The myth of coax length.

[Tallman]

Mmm, power factor, don't see people talking about that much when it comes to antennas, or coax for that matter...
I am curious as to what part of the antenna system you think we need to worry about the power factor?
The DB

Coaxial cable is reactive with both inductive and capacitive components and phase shifts do happen along transmission. Mostly at higher frequencies though.
Antenna tuners help cancel out these and other reactive components and seem resistive when properly tuned. The power factor thing was just bait.

 

[LeapFrog]

Coaxial cable is reactive with both inductive and capacitive components and phase shifts do happen along transmission. Mostly at higher frequencies though.
Antenna tuners help cancel out these and other reactive components and seem resistive when properly tuned. The power factor thing was just bait.

And it worked! Although I had not a clue what you meant about "power factor" I liked your edit about the measured rf energy at a certain distance, and a low swr may not always reflect radiated power! I took the bait so jokes on me.. Haha.

 

[Beetle]

The term "power factor" is generally only used when the subject is AC power generation, metering and switchgear at frequencies up to 400 Hertz. RF has its own language and behavior.

 

[cbphreaker]

This is great. So many ideas

Here is a quote from

Antenna -- Feed line Matching Simplified ByDonald Sanders W4BWS

"Impedance


The use of a half wave, or multiple thereof, coax or parallel conductor feed line connected to the antenna will allow the antenna to be raised to its operating position for measuring impedance. An RF Bridge should be used to measure the exact length of the feed line. It will be close but not exactly equal to 984VF/F (MHz) for 1 wave length or 492VF/F (MHz) for 1/2 wave length due to the velocity factor of the line. The RF Bridge will take this factor into account.

An electrically equivalent 1/2 wave feed line or multiple thereof will reflect the impedance at one end to the other end at the exact frequency of 1/2 or 1 wave length. The RF Bridge will then measure the impedance of the antenna as if it was at the antenna"

Source
http://www.hamuniverse.com/w4bwsmatching.html

W4BWS directly conflicts with


Part 2 - Countdown for a Journey
From Mythology to Reality

23) If SWR readings change significantly
when moving the bridge a few feet one way or the
other in the line, it probably indicates "antenna"
current flowing on the outsideof the coax, or else
an unreliable instrument, or both, but it is
not because the SWR is varying with line length. Some
writers insist the bridge must be placed at a half-
wave interval from the load to obtain a correct
reading.This is incorrect. All readings are invalid
if they change significantly along the line, even
though they may repeat at half-wavelength
intervals
(ref. 2, pp. 101, 106, and 132)

Source:
ANOTHER LOOK AT REFLECTIONS
By: M. Walter Maxwell, W2DU/W8HKK

Reflect_rev1.pdf

Unreliable insturment may mean a simple diode based vswr meter. It's unfortunate there is no description of the instrument specifications required.

 

[bob85]

No they don't conflict, both are true,
you don't understand the difference between impedance looking into a mismatched line and vswr at any point along that line,
two different instruments two different measurements,

you said
"This is exactly how misconceptions are started. It's ALWAYS best and prudent to verify any and all information from respected sources before committing them as fact in your head"

yes i agree,

 

[Tallman]

This is great. So many ideas

Here is a quote from

Antenna -- Feed line Matching Simplified ByDonald Sanders W4BWS

"Impedance


The use of a half wave, or multiple thereof, coax or parallel conductor feed line connected to the antenna will allow the antenna to be raised to its operating position for measuring impedance. An RF Bridge should be used to measure the exact length of the feed line. It will be close but not exactly equal to 984VF/F (MHz) for 1 wave length or 492VF/F (MHz) for 1/2 wave length due to the velocity factor of the line. The RF Bridge will take this factor into account.

