Co-phasing?

[Shockwave]

"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."
Sorry, not hardly. There are conditions that were left out of that.
- 'Doc

Can you elaborate on what "conditions" you are talking about? The vague answer alone that contradicts what I've said is not helpful. What would be helpful is to explain the "why" behind your statement so we can determine if I've overlooked something or if you've misunderstood something.

The only valid ground I can find to support your idea is if the antennas were not properly tuned to match the 50 ohm output of the harness in the first place. In the commercial broadcast field we usually work with a power divider that contains the matching cables within a rigid metal tube that has connectors on both ends.

All of the connectors on these power dividers are 50 ohm ports just like on your co-phase harness. They can be used with a wide assortment of antennas that would have different spacing and tower standoff distances. We make cables to reach the individual antennas just like I said, equal 50 ohm lines in all cases.

The exception is when we must employ beam tilt or null fill. In these cases we will lag the upper bays by slightly increasing their cable lengths. The phasing techniques used on VHF also work on HF, just the phasing sections are longer.

 

[Marconi]

Once you get to the end of the pair of 75 ohm cables, the impedance match is done. At this point the only important thing is to keep the phase matched. The only requirement for this is that the 50 ohm extension cables must be equal length. The actual length matters none as long as they are equal and in phase.

Another option has already been pointed out and that is to extend the 75 ohm cable to 3/4 wavelength. Eliminating two connections in the harness. In this case the actual length of cable used is very important. This is because if you want the extended 75 ohm cable to reflect the 50 ohm impedance, you must add an exact 1/2 wavelength of 75 ohm cable to the existing harness.

The tradeoff here is that to extend the harness with 75 ohm cable requires you get two more lengths of cable precisely tuned at an electrical 1/2 wavelength. Errors here will reduce performance just as easily as adding two more splices in the cable to extend with 50 ohm cable of equal but random lengths.

If you think about this, Shockwave's comments make perfect sense, how to make the CP harness longer.

 

[W5LZ]

"Can you elaborate on what "conditions" you are talking about?"
I can, but I'm not going to. Do some looking/thinking about it and then you tell me.
- 'Doc

What's the difference between using an electrical 1/2 wave of 50 ohm cable and just findind an electrical 3/4 wane length of 75 ohm stuff? ... None.

 

[Marconi]

"Can you elaborate on what "conditions" you are talking about?"
I can, but I'm not going to. Do some looking/thinking about it and then you tell me.
- 'Doc

What's the difference between using an electrical 1/2 wave of 50 ohm cable and just findind an electrical 3/4 wane length of 75 ohm stuff? ... None.

I don't think there is a big difference if at all, but the 3/4 wave is how I've always seen it done.

 

[Shockwave]

"Can you elaborate on what "conditions" you are talking about?"
I can, but I'm not going to. Do some looking/thinking about it and then you tell me.
- 'Doc

What's the difference between using an electrical 1/2 wave of 50 ohm cable and just findind an electrical 3/4 wane length of 75 ohm stuff? ... None.

The difference was already explained and I agree it is very little. The comment we are in disagreement over and the one I asked you to elaborate on is the one quoted below. I understand that you've declined to explain it beyond just making the claim. I couldn't defend that statement either which is why I wondered how you would handle the question.

"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."
Sorry, not hardly. There are conditions that were left out of that.
- 'Doc

 

[W5LZ]

I'm not going to try to explain it adequately, it's too much trouble. Using whatever nethod you want, combine the impedance of a 1/4 wave of 75 ohm coax with the input impedance of an antenna, what do you get? It isn't 50 ohms or even close. Now combine that resulting impedance with that of a 1/2 wave of 50 ohm coax and what do you get? It isn't 50 ohms, or even close. It isn't that the idea isn't going in the 'right' direction, it's that it's only 'part way' there, it's incomplete.
- 'Doc

 

[Shockwave]

I'm not going to try to explain it adequately, it's too much trouble. Using whatever nethod you want, combine the impedance of a 1/4 wave of 75 ohm coax with the input impedance of an antenna, what do you get? It isn't 50 ohms or even close. Now combine that resulting impedance with that of a 1/2 wave of 50 ohm coax and what do you get? It isn't 50 ohms, or even close. It isn't that the idea isn't going in the 'right' direction, it's that it's only 'part way' there, it's incomplete.
- 'Doc

You fail to understand that at the ends of the 1/4 wavelength of 75 ohm cable, the impedance has already been transformed back to 50 ohms. Any length of 50 ohm cable after that point will maintain the same impedance so long as the load is also 50 ohms. This is basic stuff Doc. Please look it up before you confuse others.

