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Dipole feed line length question

brizzotheizzo

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I am building a dipole antenna. I know that the feedline needs to be approximately 1/2 wavelength in length (or factors there of) for best SWR. However, there is one item of confusion. I am making an RF choke by coiling the feedline at the base of the antenna. Do I include the length of coax in the coil as part of my “feed line length“ when calculating the 1/2 wavelength feedline length?

what about if I use a premanufactured one to one Balun/choke. Some are made by “coiling” coax Inside. This unit will sit between the feedline and the antenna connector. Technically, I would consider this part of feedline length. But I’m just not sure when calculating.

my plan is probably to just make my own air balun / choke by coiling the feedline.

I have included a picture below just for reference. This is not mine. I am still in the planning phases.CCECED93-FFA1-47CD-A3F6-180EC498C5A6.jpeg
 

prcguy

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Is this for HF or VHF/UHF? Coiling the coax is nearly useless, if you get the exact needed turns it can sort of act like a choke but its reflective and will bounce any VSWR problem back and forth between the antenna and choke. Instead use a ferrite choke designed for the frequency you are using, that will absorb reflections rather than reflect them.

Coax length should not be critical, just use the amount of coax that will reach your radio. The only time coax length matters is when the coax is a radiating part of the antenna and it can be tuned. You don't want that and a ferrite choke will help this all work.
 

jaspence

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Coax length is not a big factor as long as you don't have a great deal of extra on receive. It is more important when the antenna is also used to transmit.
 

brizzotheizzo

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Is this for HF or VHF/UHF? Coiling the coax is nearly useless, if you get the exact needed turns it can sort of act like a choke but its reflective and will bounce any VSWR problem back and forth between the antenna and choke. Instead use a ferrite choke designed for the frequency you are using, that will absorb reflections rather than reflect them.

Coax length should not be critical, just use the amount of coax that will reach your radio. The only time coax length matters is when the coax is a radiating part of the antenna and it can be tuned. You don't want that and a ferrite choke will help this all work.
This will be used on an 11 meter transceiver. The reason I was going to use the air choke instead of the ferrite was to save money. I’m trying to do this on a budget and I have plenty of coax. The ferrite choke would add to that amount. I also have a good reference to show that the air choke does work exceedingly well. Here’s the study done by g3txq.


here is an article referencing the importance of getting feed line length at 1/2 wave intervals


so once again I go back to my original question. Do I count the turns in the coil as part of the overall coaxfeedline length ?
E117AD64-1FEA-47F6-9EAE-8F14A1A281AF.jpeg
 

brizzotheizzo

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Coax length is not a big factor as long as you don't have a great deal of extra on receive. It is more important when the antenna is also used to transmit.
I know that it’s important on a dipole because of the change in ohms. . 50 ohm radio, 50 ohm coax, 75 ohm antenna. If not cutproperly, the radio will see high SWR due to coax impedance, and go into low power protection mode. That is what I’m trying to avoid. And yes, this antenna will be used for transmit and receive.

 

prcguy

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Needing a specific coax length is an old CB tale, please purge that from your memory. If your antenna is broken or not designed right then the length of coax can make the SWR meter read different because the coax can become a radiating part of the antenna. Don't use a broken or poorly designed antenna and you can ignore the article. Using an effective ferrite choke balun at the antenna will also isolate the antenna from the feedline further reducing the need for a specific coax length.

A dipole is about 72 ohms in free space. We don't live in free space and a dipole 10-20-30ft off the ground will be somewhat lower in impedance and close enough to 50 ohms so you don't have to worry about it. At exact 1/2 wavelengths above ground it will be at its higher impedance point, so 18ft, 36ft, etc, off the ground it will be above 50 ohms but still probably lower than 72 ohms. At 9ft, 27ft, etc, it will probably be lower than 50 ohms. When you trim the dipole to resonance it might come out a little long or short to make the SWR perfect and part of that exercise is tweaking the impedance near resonance to obtain the best match.