An electrically equivalent 1/2 wave feed line or multiple thereof will reflect the impedance at one end to the other end at the exact frequency of 1/2 or 1 wave length. The RF Bridge will then measure the impedance of the antenna as if it was at the antenna"

Source
http://www.hamuniverse.com/w4bwsmatching.html

W4BWS directly conflicts with


Part 2 - Countdown for a Journey
From Mythology to Reality

23) If SWR readings change significantly
when moving the bridge a few feet one way or the
other in the line, it probably indicates "antenna"
current flowing on the outsideof the coax, or else
an unreliable instrument, or both, but it is
not because the SWR is varying with line length. Some
writers insist the bridge must be placed at a half-
wave interval from the load to obtain a correct
reading.This is incorrect. All readings are invalid
if they change significantly along the line, even
though they may repeat at half-wavelength
intervals
(ref. 2, pp. 101, 106, and 132)

Source:
ANOTHER LOOK AT REFLECTIONS
By: M. Walter Maxwell, W2DU/W8HKK

Reflect_rev1.pdf

Unreliable insturment may mean a simple diode based vswr meter. It's unfortunate there is no description of the instrument specifications required.

Yep there is truth in there, but the 1/2 wave must be the electrical length in coax because of the Velocity Factor of coax.(Mostly 66%) For amateur radio it is crazy to think of all of the coax segments required to operate on all of the bands we use. But it is better to attach your antenna analyzer directly with less than one foot jumper cable.

 

[The DB]

Coaxial cable is reactive with both inductive and capacitive components and phase shifts do happen along transmission. Mostly at higher frequencies though.
Antenna tuners help cancel out these and other reactive components and seem resistive when properly tuned. The power factor thing was just bait.

It is where I normally expect people to be talking about power factor, although you seem to be saying something unrelated to what people I have seen talk about power factor and coax typically claim, and that is the erroneous claim that reflected power from the antenna are voltless watt amperes.

At least through the range of frequencies that I have easy access to, power factor is not a problem. You are correct that coax is reactive, although only to a point. I can use a length of coax to simulate any inductance or capacitance value I need, and this is how it is used when it comes to stub tuning. However, when used as a transmission line, said capacitance and inductance are balanced to provide 50 ohms of impedance, or whatever the value of the feed line happens to be.

Your statement that this happens at higher frequencies does make sense. As an example, with parallel feed line such as ladder line, many sources say the the distance between the two elements should be no more than a small fraction of a wavelength apart. The same would be true using a coax, although the frequencies that such a problem will happen with coax, it would have been much more efficient to switch to something like a wave guide long before you get to that point.

Did you know that power from the power company is capacitive? The reason is many if not most things that are powered off of the power grid are inductive in nature. Think about it, power supplies, electric motors, ect. Electric companies need to take this into account, and compensate for the prevalence of inductance with capacitance.


The DB

 

[LeapFrog]

Yep there is truth in there, but the 1/2 wave must be the electrical length in coax because of the Velocity Factor of coax.(Mostly 66%) For amateur radio it is crazy to think of all of the coax segments required to operate on all of the bands we use. But it is better to attach your antenna analyzer directly with less than one foot jumper cable.

So multiples of 1/2 wave electrical lengths (in coax) will allow me to analyze the antenna from inside the shack, as if I was at the antenna!?

Wow I'm going to need some coffee after this thread!

-Leap

 

[Tallman]

Theoretically yes.

 

[Tallman]

Did you know that power from the power company is capacitive? The reason is many if not most things that are powered off of the power grid are inductive in nature. Think about it, power supplies, electric motors, ect. Electric companies need to take this into account, and compensate for the prevalence of inductance with capacitance.

That can't be right. Voltage and Current should be in phase from the power company.
The frequencies I'm talking about LMR-400 is the only coax used. Circuit boards are double sided with only one side being used for the traces and component placement. The other side is strictly used as a ground plane to cancel or reduce inductance of long traces. Self destructive oscillation is always a hazard.

 

[The DB]

The center conductor of the coax and the shield do not change polarity for the cycle.

