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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 03, 2019 3:48 pm 
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Location: Livermore, CA
Five FT-240-43 taped together then as many loops of coax as you can get will work better then a boat load of snap on beads in my experience. Also had good luck with RG-58 c/u rapped around a 4" ABS pipe need to use c/u coax because the center conductor will migrate through foam dialectic when heated.


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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 03, 2019 4:36 pm 
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I have a possible alternative that should not only be light weight but would not have the heating losses at the apex of the multi-dipole.

A balanced feeder. The requirement is to know what sort of impedance, even an educated guess at to what the multi-dipole is doing at the desired transmit frequencies. A single dipole is 72 that can be achieved with #14 strand, 600v insulated, choose a UV proof insulation, a pair, twist 1 turn/inch or so. All bets off if over 100 watts. At the terminal end connect to balun in the eave then the coax to the shack directly ground the coax at the balun conversion.

If the estimate of multi-dipole impedance falls into a commercial product such as 300/600/1200 ladder or open wire can be used. If it is substantially off the line can be home made. Plastic UV proof spreaders are very light. Regardless, twist the pair1/2 turn /ft. or so, that distributes wind effect to reduce the cycling whipping action.

Yeppers, some homework required.

Do take in account the materials and methods, that will contribute to the antenna remaining aloft for many years.

There will be, as always, losses off resonance, they will occur as heating in the balun. Further, off resonance will bring into play radiation from the balanced feed-line. This component be have a positive or negative effect on the pattern, depending where one desires in the world to work and propagation... There should be no RF in the shack.

If the balance line is brought into the shack and at the point of entry a balanced to unbalanced tuner used. This could improve the off resonance radiation strength an allow manual tweaking. The trade-off is cost, hassle of tuning and a homemade spark gap/lightening arrestor outdoors.

YMMV

Chas

WA1JFD

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 03, 2019 4:44 pm 
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Thanks guys and Chas, you now have my head hurting. I am running up to 600 watts and might have to go to a wire coat hanger with spark plug on the end. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Sun 09, 2019 6:43 pm 
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Sorry but operating off resonance does not cause radiation on the feedline! Where do these ideas come from.
Only unbalanced currents on the feedline cause radiation. Operating off resonance causes standing waves to occur, resulting in high SWR, but is not a cause of radiation.

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 10, 2019 5:46 am 
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Greetings to the Forum:

I must respectfully disagree with Phil.

First, a couple of facts: Most antennas (dipoles, yagis, etc.) are BALANCED antennas. They require a BALANCED feed to operate properly. Coaxial cable is by definition an UNBALANCED feedline. Therefore, to properly match the antenna, if feeding a 75 ohm antenna with 75 ohm coaxial cable, a 1:1 balun to convert from the unbalanced feed line to the balanced driven element is required. This can take the form of a transformer or a choke on the shield; they are functionally equivalent.

Let us consider the case where the antenna is in resonance and therefore presents a resistive load to the end of the feed line. If you are using 75 ohm coax and you put a 75 ohm resistor across the end of it, virtually all the energy sent up the feedline (baring losses) ends up in the resistor and is dissipated as heat. For a resonant antenna, it is equivalent to the resistor and therefore all of the energy goes into EM radiation (neglecting losses) and therefore there is none to be reflected.

One more fact we need: When using coaxial cable, all of the RF current in the cable is on the INSIDE of the shield and the OUTSIDE of the center conductor. This is the nature of and the advantage of, unbalanced feed line. Because there is no current on the outside of the shield, one can run coax over obstacles, including metal roofs without difficulty. This is in contrast to balanced line, which must be stood off from any nearby objects, especially metallic ones if it is to retain its design characteristics.

What happens when the antenna is non-resonant, i.e. some energy is reflected from the antenna? That changes everything. Due to skin effect, RF flows on the surface of a conductor. When properly employing an unbalanced line (i.e. coaxial transmission line) the energy is transported on the inside surface of the shield conductor. But when energy is reflected from an antenna, it ends up on the OUTSIDE of the shield... skin effect, path of least resistance.

If a 1:1 transformer balun is employed, the reflected energy is sent back into the transmission line the same way it was at the transmitter and appears on the INSIDE of the shield and the OUTSIDE of the center conductor. No radiation from the shield results. In order to confine the reflected energy to the inside surface of the shield, an alternative to a 1:1 transformer balun is to apply ferrite beads or wrap the feed line in a coil to provide inductance to the outside shield surface path. This forces the reflected energy into the normal transmission mode for unbalanced line and removes current from the outside of the coax shield.

