| Author |
Message |
    
vernonott
| | Posted on Friday, August 24, 2001 - 9:14 pm: | 
|
Just playing around and experimenting I installed a cophase antenna system on my Toyota truck to see how it would perform.The antennas are 28" long ,fiberglass , with copper wire wrapped around them.The package says full wave.After installing them I clipped 1/8 " off each antenna copper wire until I got the swr down to 1.5 on each antenna by itself .Now when I hook both of them up the swr goes up to 3.2.Each antenna by itself will read 1.5. |
ChillyDog
| | Posted on Saturday, August 25, 2001 - 3:03 am: |
|
When you tuned the individual antennas, did you use the co-phase harness or did you use a regular 50 ohm coax running directly to the antenna? If you used the harness, did you terminate the end without an antenna with a 50 ohm dummy load?
If you tuned the antennas on the co-phase harness but left the opposite side of the harness open, you may have mistuned the antennas.
I've only installed a few co-phase systems, so I have just a little experience with them, but I've had the best success using this process:
1. Physically install the antennas and run the co-phase harness.
2. Run a temporary 50 ohm coax (ideal length 1/2 wavelength, considering coax velocity factor) from the radio/meter (or antenna analyzer) to one antenna. Check SWR and tune the antenna.
3. Remove the 50 ohm coax from the first antenna and run it to the second. Again, check SWR and tune the antenna.
4. Remove the temporary 50 ohm coax and hook up the co-phase harness. Check SWR. It should be pretty close at this point.
5. If the SWR is not good, remove one antenna and replace it with a 50 ohm dummy load. Check SWR and tune the remaining antenna.
6. Remove the antenna you just tuned and put the dummy load in its place. Install the other antenna, check SWR and tune it.
7. Replace the dummy load with the antenna. Check SWR. Should be good now.
If there still is a problem do the same checking for shorts and opens, damaged coax, etc., that you would do for any antenna installation.
If you don't have a dummy load you can leave both antennas on while you tune one, but I haven't had very good results using that technique.
If anyone else has other suggestions or procedures for installing co-phased antennas I'd like to learn them!
Good luck, Vernonott!
Bob |
Robbie
| | Posted on Saturday, August 25, 2001 - 8:44 am: |
|
You have to adjust them at the same time - the co-phase harness has a higher ohm than a non-co-phase coax does. You are testing each one individualy, instead hook them both up, then adjust - you'll get much better results that way. Make sure the doors are closed and such. I have Co-phase on my truck also, and my meter just barely moves now - and I get much better reception. I've got dual 4' Firestik IIs on my truck..... |
Bigfoot
| | Posted on Saturday, August 25, 2001 - 12:12 pm: |
|
I am no tech, but I do know that you won't see much benefit from duals, unless you can get them at least 110 inchs apart. as for the cophase, is you coax 75 ohm from the splitter to each antenna?
How are you splitting the coax? |
Skullman
| | Posted on Saturday, August 25, 2001 - 5:26 pm: |
|
Bigfoot is correct, dual antennas need to be 110 inches apart to keep them from cross radiating on each other. Unless you're in a big truck, it won't work. My advice ditch the dual antennas, they look cool I know, but you can get way more performace out of a whip or single antenna.
CEO of DEADMAN INC. |
vernonott
| | Posted on Saturday, August 25, 2001 - 8:16 pm: |
|
I bought these antennas as a kit with cophase harness supplied.As far as the 110" goes the truck isn't much longer than that. |
Bigfoot
| | Posted on Wednesday, August 29, 2001 - 7:09 pm: |
|
You could mount the antennas on each bumper, but you signal will be directional with the duals. Like football goalposts and your signal is the ball on a field goal. |
bruce
| | Posted on Thursday, August 30, 2001 - 12:11 am: |
|
if you corectly change the phase the ants wil act as a broadside, endfire or beam hygain put out a great brochre on how to do it somewhere i have a coppy maby MFJ that bought them still has some |
marconi390@pdq.net
| | Posted on Saturday, October 06, 2001 - 6:50 am: |
|
Hellow ChilliDog Bob,
First I also discourage the use of co-phase setups unless you are able to effect the proper phasing via proper spacing and are able to tune the harness correctly. Some may disagree but close spacing can also work with modest losses in gain.
I agree with what you say up through item 4. You are absolutely correct using a true ½ wave line (assuming you can produce a true ½ wave line) to tune the antennas individually first is the way to go. However, it is a little more complicated than is often discussed.
One needs to question the harness first off. Co-phase harnesses are tuned devices and should be near a true electrical ¼ wave length from the harness feed point to each antenna. It must also be assumed that both antennas present similar loads. You never really know what was done in this regard and that can make for problems for tuning right-off-the-bat.
