Coax For Cophase Setup

[W5LZ]

And if you really wanna have some fun...

Start that impedance matching from the antenna end with a typical 1/4 wave antenna input impedance of something on the order of 35 ohms (IF you're lucky). Do all that 'conjugate' matching with that 75 ohm coax, parallel it with an identical antenna/coax from the other side of the vehicle. Tell me what the impedance is at that junction point, the 'T'.
Now go back and do the same thing again with an input impedance at that antenna of closer to 30 ohms. One more time with an antenna input impedance of about 20 ohms. And if you really want to cover all the bases, try it with 40 ohms antenna input impedance. That pretty well covers the possible range for mobile antenna input impedances. Not very close to 50 ohms that the radio wants to see is it?
And if you want to see what would make the most difference, since the input impedances of the typical commercial mobile antenna just isn't gonna get real close to 50 ohms anyway... try all of the above with 100 ohm characteristic impedance coax. That '100 ohms coax' was actually closer to 93 ohms, but who cares.
Wonder why 'they' don't make 'dualy' antennas with an impedance matching device at the input to make all this @#$ easier? Make that impedance matching device just a little bit adjustable and it ought'a work for just about anybody, wouldn't you think??

OH! And to make it even more fun! Adjusting the length of an antenna to change it's input impedance is NOT a good way of doing it. It introduces reactance into that SWR which is always a terrible idea.
I just love this $#^+, don't you??
- 'Doc

 

[mackmobile43]

hahahahahahahahahaha

Not very helpful

 

[Shockwave]

DJ, let me save you the time here. Don't bother trying to do this with under 70 inches of separation, a kilowatt of power, and no ground plane under the antenna. Complete waste of time in terms of performance! The antennas are so close their patterns will overlap rather then creating two individual patterns that combine. Mirror mounts are the worst for any type of high power because it places the antenna in a location with no ground plane underneath. You'll go nuts trying to get the VSWR right and when you do, it will be nothing but RF problems because of the poor ground. You'll likely get RF burns off the mic, radio, or amp.

It has already been mentioned here but I'll say it again. Put one antenna in the center of the cab and it will beat anything you can do on the mirrors. I know you can't put the same length antenna in this location but even a shorter one will beat what you're trying to do. The difference is one you can notice. Even a Wilson 1000 mag mount is a better choice here. Lose the magnet and drill a hole if you want the best ground.

Furthermore, the co-phase harness discussion here has almost completely overlooked the velocity factor of the coax used. You do not use two 75 ohm 1/4 wave lines. The length of the coax needs to be an ELECTRICAL 1/4 wave not a PHYSICAL 1/4 wave. The velocity factor of coax will make the length shorter then you expect and must be calculated using the given VF of the coax used.

 

[jazzsinger]

Furthermore, the co-phase harness discussion here has almost completely overlooked the velocity factor of the coax used. You do not use two 75 ohm 1/4 wave lines. The length of the coax needs to be an ELECTRICAL 1/4 wave not a PHYSICAL 1/4 wave. The velocity factor of coax will make the length shorter then you expect and must be calculated using the given VF of the coax used.

I already gave him all the lengths necessary with the vf accounted for in both the q sections and halfwave sections for both foam and solid dielectric cables.

 

[Marconi]

DJ, let me save you the time here. Don't bother trying to do this with under 70 inches of separation, a kilowatt of power, and no ground plane under the antenna. Complete waste of time in terms of performance! The antennas are so close their patterns will overlap rather then creating two individual patterns that combine. Mirror mounts are the worst for any type of high power because it places the antenna in a location with no ground plane underneath. You'll go nuts trying to get the VSWR right and when you do, it will be nothing but RF problems because of the poor ground. You'll likely get RF burns off the mic, radio, or amp.

It has already been mentioned here but I'll say it again. Put one antenna in the center of the cab and it will beat anything you can do on the mirrors. I know you can't put the same length antenna in this location but even a shorter one will beat what you're trying to do. The difference is one you can notice. Even a Wilson 1000 mag mount is a better choice here. Lose the magnet and drill a hole if you want the best ground.

