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Old 08-24-2013, 11:48 AM
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Default CB base station antenna tips?

I'm within city limits so no ugly antenna or tower.
I'd like to do this w/ $0 using 102" stainless whip and parts around house.

Since I'm in a valley my thought is to use my TV antenna as mount location, highest most inconspicuous location, base of whip approx 20'-25' high. <--- I don't mind if that causes interference w/ TV signal.

Question is, would the TV antenna act as my ground plain (I don't know if its grounded, hoping you guys do?), or should I insulate the mount and put rubber ducky below for a vertical dipole? Or will all this mess of metal throw my SWR off so bad I'd be better installing it in attic?

I've not tried anything yet, I'm only assuming highest point is best since truck CB won't reach very far.

Thanks for your help!
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Old 08-24-2013, 6:23 PM
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The TV antenna booms are usually grounded. It should work as a ground plane.

However, you'll get better performance if you make a ground plane that is designed for your use. All you would need is a couple lengths of metal rod, tubing, or something similar. Fabricate a bracket to hold them out horizontally and evenly spaced around the base of the antenna. Make sure the shield of the coax is connected to them. The material could be anything conductive, even other 102" whips. sections of copper pipe or conduit would work just fine. Steel rod, etc would work, also.

The new ground plane needs to be horizontal, or sloping down a little. That will create a ground plane under the antenna and give you a much better radiation pattern.
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Old 08-24-2013, 7:35 PM
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The only thing I'll say about your ideas is that the one about using a rubberducky for the other half of the antenna is -not- such a good idea. It won't provide the necessary requirements for an HF antenna.
- 'Doc
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Old 08-24-2013, 7:43 PM
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The TV antenna is a start on a ground plane but its only going to be a fraction of size needed. If you put the 102" whip on a mirror type mount above the TV antenna and add a dangling wire counterpoise around 9ft long running parallel but several inches away from the mast you should be able to tune the counterpoise for a good overall match.

The end result will be basically a half wave vertical dipole. If there are TVs connected to the antenna you could get some interference radiated from the TV antenna to the CB antenna.
prcguy
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Old 08-26-2013, 3:44 PM
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i made one almost identical to one that's show on a you tube video, do a google search for "the 102" whip cb base antenna", you should find the video, it was done by a guy that goes by cryptic cricket, mine works very well, swr's are at 1.1, and it was easy to make................oldcb
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Old 08-26-2013, 5:15 PM
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Default CB base station antenna tips?

Do you need the downward sloped ground radials? It seems like the top rail post would act like the ground side of a dipole. Please tell me if I'm missing something here.
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Old 08-26-2013, 5:44 PM
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The characteristic impedance of a vertical dipole, in other words, a 102+ inch whip pointing straight up and the top rail post going straight down at 180 degrees to the radiator, it is around 72 ohms. Not a big deal, but a mismatch.

If the radials are out at 90 degrees from the radiating element (the 102+ inch whip), the impedance drops to 42 ohms or so.

A perfect 50 ohm impedance is achieved with some amount of downward slope in the radials.

If you don't mind the mismatch and can be happy with a slightly higher SWR, then it would work.

Not sure if I explained that right. It makes sense in my head, but typing it out makes me second guess myself.
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Old 08-26-2013, 6:15 PM
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Default CB base station antenna tips?

