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Questions about building a dipole antenna

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Ok, I understand the basic idea of what I want to build and how its done. I know that with 4 watts I am in no danger of burning the wire up. My concern is how big does it have to be for durability? Would I be crazy to use some #2 braided wire I have laying around? Would that negatively affect the functionality? And would I need to remove the insulation from the wire? I know that it would be able to stand up to a hurricane lol. I keep hearing people talking from all over the US and parts of Canada, but the swr is way too high on my current setup to risk transmitting, so it would be nice to talk back. Thanks in advance for any help.
 
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SCPD

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Although the braided wire would work, I'm worried it's gonna stretch on you quite a bit, and there goes the match. You don't need to remove the plastic insulation. But any soft copper wire is going to stretch while it's hanging, and like I said, throw the match off. There's a wire made for dipoles called Copperweld, that won't stretch.

Do you want to share what the problem is with your current setup? Maybe we can fix that.
 
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I dont have the funds right now to buy the "proper" materials, so I am trying to use things I have laying around. This would hopefully not be permanent. I was actually considering mounting it in the attic of a detached garage. The house has a steel roof, so I know that would never work, but the garage is a normal shingle roof, and I have read about success with attic antennas. My plan would be to run it along the rafters in an inverted V. I would like to have it out of the elements. If properly secured to the rafters, would stretch really be a major factor?

And as for the current setup, its a 48" antenna that hung on a mirror mount on a 78 Ford truck that hasnt run in over a decade. I shoved it at the top of a 15' aluminum pole. No ground plane, swr numbers off the chart, and enough corrosion to pass for a shipwreck lol.
 

K7MEM

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Your right, a antenna under a steel roof wouldn't work very well. I have a steel roof on my house and my barn so I have to get everything fare away from both of them.

There is nothing wrong with using whatever materials you have on hand. The braided wire you mentioned would work just fine. Just remember, your not tuning a guitar, so don't put too much tension on it and you won't have a problem.

An attic antenna does work, but don't expect big performance. But if that's what you have to work with, go for it. A single band dipole is a pretty simple to construct and will give you better performance than your current antenna. Initially cut it a little long and then trim it until you are getting a good match. You should be able to get a good match across the band.

Martin - K7MEM
 

AC9KH

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A half-wave dipole for 11 meter is only 17.1 feet long for the antenna to be resonant.. Not much danger of the wire stretching and throwing the SWR off at that length. Now, if you were talking a 160 meter antenna at 260 feet, then you might want to use hard drawn wire, but few hams do. Because they use an antenna tuner than don't care how long the antenna is, and feed it with ladder line with a balanced output on the tuner, or a 4:1 balun.

Also be aware that a half-wave horizontally polarized dipole has a high takeoff angle and is best for NVIS unless you have it at least 34 feet off the ground. If you want lower radiation angle for better long distance propagation, a vertical dipole is better. And for 11 meter it's easy to make one. Here's a video I did on a simple PVC vertical dipole that I built for 11 meter sideband

11 meter Vertical Dipole Antenna - YouTube

The impedance on the vertical dipole is 72 ohm and your transmitter is 50 ohm. So using a 50 ohm feedline you're going to get a 1.44 SWR if the elements are cut for resonance. I was able to tune mine down to 1.3 SWR by shortening the elements excessively, and the center feed should be perpendicular to the antenna for less RF flowing in the shield on the coax. A vertical dipole has 2.19 dBi gain.

This quarter wave elevated ground plane antenna is a much higher performance antenna with better gain than a dipole. And with the radials angled at 30 degrees gives a perfect 1:1 match on the feedline without a tuner

10 Meter Ground Plane Antenna - YouTube
 
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A lot of info to digest for a redneck lol. I know the attic is not ideal, but I would like to just have something simple I can use until I can afford the setup I want. I assume I am not the only one who has been there.

