Co-phasing?

[W5LZ]

Odd! I don't remember attributing it to the ARRL, only that it was in their '67 Hand Book. If you'll notice, the ARRL doesn't 'discover' much at all. They do refer to other author's 'discoveries'. Why the 1967 reference? Because I don't have an earlier one handy. I think you'll also notice that I didn't claim to 'discover' anything either. I did give a reference to published information (not mine, by the way).
- 'Doc

 

[KF5FUR]

I don't mean to butt in here with something s small bit off subject, but I didn't want to start a new thread just for this, and this may be for co-phasing of some sort idk. but what is it for?'



And as to the thread topic, I new you could use 50 Ohm cable after the 75 ohm harness. I have a co-phased setup using 2 wire wound big rig antennas with the proved harness with the kit I bought a while back and I have 60ft of RG-8/X that I can run up to it's harness on its mast. Excellent, I really didn't want to buy another cable. I will start a new thread here soon with pics in it, I will name it "Rotatable co-phased 10m/11m base antenna" so search that.

 

[rabbiporkchop]

co phasing

I drive a '00 Peterbilt 379 with sleeper. If I were to use two antennas, and they were both tuned for say like 1.2 give or take 1, would I still have to have an 18' co-phase coax to make this work? What I'm trying to ask is will a shorter co-phase coax work or do I have to use an 18' co-phase coax? Anybody confused yet? I think I am, lol!

You need at least a 90 degree phase angle per side to achieve 50 ohms in theory.
I have never been able to make 90 degrees work, and your only other option would be 270 degrees per side. To calculate phase angle 300 divided by frequency equals 360 degrees through a vacuum. Multiply by the velocity of propagation for the feedline you are using. Then multiply by .75-----this will tell you how long in meters to cut your 75 ohm phasing lines.

 

[dennis4040]

Using 50 ohm coax between antennas and co-phase harness

I know this is an old thread but it is the best description of how to build a co-phase harness I have found.

My question is can you use equal lengths of 50 ohm from each antenna in an array to the 75 ohm co-phase harness? I have read that as long as the 2 pieces are the same length (any length need to reach), it will work.

Marconi and others, can you tell me if this is true?

I use to help build tuned co-phase harnesses years ago. We didn't understand much about VF and specs on cable back then was dubious. So, we tuned the harnesses to work at 27.205 for CB and recommend their use on P/U trucks rear bumpers with 102" whips. Tuning the harness accounted for the variability of VF in the cable used.

This tuning process took care of whatever the VF was for the coax we used. In tuning both RG6 & RG59 we noticed at times the tuned length of the harnesses did change a little, so we figured the cable's VF was the difference.

Each harness was hand made and tuned. We did not build these in the traditional 18' foot length, which is electrically about the same with 3 times the side length of our shorter custom harnesses.

We started with a length of cable about 13' feet long, doubled it over and carefully cut a 2" space in the middle of the coax to expose the shield. We then carefully spread the shield apart and fished-out the center conductor with all wire left hole and intact, with no nicks either. We then carefully removed and inch of insulator without affecting the center wire and created a short twisted pig-tail for each conductor, being careful to keep both uncut with no nicks. This pig tail was installed within a coax connector and that would accommodate a standard barrel connector and soldered well. This end was later connected to the random length working feed line to the radio.

The open ends of this harness were also stubed-out and attached to the antennas and tested for resonance. These ends were made a bit long and then trimmed down to center frequency for CB. Each end ended up being about 6' feet long per side and would easily reach across the back end of a P/U truck.

If you want to get closer to start you can use the following formula.

492 x VF / frequency = total harness length.
492 x .66 / 27.205 = 11.94' which is then folded over.
11.94' / 2 = 5.97' x 3 = 17.9' this is where the 18' harness comes from and is electrically similar to the shorter harness.

Again to be safe make your cable a little long in case the VF for the cable used is not what is published. Then fold this in half and follow the instructions noted above.

We also water proofed all cable ends. You will note when finished that you will have 2 x 1/4 wave tuned wires that help to transform the low impedance match for each whip upward closer to 50 ohms when combined in parallel. The best place to test the SWR before installing is right at the pig tail end. Make sure the truck is in the clear for testing.

I make no claim for this working as intended on the big truck, but your phasing harness does not need to be 18' feet long, unless you need the added length to reach.

We found that Field Strength was improved noticeable using the shorter harness vs. the 3x cable when tuned properly for the antennas. This is due to the increased cable losses in transformation. You may also find this setup to be rather narrow banded.

With a big truck one antenna may actually work better and possibly provide added bandwidth to accomadate modern broad-band radios. Keep me posted on your progress.

 

[W5LZ]

Quick-n-dirty answer is no you can't.
- 'Doc

 

[Marconi]

4040, some argue that you can do close to what you suggest, but I wouldn't. IMO, we need to tune the harness to get it right.

If the harness was say 12' feet long on each side, and somehow there was enough loss in the feeder system, then the match (SWR) might look good, but the co-phase setup would not be very effective IMO. I've seen and heard guys make similar claims about such setups though.