Ok, that sounds good to me. I felt like there had to be a reason to add them. I'm just trying to make sense out of all this new information bouncing around inside my skull.
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Old 08-26-2013, 6:34 PM
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Quote:
Originally Posted by jeepinjeepin View Post
Ok, that sounds good to me. I felt like there had to be a reason to add them. I'm just trying to make sense out of all this new information bouncing around inside my skull.
not sure if you had a chance to find the video i mentioned, but i cut my radials at 108" using #6 solid copper wire available at most hardware stores, i really does do the job for a 1/4 wave set up.................oldcb
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Old 08-26-2013, 6:47 PM
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There are two things that make an antenna perform well. One is resonance, the other is matching impedance with the rest of the antenna system. I think resonance is the biggy. That means that the antenna is radiating as well as it can at the frequency of interest, good efficiency. That other quality, impedance matching or SWR is a secondary consideration. Impedance matching means you get the best transfer of power from the feed line to the antenna. Of course it's nice to have, but it isn't all that critical really.
The primary way of making an antenna resonant is finding the right length for the radiating parts of the antenna. That involves knowing about reactance which comes in tow varieties, 'plus' and 'minus'. They are best when they equal each other so that they sort of 'disappear' reactance is equal to zero. That means that everything that get's to that antenna is radiated, none of it is turned back (reactance does not radiate anything) so it goes back to where it came from, sort of).
There are a number of ways to manipulate impedance. One of them with a groundplane antenna is by the angle of the 'dangle' of the radials. As the angle between the vertical element and the radials get's larger so does the antenna's input impedance. At 90 degrees, a typical 1/4 wave groundplane antenna has an input impedance somewhere around 35 ohms. As you start to 'droop' those radials the input impedance goes up. When it get's to abut 50 ohms, that's the 'sweet spot', so quit drooping the radials at that point. And the best news is that each and every installation is going to be slightly different. Which means that at one place 42 degrees is just right. At the next place, 46 degrees may be just right. And it varies for each and every place you can think about putting an antenna. There ain't no 'one size fit's all' droop for radials. Close, but no cigar sort of thingy.
How'z 'at for complicating things? Doesn't matter, it's a part of tuning all antennas. Gotta make them 'fit' where you place them.
- 'Doc
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Old 08-26-2013, 8:29 PM
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Default CB base station antenna tips?

Quote:
Originally Posted by oldcb View Post
not sure if you had a chance to find the video i mentioned, but i cut my radials at 108" using #6 solid copper wire available at most hardware stores, i really does do the job for a 1/4 wave set up.................oldcb
I did. I had started watching it one time before. Unfinished watching it this afternoon. I keep seeing "solid" wire. I don't want to go buy wire. I want to use the 10 or 12 gauge stranded wire I already have. Oh well, hopefully eBay will be kind to me over the next few days and I can get some new toys to play with.
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Old 08-26-2013, 8:32 PM
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Default CB base station antenna tips?

Quote:
Originally Posted by LtDoc View Post
How'z 'at for complicating things? Doesn't matter, it's a part of tuning all antennas. Gotta make them 'fit' where you place them.
- 'Doc
Yeah, I'm learning. Thanks for the explanation.
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Old 08-26-2013, 11:07 PM
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Quote:
Originally Posted by jeepinjeepin View Post
I did. I had started watching it one time before. Unfinished watching it this afternoon. I keep seeing "solid" wire. I don't want to go buy wire. I want to use the 10 or 12 gauge stranded wire I already have. Oh well, hopefully eBay will be kind to me over the next few days and I can get some new toys to play with.
No problem with that. Cut the wire to 108 inches. Connect one end to the antenna base, tie a piece of line to the other end. Stretch each one out and tie it off. Might not look perfect, but it'll work.

Good on you for wanting to do this on the cheap. It doesn't take much to go out and buy a brand new antenna, but you'll learn a lot more from building your own.

Other thing you could do is search for 10 meter antennas. Likely you'll find a lot more info on those. The design will be similar, but the lengths will be too short. There should be formulas that you can use to calculate the correct dimensions for the CB band. A 1/4 wave antenna is nice, but if you search some of the other designs you'll find some good options. 5/8th's wave isn't that much harder to build, and you'll get some gain. 1/2 wave won't need a ground plane. There is a lot of good info out there. Good luck.
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Old 08-27-2013, 1:30 PM
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Default CB base station antenna tips?

Quote:
Originally Posted by mmckenna View Post
A 1/4 wave antenna is nice, but if you search some of the other designs you'll find some good options. 5/8th's wave isn't that much harder to build, and you'll get some gain. 1/2 wave won't need a ground plane. There is a lot of good info out there. Good luck.
1/2 wave doesn't need a ground plane? Is that on dipoles only or end fed antennas as well?

Bear with me. I'm shooting from the hip.