Also, I was wondering what you guys think about this:
Amazing CB 11 Meter Dipole Antenna Built for Distance High Power | eBay

It wouldnt be terribly expensive, and would save time. But I want to know if this is a rip off before I do anything crazy.
 

SCPD

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that is the basic wire dipole.
i have a homebrew indoor dipole made from the coax itself.
a lot of checking the meter to setting the swr.
one trick I learned was not to cut the wire to tune the swr.
simply fold the ends back as if you are making a loop on the ends.
of course with it being indoors and lower to the ground than it should be, it picks up a lot of electrical noise.
 

prcguy

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I'd like to make a few comments on your post here.
For most people putting up a horizontal dipole for CB, it will probably be 20ft or so in the air, which is close enough to a half wave high and will be fine for some low angle take off. 34ft is close to a full wavelength high and would be slightly better than a half wavelenght up but you may not notice the difference.

There is no NVIS at 27MHz and it rarely happens above about 10MHz so you will not be raining signals down from high takeoff angles and covering an area out to a couple of hundred miles on CB via NVIS.

A half wave dipole has slightly more gain than a 1/4 wave ground plane. This is because both halves of the dipole radiate equally and only the vertical element of the ground plane radiates because of equal and opposite RF currents flowing in the radials, which cancels their radiation.

A 1/4 wave ground plane with 3 or 4 radials at 90 deg to the vertical element will match perfectally and you don't need to bend the radials down. This myth comes from a vertical monopole over an infinate perfect ground having an impedance around 35 ohms. Lessen the ground plane effect by going from an infinate ground to just a couple of radials and the feedpoint impedance goes up and pretty close to 50 ohms in most cases. A little tuning of the vertical whip and it will match just fine.

A half wave dipole should have 2.14 dB gain over an Isotropic antenna, but who's counting. Well, me of course.
prcguy






A half-wave dipole for 11 meter is only 17.1 feet long for the antenna to be resonant.. Not much danger of the wire stretching and throwing the SWR off at that length. Now, if you were talking a 160 meter antenna at 260 feet, then you might want to use hard drawn wire, but few hams do. Because they use an antenna tuner than don't care how long the antenna is, and feed it with ladder line with a balanced output on the tuner, or a 4:1 balun.

Also be aware that a half-wave horizontally polarized dipole has a high takeoff angle and is best for NVIS unless you have it at least 34 feet off the ground. If you want lower radiation angle for better long distance propagation, a vertical dipole is better. And for 11 meter it's easy to make one. Here's a video I did on a simple PVC vertical dipole that I built for 11 meter sideband

11 meter Vertical Dipole Antenna - YouTube

The impedance on the vertical dipole is 72 ohm and your transmitter is 50 ohm. So using a 50 ohm feedline you're going to get a 1.44 SWR if the elements are cut for resonance. I was able to tune mine down to 1.3 SWR by shortening the elements excessively, and the center feed should be perpendicular to the antenna for less RF flowing in the shield on the coax. A vertical dipole has 2.19 dBi gain.

This quarter wave elevated ground plane antenna is a much higher performance antenna with better gain than a dipole. And with the radials angled at 30 degrees gives a perfect 1:1 match on the feedline without a tuner

10 Meter Ground Plane Antenna - YouTube
 

AC9KH

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There is no NVIS at 27MHz and it rarely happens above about 10MHz so you will not be raining signals down from high takeoff angles and covering an area out to a couple of hundred miles on CB via NVIS.

Actually NVIS does work on 11 meter. I've talked from northern Wisconsin to points in Ontario many times when the conditions are right. The probability of reliable NVIS is best from 160 meter to 40 meter. And at 10 meter it is very, very low. But it does happen.

The main thing with 11 meter radios that have very limited power output is to get a horizontal dipole antenna high enough or the radiation angle will be almost straight up. That's why NVIS antennas are less than 1/4 wavelength off the ground. The first inclination of many folks playing with CB radios is to stretch a wire between a couple trees at eye level, or stretch one in the attic, and hope it works. It don't and will barely talk to somebody on the next block.