First you must make sure the antennas are similarly tuned for resonance and match so as to present the same load. I question there being such a thing as a fit all harness.
I start off with a physical ½ wavelength line so I can trim down. Then the harness must be tuned for the load. Since you cannot always believe the specs on coax you must tune via the old trusted method of trial and error. The math can certainly get good results but it depends on good information as to the velocity factor and other line characteristics you are working with. In the good old days we didn’t have any idea what the specs were, but we knew something was working against our best judgments as to the length. In modern times the problems presented by cost minded producers of coax can still make a mess of things when simply using the math.
I may be making too much out of this, but good results demand a good efforts. I tune the antennas just as you note, using a true ½ wave line. Then I tune the harness to both antennas by trimming the antenna ends of the coax. I do not use tee connectors. I divide by bending over a ½ wave line and make a pigtail for the harness feed point. The cable is carefully bent-over, parts seperated, stripped out exposed, and tinned. It can also be installed into a coax connector. I personally use a well-protected physical connection to the balance of the line to the radio and I prefer physical connections at the antennas as well. These areas of the line are hard to protect and protection of your coax line is critical for any type of mobile installation.
My exception to your approach is, that I am not to sure about using a dummy load to make adjustments to an antenna on a co-phase harness setup. I have heard this topic discussed but I am not convinced. The dummy load may do one of two things. Considering a dummy load should properly show a short, it is appropriate for testing the shunt effects of a ¼ wave line. However a dummy load may not show the same load as the antennas, so you will be off somewhat and that could cause a problem. Most installers attempt to tune the antennas to the line rather than tune the line to pre-tune antennas. I believe there is a difference.
Just some thoughts on the subject. Good topic for developing better understanding of things.
Marconi 39ZERO |
ChilliDog
| | Posted on Monday, October 08, 2001 - 11:27 am: |
|
Hi, Marconi!
I very much appreciate you sharing your thoughts and experience. I do have limited background with dual antennas, and your comments help to expand my knowledge!
I strongly agree with you in discouraging use of dual antennas for most applications. As you noted, optimal directional results come from proper (1/4 wave) antenna separation, a parameter typically impossible to meet on most vehicles. Still, I have made some field strengh measurements that indicate somewhat acceptable omnidirectional results with less than ideal separation. More about that later.
In my response to Vernonott I made the assumption that he was using a factory developed co-phase harness. I have only used factory co-phase harnesses, and have had good SWR results without trimming the coax. I no longer have specific notes, but as I recall with 6' Firesticks mounted high on the rear sides of a Ford F250 extended cab (approximately 6' apart) I was seeing SWR of around 1:1.8 at the CB band limits.
I have never attempted to fabricate a co-phase harness. Your instructions on how to do so are interesting. I may try to follow your process just to see if I can!
As far as the directionality of a dual antenna system with less than 1/4 wave separation, after the same F250 install I took a powered field strength meter and did a field survey of the radiation pattern. I was surprised. I expected a really crappy pattern, but actually found very usable results.
In a large empty parking lot (one of the old abandoned Ontario Motor Speedway lots in So. Cal.) I tied a 100' piece of twine to the meter and then tied the other end to a magnetic mount (without antenna) set in the center of the cab roof. This kept my distance from the Ford consistant. I then moved all around the vehicle, plotting the field strength measurements on polar graph paper.
Again, I no longer have notes from that time, but I remember the radiation pattern showed an approximate oval shape, close to round but with a _little_ more signal strength to the front and rear. There were two sharp nulls, extending from about 100 degrees and 260 degrees, referenced to the front of the truck as 0 degree. (In other words, the nulls seemed to come from the back edges of the cab.) I have no doubt that a single antenna mounted in the center of the cab would have a more omnidirectional pattern, but the duals did better than I expected. (I lost a bet on that one!)
Here's a point where I disagree with you. A dummy load should not show a short. It should represent a perfect (50 ohm) match, the desired impedance of the properly tuned antennas. That's why when having problems tuning the dual antenna system it comes in handy as a substitute for one of the antennas. It assures at least one leg of the co-phase harness is looking into the correct impedance.
That said, you are absolutly right that the 75 ohm legs of the harness have to be as close as possible to 1/4 wave electrical length for proper impedance transform from dual antennas to 50 ohms. I guess in my limited dual antenna installation experience I've been lucky that the harness manufacturer did a good job of setting that length!
Very Best Regards ...
Bob |
Hamcber
| | Posted on Tuesday, October 09, 2001 - 1:08 am: |
|
What you want to see is 100 ohms, zero reactance at each end of the co-phase harness. Each antenna should be a little longer to bring the impedance up from 50 ohms to near 100 ohms.