Could it still work? I believe it would, so-so, if the match was good enough at the radio.

 

[dennis4040]

Thanks for the quick response Doc and Marconi.
This is my first co-phase project so I better make sure I understand

If I stack 2 antennas and the SO-239s are 32' apart and I am using LMR-240 75 Ohm my harness would have to be:
((492x VF .84/27.205) /2)x3 = 22.787'? 45.57' end to end?
Have I got this right and would this be the most efficient way to do it?
Thanks for your help, dennis

4040, some argue that you can do close to what you suggest, but I wouldn't. IMO, we need to tune the harness to get it right.

If the harness was say 12' feet long on each side, and somehow there was enough loss in the feeder system, then the match (SWR) might look good, but the co-phase setup would not be very effective IMO. I've seen and heard guys make similar claims about such setups though.

Could it still work? I believe it would, so-so, if the match was good enough at the radio.

 

[dennis4040]

Also, if i used an MFJ or similar analyzer would I be able to tune the harness using 50 Ohm lengths from antennas to 75 Ohm harness or is there just no good way to tune it when setup that way?
thanks again, dennis

4040, some argue that you can do close to what you suggest, but I wouldn't. IMO, we need to tune the harness to get it right.
.

 

[Marconi]

Thanks for the quick response Doc and Marconi.
This is my first co-phase project so I better make sure I understand

If I stack 2 antennas and the SO-239s are 32' apart and I am using LMR-240 75 Ohm my harness would have to be:
((492x VF .84/27.205) /2)x3 = 22.787'? 45.57' end to end?
Have I got this right and would this be the most efficient way to do it?
Thanks for your help, dennis

Dennis, I think you're right. I would think 22.8' divided in half would give you the 1/4 CP harness you need for each antenna, but your harness will be too short.

So, you need to add a tuned 1/2 wave length to each end of the harness, or make the CP harness 3 x 1/4 wave on each side...to make it long enough. Then you end up with 22.8' on each side or about 45.6' end to end...just like you've note. I would add a couple of inches and then tune the harness, but it is more work. Just remember, it's hard to add short pieces of coax.

I think you will be close, but try and check the harness with an analyzer and a couple of good dummy loads that read close to the same on your analyer when measured seperately. If your loads are close and your antennas match close, then you should be good to go.

This is not a simple setup to make, so be ready to do some checking before you fully install the antenna, and then hope the match does not change for the worse on raising. Be careful.

 

[dennis4040]

Marconi,
So I would add a tuned 1/2 WL 50 Ohm line between each antenna connector and the CP harness?

I would tune each 1/2 WL and the CP harness and then double check everything together?

I have not used an analyzer yet but have a friend that is willing to loan one to me for the project.
Would I need to make a choke out of the excess?
Thanks again, dennis

Dennis, I think you're right. I would think 22.8' divided in half would give you the 1/4 CP harness you need for each antenna, but your harness will be too short.

So, you need to add a tuned 1/2 wave length to each end of the harness, or make the CP harness 3 x 1/4 wave on each side...to make it long enough. Then you end up with 22.8' on each side or about 45.6' end to end...just like you've note. I would add a couple of inches and then tune the harness, but it is more work. Just remember, it's hard to add short pieces of coax.

I think you will be close, but try and check the harness with an analyzer and a couple of good dummy loads that read close to the same on your analyer when measured seperately. If your loads are close and your antennas match close, then you should be good to go.

This is not a simple setup to make, so be ready to do some checking before you fully install the antenna, and then hope the match does not change for the worse on raising. Be careful.

 

[Marconi]

Regarding your first statement above...I wouldn't exactly say it that way. Maybe I should have left the idea of a 1/2 wave...completely out of this discussion.

First off you can't successfully use LMR240, because I think it is 50 ohm coax. From your antennas to where it connects to the regular feed line must be some odd 1/4 wave multiple(s) of 75 ohm coax on both sides of the harness starting with 1.

Remember the formula you used is for a 1/2 wavelength, but in the math we fold the wire in half to make the harness, so now we have a 1/4 wavelength. From there on to the radio you can use any length of 50 ohm line necessary to reach the meter, amp, or the radio.

Dennis, why don't you just buy a cheap CP harness for a mobile or order an 18' foot harness from Maco. Fiddle with it as best you can and test it out with two antennas of your choice. IMO it is best using a 102" ss whips. Do this before you go spending a lot of money on your stacked base antenna idea, because if you get stuck with a bad match you might not get the help you really need to learn about how and why.

After you see how that goes, then try building your own CP harness out of whatever you desire, and if you can get that to working as you wish, then consider to tackle the big job maybe.

I won't say you can't do the job, you have the numbers, but you might be biting off more than you can chew at this point.

Forget the analyzer idea if you or your buddy don't have some good informed experience, because otherwise it might only serve you as an expensive SWR meter. Just use your radio a feed line, a dummy load, inline meter, and your radio. Do some setup testing near the ground, see if you can get a CP match that looks good enough to work your radio.

