if you want.



Moleculo


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[W5LZ]

Hossless,

Is it necessary to use 75 ohm coax instead of 50 ohm coax? Yes, you can't 'co-phase' antennas using the same impedance feed line. It isn't strictly necessary to use 75 ohm coax, though. If you can find it, 93 ohm coax would work too, or almost any impedance larger than 50 ohms. The bigger the difference is, the 'better' it will work, (sort of). You also have to consider the 'matching transformer's other characteristics, such as power handling ability (break down voltage, actually), how easy it is to find, what it costs, etc. Since 75 ohm coax is so common, it's the 'usual' choice.

- 'Doc



PS - You are basically dealing with a parallel resistive circuit (handle impedance like you would resistace, in this case). If you analyze 'how' the different values affect the resulting resistance/impedance in a parallel circuit you'll see what I mean. To my knowledge, there are no commonly available coaxs that will result in 50 ohms, they all result in some impedance that is 'close', but not 'on the nose'. In most cases, 'close' is good enough...




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[hossless]

OK, that's what I figured. Funny how many so called "co-phased" harnesses get away with using 50ohm. I want to do it right, otherwise whats the use in wasting time, effort and money on co-phasing?? I'll get some good RG-59 coax a T-connector, and do some reading on how to come up with the proper length for antenna leads. I'll keep ya'll posted on the end results, and ask for some more help along the way if needed.



Thanks Moleculo, Doc and Freecell,<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/smile.gif ALT=""> Hossless


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[freecell]

"All of the co-phasing harnesses I have ever seen have been made up of only standard 50ohm coax."

then they weren't phasing harnesses........

Several times a year we get tech calls from installers who place dual antennas on their vehicle and run RG-58 or RG-8X from each antenna to a T-connector at the back of their radio only to find that the system "doesn't get out". You should not use 50 ohm coax on a ground plane dependent dual antenna installation ... it MUST be 72 ohm RG-59 type coax. These flawed installs can be misleading because SWR tests can show exceptionally low SWR, making the installer think that all is well. However, the impedence of the antenna system does not match the requirements of the radio and therefore, the output power of the radio is greatly reduced. In several tests, we found that a 4 watt radio would only generate 1.75 watts of output power which is the equivalent of having an SWR reading that exceeds 6.0:1. in addition, there is no need for a t-connector as a pair of 59 lines fit quite nicely into the rear of a PL-259 connector.

nothing new here.

http://www.firecommunications.com/antennafaq.txt

 

[hossless]

How many watts will the 75ohm coax handle?

Also, is it absolutely necessary to have an exact predetermined length of the 75ohm to have a good matching antenna system?





Thanks, Hossless


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[hossless]

Moleculo, YES, if you can locate the co-phase "box" please let me know.



Thanks, Hossless


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[hossless]

I'm having second thoughts on co-phasing then. I will be pushing at least 1400w, so maybe it's not such a great idea?

Is there anything else other than 75ohm that would work and be able to handle the power requirements?



Thanks Hossless


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=hossless>hossless</A> at: 5/6/04 6:12 pm

 

[freecell]

" btw, if you cophase you are splitting up the power between 2 lengths of coax." no kidding, really?

yeah, so what's your point? this statement is meaningless and the fact presented is obvious to anyone who is capable of rational thought. i have heard this before and most who utter it seem to think it's a "bad thing." i have heard it used as an excuse and a reason to support the line that a single antenna works better than dual or phased antennas. poppycock. this is simply part of some concocted spiel used by the same individuals in so-called cb shops around the country who have been constructing phasing harnesses from 50 ohm cable for years. if this is the case then i would also agree that the single antenna will work better than the duals. the real problem is that they don't know how to properly install dual antennas in the first place.