A half wave dipole has slightly more gain than a 1/4 wave ground plane. This is because both halves of the dipole radiate equally and only the vertical element of the ground plane radiates because of equal and opposite RF currents flowing in the radials, which cancels their radiation.

The pattern of a 1/4 wave monopole with a good ground plane is identical to the top half of a dipole pattern, with its maximum radiation in the horizontal direction, perpendicular to the antenna. Because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of twice the gain of a similar dipole antenna, and a radiation resistance half that of a dipole. The ground plane antenna "mirrors" an image of the vertical dipole's ground element in its ground plane.

Excuse me, yes, the gain of a dipole over isotropic is 2.14 or 2.15 dBi, depending on which text book you study. The gain of a 1/4 wave monopole with a good ground plane is twice that, or +3 dB. When you get into gain there is three types:
dBi - gain over isotropic
dBd - gain over a dipole
dBadv - huge random numbers thought up by antenna manufacturers
 
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Jimru

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The OP mentioned setting up his wire as an inverted V in his garage. Will that angle then be sufficient for him to be able to radiate a good enough signal?
 

AC9KH

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The OP mentioned setting up his wire as an inverted V in his garage. Will that angle then be sufficient for him to be able to radiate a good enough signal?

The main thing with 11 meter (CB) radios is the height of the antenna and obstructions, more than what type of antenna you use. These are very low power radios, with often quite poor quality receivers. They will talk over the radio horizon much better if you get the antenna as high as you can (within the legal limits specified in the FCC "rules").

So basically, setting up an inverted V dipole in the garage is not a good setup. A mobile antenna sitting on the peak of the roof outside will work much better.
 

Jimru

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The main thing with 11 meter (CB) radios is the height of the antenna and obstructions, more than what type of antenna you use. These are very low power radios, with often quite poor quality receivers. They will talk over the radio horizon much better if you get the antenna as high as you can (within the legal limits specified in the FCC "rules").



So basically, setting up an inverted V dipole in the garage is not a good setup. A mobile antenna sitting on the peak of the roof outside will work much better.


Thank you!
 

prcguy

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Not sure how long you've been playing with antennas but I can remember putting an inverted V dipole for CB on the wall of my garage about 40yrs ago and it worked surprisingly well, better than any short loaded mobile on a car in the same garage.

No need to state dBd or dBi explanations here as I've been referencing them all my professional career and at one point was the range master responsible for all customer measurements at our indoor antenna range when working for the largest aerospace company in the US.

I can tell you from textbook and measurements that a half wave dipole has slightly more gain than a quarter wave monopole over some ground radials when measured under the same conditions.

I can also say if you ran an Ionospheric Sounder 24/7 with antennas pointing upward for years you might log a few ultra rare NVIS openings at 27MHz, but to say you made a contact in that mode, especially when 99% of all CB antennas are vertical pol would be streatching things. There would have to be another explanation for a couple hundred mile contact on CB. Like maybe Aurora being in your northern area?
prcguy



Actually NVIS does work on 11 meter. I've talked from northern Wisconsin to points in Ontario many times when the conditions are right. The probability of reliable NVIS is best from 160 meter to 40 meter. And at 10 meter it is very, very low. But it does happen.

The main thing with 11 meter radios that have very limited power output is to get a horizontal dipole antenna high enough or the radiation angle will be almost straight up. That's why NVIS antennas are less than 1/4 wavelength off the ground. The first inclination of many folks playing with CB radios is to stretch a wire between a couple trees at eye level, or stretch one in the attic, and hope it works. It don't and will barely talk to somebody on the next block.