Actually, they do. The current flowing on the center conductor induces a field around said conductor. That field interacts with the inside of the shield, and thus induces current on said conductor. Also, just because one conductor is connected to ground does not mean that the only potential on said conductor is ground. It is possible to have an AC signal and a DC signal present on both wires of a coax at the same time, and in fact devices are made that do just that.

If you look at a 120 volt ac power plug you have three colors of wires. Green, White, and Black. The only wire that supplies the power is the Black wire. It and only it oscillates between positive and negative voltage. The white is the Neutral wire and is at ZERO AC voltage potential and is the return line for the power.
The Green wire is EARTH GROUND and is at ZERO Volts. The safety chassis ground cable which is in no way a suitable Station ground.

How is the power as it enters your house?. Three wires again, but at that point you have a neutral, and two wires carrying AC voltage. The voltage and current on those wires are 180 degrees out of phase. You are correct that there is no voltage induced on the neutral wire in your house. If you tried to do that at 27 MHz, or even far blow that, how far do you think the wiring in your house would get before it tried to radiate the signal on said wires. Bonus points if you can tell me why that isn't a problem with house power.

If radio transmitter outputs truly changed in that fashion you would need Dual Gamma matches on every beam antenna.

No, you wouldn't. All you would need to do is match the impedance to the feed point. That being said, I have seen such a setup, what was it called, something like a T match?


The DB

 

[The DB]

That can't be right. Voltage and Current should be in phase from the power company.

You might be right, I am trying to remember where I heard that. I'll have to look into that more I guess.


The DB

 

[Tallman]

Actually, they do. The current flowing on the center conductor induces a field around said conductor.
True, but the braid does not go positive. In coax only the center is conducting the power. The voltage is induced in the other element.

You are correct that there is no voltage induced on the neutral wire in your house.
I stand by what I said, the power, voltage and current are in phase with each other, even the other phase that you said is 180 degrees out of phase with each other. Voltage and current are in phase.

No, you wouldn't. All you would need to do is match the impedance to the feed point. That being said, I have seen such a setup, what was it called, something like a T match?
There is a Wilkerson circuit that allows a 50 Ohm transmitter to drive two 50 Ohm antennas with only a 50 ohm single input

The DB

 

[Tallman]

You might be right, I am trying to remember where I heard that. I'll have to look into that more I guess.
The DB

Three Phase power is 120 degrees in relation to each other.
Each phase voltage and current is in phase with it self.
Phase A:120 VAC in phase with self
Phase B:120 VAC 120 degrees behind Phase A
Phase C: 120 VAC 240 degrees behind Phase A

 

[222DBFL]

Bias T's are used to inject DC voltage into a coax line to power a mast head amp at the top of a tower. This is something I had to deal with myself. I won't get involved too mhch in this thread as it's got lots of great info and also some myths that needed to be debunked! I am with Tallman and all the others that believe the best way to tune an antenna with an a analyzer is by using a quality piece of coax 12 inches or less in length or connect directly to the feedpoint to tune the antenna! Yes you can use an electrically tuned 1/2 wave coax or a multiple of one, but connecting directly to the feedpoint removes any and all possibly issues other then having a bad analyzer! JMHO's.
Every antenna install is different and needs to be approached as such!! Again, JMO. To all have a great day and wish me luck as it's going to get pretty nasty here around Daytona Beach, FL for the next few days. We are already under a hurricane watch and I am sure that will change to a warning by today as I am not more than 30 miles from the Brevard County line where I live. Just south of Daytona Beach, a place called Port Orange, FL. Done all I can do for the moment to my antenna to keep it upright. If I get home early enough and it's not pouring down rain I may try to lower it!! But for now it's where it is at 27ft to the base with a mix of 15+ ropes and ratchet straps to keep it in place. Only thing I can't brace is the cone and the center radiator!! My fingers are crossed and God willing, my antenna will make it!!! Cause I don't have the money to replace it at the moment!!
Again, to all, have a great day!! Sean.