There is no resistive choking with coax coiled up as a choke and the thing you really want is resistive choking. Look at your G3TXQ chart again and you will see a single FT-240-61 mix ferrite core wrapped with about 8 turns of coax will give a huge resistive choking impedance across the CB band. That core costs maybe $10. If buying the ferrite core causes the kids to miss a meal or two then I will send you one for free.


This will be used on an 11 meter transceiver. The reason I was going to use the air choke instead of the ferrite was to save money. I’m trying to do this on a budget and I have plenty of coax. The ferrite choke would add to that amount. I also have a good reference to show that the air choke does work exceedingly well. Here’s the study done by g3txq.


here is an article referencing the importance of getting feed line length at 1/2 wave intervals


so once again I go back to my original question. Do I count the turns in the coil as part of the overall coaxfeedline length ?
View attachment 87134
 

brizzotheizzo

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Needing a specific coax length is an old CB tale, please purge that from your memory. If your antenna is broken or not designed right then the length of coax can make the SWR meter read different because the coax can become a radiating part of the antenna. Don't use a broken or poorly designed antenna and you can ignore the article. Using an effective ferrite choke balun at the antenna will also isolate the antenna from the feedline further reducing the need for a specific coax length.

A dipole is about 72 ohms in free space. We don't live in free space and a dipole 10-20-30ft off the ground will be somewhat lower in impedance and close enough to 50 ohms so you don't have to worry about it. At exact 1/2 wavelengths above ground it will be at its higher impedance point, so 18ft, 36ft, etc, off the ground it will be above 50 ohms but still probably lower than 72 ohms. At 9ft, 27ft, etc, it will probably be lower than 50 ohms. When you trim the dipole to resonance it might come out a little long or short to make the SWR perfect and part of that exercise is tweaking the impedance near resonance to obtain the best match.

There is no resistive choking with coax coiled up as a choke and the thing you really want is resistive choking. Look at your G3TXQ chart again and you will see a single FT-240-61 mix ferrite core wrapped with about 8 turns of coax will give a huge resistive choking impedance across the CB band. That core costs maybe $10. If buying the ferrite core causes the kids to miss a meal or two then I will send you one for free.
according to this chart, 20 turns of RG 58 around a 4.24 inch diameter does have affective resistive RF choking abilities. In fact, If you look across the 11 m band, the air cored choke is better than than an actual ferrite core.
022641BF-6974-4C06-9619-2EDD4A4E4AE9.jpeg
 

prcguy

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The black line is resistive and an air wound choke has none. It bounces the problem around and doesn't really fix it. This is not the first time I've visited RF feedline chokes. According to the chart an FT-240-61 ferrite core with 8 turns of coax is the best choice for 27MHz.

according to this chart, 20 turns of RG 58 around a 4.24 inch diameter does have affective resistive RF choking abilities. In fact, If you look across the 11 m band, the air cored choke is better than than an actual ferrite core.
View attachment 87139
 

gh6406

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Brizzo, I think you are seeing the Chart wrong. The chart is in MHz not Meters. 20 turns of RG 58 around a 4.25 inch diameter is good for 9 to 13 MHz, not 11 meter band.
 

brizzotheizzo

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some people actually mentioned not even using a choke at the feet point. However, this will make a tripole not a dipole. And the antenna software proves it.

no chokeC2E28DE8-4594-40E5-A987-500235162CE8.png

chokeAF4C4D5D-CB91-40DE-8085-42A154585399.png
 

brizzotheizzo

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The black line is resistive and an air wound choke has none. It bounces the problem around and doesn't really fix it. This is not the first time I've visited RF feedline chokes. According to the chart an FT-240-61 ferrite core with 8 turns of coax is the best choice for 27MHz.
This could possibly be my confusion. I was under the assumption that the colored key chart on the bottom right showed green as being highly resistive. Is this incorrect?
 

brizzotheizzo

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Brizzo, I think you are seeing the Chart wrong. The chart is in MHz not Meters. 20 turns of RG 58 around a 4.25 inch diameter is good for 9 to 13 MHz, not 11 meter band.
Yes, I just caught that. Thank you. However, if you go to the 27 MHz line. It’s still showing the air cored choke as being an effective solution at this frequency as noted by the “green color“. If I’m reading that part right also. Very unsure of myself at this point. LOL
 

prcguy

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A common mode choke has more than one attribute and it can reduce RF on the feedline by reflecting it back to the source, or it can absorb it. A coiled up feedline with just the right amount of inductance can reflect most of the common mode current back to its source and in effect, reduce feedline radiation, but its still there and will most likely get reflected back to the choke and back to the antenna then back to the choke in an endless cycle while transmitting. The preferred method is to absorb the RF on the feedline and only ferrite can do this effectively, and easily and cheaply.