Having one antenna at 150 ohms and the other at 25 ohms could work well on an SWR meter, but what will happen is the RF pattern will skew toward the higher impedance antenna since the current will be lower and thus the ratio will be different.
It certainly seems appropriate to tune the antennas one at a time as long as you put a 100 ohm resistor across the unused end of the cable harness with no antenna connected there. Otherwise, I suggest tuning the antennas at the same time, making the exact same adjustments to both antennas at the same time. |
ChillyDog
| | Posted on Tuesday, October 09, 2001 - 4:49 pm: |
|
Hey, Hamcber,
I've got to disagree with you regarding the impedances you will see in a "standard CB" dual antenna co-phased system; in other words, in a system using two regular 50 ohm antennas coupled by 75 ohm 1/4 wave matching sections to a 50 ohm source.
If we were not discussing dual antennas, your analysis is correct; a single 75 ohm 1/4 wave section will match a 100 ohm load to a 50 ohm source. But we're discussing dual antennas.
The individual antenna impedances should be 50 ohms, not 100. When you put two 50 ohm antennas in parallel their impedance halves to 25 ohms, just like two resistors in parallel. Therefore: the matching is from 25 ohms to 50 ohms, not 100 to 50.
So how can 75 ohm coax match 25 to 50? Calculations indicate that you need a 37.5 ohm 1/4 wave section to match that ratio, not 75 ohms.
Well, a co-phase harness actually has two 75 ohm sections in parallel; the parallel 75 ohm impedances half to 37.5 ohms, again, just like resistors in parallel. That 37.5 ohm impedance is exactly what is needed to match the 25 ohms to 50.
That's why a 50 ohm dummy load is helpful in substituting for a removed antenna when adjusting SWR in a co-phase system. In this case, a 100 ohm load will provide a 1:2 mismatch on that leg.
You're right about wanting zero reactance. I think Marconi's extra efforts in trimming the co-phase matching sections not only optimize the harness length to an electrical 1/4 wave, but also serve to trim out the reactance of the antennas.
As always, Very Best Regards to all!
Bob |
Hamcber
| | Posted on Tuesday, October 09, 2001 - 10:35 pm: |
|
Bob.
You are incorrect on this one thing. The 2 50 ohm antennas in PARALLEL such as in a co-phase situation will equal only 25 ohms load at the common point regardless of the cable carrying the current to (and from) the radiators. You need more impedance at the feedpoints since the two antennas are indeed parallel. On this point, I am positive (I do this for a living).
If you match each antenna separately, use a 100 ohm resistor instead of a 50 ohm resistor in place of the antenna not being tuned at the moment.
Everything else you got right on the money. |
Marconi
| | Posted on Tuesday, October 09, 2001 - 9:42 pm: |
|
Hey Bob, email me. I would like to discuss your ideas off forum.
Marconi |
ChillyDog
| | Posted on Wednesday, October 10, 2001 - 7:13 pm: |
|
Marconi,
I'm sure it's my ignorance, but I don't seem to be able to find a way to send an e-mail to you directly through this forum without actually posting my e-mail address on the forum.
I'm reluctant to display my e-mail address for all to see and some to abuse, but I am very interested in hearing your perspective; can you suggest a resolution to my dilema?
Hamcber,
Please don't don't think I'm saying you're wrong; but your last comments go against my understanding. I have great respect for experience vs. theory, and I'm working more on theory here!
In order to expand my knowledge from theory to practice, this is what I'm going to do: I left work early today (I'm a bad boy, boss!) and picked up the materials to make some "calibrated" mis-matches.
By that I mean I'm going to make a 100 ohm load and a 25 ohm load, to use with my existing 50 ohm dummy loads in evaluating transmission line concerns. I already have (somewhere in my big pile of stuff!) a CB co-phase matching harness.
With some matched 50 ohm loads and calibrated mis-matches I can make some real measurements on the co-phase harness to support (or contradict!) the theory. This is something I've meant to do for a while, now I'm motivated to get my butt in gear!
I'm not sure when I'll be able to find the harness and get to the measurements, but what I plan to do is take SWR measurements at different points on the harness with different value loads substituting for antennas.
I still believe that the antennas should show a 50 ohm impedance and because of that the load substituted when setting up the co-phase harness should be 50 ohms, not 100.
It will be interesting to see what actual measurements show!
Life is good when there are questions to investigate!
Bob |
marconi390@pdq.net
| | Posted on Friday, October 12, 2001 - 1:12 am: |
|
Bob, this forum allows you to send me an email quite nicely. I will include my email address in this post and it will show up in the left hand column as a clickable email address instead of my name.
All you have to do is click on it and it will bring up your own email system. I guess you know the details for that. Your email address should be preserved as best I can tell. If you are concerned doing this inside the forum, then use my address to email me from within your own email system.