Furthermore, the co-phase harness discussion here has almost completely overlooked the velocity factor of the coax used. You do not use two 75 ohm 1/4 wave lines. The length of the coax needs to be an ELECTRICAL 1/4 wave not a PHYSICAL 1/4 wave. The velocity factor of coax will make the length shorter then you expect and must be calculated using the given VF of the coax used.

I agree with Shockwave and this is why I recommend trimming the co-phase harness to tune in order to get the best match possible. If you know the feed point impedance of both antennas in parallel and the true VF of the two sections of coax being used, then you can get really close using the math. But getting the best tune possible while tuning side-steps the need to know these factors for sure.

You might be surprised however, to find that the feed point impedance at the end of a well grounded 1/4 wave element is near 15-25 ohms or less at resonance. If you don't consider resonance in your thinking, then you're making a mistake. True resonance is important to consider unless you like to operate your antenna system with high levels of reactance---and like our friend 'Doc tells us, "REACTANCE IN YOU ANTENNA'S FEED POINT DOES NOT RADIATE."

So, if you are seeing a 1.1:1 SWR match with a 1/4 system, you are likely seeing reactance (loss) at work big time and then you should consider ('Doc's) advice about reactance. You would likely make a better signal if your system showed you some SWR like 1.3 - 1:7 at the feed point. This happens with parallel feed lines in a cophase harness because you are working with a transformer and it is part of the antenna. If you have it right though, it will show a little SWR when you get the reactance tuned out of such a system and in my mind that is a good thing.

If you tune the harness, as in this case, you will compromise adjust for VF using the tuning process. To some degree you will also be affecting the resonance a bit, because the feed lines will effectively add a little length to the antennas. With a 102" whip on most vehicles the better the ground the lower in frequency you will likely see and the better the ground with 102" elements the lower the reactance (X=0) should be in the CB band, and this all tends to show a little lower in frequency than a 102" element should show.

In my experience we find a resonant match closer to 1.3 - 1.5 SWR in a co-phase setup---makes for a better signal. This is because you will not typically be able to reach a purely resistive load of R=50, X=0 ohms without adding losses in adding some sort of matching device near the feed point of a 1/4 wave element. This is true because we are dealing with an element located near Earth ground that will naturally show approximately 38 ohms of feed point resistance at resonance.

IMO, the closer your match is to 1.1:1 the more loss you will have in such a system, because you will probably never attain a 50 ohm match at the junction of the two short coax feed lines from closely space parallel antennas.

I will also add that you will find a properly tuned and setup co-phase system will show a very narrow bandwidth (<0.6 mhz) and that is a sure sign of such working correctly. If you try to operate very far from resonance you will see a sharp rise in SWR and considerable attentuation of both TX/RX signals. This is also what happens if you're system is not setup correctly and instead showing a broad bandwidth of (>1.0 mhz) or more.

A co-phase setup is a tuned device and is going to be part of the radiating antenna and it will be shorter than you think.

 

[jessejamesdallas]

You guys may as well save your breath trying to talk him out of this...Seeing how he has already had it done...And why he was asking all these questions on "what kind of coax to use, and lengths" is beyond me, since he had a shop do "ALL" the work anyway!

 

[W5LZ]

I guess it's in the eye of the beholder, but I'm glad it's his eye and not mine.
- 'Doc

 

[mackmobile43]

You guys may as well save your breath trying to talk him out of this...Seeing how he has already had it done...And why he was asking all these questions on "what kind of coax to use, and lengths" is beyond me, since he had a shop do "ALL" the work anyway!

As I stated before, it's a million wonders he does not get beat up and stop lights driving around with that monstrosity.

 

[Marconi]

Hey Mac, is that what you would try do to DJ if you met him on the street? You're just lucky, you're dreaming boy.

 

[bigred222]

suburbans run with big coily F$&^rs on the roof all the time which he would be better off running but if he really wants to run two antennas that bad whatever dude. My advice. listen to shockwave and marconi. i know shockwave knows what hes talking about and marconi does too.

PUT ONE IN THE CENTER OF THE ROOF AND BE DONE WITH IT