The pattern of a 1/4 wave monopole with a good ground plane is identical to the top half of a dipole pattern, with its maximum radiation in the horizontal direction, perpendicular to the antenna. Because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of twice the gain of a similar dipole antenna, and a radiation resistance half that of a dipole. The ground plane antenna "mirrors" an image of the vertical dipole's ground element in its ground plane.

Excuse me, yes, the gain of a dipole over isotropic is 2.14 or 2.15 dBi, depending on which text book you study. The gain of a 1/4 wave monopole with a good ground plane is twice that, or +3 dB. When you get into gain there is three types:
dBi - gain over isotropic
dBd - gain over a dipole
dBadv - huge random numbers thought up by antenna manufacturers
 

JayMojave

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Hello All: A good NVIS explanation is at
https://www.txarmymars.org/downloads/NVIS-Theory-and-Practice.pdf

The NVIS might not work at 12/11/10 Meters, but one thing is for sure, antennas with the first and or second elevation lope or pattern, does have a advantage when the skip signals are coming in at that angle. That is a 5/8 wavelength may hear a DX Station were a 1/4 wavelength antenna may not, and vis versa. Different length antennas will have different elevation patterns.

Jay in the Mojave
 

AC9KH

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I can also say if you ran an Ionospheric Sounder 24/7 with antennas pointing upward for years you might log a few ultra rare NVIS openings at 27MHz, but to say you made a contact in that mode, especially when 99% of all CB antennas are vertical pol would be streatching things. There would have to be another explanation for a couple hundred mile contact on CB. Like maybe Aurora being in your northern area?
prcguy

Done with a horizontal wire dipole mounted 8 feet off the ground during the day. Got probably a couple dozen QSL cards from contacts in Ontario, all done on 11 meter sideband.

Yes, real world gain can vary quite a bit depending on conditions, quality of the ground plane, etc.. I am quoting dBi and in the real world for local communications I have always had much better performance from 1/4 wave elevated ground plane antennas than vertical dipoles on 11 meter. I've also built several end feed 1/2 wave ground plane antennas with inductors. They work OK, but in the real world usually only show about 0.3 dB gain over a 1/4 wave.
 

AC9KH

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Thanks for that. Really interesting article.

Despite popular concensus that NVIS doesn't work on 11 meter, I been working it for 35-40 years on 11M sideband. The reason it don't work for most people is because they run vertically polarized antennas. But when Dec and Jan get here (when it works pretty reliable at our latitude) I flip the coax switch to my "short skip" antenna and I've gotten a pretty good collection of QSL cards on it over the years.

Mostly to the north in Ontario where there's more hams that mess with it on 11 meter. But I also got one from Illinois and one from North Dakota.
 

K7MEM

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Did I inadvertently make one of those posts that causes debate between people? lol

No, you did nothing wrong. Just ignore them.

The antenna you linked to on Ebay appears to be a simple dipole. You should be able to run it as a horizontal dipole or a inverted-V and it would work just fine. However, I would be a little wary of a product that has so much hype attached. It is really overboard.

Just take the wire that you originally talked about and make a dipole or inverted-V. If you have to do it in your garage, that's OK. It's probably going to be worlds better than what your currently using. But you won't know until you try.
 

AC9KH

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The antenna you linked to on Ebay appears to be a simple dipole. You should be able to run it as a horizontal dipole or a inverted-V and it would work just fine. However, I would be a little wary of a product that has so much hype attached. It is really overboard.

It is a simple half-wave dipole. There's nothing to really be wary about. As long as it's put together right at the center feed where the SO-239 is it'll work.

Just be aware that the feedpoint impedance on a dipole is 73 ohm. And you're probably going to use a 50 ohm feedline and your transmitter is 50 ohm. Installing the dipole closer to the ground will lower the feedpoint impedance. If you buy it I'd throw the SWR meter on it and check it because who knows how close the elements were cut for 11 meter? The specs on it says it's "16-17 feet long". Well, the difference between 16 and 17 feet is huge when it comes to tuning a half-wave antenna for 11 meter.
 
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