| Author |
Message |
    
Scoobydoo
Junior Member
Username: Scoobydoo
Post Number: 48
Registered: 8-2005
| | Posted on Friday, January 20, 2006 - 6:17 pm: | 
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Hi everyone, I bought a Antenex dual band antenna that covers 150-170 and 450-470mhz and they make a ground plane kit that converts it to a base antenna. What I am trying to figure out is what length to cut the ground plane radials. The radials are 22 1/2 inches and there are four of them. The instructions say for 406-512mhz use cut the radials to 8 inches but then wouldnt they be to short for 150mhz? Im thinking they never thought someone would use this kit with a dual band antenna. I looked around on the internet and found a few homemade dual band base antennas that had four radials with two being short for the uhf portion of the antenna and two long radials for the vhf portion of the antenna, this antenna was a 1/2 wave on vhf and a 5/8 wave on uhf. My antenna is also 1/2 wave on vhf but is 5/8 over 1/2 on uhf or so the instructions said. Im sure i need to take all of this into account when cutting the radials but I am stumped when it comes to figuring it out. Antenex hasent been any help, they dont reply to there emails. Please, any antenna Guru's could you please help me? Thanks in advance. |
Tech833
Moderator
Username: Tech833
Post Number: 1161
Registered: 8-2002
| | Posted on Friday, January 20, 2006 - 11:01 pm: |
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Normally, people don't use those to convert dual band mobile antennas to base station use.
If you cut the radials to 19 inches, they will work great for 2m and very well for UHF. |
Scoobydoo
Junior Member
Username: Scoobydoo
Post Number: 49
Registered: 8-2005
| | Posted on Saturday, January 21, 2006 - 3:18 am: |
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If you dont mind me asking , is there a formula you do to come up with the 19 inches? I read some more about antennas on the internet and found that 1/2 wave antennas dont require a groundplane, would this be true in my case since on vhf its a 1/2 wave antenna? I'll go ahead and cut my radials to 19 inches. I guess I should invest in a swr meter so I can check the antenna. What would happen if I used my CB swr meter? Thank you very much for the help. |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1033
Registered: 6-2004
| | Posted on Saturday, January 21, 2006 - 12:44 pm: |
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234/frequency in MHz gives you an approximation for 1/4 wavelength in metal.
1/2 wave antennas don't require radials to be resonant, but that doesn't meant that radials can't be used to improve the radiation pattern or to decouple RF from the feedline.
Your CB VSWR meter might work okay at 2 meters, but probably won't on UHF. |
Bruce
Senior Member
Username: Bruce
Post Number: 3438
Registered: 9-2003
| | Posted on Saturday, January 21, 2006 - 1:16 pm: |
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19 inch will work as 3/4 wave on 440 and there is nothing wrong with that
Some CB meters WILL work ok on 440! |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 50
Registered: 8-2005
| | Posted on Saturday, January 21, 2006 - 10:20 pm: |
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Will using it from 460-470 change the length or will 19 inches still be close enough? Im thinking it would need to be shorter for that frequency range right? Thanks for the help. |
Bruce
Senior Member
Username: Bruce
Post Number: 3447
Registered: 9-2003
| | Posted on Saturday, January 21, 2006 - 10:59 pm: |
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yes 18 in at 460 but why 460 unless your on gmrs .... |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 52
Registered: 8-2005
| | Posted on Saturday, January 21, 2006 - 11:31 pm: |
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Ok, thank you very much for the help. |
Tech833
Moderator
Username: Tech833
Post Number: 1164
Registered: 8-2002
| | Posted on Sunday, January 22, 2006 - 2:24 am: |
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Just cut them to 19 inches and leave it alone. It will work fine.
Ground planes longer than 1/4 will still work, but ground planes shorter than 1/4 will not do their job properly. |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 53
Registered: 8-2005
| | Posted on Sunday, January 22, 2006 - 4:57 pm: |
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19 inches it is! Thank again everyone for the help. |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 55
Registered: 8-2005
| | Posted on Thursday, January 26, 2006 - 3:52 am: |
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Well, looks like this ground plane kit was'ent made for dual band antennas. I cut the radials to 19 inches but my buddy couldnt even hear me 3 miles away and i could hear him perfect on his handheld. I went ahead and tried cutting them down to 8 inches like the instructions said for 406-512mhz I then was barely able to break his squelch. Oh well, live and learn, next time I'll buy the proper antenna instead of trying to save a buck. Thanks again for the help everyone. |
Bruce
Senior Member
Username: Bruce
Post Number: 3463
Registered: 9-2003
| | Posted on Thursday, January 26, 2006 - 7:37 am: |
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Something is wrong how high is it and what are you feeding it with ?????
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Scoobydoo
Member
Username: Scoobydoo
Post Number: 56
Registered: 8-2005
| | Posted on Thursday, January 26, 2006 - 9:01 pm: |
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Its at least 20 feet form the ground and im feeding it with Radio Shack RG8/U. |
Bruce
Senior Member
Username: Bruce
Post Number: 3467
Registered: 9-2003
| | Posted on Thursday, January 26, 2006 - 10:20 pm: |
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Should work better than that my V-2000 is up 25 foot and i can work 30 miles to another base |
Marconi
Advanced Member
Username: Marconi
Post Number: 563
Registered: 11-2001
| | Posted on Thursday, January 26, 2006 - 11:41 pm: |
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Mine too I have one up at 24' to the tip and it is the hottest I have ever had. It goes out about 37 miles before it peters out and I cnat haire nothing ecxept atatic. I will try one of the 5000 watt better machers and see if it won't make me get out about 40 miles or more. I got a real short feed line so that should help hold all the power. |
Bruce
Senior Member
Username: Bruce
Post Number: 3469
Registered: 9-2003
| | Posted on Friday, January 27, 2006 - 7:38 am: |
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Last Weds on out 2 meters net using the Diamond vert i easly could hear checkins from Bradinton 30 miles away and on the beams out to 40.
Even THAT simple antenna Scoobydoo has should be good for 10-20 miles |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 57
Registered: 8-2005
| | Posted on Friday, January 27, 2006 - 1:47 pm: |
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Yeah, This is mind boggling for me, The recieve was excellent but transmit was horrible. Should the coax show a short when connected to the antenna? There is no short when not connected to the antenna. Will the thickness of the groundplane wire make a difference? What about the little balls on the ends? I bought some more music wire when I was messing around with different lengths and I used a slighty smaller diameter, .42 instead of .55. I finally found a company that does sell an base antenna that covers the frequencies I use, Its made by Comet, Its something like 2 5/8 waves on vhf and 4 5/8 waves on uhf. If I cant get this one to work properly I'll order it. Thanks again for the help. |
Tech833
Moderator
Username: Tech833
Post Number: 1173
Registered: 8-2002
| | Posted on Saturday, January 28, 2006 - 12:50 am: |
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Your coax is the problem. For VHF and especially UHF, you need double-shielded coax. It is not really an option, it is almost mandatory.
Copper sells LMR-400 which would be a superb choice. |
Scoobydoo
Member
Username: Scoobydoo
Post Number: 58
Registered: 8-2005
| | Posted on Saturday, January 28, 2006 - 1:17 am: |
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Wow, did not know that, But the weird thing is, it works better on UHF than VHF. Goofy Antenna! LOL! |
Bruce
Senior Member
Username: Bruce
Post Number: 3474
Registered: 9-2003
| | Posted on Saturday, January 28, 2006 - 6:49 pm: |
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Something wrong My DIAMOND V-2000 is a standard ham vertical and it works about the same on all 3 bands which givin it's gain is normal |