I'm assuming we are discussing straight antennas with no wound wire or loading coils. A 1/4 wave would be the shortest antenna that would function? And for it to function it requires a ground plane that simulates the other 1/4 wave in essence making it a 1/2 wave dipole? And if you use 1/2 wave or 5/8 wave antennas to build a dipole? Granted they would be quite long, but does the theory work out?

And end fed antennas... I've seen commercially made antennas in 1/4 wave whip and wire wound versions of 1/2, 5/8, and 3/4 wave, but not full wave. I'm think I've heard that a full wave antenna isn't possible?
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Old 08-27-2013, 11:48 PM
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Particular length have a useful purpose. a 1/4 wave produces a useable radiation pattern an it's input impedance is fairly easy to match. (No, it's not 50 ohms but can be made 50 ohms reasonably easily.) A 1/2 wave if center fed is some impedance close to 50 ohms, sort of. More like 70 - 75, which works out to less tha about 1.5:1 SWR. If end fed, a 1/2 wave's input impedance isin the neighborhood of 1000 to 2000 ohms. that's quite a jump from 50 ohms and while not impossible by any means, isn't as easy to handle as a lower impedance, you know? An end fed or center fed 1/2 wave antenna will perform the same. Small differences but nothing huge. The biggest difference in use is getting a reasonable impedance match (SWR).
As for mobile antennas claiming to be a larger wave length than they physically measure... you know that stuff you spread on gardens to increase growth? It's the same stuff. The physical length of an antenna determines what it will radiate like. The amount of wire wound on that 'stick' doesn't make any difference in how it will perform. It's actual length determines how it will shape it's radiation pattern. If it's real close to a 1/4 wave length long, then it will radiate like a 1/4 wave. If it's length is less than a 1/4 wave length then it's radiation pattern will be 'less' tha a 1/4 wave length's radiation pattern. The physical size it the biggy.
The vsriouds sizes of antennas, 1/4 w, 1/2 w, 5/8 w, or what ever have genral shapes of radiation patterns. If a particular length's radiation pattern fits or exceeds your needs, use it.
- 'Doc
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Old 08-31-2013, 8:30 AM
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Few questions regarding the video
1: He is using 2 grounds @ 108"
.....1a) His signal would look like a figure 8 in those same directions, correct?
.....1b) If using 4 ground radials do you still need 108" or would proper length then be 54"?
2: Lots of talk about impedance in your replies. Is this something I can measure using DVOM or SWR meter? If not is there a DIY way other than fancy equipment?
3: Do the ground radials need to be at the base of the antenna, or could I mount them lower on the mast for proper angle 'droop'

Thanks everyone!
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Old 08-31-2013, 8:36 AM
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Default CB base station antenna tips?

Quote:
Originally Posted by TheDude1969 View Post
Few questions regarding the video
1: He is using 2 grounds @ 108"
.....1a) His signal would look like a figure 8 in those same directions, correct?
.....1b) If using 4 ground radials do you still need 108" or would proper length then be 54"?
2: Lots of talk about impedance in your replies. Is this something I can measure using DVOM or SWR meter? If not is there a DIY way other than fancy equipment?
3: Do the ground radials need to be at the base of the antenna, or could I mount them lower on the mast for proper angle 'droop'

Thanks everyone!
1b. I feel sure they will always need to be 108"(if made of copper) whether there are 2, 4, 8, 16, etc.

2. I think the only way to directly measure impedance is with an antenna analyzer. You get an indirect measurement by your power output.

3. I'm gonna say they should be at the base of the antenna.
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Old 08-31-2013, 6:34 PM
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I love my A99, could talk 20 Some miles on 35-50 Watts. Probably not what you want though.
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Old 08-31-2013, 8:37 PM
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Quote:
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I love my A99, could talk 20 Some miles on 35-50 Watts. Probably not what you want though.
I really do want to get out of this neighborhood so that I can put an A99 or Imax2000 up on a pole.
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Old 08-31-2013, 8:39 PM
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Yeah, both are great for about $100. What about a Flag Pole Antenna? I know the HAM License allows you to put up antennas even if the "gated" communities say no. At least in our area that's how it is.
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