I can't tell you how many times friends and acquaintances have had RF problems on the coax and they had "the right number" of coils in the coax at the antenna and I stuck a ferrite choke balun in line and it immediately fixed the problem. I'm attempting to save you some frustration and time doing it right the first time so you don't have to discover by experimentation when the coiled coax didn't do what you expected.

I mentioned in another thread there are literally tens of thousands or more dipoles out there with no choke anywhere in the coax and they work just fine and don't have any feedline radiation. It depends a lot on the length of the coax and if it is a 1/4 or 3/4 wavelength resonant length or a 1/2, full, etc, wavelength length or something in betweeen.

A 1/4 or 3/4 resonant length will present a low impedance at the feedpoint inviting current to flow on the coax just like it does on the grounded side of the dipole. The currents can roughly divide between the dipole element and the coax and the coax can radiate. Coax cut to a 1/2 wave or multiple resonant length will present a high impedance at the feedpoint and virtually no RF current will flow on the coax shield. I suspect most dipoles with whatever random length of coax used falls more in the higher impedance range as it can be a sharply tuned length to hit exactly 1/4 or 3/4 wavelength.

Then you have some dipoles that are grounded to a metal mast and that mast can become a radiating element or not based on it being a resonant 1/4 wave or 3/4 wave multiple. If the mast and coax are just the perfect wrong length then RF will divide between all of them, however they might not radiate if they are heading out from the feedpoint in opposite directions, which can cancel radiation as it does with tuned radials on a ground plane.




Yes, I just caught that. Thank you. However, if you go to the 27 MHz line. It’s still showing the air cored choke as being an effective solution at this frequency as noted by the “green color“. If I’m reading that part right also. Very unsure of myself at this point. LOL
 
Last edited:

brizzotheizzo

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A common mode choke has more than one attribute and it can reduce RF on the feedline by reflecting it back to the source, or it can absorb it. A coiled up feedline with just the right amount of inductance can reflect most of the common mode current back to its source and in effect, reduce feedline radiation, but its still there and will most likely get reflected back to the choke and back to the antenna then back to the choke in an endless cycle while transmitting. The preferred method is to absorb the RF on the feedline and only ferrite can do this effectively, and easily and cheaply.

I can't tell you how many times friends and acquaintances have had RF problems on the coax and they had "the right number" of coils in the coax at the antenna and I stuck a ferrite choke balun in line and it immediately fixed the problem. I'm attempting to save you some frustration and time doing it right the first time so you don't have to discover by experimentation when the coiled coax didn't do what you expected.

I mentioned in another thread there are literally tens of thousands or more dipoles out there with no choke anywhere in the coax and they work just fine and don't have any feedline radiation. It depends a lot on the length of the coax and if it is a 1/4 or 3/4 wavelength resonant length or a 1/2, full, etc, wavelength length or something in betweeen.

A 1/4 or 3/4 resonant length will present a low impedance at the feedpoint inviting current to flow on the coax just like it does on the grounded side of the dipole. The currents can roughly divide between the dipole element and the coax and the coax can radiate. Coax cut to a 1/2 wave or multiple resonant length will present a high impedance at the feedpoint and virtually no RF current will flow on the coax shield. I suspect most dipoles with whatever random length of coax used falls more in the higher impedance range as it can be a sharply tuned length to hit exactly 1/4 or 3/4 wavelength.