I am working on a piece that may be of interest to you. I was going to send it to you via email, but I may just post it here instead.
Marconi |
Hamcber
| | Posted on Friday, October 12, 2001 - 1:54 pm: |
|
Hey Bob,
No worries, dude! I do not get angry when people disagree with me! I enjoy sharing my experience with others on this board even if it unfortunately goes against what they believe to be true. I highly suggest you do your own experiments and create your own conclusions. That's how I learned too. My book collection doesn't cover 10% of what I have to know in order to build some of the systems I have. The theoretical basics get you started, the rest is done in the lab and on the test range.
Make sure you conduct your tests with adequate ground under antennas if you use them. Also, a standard SWR meter will not show you the resistance or reactance. You will need to use an OIB (Operating Impedance Bridge) to get readings that mean anything.
Good luck! I can't wait to read about your experiment! Experimentation is truly the most fun part of my job and I enjoy time in the lab VERY much myself as well. |
Marconi
| | Posted on Thursday, October 18, 2001 - 8:12 am: |
|
Hey Bob and Hamcber, there is an interesting thread developing you can click-on regarding the subject of Line Transformers
Thought you might be interested.
Marconi |
Hamcber
| | Posted on Thursday, October 18, 2001 - 10:47 pm: |
|
That link pops up an error. |
Marconi
| | Posted on Friday, October 19, 2001 - 4:51 pm: |
|
Hamcber, Uncle Jim's forum is down, that is why you got the error. At this point it looks like the thread may not go anywhere. I suspect the guy that posted the report is not going to be asked any questions and the one guy that did ask some questions and give his point of view has gotten quite.
Sorry for the false alarm.
Marconi |
Marconi
| | Posted on Saturday, October 20, 2001 - 7:10 am: |
|
Hamcber, I am trying this post to see if I can ues the edit feature. Although I tested the link noted above, it may have changed for some reason and that may be why it is not working for us. I will try it again here.
Line Transformers |
Hamcber
| | Posted on Saturday, October 20, 2001 - 6:51 pm: |
|
Hey Marconi.
Those guys in that thread are all over the road, but they are at least circling the correct block!
For instance, 'Big A' wrote: "I agree that there will be unusual currents along the pieces of coax that make up the "transformer", but the final objective was to make the box/radio see a desirable load and not go into self destruction. I think I accomplished that."
Your clue should be when he states: "And finally, the Wilson now works as well as the 102 (inch whip -ed)."
NEWSFLASH*** Loaded (shortened) 1/4 wave antennas do NOT perform as well as full length 1/4 wave radiators.
As much as I would like to believe in voodoo too, theory is almost always right. In this case, theory tells you that you are only fooling the SWR meter or bridge, and not improving the substandard installation so that it performs as well as a properly done installation. That is exactly what 'Big A' said in his quoted paragraph above.
It is true that using different lengths of coaxial cable can affect reactance. However, coaxial cable alone cannot affect feedpoint impedance. If the resistance at the antenna feedpoint was low, I suggest an L network to bring the feedpoint resistance up and then tuning out whatever reactance may be result. Then, power would not be lost in the coax debacle mentioned in the post. Since the antenna is mounted behind the cab of the truck on a 'toolbox', then getting at the feedpoint below the antenna mounting point would be no trouble. That would also keep the L network components out of the elements.
Thank you for the report! It was interesting. |
Marconi
| | Posted on Saturday, October 20, 2001 - 9:08 pm: |
|
Hamcber, your circle analogy was a great description of that effort. I was expecting something different, of course. There was two threads about this issue. One is down lower in the topics titled "MFJ confirms the cable length/Transformer debate" You will have to go look soon as UJ's forum deletes things as they get to the bottom.
The more recent one at the top of threads is titled "Marconi statement on coax Transformers"
Marconi |
Hamcber
| | Posted on Sunday, October 21, 2001 - 1:38 am: |
|
I saw both of the threads.
The info is out there for people, they just have to look.
I'm not preaching something new or something I made up. Open any advanced antenna design spec and you will not only find what I use every day in my professional life, but you will also find stuff that contradicts about everything CBers believe to be true about antennas. |
Marconi
| | Posted on Thursday, October 25, 2001 - 2:44 am: |
|
Hey ChillyDog Bob, how is you investigation coming along?
Marconi |
ChillyDog
| | Posted on Thursday, October 25, 2001 - 12:25 pm: |
|
Hi, Marconi!
I haven't been able to complete the experiment since my last postings.
There have been a number of personal and professional issues that have prevented me from even logging into the forum recently. That's unfortunate, since this topic is getting attention.
Although I would rather spend my time with radio concerns, right now I have other things to take care of. At some point (I hope soon!) I'll carry on with my tests, and you and HamCBer will be the first to hear the results!
Bob |
|
