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Message |
    
Dxuall
Junior Member
Username: Dxuall
Post Number: 26
Registered: 10-2002
| | Posted on Wednesday, December 08, 2004 - 10:53 pm: | 
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I installed a 102'' whip as my base antenna but the only thing i get is high swr. I had 3swr and lower it to 2swr by adding ground that's the lower i can go. WHAT CAN I DO???????? I trimmed the antenna to 102''(from the base of the braket to the tip of the antenna) and mounted it on a mirror mount braket. does it need radials to get the swr down? if it does how long how many. is anything wrong with this set up? thank you for your help. |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, December 09, 2004 - 7:08 am: |
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Yes, you need radials to complete the antenna. A 102" whip is a mobile antenna, it's not really intended to be a base antenna. When the 102" whip is installed on a car the body of the car acts as the counterpoise and completes the antenna.
You could get by with adding one radial straight down, but you would get a better match if you use 3 or more at an angle of 135 degrees from the vertical element. The radials should be the same length as the driven element. Basically you would be making a ground plane antenna.
There were brackets available to make ground plane antennas from mobile whips at one time, but I don't know if they are still sold. They came predrilled for the vertical and 3 horizontal elements with the appropriate connectors and mounting bracket. |
Mike0228
Junior Member
Username: Mike0228
Post Number: 19
Registered: 6-2004
| | Posted on Thursday, December 09, 2004 - 7:19 am: |
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A 1/4 wave antenna needs a groundplane. Can you put 3 guy wires attached to the mounting bracket about 8.5 to 9 ft long? If they have to be longer you can break the wire at the required length using an egg insulator. You would also want to droop the wires at about a 45 degree angle. Doesn't have to be perfect. You could also try a single wire hanging straight down to form a vertical dipole then try trimming the wire to get swr down before trimming the antenna. Start out a little long and trim about 1/2" at a time on the wire only to get to your lowest swr. I did a similiar setup as a kid back in the 60's.
Mac |
Barracuda
Intermediate Member
Username: Barracuda
Post Number: 279
Registered: 3-2003
| | Posted on Thursday, December 09, 2004 - 5:38 pm: |
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DXuall,
You didn't say how you have the antenna mounted (to what), but if it is high enough you may be able to add a second 102" whip pointing opposite the first and connected to the coax shield to form a vertical 1/2 wave dipole with the first. Some company(s) makes a bracket specifically for doing this. I'm not sure if Copper sells them, but they are readily available with a quick search. |
That Elco guy (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, December 09, 2004 - 6:43 pm: |
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I hope I do not get you confused. So here goes...
all of the above postings are correct in that you need a ground plane to act as a ground field.
Here is the scoop.
IF you were to mount that antenna within a few inches off the ground itself the proximity to the dirt/ground would be a Natural Ground. The SWR would be a function of the length of the vertical. BUT keep on mind that natural dirt is a very bad ground field due to natural resistance. And even if you were to mount it very close to the ground without running a series of ground wires just under the surface of the dirt (which is advisable on all ground mounted verticals) the efficiency in both hearing and transmission would not be optimized.
SOOO lets raise that quarter wave antenna far above natural ground and put it on a telescopic pole or such supporting device. The loss of ground field would cause your SWR to be at least 3 to 1. To cure this add 3 or 4 ground field radial wires that are 5% longer than a quarter wave and attach them slightly below the feedpoint and angle them in slightly downward position. You can adjust SWR at feedpoint by adjusting the angles of the ground field wires.
That ol Elco Guy
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Tech291
Moderator
Username: Tech291
Post Number: 4
Registered: 11-2004
| | Posted on Thursday, December 09, 2004 - 7:16 pm: |
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Dxuall,you said in your post you trimmed the whip to 102".1/4wave in the middle of the cb band would put it about 108.25".try adding a spring to lenghen it and see what the swr does.this is in addition to the groundplane as others suggested above.you may with a swr that high only see a very slight difference but;
If swr is higher on ch1 than 40 you have to lenghen the whip.
If swr is higher on ch40 than 1 you have to shorten the whip.
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Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, December 09, 2004 - 8:13 pm: |
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Stainless steel is not as good a conductor as copper, because of that 102" should be fine. I've run a 102" both with and without a 6" spring, and the difference in the CB band is not noticeable. It does change the antenna's center frequency, but we're talking about an antenna that has a useable bandwidth of about 4 MHz and it's not enough to make it unusable in the Citizen's Band. |
Dxuall
Junior Member
Username: Dxuall
Post Number: 27
Registered: 10-2002
| | Posted on Friday, December 10, 2004 - 1:26 pm: |
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ok thank you for the replies i'm gonna try the 3 radials this weekend let;s see what happen. (the bracket is mounted on a fence pipe 30ft high) I've noticed that now i can receive very weak signals that i didn't hear before with the skylab. Can this be true? Does the 102'' antenna gives any gain? Or maybe i didn't pay atention to those stations before lol. The 102'' set up and the skylab is the same thing? |
Starface
New member
Username: Starface
Post Number: 9
Registered: 1-2005
| | Posted on Tuesday, January 18, 2005 - 7:45 pm: |
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Remember, all transmitting antennas need ground plane (counterpoise). Base antennas, much like "no ground plane" antennas, build it in. Do not use mobile antennas for base station applications unless you know how to build your own ground plane.
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Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Tuesday, January 18, 2005 - 8:40 pm: |
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A counterpoise is needed by a 1/4 wavelength antenna to complete the antenna so that it can be resonant. In the case of using a mobile antenna as a base you are correct, because mobile antennas are 1/4 wavelength whips they need a counterpoise to complete the antenna and make it resonant.
There are types of antennas that don't have a counterpoise, don't need one, and will function just fine without one. End fed 1/2 wave and 5/8 wave antennas for example. They are 1/2 wavelength or longer and are capable of being resonant radiators. |
Kc8tyk
New member
Username: Kc8tyk
Post Number: 7
Registered: 1-2005
| | Posted on Wednesday, January 19, 2005 - 7:19 pm: |
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do what That Elco guy said he gave you the correct answer. 5% longer than a quarter wave attached just below the feedpoint. adjust radial pitch to get your feedpoint closer to 50 ohm.
73 Tim |
Steve Black (Unregistered Guest)
Unregistered guest
| | Posted on Friday, January 21, 2005 - 11:10 am: |
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TECH 291... I'm curious where you got that lenth of "108.25" from??? The correct 1/4 wave antenna lenth for the middle of the 40 channel CB band should be around "103.25" NOT "108.25" |
Tech291
Moderator
Username: Tech291
Post Number: 50
Registered: 11-2004
| | Posted on Friday, January 21, 2005 - 9:46 pm: |
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Steve Black,
check the arrl handbook for one.
492/f(*n)=wl in feet(*12)=wl in inches
that the one that works for me |
Racer X (Unregistered Guest)
Unregistered guest
| | Posted on Saturday, January 22, 2005 - 9:54 pm: |
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Tech291 - 492/f in MHz is the formula for free space. RF propogates at different rates in different materials.
I believe the starting point most folks use for making a 1/2 wave antenna is 468/frequency in MHz. Depending on the material and what's nearby it may need to be shorter.
To find the length of a 1/4 wave whip divide 234 by the frequency in MHz.
102" is nearly perfect at 27.405 MHz. |