I used to run directly fed dipoles with RG-58 back when I first got my ham ticket. I never seemed to be able to get one to resonate properly and of course I was too poor to buy ferrites or baluns and didn't know about the wind the feed line into a coil trick. At this time, I was running a Gonset G-77 transmitter. Look it up on the RigPix web site if you are not familiar with it. It was intended for mobile operation when chrome was the fashion for cars and it had polished aluminum knobs and a chrome front panel.... all metal. I used to routinely get RF burns off the front of that transmitter because of the reflected RF traveling back down the outside of the coaxial cable shield and ending up on the outside of the rig. So I know from both theory and practice that RF does indeed end up on the outside of the coax shield when the antenna is mis-matched (non-resonant) and in such cases the feed line (and in my case, even the transmitter cabinet) becomes a part of the antenna system and radiates. Usually, this is undesirable, as a proper antenna almost always behaves better than radiating feedline (except in antenna designs where a portion of the feed line is intentionally part of the radiating element).

Therefore, a ham using coaxial feed line has three choices:
1. Use a balun or choke to isolate the shield.
2. Operate only on the narrow range of frequencies where the antenna presents an exact match to the feedline.
3. Live with shield radiation and its consequences.

Regards,

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 10, 2019 11:50 am 
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Interesting Jim, thanks, Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Mon 10, 2019 3:05 pm 
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Thanks Jim,

That is the most informative explanation of transmitting antennas and the relationship to their feed-lines I have ever read.

It should be repeated over and over when like questions come up about transmission lines!

Certainly wish I knew that when I was a Novice struggling to get out with my Knight T-50. Fortunately the 807 and the Pi network were robust enough to handle the miss-match :shock:

Thanks Again!

Chas

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Tue 11, 2019 1:40 am 
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Except that it's wrong wrong wrong.

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Tue 11, 2019 4:26 pm 
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Let me explain. No where have I ever read that standing waves cause common mode currents on coax. Remember that before these magic ferites came along we connected coax directly to our dipoles and almost no one ever had rf in the shack. A properly functioning feedline will be invisible to the antenna. OCF dipoles are unbalanced and need to be choked. Other causes of unbalance are feedlines not coming directly away from the antenna, or metallic objeccts near to non coaxial lines.

Also, coax is not unbalanced. Antenna current(dipole) and transmitter currents are both balanced. Coax does not unbalance them. Also remember that equal and opposite currents on feedlines prevents radiation from occurring, That's why parallel lines are so effective. They do not radiate unless there is an external unbalance somewhere.

So. Where does it read that standing waves cause imbalance or radiation?

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Tue 11, 2019 4:47 pm 
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Why is 6 or more turns of the coax at the antenna feed point called a balun or chock/balun. I only see it as a chock. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Wed 12, 2019 2:15 am 
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Greetings to the Forum:

I will try one more time and then I give up.

Please refer to the drawing in the pdf file below for the following discussion.

Attachment:
Balanced and Unbalanced Feed Lines.pdf [5.49 KiB]
Downloaded 10 times


In the first case (drawing on the left), we see a dipole fed with balanced line (open wire line or similar). We are not concerned with the line dimensions; we assume that the line is matched to the antenna impedance at the frequency of interest. Because the system is highly symmetrical, there is little or no unbalanced current on the transmission line, regardless of VSWR.

Now, let us turn our attention to the drawing on the right. Here we see why coaxial cable is correctly defined as UNBALANCED line. Coaxial cable at RF frequencies is actually 3-conductor cable. The shield inner surface and the shield outer surface are not the same conductor due to skin effect. Therefore, there is a second piece of wire connected to one side of the antenna. This second piece of wire unbalances the antenna and results in outer shield current.

Now, there is ALWAYS some outer shield current. For one thing, any piece of wire in the near field of a transmitting antenna will have a voltage induced in it because of the radiation of the antenna itself. Even if the feedline is led straight away from the antenna at right angles, this effect still exists; in some cases (such as inverted V's, it is not even approachable in practice. Therefore there is always some induced shield current from this cause alone.