the fact that the power is divided between the two antennas is of no consequence. the interaction between the pair of antennas re-creates a single pattern utilizing the combined power radiated by both antennas in the final result and also creates advantageous radiation patterns that produce gain in the favored directions when compared to a single antenna of the same type and design, all of which can be controlled and manipulated by leading or lagging phase relationships between the two lines connected to each of the antennas. another advantage inherent in dual antenna systems is that they are decidedly much less dependent on large areas of counterpoise or ground plane surface for adequate performance and matching, as long as at least the top 2/3's of each of the antennas have unobstructed line-of-sight to each other.

as can be seen by the mistakes (or related questions) made when attempting to properly combine a pair of antennas in the previous posts with respect to the proper type and lengths of feedline involved, there are very few (if any) situations where a single antenna would outperform a dual antenna system in one or more directions.

the dual antenna system is basically a pair of identical antennas at a physical spacing of a 1/4 wavelength being fed simultaneously with an electrical 1/4 wave phasing harness constructed of quality 75 ohm coaxial feedline to provide the necessary parallel impedance transformation. the pattern generated is roughly that of a figure 8 with primary lobes at right angles to the vertical plane of the antennas. contrary to undocumented opinion the directional effect is not entirely diminished at closer spacings. the forward and rearward lobes do recede slightly and there is some added response in the side lobes but the overall effect is nonetheless still the same. at the lesser than formula spacings using top-loaded antennas i have seen radios alone capable of communications over distances in excess of 50 miles in the favored direction/s (front and rear) of properly configured and installed dual antenna systems. in one instance that comes to mind, in the case of a 97 KW with dual stacks, for any of you familiar with that particular truck, you know that there's no way you're going to achieve full 1/4 wave spacing. the results quoted above apply to this and other similar situations encountered in vehicles such as this. on the other hand, i have never been able to duplicate these results in a single antenna scenario. if anyone has, i'd sure like to hear about it.

http://www.firecommunications.com/antennas.txt

 

[freecell]

"Phasing antennas only has one benifit, it makes the antenna system directional. It doesn't mean you can increase your actual transmitted power, just that the power radiated can be focused in particular directions."



if we take signal away from one or several directions and "focus" it into other directions then we have increased the actual amount of transmitted power in those directions. (use the word radiated if you like, that may be less confusing) this in essence is the definition of the term "gain" as it refers to antennas and especially antenna systems. (2 or more) i believe i mentioned the several benefits in previous posts.

one other advantage i failed to mention is that since the transmitter (or amplifier) power generated IS divided among 2 antennas and feedlines, this will generally save money normally spent on feedline with larger power-handling capabilities, not to mention the antenna systems improved ability to handle higher power levels as compared with the same power level used with a single antenna/feedline of the same type and caliber. here's another graphic example of another of the benefits of dual antennas and knowing what to use and when to use it,.



www.firecommunications.com/install.txt



"The radiation pattern of a mobile antenna system is not constant."

the initial pattern generated by the antenna system is always a constant unless the antenna configuration itself changes. referring to the near and far-field aspects the only times the above statement applies is when we are dealing with changing altitudes in the ionospheric layers in skywave applications where varying heights affect the angle that the transmitted signal arrives at and leaves from the associated layers.

another instance where this may apply is when surrounding metal objects closer than 1 wavelength (or extremely large metal objects at 1 wavelength or farther out) are in the near field of the antenna system. local terrain can be included here too, including soil conductivity.



"Phased antenna systems are NOT imune to the available 'ground' of the vehicle they are on." no one said they were immune or operated totally independent of the vehicle ground. (this includes the capacitively coupled earth ground as well)



here's what was said:



another advantage inherent in dual antenna systems is that they are decidedly much less dependent on large areas of counterpoise or ground plane surface for adequate performance and matching, as long as at least the top 2/3's of each of the antennas have unobstructed line-of-sight to each other.



