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  #1 (permalink)  
Old 02-07-2018, 1:22 PM
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Default Help with a high SWR

I put a regular wire dipole in my attic, horizontal, and I get 2:1 on 1 and 3:1 on 40, so that one works, not optimally, but works..

Now I put a firestick dipole in the attic, vertical, and can't get it to work.
The firestick gives me way over 3, best guess, 5:1 across the board, ch. 1, 19 and 40....

I tried it horizontal just for the heck of it and it stayed the same.

Just to make sure it wasn't the coax, I switched them and use the one from the other antenna, and nothing changed, the wire dipole was still 2:1 and the firestick was 5:1, so the coax is good.

There is no ground on either antenna, but I figure if the wire dipole doesn't need one, the firestick dipole doesn't. The one I'm copying doesn't have a ground either, I'm copying the design from here A Stealthy Homebrew Vertical Dipole Antenna Using Mobile CB Antennas

Both firesticks are 4'
I have a 4' firestick on my car, so to do more troubleshooting I took that one off the car and used if for the hot/feed antenna, in case I had a bad antenna, but it stayed the same, 5:1 across the board. I switched the antenna's all the way around, tried everyone in every spot, nothing changed, 5:1

I didn't make my bracket for the dipoles like in the link I posted, I bought one that was made for truckers, but I still took it apart to make sure there was no short. My ohm meter showed everything to be perfect, the stud going to the down ground antenna was shorted to ground and the hot end had no continuity to ground, but it did have continuity to the center of the coax connector, so the bracket is all good...

One think I'm thinking of is that my bracket, the base of each antenna is at the same spot and they are offset buy 2", and if you look at the home made one, they are directly in line with each other and there is about 5" between the antenna's, I"m wondering if that could be causing an issue, except the bracket I have was actually made for this, it's not home made....

Right now I'm headed to Home Depot to get some ground wire, I'm thinking of running ground wires between the antenna's making it like a ground plane, I don't know what to do, this boggles my mind, it should just work.
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Old 02-07-2018, 3:15 PM
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This was copied and pasted from the article that you linked to:

Antenna Tuning - High SWR
After getting everything installed, I realized the antenna was giving me high SWR readings. The SWR was in the high 2's on Channel 01 and 20, and 3+ on Channel 40. It was time to start tuning the antenna. In my case, the SWR on channel 40 was greater than on channel 1, so the antenna was considered "LONG" and a reduction of conductor length is what was needed to correct it. There's a tiny wire that wraps around the Firestik® fiberglass antenna forming a coil and goes all the way up to the tip. You can actually see it through the plastic sleeve of the antenna when looking at it closely. This wire is what needed to be shortened, not the fiberglass itself. Here's a link to FireStik's website with information about tuning an antenna: Setting the SWR of Your Antenna© - Firestik® Antenna Company

Just might need to make a few snips.
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Old 02-07-2018, 3:18 PM
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Don't do a dipole for CB. A CB dipole wants to be about 22 feet long. Do a 1/4-wave ground plane instead. It will have the correct impedance (50Ω vs 75Ω for a dipole), and because the physical length will be closer to the electrical length, it will perform much better.
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Old 02-07-2018, 5:17 PM
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Quote:
Originally Posted by jonwienke View Post
Don't do a dipole for CB. A CB dipole wants to be about 22 feet long. Do a 1/4-wave ground plane instead. It will have the correct impedance (50Ω vs 75Ω for a dipole), and because the physical length will be closer to the electrical length, it will perform much better.
If you check out this page, you'll see it can be done A Stealthy Homebrew Vertical Dipole Antenna Using Mobile CB Antennas
It just didn't work for me and I wonder why....
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Old 02-07-2018, 6:35 PM
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Antennas that use coils or are shortened are poor performers when used in a dipole configuration. Antennas that are 1/4 wavelength (such as 102 to 108 inch whips) will work much better. If oriented vertically the coax should leave the antenna at 90 degrees for at least 1/4 wavelength.
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Old 02-07-2018, 7:28 PM
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Quote:
Originally Posted by WA0CBW View Post
Antennas that use coils or are shortened are poor performers when used in a dipole configuration. Antennas that are 1/4 wavelength (such as 102 to 108 inch whips) will work much better. If oriented vertically the coax should leave the antenna at 90 degrees for at least 1/4 wavelength.
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This is going in my attic, 17' of vertical antenna is not doable, I only have 9' to work with.
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Old 02-07-2018, 8:34 PM
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A 1/4 wavelength vertical ground plane would be a better choice. The ground plane radials could be constructed from 3 or 4 nine foot whips or copper electrical wire. Be sure to keep it as far away as possible from electrical wiring, duct work and foil backed insulation.
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Old 02-07-2018, 9:28 PM
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Quote:
Originally Posted by jonwienke View Post
Don't do a dipole for CB. A CB dipole wants to be about 22 feet long. Do a 1/4-wave ground plane instead. It will have the correct impedance (50Ω vs 75Ω for a dipole), and because the physical length will be closer to the electrical length, it will perform much better.
I don't really understand what you are saying here.