However, in addition to this current, we also have a current flowing due to the inherent imbalance in the coaxial line. It is this second current that we attempt to suppress, either with a 1:1 balun or by choking the outer shield path which is functionally equivalent. It is also possible to place a 1/4 wave sleeve over the feedline and connect it to the outside of the coax shield which will make the outside of the shield where it connects to the antenna look like an open circuit. This technique tends to broaden the frequency response of the antenna somewhat because as the frequency is lowered and the antenna looks capacitive, the sleeve looks inductive. When the frequency is above resonance, the antenna looks inductive, but the stub looks capacitive, so the usable bandwidth of the antenna is improved somewhat as well as the original goal of removing shield current. However, this technique is not usable for multi-band antennas.

Normally, the shield current (and induced voltage) is fairly small and can be ignored. However, when the antenna is not resonant and presents a reflective load to the coaxial transmission line, then some of the VOLTAGE (note VSWR stands for VOLTAGE standing wave ratio) appears on the third conductor, i.e. the coaxial shield outer surface.

Lets digress for a moment and look at our dipole in theory. Is VSWR on the dipole a good thing? You bet your sweet bippy (as Rowan and Martin used to say). If there was not a standing wave on the antenna, it would be a transmission line and would not radiate. In fact, at the end of our dipole, a voltage antinode (or voltage point) exists where the standing wave on the antenna creates a high voltage. Because the end of the wire connects to an insulator, there is no appreciable current in the antenna at this point. Where is the current? It is back near the feed point. This is why end-fed antennas are also called voltage fed antennas and center-fed antennas like our dipole example are called current-fed antennas.

So, now let us see what is happening with our coax-fed non resonant antenna. Because the reflected power divides between the coax shield inner and outer surfaces, the VSWR on the OUTSIDE of the coax increases. Remember that the voltage ratio is a function of not only the degree of mismatch but also the amount of power present. The magnitude of the voltages on a transmission line increases with increasing power. So now, because we have an increasing amount of power appearing on a single conductor, that conductor now becomes an antenna. Not a very good one, I'll grant you, but an antenna nevertheless. And one of the ways that you can determine the power in an antenna is to measure the voltage on it.

Now back to poor Jim in his early ham days with his highly reflective antenna. Some of that reflected power comes back down the inside of the coax and appears as increased voltage in the PA tank circuit. Because the PA can match a fairly wide range of impedances, the transmitter is still able, reluctantly, to cram power into the line.

However, since I didn't have a station ground and the transmitter was setting on a wooden desk, the outside of the coaxial shield terminates in a big blob of metal sitting on an insulator. Just like the end of our dipole, this is a voltage point, and poor Jim gets a tingle every time he tries to tune the transmitter.

Now, if I had had a very good station ground (a wide copper strip connected to a good earth ground system close to the transmitter), then the transmitter case would have been a current point... which simply moves the voltage point to some other place on the outside of the coax.

The point being, when you have voltage and current maxima and minima on a single conductor, that conductor is an antenna and radiation results. The more power present on that conductor, the stronger the radiation. The outside of a coaxial cable shield is electrically a separate third conductor and for best results, especially with mismatched antennas, the shield outer surface must be isolated from the antenna. Hence the choke or a 1:1 balun.

BTW, for Cwebs, since the two techniques are functionally equivalent, the choke technique is sometimes refered to a choke balun. You will also sometimes hear the choke technique referred to as a "current balun" while the 1:1 transformer is referred to a "voltage balun".

Regards,

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Wed 12, 2019 2:51 pm 
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Thanks again Jim for your time and explanation, Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Wed 12, 2019 10:09 pm 
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My ADD kicks in very rapidly with long posts. So far you haven't answered my question as to standing waves and common mode currents. Your referenced diagram does not solve anything other than to show when common mode currents occur.

When there are common mode currents it is exactly like having an extra wire at the antenna. This extra wire does detune your antenna from resonance(just like any extra wire does). That's what your diagram shows. However, common mode currents are an exception, not a given.

For those trying to follow along with this thread, My comments still stand. Common mode currents are caused by an unbalance in the system, not because of the coax itself.

OCF dipoles, for example, will create an unbalance at the feedpoint and requires a choke to rebalance the currents.

Notice that the diagram does state that the currents on the center conductor and the inside of the shield are equal and opposite. This is the important part. Coax is not unbalanced if the currents are equal and opposite. Coax DOES NOT require choking(ie a balun) if there are no common mode currents.

Standing waves on the coax do not cause imbalance as I stated some time ago. This is very erroneous and totally misleading.