Just be careful with some matching that may be way off going into your radio.

If you have some questions in the process of working with your CP setups, then let us know the specifics.

 

[Shockwave]

The Co-Phase harness you need to stack the antennas at 34 feet apart is no different than what you would use at closer spacing. All you have to do is add equal 50 ohm lines off the 75 ohm harness to reach the pair of antennas. Once you pass through the 1/4 wave of 75 ohm coax lines, you're back to 50 ohms. It doesn't matter if the antennas are plugged directly into the harness here or pass through any random equal lengths of 50 ohm coax first.

Another problem people usually face when phasing antennas together is called "Mutual Coupling". The effects of mutual coupling cause the resonant frequency of antenna to lower when combined together. For example, on the FM broadcast band, a dipole tuned to 98.1 Mhz. as a single antenna will usually drop its resonant frequency down to 97.9 Mhz when combined with second dipole that was tuned at 98.1 Mhz. by itself.

To compensate for this we simply tune the individual antennas to be resonant 200 Khz. higher than the desired frequency. The offset in frequency will probably be a little different on 11 meters with two yagi's but the effect and direction of frequency shift will be the same. For maximum performance it's important that we correct this problem by tuning the antennas and not mistake this as needing to compensate by adjusting the length of the phasing harness.

 

[W5LZ]

Which is handier, 35 feet of 75 ohm coax, or, about 11 feet of 75 ohm coax and 35 feet of 50 ohm coax. Why not keep it simple and use all 75 ohm coax to that 'T', then whatever 50 ohm coax to reach the radio? Your choice,
- 'Doc

 

[Marconi]

The Co-Phase harness you need to stack the antennas at 34 feet apart is no different than what you would use at closer spacing. All you have to do is add equal 50 ohm lines off the 75 ohm harness to reach the pair of antennas. Once you pass through the 1/4 wave of 75 ohm coax lines, you're back to 50 ohms. It doesn't matter if the antennas are plugged directly into the harness here or pass through any random equal lengths of 50 ohm coax first.

Another problem people usually face when phasing antennas together is called "Mutual Coupling". The effects of mutual coupling cause the resonant frequency of antenna to lower when combined together. For example, on the FM broadcast band, a dipole tuned to 98.1 Mhz. as a single antenna will usually drop its resonant frequency down to 97.9 Mhz when combined with second dipole that was tuned at 98.1 Mhz. by itself.

To compensate for this we simply tune the individual antennas to be resonant 200 Khz. higher than the desired frequency. The offset in frequency will probably be a little different on 11 meters with two yagi's but the effect and direction of frequency shift will be the same. For maximum performance it's important that we correct this problem by tuning the antennas and not mistake this as needing to compensate by adjusting the length of the phasing harness.

SW may be right, but I've never seen it done that way, with both 75 ohm and 50 ohm mixed to make up the added length for the harness itself. From the harness back to the radio with 50 ohms at any reasonable length is OK however.

I would think that if what SW says is possible and you can get a good tune...then the 50 ohm line extension used to make the 30'+ gap would have to also be a tuned length 1/2 wave line, and not just any old length. Here is a YouTube video I did after talking to SW about the subject of co-phasing two antennas some time back. In this video I cophase two of my base antennas, using their feed lines and a T connector. This setup showed >2.0:1 SWR on my radio and the radio's cut-back was working, so I switched on my auto-tuner to take care of the mismatch. So, we see that SW is right, we can use a random length for our harness, and we can even use 50 ohm coax, without a phasing harness, but we won't get a workable tune.

Marconi Testing Co-Phased Base Antennas Video 1 - YouTube

I've never extended a real CP harness myself, but I know adding random feed line to any antenna setup can skew the results in many ways...if the feed point match (load) is not very close to perfect.

All one has to do to check these ideas out, is make the setups and check the results out...at the termination point on the radio end of the harness connected to the antennas, and to two accurate 50 ohm resistive dummy loads.

If the dummy loads are accurate I would think they should permit a good match with a random length 50 ohm extension to the harness.

Since the antenna loads are often not perfect, such an addition of a random length 50 ohm line to the harness, could skew the results badly, so any such setup as a CP harness needs testing.

SW, I just didn't think that Dennis was up to checking out such a big setup without first getting familiar with a mobile CP harness, but you are right, I think tuned 1/2 wave 50 ohm jumpers could be added to the harness to extend the length, so it would reach in his case.

I was suggesting he use a tuned 1/2 wave length of 75 ohm instead to make up the space, and if he did this setup needs checking out.

So Dennis, take your pick.

 

[The DB]

Once you get both sides of that harness to a 50 ohm match, and as long as you keep to half wavelength (accounting for velocity vactor) multiples of feed line beyond that point, it doesn't matter what the impedance of the feedline is. 50 ohms, 75 ohms, or even 100 ohm feedline should all work equally well...


The DB