A CB dipole does not "want to be" about 22 feet long. A half wave dipole cut for channel 20 will be about 17.21 feet long.

As for the mismatch of a dipole to coax, the ground plane has no particular advantage there. Yes, a straight dipole has an impedance of greater than 50 Ohms, 65 to 70 is pretty typical. But a 1/4 ground plane with the "plane" elements perpendicular to the vertical element has a very similar impedance. It is not until you bend the radials down about 45 degrees that the 1/4 wave impedance gets close to 50 Ohms. You can do the same thing to a dipole by bringing the ends down and forming an inverted V.

And that last part about using a 1/4 wave ground plane because the physical length is closer to the electrical length... what? Can you clarify that just a bit, because I am having trouble visualizing what you might mean.

The main problem with using a half wave dipole for CB is that most such antennas get configured horizontally, and most local CB operations are vertically polarized. So there might be a bit of cross-pol loss for local comms. For DX this is much less a problem, as reflected signals tend to be chaotic in their polarization. But if you install a 1/2 wave dipole as an inverted V, as I said above, then the pattern becomes more omni, and contains more of a vertical element.

A wire inverted V is a fine quick and dirty CB antenna for installation in someplace like an attic. A quarter wave ground plane may be better, mostly because of the polarization issue, but it may not be practical in an attic.

T!
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Old 02-07-2018, 10:03 PM
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Firesticks are 5\8 wave whips so you didn't make a dipole.
......
You must use 1\4 wave whips to make a dipole.
.....
What you made is a center fed zepp which has a very high impedance
likely near 260 ohms since the swr is 5 to 1
....
You would need a 4 to 1 balun to bring the swr down but even then it
likely wouldn't perform well.
.....
If you didn't use the exact same design as the article that could be an issue

Last edited by MOTEX; 02-07-2018 at 11:39 PM..
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Old 02-07-2018, 10:23 PM
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A Firestick is not really a 5/8 wave, its more like 3/4 wavelength of wire to resonate near 50 ohms because its designed to work with typical vehicle mounts. Two long Firesticks work ok as a dipole, not much different than a 1/2 wave dipole made from wire or whips. Use shorter ones like 3 or 4ft and efficiency goes further down the toilet.
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Quote:
Originally Posted by MOTEX View Post
Firesticks are 5\8 wave whips so you didn't make a dipole.
......
You must use 1\4 wave whips to make a dipole.
.....
What you made is a center fed zepp which has a very high impedance
likely near 300 ohms.
....
You would need a 4 to 1 balun to bring the swr down but even then it
likely wouldn't perform well.
...
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Old 02-07-2018, 10:34 PM
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Quote:
Originally Posted by MOTEX View Post
Firesticks are 5\8 wave whips so you didn't make a dipole.
......
You must use 1\4 wave whips to make a dipole.
.....
What you made is a center fed zepp which has a very high impedance
likely near 300 ohms.
....
You would need a 4 to 1 balun to bring the swr down but even then it
likely wouldn't perform well.
...
If that's true, then how did this work? A Stealthy Homebrew Vertical Dipole Antenna Using Mobile CB Antennas
He used the same setup I have, this is why I don't understand what happened. If I was just experimenting, I would get it, I screwed up, but this is something that's been done.
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Old 02-07-2018, 10:39 PM
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Quote:
Originally Posted by Token View Post
You can do the same thing to a dipole by bringing the ends down and forming an inverted V.

But if you install a 1/2 wave dipole as an inverted V, as I said above, then the pattern becomes more omni, and contains more of a vertical element.

A wire inverted V is a fine quick and dirty CB antenna for installation in someplace like an attic. A quarter wave ground plane may be better, mostly because of the polarization issue, but it may not be practical in an attic.

T!
I have a question, does the inverted V have to be 90 degrees? Could I just follow roof line at whatever angle that is?
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Old 02-07-2018, 11:02 PM
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Quote:
Originally Posted by Token View Post
And that last part about using a 1/4 wave ground plane because the physical length is closer to the electrical length... what? Can you clarify that just a bit, because I am having trouble visualizing what you might mean.
You have limited space in the attic.

You can make an antenna physically shorter than its normal resonant length by incorporating an inductor into the design, but doing so reduces usable bandwidth, and reduces efficiency by approximately the degree the antenna is physically shortened. So if you take a dipole and add loading coils to shorten it enough to fit in your attic, it will have less bandwidth than a 1/4-wave ground plane designed to the same overall length.
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Old 02-07-2018, 11:05 PM
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Quote:
Originally Posted by Bryan_G View Post
If that's true, then how did this work? A Stealthy Homebrew Vertical Dipole Antenna Using Mobile CB Antennas
He used the same setup I have, this is why I don't understand what happened. If I was just experimenting, I would get it, I screwed up, but this is something that's been done.
A simple google search of the firestick web page will show the ones you have are likely 5\8 wave since only the 7 ft long model is 3\4 wave all others are 5/8 wave and a 14 ft high attic space is pretty rare.
.....