 

[W5LZ]

No, it hasn't been transformed to 50 ohms. It been transformed to something near 100 ohms. It only gets to 50 ohms when that one 'side' is paralleled with an identical 'other' side of 100 ohms yielding 50 ohms (or close, if you're lucky).
- 'Doc

 

[Shockwave]

No, it hasn't been transformed to 50 ohms. It been transformed to something near 100 ohms. It only gets to 50 ohms when that one 'side' is paralleled with an identical 'other' side of 100 ohms yielding 50 ohms (or close, if you're lucky).
- 'Doc

You are thinking about the wrong end of the wire where they to join together at 100 ohms. I'm talking about the other two ends being transformed to 50 ohms. if you doubt this connect two 50 ohm loads and prove yourself wrong when you see a perfect match.

 

[W5LZ]

You've managed to totally loose me on that one. What other two ends?
- 'Doc

 

[Shockwave]

What I'm trying to say is that the antenna ends of the phasing harness operate at 50 ohms. It's only once they pass through the 1/4 wavelength of cable that each leg is transformed to 100 ohms. At this point they are paralleled so that the input and both outputs can match the standard 50 ohm impedance.

To demonstrate where we disagree, we can take something you already understand and work the problem backwards. We agree you can extend the common co-phase harness by simply adding another 1/2 wave of 75 ohm cable to each leg. If you know the reason why the 75 ohm extension would have to be a multiple of a 1/2 wavelength, it would be clear any two equal lengths of 50 ohm cable would work.

The reason you must use a 1/2 wavelength multiple of 75 ohm cable here is because we need that part of the cable to deliver the antennas 50 ohm impedance to the 1/4 wave matching section where it is stepped up to 100 ohms and paralleled with another 100 ohms to get back to 50 on the input side.

The key here is that you recognize the 1/2 wavelength extension of 75 ohm cable is simply reflecting the antennas impedance back to the 1/4 wave matching section. That being the case, if we were to switch to 50 ohm cable here, the actual length would not have an impact on the impedance being delivered to the 1/4 wave matching sections. Your response to this was "Sorry, not hardly". If random but equal extension lengths of 50 ohm cable shifted the impedance, that just confirms the antennas were not tuned correctly to begin with.

 

[W5LZ]

And the antennas in question are typical 1/4 wave mobile antennas?
- 'Doc

 

[Shockwave]

I think I see where this is going.....Perhaps those may have been the original antennas in question 2.5 years ago but this month on page 3 Dennis moved us onto questions like "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?" Where you quickly replied with "Quick-n-dirty answer is no you can't."

The reality is he certainly can do exactly what he suggests with good results. Dennis goes on to say he's stacking two antennas 32 feet apart so I think it's safe to assume we are talking about base antennas that will be tuned much closer to 50 ohms than the 36 ohms you might have been thinking with a 1/4 wave mobile. Even if the antennas were 36 ohms, both methods of extending the harness would produce a mismatch. You're just changing the location in the harness where the mismatch occurs.

 

[W5LZ]

Combine the impedance of the antenna's input with the 75 ohm 1/4 wave section. What do you get? Use the formula for conjugate matching. Now, combine that impedance with the effect of a 1/2 wave section of coax, what do you get? Assuming that there are going to be two identical 'sides' to this, what's half of that impedance you found earlier? Half because of paralleling two identical impedances. Now, express that impedance in 'R'=/-'J'. What have you got?
- 'Doc

 

[Shockwave]

Combine the impedance of the antenna's input with the 75 ohm 1/4 wave section. What do you get? Use the formula for conjugate matching. Now, combine that impedance with the effect of a 1/2 wave section of coax, what do you get? Assuming that there are going to be two identical 'sides' to this, what's half of that impedance you found earlier? Half because of paralleling two identical impedances. Now, express that impedance in 'R'=/-'J'. What have you got?
- 'Doc

That's a bit funny Doc. Are you trying to stump me or help others learn here? You must know the most important factor in answering your question is missing. That would be the matching characteristics of the actual antennas being combined.

Let's put the shoe on the other foot and let me ask how you assumed it would be inductively reactive by asking me to " express that impedance in 'R'=/-'J'." The "-J" designates inductive reactance. Obviously we would like to see R = 50 and J = 0 but the perfect conjugate match is rarely achieved or required.

How about putting less effort into confusing people and accept the fact that both methods of extending a co-phase harness function properly? What one you use depends on your situation. If you're making your own, just make the 75 ohm legs longer. If you're extending a pre made one, you might as well just use 50 ohm cable since you'll need barrel connectors anyhow.