Co Phase project

[StrykerLiker]

So I'm a fan of single antenna. But all my co workers are stuck on running dual antenna.

I've decided to set up a cophase system on my truck so that I can help them with theirs.

My first hiccup is, I run an amp, so I have the dual rg59 between antenna and amp, and rg58 between radio and amp.

Does that cause any kind of mismatch? If everything is set up properly, does the rest of the system just see it (the dual rg59) as a 50 ohm line.

And if that's acceptable, does that mean it would be ok to run say, 9 feet of rg58 to a male male to dual 18 foot rg59?

Some of my friends drive pick ups and need a way to extend the coax to make it to the back of their radio...

So far I am not at all impressed with the dual set up, alot of hassle for minimal benefit, if any. But I can't help my friends if I don't understand what I'm doing

[The DB]

Essentially what the cophaseing harness does is convert the antenna system's impedance to near 50 ohms. Once this happens, as long as everything before said cophasing harness is 50 ohms you are good.

It doesn't matter that you have an amplifier in the system, or how long the coax is before the amplifier.

So far I am not at all impressed with the dual set up, alot of hassle for minimal benefit, if any. But I can't help my friends if I don't understand what I'm doing

The gains you get from cophasing on a vehicle at CB frequencies are insignificant, unless there is some other factor involved, the cophasing in and if itself will not be noticeable over a single antenna in the middle of the vehicle. I wouldn't worry about it unless I absolutely had to.

It is possible to get pretty significant gains in a cophased setup, but the antennas would need to be more like 18 to 22 feet apart...


The DB

[MDYoungblood]

Not to go into the math, the two runs of RG59 makes the Ohmage at the radio end connector 50ohms, anything on that side of the connector has to be 50ohms so your right on using RG58 or better on that side. When adjusting co-phased antennas both antennas need to be adjusted equally so make sure everything is the same at the start. Most times when I've made adjustments I've found the the antennas are short so most need added length, another thing is open coil antennas react differently to co-phase compared to fiberglass antennas. Happy experimenting.

3's

Greg

[StrykerLiker]

Thanks for the replies.

On my mfj 225, my cophase set up shows x=2 . SWR= 1.12 . R=30

On my mfj 259b the R=51

So I'm gonna assume the 225 analyzer doesn't represent the R properly in this cophase set up.


Pretty sure I'm just gonna drill the roof of my Peterbilt.





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[The DB]

Thanks for the replies.

On my mfj 225, my cophase set up shows x=2 . SWR= 1.12 . R=30

On my mfj 259b the R=51

So I'm gonna assume the 225 analyzer doesn't represent the R properly in this cophase set up.


Pretty sure I'm just gonna drill the roof of my Peterbilt.

R=30 X=2 would be closer to an SWR of 1.6. Something is off in that meter. Your SWR reading is much closer to an R of 51.


The DB

[StrykerLiker]

Thanks for the replies.

On my mfj 225, my cophase set up shows x=2 . SWR= 1.12 . R=30

On my mfj 259b the R=51

So I'm gonna assume the 225 analyzer doesn't represent the R properly in this cophase set up.


Pretty sure I'm just gonna drill the roof of my Peterbilt.

R=30 X=2 would be closer to an SWR of 1.6. Something is off in that meter. Your SWR reading is much closer to an R of 51.


The DB

I think your right. I've been very dissatisfied with this 225 meter.

I'm thinking about going with a RigExpert AA30.



The other issue I am having with this cophase setup, is that I seem to be having much more variance in SWR. It seems to be reactance from reflection or possibly RF interference.

If I go out to a good location, I can always get a very low mfj reading. But if I check it in a truck stop or parking lot, the SWR can go from 1.1 to as high as 2.3

I have always had variance in antenna tuning, but more like 1.1 to as high as 1.5, dependant on location.


Is a cophase setup more sensitive to reflection and interference? Is it the radiation pattern that causes this or something else?

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[MDYoungblood]

Check the 225 analyzer to a dummy load and take into account for the coax and see what you get, could need to be recalibrated.

3's

Greg

[The DB]

The other issue I am having with this cophase setup, is that I seem to be having much more variance in SWR. It seems to be reactance from reflection or possibly RF interference.

If I go out to a good location, I can always get a very low mfj reading. But if I check it in a truck stop or parking lot, the SWR can go from 1.1 to as high as 2.3

I have always had variance in antenna tuning, but more like 1.1 to as high as 1.5, dependant on location.


Is a cophase setup more sensitive to reflection and interference? Is it the radiation pattern that causes this or something else?

A cophased setup having noticeably higher SWR swings than a single antenna is normal. With one antenna you have the environment affecting one antenna. With a cophased setup you have the environment affecting both antennas, and not always to the same degree. A vehicle passing on one side of your truck, for instance, will be much closer to one of the antennas than the other, thus knocking the antennas out balance. This balance is important to such a system, and it shows in the tuning changes as your vehicle moves around.

To compound the situation, the cophasing harness, in and of itself, will narrow the useable bandwidth a cophased antenna system has over a single antenna. They work very well when the antennas are perfectly tuned, however, the variances in tuning you typically see as you move around in a mobile environment are "amplified" by these devices. There really is no way around this. Using wider bandwidth antenna will help some, but in the end, the harness still ends up doing the same thing.

Check the 225 analyzer to a dummy load and take into account for the coax and see what you get, could need to be recalibrated.

MFJ devices do occasionally need to be recalibrated. MFJ will do it, I'm not sure if there is a fee involved. I have also seen instructions online on how to do this if you want to dig them up...


The DB

[StrykerLiker]

MFJ devices do occasionally need to be recalibrated. MFJ will do it, I'm not sure if there is a fee involved. I have also seen instructions online on how to do this if you want to dig them up...


The DB

I tried to look up the calibration for the MFJ 225, it says it has an auto calibration. But couldn't find the instructions to engage the auto calibration.

I am pretty over the 225 anyways, the batteries go under the led screen. So you basically have to disassemble half the unit to get the batteries in. The rechargeable batteries have corroded inside it, so I took it apart, cleaned it up and left the batteries out. It has USB power but that makes it less portable.

The frequency knob is jumpy at best. Some times you cycle up, and the freq goes down, sometimes it does nothing. It seems more vulnerable to interference than the 259b. Etc. Etc.


Cophase project complete. Dual antennas coming off and going into the collection. Now it's time to man up, put a hole in the roof of my sleeper and do it right.