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Post by doctor on Sept 1, 2015 7:34:17 GMT -5
I was watching a video on building a cb antenna using a 108 inch whip, easy enough but when it came to putting radials on it, they were 108 inches, not the length that most of the prebuilt antenna with radials are, we all know much shorter.
In digesting this, the 108 inch radials make more sense than putting on the commercial radials at least half the size. Any explanations on this...
DOCTOR/795
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Sandbagger
Administrator/The Boss 
Posts: 6,250 
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Post by Sandbagger on Sept 1, 2015 9:12:40 GMT -5
I was watching a video on building a cb antenna using a 108 inch whip, easy enough but when it came to putting radials on it, they were 108 inches, not the length that most of the prebuilt antenna with radials are, we all know much shorter. In digesting this, the 108 inch radials make more sense than putting on the commercial radials at least half the size. Any explanations on this... DOCTOR/795 The most common ground plane antenna to build is a simple 1/4 wave. That consists of a single vertical 108" radiator section, and 3 or 4 108" radials drooped at a 45 degree angle (to provide the proper 50 Ohm feedpoint impedance). Any ground plane "should" have 1/4 wave (108") radials for the most effective ground plane. However, many 1/2 wave and other designs have used 1/8th wave (54") radials. Those would seem to work to some degree, but I would think that full 1/4 wave radials would work better. |
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Post by doctor on Sept 2, 2015 8:13:29 GMT -5
I WOULD AGREE FULL LENGTH RADIALS IS THE BEST WAY TO GO. DOCTOR/795  |
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Post by 2600 on Sept 7, 2015 22:02:57 GMT -5
The traditional "weeping willow" ground plane uses that 108-inch vertical stinger and three radials the same quarter-wave length. As a rule you'll need to bend the radials downward to about 45 degrees to get the SWR right.
And that's how we 'tuned' that antenna back in the day, by bending the radials up or down for lowest SWR. Nothing else on that antenna was adjustable.
Making one out of a full-length steel whip should work just fine. Remember that the "102-inch" whip was meant to have a six-inch spring at the bottom, for a total length of 108 inches.
The radials seen on half-wave and 5/8-wave vertical base antennas are usually not the full 108 inches in length, and they're not usually bent downwards at 45 degrees, either. Those types of base antenna are just different from the quarter-wave type.
73
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Post by doctor on Sept 8, 2015 8:38:00 GMT -5
THANKS for the comments, appreciate it. Interesting , yesterday I put 2 more radials on the setup now have 4 radials in a h type of pattern, now my swr climbed 1.3 to 1. I then removed the 2 additional radials and put them over on the same post as the other original , one each on original post, checked swr 1.1 to 1, so left them as is. ANTENNAS are interesting, fun to experiment. HAVE A GOOD ONE DOCTOR/795 |
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roadrunner
Mudduck
Just got here today. Looking forward to talking to anyone about RF radios and antennas.
Posts: 37
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Post by roadrunner on Oct 31, 2015 12:40:33 GMT -5
![]() Attachment DeletedHere is a Antenna that i think your talking about. This is a home made 1/4 wave aluminum tube Antenna that is turned to the center frequency of 27.205. Transmitting element has 5/8,1/2,and 3/8 telescopic tubes at 8.600 long for channel 20. (11 Meters) The Ground radial length is 102'' with four elements that are telescopic as well. The ground radials are bent at 45% and the match was Almost 1 to 2 but i am going to test it with the ground radials at a horizontal position and the same length. I have read that ground radials bent down at 45% will give you a better match. It Maybe true but i will find out soon. Roadrunner |
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Sandbagger
Administrator/The Boss
Posts: 6,250
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Post by Sandbagger on Oct 31, 2015 15:27:41 GMT -5
![]() View AttachmentHere is a Antenna that i think your talking about. This is a home made 1/4 wave aluminum tube Antenna that is turned to the center frequency of 27.205. Transmitting element has 5/8,1/2,and 3/8 telescopic tubes at 8.600 long for channel 20. (11 Meters) The Ground radial length is 102'' with four elements that are telescopic as well. The ground radials are bent at 45% and the match was Almost 1 to 2 but i am going to test it with the ground radials at a horizontal position and the same length. I have read that ground radials bent down at 45% will give you a better match. It Maybe true but i will find out soon. Roadrunner A direct fed 1/4 wave ground plane should have the radials at a 45 degree angle, in order to come close to a 50 Ohm feedpoint impedance. Placing the radials in a horizontal plane will give you a much worse SWR unless you employ some sort of beta match like Radio Shack used on its 1/4 wave GP. |
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