Last edited by MOTEX; 02-07-2018 at 11:42 PM..
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Old 02-07-2018, 11:13 PM
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At this point it would be cheaper and easier to simply use the 5\8 wave firestick with ground radials under it
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Old 02-07-2018, 11:40 PM
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Quote:
Originally Posted by MOTEX View Post
A simple google search of the firestick web page will show the ones you have are likely 5\8 wave since only the 7 ft long model is 3\4 wave all others are 5/8 wave.
......
Another simple google search of zepp antenna design will show that they are fed with 450 ohm ladder line not coax.
.......
And the 5 to 1 swr also confirms my statements.
.....
I understand that, and yes, mine are 5/8 wave, but someone else already did this using the same coax and same antennas, the link I posted.

I've given up on the firestick dipole, I just want to know why someone else got it to work with the same exact parts I have, but I can't get it to work.

I have a 1/2 wave horizontal dipole that kinda works. It's about 17' 2" on each leg. It seemed to work better last week, but that was before the roof got covered with a foot of snow, I think it will work again once the snow melts. The SWR is 2:1 on ch 1 and 3:1 on channel 40, so I can work that down by shortening it (plus, I was thinking of adding a ground wire where the coax connects). Also, I think I'll take Token's advice and turn it into an inverted V.

As far as the firestick's, I'm going forget about the vertical dipole and just make a ground plane out of one of them. Another fun project that probably won't work. I'll take any advice you or anyone else has. Right now I mounted the firestick stud mount on a 1' x 1' piece of aluminum and I plan to run wires out from each corner. I'm just trying to figure out the lengths. The tech at firestick said the longer the better but did not give me a minimum length.
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Old 02-07-2018, 11:50 PM
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You mention your dipole is 17' 2" on each leg? As in 34' 8" total? A 1/2 wavelength dipole would be 1/2 wavelength total length and fed in the center for about 8' 6" or so each leg.
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Quote:
Originally Posted by Bryan_G View Post
I understand that, and yes, mine are 5/8 wave, but someone else already did this using the same coax and same antennas, the link I posted.

I've given up on the firestick dipole, I just want to know why someone else got it to work with the same exact parts I have, but I can't get it to work.

I have a 1/2 wave horizontal dipole that kinda works. It's about 17' 2" on each leg. It seemed to work better last week, but that was before the roof got covered with a foot of snow, I think it will work again once the snow melts. The SWR is 2:1 on ch 1 and 3:1 on channel 40, so I can work that down by shortening it (plus, I was thinking of adding a ground wire where the coax connects). Also, I think I'll take Token's advice and turn it into an inverted V.

As far as the firestick's, I'm going forget about the vertical dipole and just make a ground plane out of one of them. Another fun project that probably won't work. I'll take any advice you or anyone else has. Right now I mounted the firestick stud mount on a 1' x 1' piece of aluminum and I plan to run wires out from each corner. I'm just trying to figure out the lengths. The tech at firestick said the longer the better but did not give me a minimum length.
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Old 02-08-2018, 6:17 AM
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Quote:
Originally Posted by Bryan_G View Post
I have a question, does the inverted V have to be 90 degrees? Could I just follow roof line at whatever angle that is?
Such a V does not have to be 90 deg, and yes, you could just follow the joist. However, an inverted V will work optimally for local (and vertical) communications when it is about 90 degrees inside angle.

The total length of the halfwave dipole is about 17.21 feet, or about 8.6 feet per side. If you have a 9 foot tall attic you should be able to secure the feedpoint of the dipole at the peak of the attic and come down at a 90 degree enclosed angle with no problem, the end points should still be well above the attic floor level.

Keep in mind no inside the attic antenna is ever great, but if it is the only option you have you definitely can make it work.

T!
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Old 02-08-2018, 6:22 AM
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Quote:
Originally Posted by jonwienke View Post
You have limited space in the attic.

You can make an antenna physically shorter than its normal resonant length by incorporating an inductor into the design, but doing so reduces usable bandwidth, and reduces efficiency by approximately the degree the antenna is physically shortened. So if you take a dipole and add loading coils to shorten it enough to fit in your attic, it will have less bandwidth than a 1/4-wave ground plane designed to the same overall length.
I don't believe anyone was talking about adding loading coils or inductors to a half wave dipole. So far it sounds like we are talking about an unloaded dipole.

T!
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Old 02-08-2018, 6:27 AM
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Quote:
Originally Posted by Bryan_G View Post
I have a 1/2 wave horizontal dipole that kinda works. It's about 17' 2" on each leg. It seemed to work better last week, but that was before the roof got covered with a foot of snow, I think it will work again once the snow melts. The SWR is 2:1 on ch 1 and 3:1 on channel 40, so I can work that down by shortening it (plus, I was thinking of adding a ground wire where the coax connects). Also, I think I'll take Token's advice and turn it into an inverted V.
prcguy hit the nail on the head here. Your TOTAL half wave dipole (or inverted V) length should be about 17 feet 3 inches as a starting point. Try starting with your antenna about 8 feet 8 inches on each side, or about 17.2 feet total. Then check the SWR and don't be afraid to tune it. Starting at 8 feet 8 inches should make the antenna a little long, but far easier to trim to tune then to add to tune.

T!
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