Save your money. Do not add choking(baluns) if the system is already balanced.

Following picture referenced by Jim above.


Attachments:
Balanced and Unbalanced Feed Lines.jpg
Balanced and Unbalanced Feed Lines.jpg [ 77.65 KiB | Viewed 574 times ]

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Thu 13, 2019 5:09 am 
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Greetings to the Forum:

Someone didn't read my diagram very carefully, I said inside currents are "NEARLY" equal. That they are not exactly equal is due to the very thing we are arguing about.

However, I promised to give up, so I will. For those of you who are interested, read up on the subject in the ARRL Antenna Book. In my edition (14th, 1983), it is in chapter 5 entitled "Coupling the Line to the Antenna".

Regards,
James K. Thorusen
Broadcast Engineer, retired
P1-11-54574

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Thu 13, 2019 5:43 pm 
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A quick reply: Your diagram states they are equal.

And what about the claim that standing waves cause common mode currents. You really need to address that as it is still an issue. Where exactly did you read that?

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Thu 13, 2019 6:38 pm 
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Greetings to Phil and the Forum:

Quote:
Your diagram states they are equal.


I quote verbatim from the text block under the right hand side illustration in my drawing:

Quote:
Transmission line is electrically THREE wire line; only currents in center conductor and shield inner surface are NEARLY equal and opposite and cancel. Current in outside surface of shield does not have a counter-current and is therefore equivalent to an additional piece of wire connected to the antenna.
This is why it is named (correctly) as UNBALANCED line.

(emphasis added for clarity)

Note that this applies only to the case where the load is balanced; an unbalanced load such as a coaxial dummy load has no current on the outside of the shield.


Quote:
And what about the claim that standing waves cause common mode currents.


I never said that either. The term "common mode" is not applicable here. The current on the outside of the shield is unique and not in common with any other current present in the transmission line. What I did say is that standing waves change the MAGNITUDE of the already present unbalanced current on the outside of the coaxial line. When the VSWR is severe, so is the radiation from the line. When the antenna is a perfect match, as the ARRL Antenna Book suggests, the current on the outside of the line can be neglected. As my own experience has proved to my painful satisfaction, when the VSWR is high, this is most certainly NOT the case.

I have now broken my promise to the Forum to shut up twice. Please do not tempt me to break it again.

Thank You,

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Thu 13, 2019 11:33 pm 
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Jim and Phil, I can see only one way to solve this issue. Stand back to back, take ten steps forward, turn and.........

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Fri 14, 2019 2:18 pm 
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Quote:
What type and size and number ferrite beads would you use on the outside of a coax going to a dipole antenna to stop RF coming back into the shack? Using RG 8X. 80 through 10 meters. Thanks, Carl

Carl-to get back to your post where all this started, you seem to want beads but I'm not sure you need those, do you?
My take on the matter is that it is not a given that every feedline needs choking action. Only if there is unbalance in the system somewhere, eg, OCF dipoles etc.

A simple test: Before adding coax, measure resonance with an analyzer. Add coax and measure again. If no change in resonance then there are no common mode currents and therefore no choking required.
(If there are common mode currents then what deleterious effects are they causing?)
----------This is the simple answer to a complex problem!-----------

If a choke(balun) is needed, it can be made with a ferite toroid and coax(eg rg58). Large size coax is not required. I actually use ferite bead(s) with small diameter coax and pump 800 watts through it. These things are extremely efficient. If they weren't then even as little as five watts of heating would cause them to heat up.

Again. This is simple and does not require complicated solutions where there is no problem to begin with.

I too am done with this.

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Fri 14, 2019 7:56 pm 
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Thanks Phil. I don't have any problems with 100 watts. Changing coax and running up to only 600 watts. Just thinking a little PM. Last time I might be able to do the work myself. Not getting around very good in my old age. Carl

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 Post subject: Re: ferrite beads on coax
PostPosted: Jun Sat 15, 2019 2:20 pm 
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Phil Coe wrote:
Sorry but operating off resonance does not cause radiation on the feedline! Where do these ideas come from.
Only unbalanced currents on the feedline cause radiation. Operating off resonance causes standing waves to occur, resulting in high SWR, but is not a cause of radiation.



The idea comes from certain antenna sellers that deliberately sell unbalanced antennas, and then just happen to sell all sorts of "cures" that may or may not (depending on any number of ambiguous reasons) eliminate what could have been prevented in the first place.

Rege


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