here's the bottom line on dual/co-phased antennas. let's say, as in hossless' case where he's running 1400 watts, (geez) let's say this is being fed into a single antenna. if he were to locate and install 2 of the same antennas in a dual antenna set-up, in the favored directions of the system the increase in signal would be such that he could cut the transmitter power in half and still produce the same signal that he did previously with the single antenna at twice the power. this is completely in line with a 2.5 - 3 db. advantage with dual antennas over a single. (in the favored directions of the system)



an added bonus also is that incoming signals to the antenna system arriving at the sides are attenuated to levels below that of a single antenna (depending on which side of the cab it's mounted on) allowing the operator to concentrate on other mobiles up and down the road to the front and the rear. this i understand is what most truck drivers are looking for. well, that

and someone who has the necessary materials and can do the job correctly to procuce the mentioned results.



as for cutting to length, the following figures should be suitable for most installations.



for 75 ohm solid polypropylene:



5.96802058445138761257121852600625 feet

5' 11.616" inches........6 feet

usable lengths: 6', 18', 30', etc..



for 75 ohm foam:



7.05311523616982172394780371255284 feet



7' 3/4" inches...........7 feet

usable lengths: 7', 21', 35', etc..



compare them to the table on fgbest, they're close enough for government work.



and just to demonstrate how non-critical the length actually is:



i have used exact 6 foot lengths (or any odd multiples) of the foam and noticed nor measured any differences in tuning or performance. best though if you stick to the figures above. all math is based on a CF of 27.205 mhz.



the information and observations above are not the result of any opinions or thoughts but instead are the result of empirical evidence and knowledge gathered and collected over the course of some 30+ years obtained through almost daily experience gained from working with and testing the configurations referred to in all of my posts on the subject.



many in the past who have attempted to duplicate the results from the information provided have not been disappointed.



"I honestly can't understand why anyone would want to use one (dual antenna system) in a 'moving' mobile." i can think of several.



which costs you more? another 700 watts or another antenna and a co-phasing harness?


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/8/04 1:38 pm

 

[hossless]

You all are great! I love this forum, so much knowledge shared, so much knowledge gained. It's almost like going to school again (did I say that?). Never did much care for school, but I love learning this radio stuff..



R-11 it shall be. Hey Doc, CRS?, no doubt - some of us truckers call it "SOMETIMERS" Get's worse with me also on a daily basis.



As soon as I can pick up another 10-k I'll "git 'er done"<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/smile.gif ALT="">



Thanks, Hossless














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[freecell]

no offense was meant towards you concrete, i was just commenting on the fact that this "statement" is often used as an excuse or reason NOT to use dual antennas. i covered that in part of a previous post.



i have heard this before and most who utter it seem to think it's a "bad thing. this is simply part of some concocted spiel used by the same individuals in so-called cb shops around the country who have been constructing co-phasing harnesses from 50 ohm cable for years.



one other advantage i failed to mention is that since the transmitter (or amplifier) power generated IS divided among 2 antennas and feedlines, this will generally save money normally spent on feedline with larger power-handling capabilities, not to mention the antenna systems improved ability to handle higher power levels as compared with the same power level used with a single antenna/feedline of the same type and caliber.



hossles, good luck with the project.


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[Moleculo]

Hossless....I found the cophase "box". FGBestCommunications is where I saw them. I think they were around $50.



Moleculo


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[hossless]

Thanks everyone!

Moleculo, I'll check it out.



Hossless


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[freecell]

and you know, the increased voltage in the matching sections would worry me too if i didn't know a few other things.



all of you "high power" guys take note.



fact: standard RG-59/U with a foam dielectric has a max rms voltage rating of 300 volts.



fact: softer dielectric materials puncture easily due to high voltage.



fact: polypropylene and other dielectric materials will handle

much higher voltages than foam will.



fact: there are many grades and types of RG-59/XXXX



fact: RG-59B/U has a max rms voltage rating of anywhere

from 2.3 - 5 KV., depending on where you get the info.



fact: there are many grades and types in the middle of this

range, between these two extremes.



fact: a pair of antennas, when located +/- 1/4 wavelength apart, fed with a properly constructed 1/4 wave phasing harness, are capable of producing a 2.5 - 3 db. increase in radiated signal strength at opposite right angles to the vertical plane of the pair of antennas........when compared to one of the same.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 5/18/04 4:03 pm