Then you have some dipoles that are grounded to a metal mast and that mast can become a radiating element or not based on it being a resonant 1/4 wave or 3/4 wave multiple. If the mast and coax are just the perfect wrong length then RF will divide between all of them, however they might not radiate if they are heading out from the feedpoint in opposite directions, which can cancel radiation as it does with tuned radials on a ground plane.
Thanks for the lengthy response. I truly appreciate it. And I will use a ferrite choke instead of an air coil to avoid the headache. I appreciate it.

Between trying to go from an unbalanced line to a balanced antenna, while trying to keep the coax from becoming radiated itself has been a headache in design. I’ve always fed unbalanced antennas with unbalanced lines. never dealt with impedance mismatches before This is fairly new to me.

I suppose the only question I still have is if I get the dipole antenna up at a half wave length in height and actually managed to make it a 75 ohm antenna, will the 1:1 ferrite choke balun fix the mismatch? I didn’t think a one to one balun would change impedance at all. I also understand that it’s highly unlikely that I would actually get 75 ohms on this antenna, but what if I did? Then what?... at this point I’m thinking the actual scientific answer is “it won’t be that bad even if it is”.

I understand that the ferrite balun/choke will stop RF from coming back down and radiating the coax feedline. But I don’t understand how it will stop the impedance mismatch between a possibly 75 ohm antenna and a 50 ohm feedline. Mainly since this is a“1:1” balun.

once again, thanks to everyone for all the help. I’m almost ready to put pen to paper and design this thing
 

prcguy

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Getting this particular antenna at 1/2 or a full wavelength above ground is a good goal to get a low angle radiation for DX or even local ground wave. I'll estimate it might be in the 60 to 70 ohm range at 1/2 wavelength high and if it were a perfect 50 ohm resistive load and you had 50 ohm coax it could achieve a really good match. But it will be a little higher than 50 ohms and when tuning your probably going to be looking for the best match and not necessarily XC or XL and the best match should be when the elements are just a little bit long, adding a little more capacitance between elements and their surroundings and lowering its impedance some and closer to 50 ohms. It will have the best match a little off resonance but it should be very usable, I would expect 1.2:1 match plus or minus a little. Its all a compromise but a very slight one in this case.



Thanks for the lengthy response. I truly appreciate it. And I will use a ferrite choke instead of an air coil to avoid the headache. I appreciate it.

Between trying to go from an unbalanced line to a balanced antenna, while trying to keep the coax from becoming radiated itself has been a headache in design. I’ve always fed unbalanced antennas with unbalanced lines. never dealt with impedance mismatches before This is fairly new to me.

I suppose the only question I still have is if I get the dipole antenna up at a half wave length in height and actually managed to make it a 75 ohm antenna, will the 1:1 ferrite choke balun fix the mismatch? I didn’t think a one to one balun would change impedance at all. I also understand that it’s highly unlikely that I would actually get 75 ohms on this antenna, but what if I did? Then what?... at this point I’m thinking the actual scientific answer is “it won’t be that bad even if it is”.

I understand that the ferrite balun/choke will stop RF from coming back down and radiating the coax feedline. But I don’t understand how it will stop the impedance mismatch between a possibly 75 ohm antenna and a 50 ohm feedline. Mainly since this is a“1:1” balun.

once again, thanks to everyone for all the help. I’m almost ready to put pen to paper and design this thing
 

KEWB-N1EXA

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Well to add to the fire.
I know i'm fooling myself but this is my home brew 10-11 Meter antenna and the coil is 21 turns around a Folgers Plastic coffee can of "yes"
RG-6 and the run is 50+feet to the basement where the old Cobra 148 lives...Using the 234/freq formula the thing loads up and runs on cb and
the lower part of the 10 meter band up to 28.500. I went with the V style for a lower impedance and a cross between Horizontal and Vertical
Polarity. The thing works great and loads up but i have to use a tuner as I go up the 10 meter ham Band.
Works the East Coast ! and the 10 meters is with a Radio Shack HTX-10 not the Cobra !
Peter N1EXA
 

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brizzotheizzo

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Well to add to the fire.
I know i'm fooling myself but this is my home brew 10-11 Meter antenna and the coil is 21 turns around a Folgers Plastic coffee can of "yes"
RG-6 and the run is 50+feet to the basement where the old Cobra 148 lives...Using the 234/freq formula the thing loads up and runs on cb and
the lower part of the meter band up to 28.500. I went with the V style for a lower impedance and a cross between horizontal and verticle
Polarity. The thing works great and loads up but i have to use a tuner as I go up the 10 meter ham Band.
Works the East Coast !
Peter N1EXA
That thing looks great ! I am kind of torn between doing a horizontal or a inverted V. I was going to do a horizontal because I love to DX on side band. However, I don’t wanna block out my local CB community either. I have always seen the “V“ styles done as an inverted V. Never thought about doing it that way. Did you use an antenna analyzer to measure for impedence or did you just adjust the “V“ angle until the SWR’s were at their lowest? (And yes, for everyone who’s going to tear that comment apart, I realize that’s not the “way to do it”)



unfortunately, due to restrictions, I have to put the dipole in my attic. I know it’s not ideal, but it’s the only option I have. I was definitely worried about RF interference due to electrical runs in the attic and the air-conditioning handling unit up there. All my house electrical wiring runs on the ceiling joists, so I thought a horizontal style antenna at the rafter peak would be better than the inverted V just to keep the RF a little further away from the electric wires. But This is just me thinking out loud. I’m also only running 4 watts on 11 meter, hopefully that helps some too. I ordered the MFJ 1 to 1 choke balun. Should be here Monday. That should at least keep my feed line “calm”.
I’m also worried about impedance mismatch. As I have learned on here, the dipole in horizontal form is a 75 ohms antenna in “free space“. Well, my attic is definitely not free space. LOL. I’m hoping I can get closer to a 50 ohm impedance on that antenna just due to location even in horizontal form. If not, my plan is to bring the legs of the antenna down until it is.
Either way, I’m sure that the “RF engineers” on this site are going to destroy me for everything that I have said on here. lol. Just trying to do the best with what I got. And yeah, it WILL NOT be a perfect antenna. And I will step aside and let the RF Army begin to pick my plans apart piece by piece. :)
 

prcguy

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You could get one of the cheap Nano Vector Analyzers and use it at the dipole feedpoint with no coax, then adjust the antenna length and angle for best match at 50 ohms. I think its overkill but you would learn a lot and have a tool for future antenna projects.

I’m also worried about impedance mismatch. As I have learned on here, the dipole in horizontal form is a 75 ohms antenna in “free space“. Well, my attic is definitely not free space. LOL. I’m hoping I can get closer to a 50 ohm impedance on that antenna just due to location even in horizontal form. If not, my plan is to bring the legs of the antenna down until it is.
 

brizzotheizzo

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You could get one of the cheap Nano Vector Analyzers and use it at the dipole feedpoint with no coax, then adjust the antenna length and angle for best match at 50 ohms. I think its overkill but you would learn a lot and have a tool for future antenna projects.
I’ve seen those things on YouTube. Those are pretty cool. I’ve also been looking at the rig expert AA-35 model. Those things are great! I’ve actually almost pulled the trigger on buying one a few times. It’s just hard to justify because I don’t consider myself a “antenna builder“ I just happen to be “building an antenna“. but the nano vector analyzers definitely make the cost a whole lot more palatable for someone like me.
 

KEWB-N1EXA

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I guess I went old school ... I had to add some electrical wire to get it to resonate on ch 38 CB and added some weed whacker string on the ends to stop the pvc elements from sagging in the sun . No analyzer just got the SWR low and talked to some guys in my city then talked to a base station 15 miles away so i figured its working....Went on 10 Meters and talked to the local net on 28.490 with the HTX-10 when the DX opened
up I talked to Carolina and Tenn so I figure its working. As for the trap I read it on line to make 21 turns..I figure it works or it does not do a thing
and the coffee container was what i had.... Don't let it get you down about experimenting ... I know I broke allot of rules but I got it to work.

Mine is in the back yard off the grape vine pipes....No way would I put it on the roof... Too High and it would look pretty Ugly... !

Peter N1EXA
 
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