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Radials on Antenna

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DaveA

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What exactly do the radials on a base scanner antenna do? Do they "pick-up" the signal? If so then if the radials are longer will that antenna pick-up signals better than the same antenna with short radials? How about the amount of radials? Will 8 radials on the antenna be better than the same antenna with ony 3 or 4 radials?
 

kb2vxa

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Hi Dave and readers,

Let's take this bit by bit.

"What exactly do the radials on a base scanner antenna do?"

A vertical antenna has the radiating (active) element working against ground as a counterpoise, the other half of the antenna system. In the case of AM broadcast the tower itself is the active element working against the Earth as counterpoise and the radials simply increase the ground conductivity being laid on top or slightly buried. Raised above ground the active element still needs a ground counterpoise so the "Earth" is raised up to it, the radials too are elevated.

"Do they pick-up" the signal?"

No.

"If so then if the radials are longer will that antenna pick-up signals better than the same antenna with short radials?"

No. The radials need be no longer than 1/4 wavelength at the lowest operating frequency, longer does no better. Rule of thumb; short radials for high frequency, long radials for low frequency so that's basically how you can tell the lowest frequency the antenna operates on.

"How about the amount of radials?"

That depends on the frequency and purpose. Those broadcast towers use typically 120 radials to increase ground conductivity, VHF and UHF antennas typically 3 or 4 as a counterpoise.

"Will 8 radials on the antenna be better than the same antenna with ony 3 or 4 radials?"

That looks like the typical CB question, I have seen ads for antennas advertizing soup to nuts mostly FOR nuts taking full advantage of ignorance of antenna theory. Take it for what it's worth, in the case of the typical scanner antenna you'll see four on a groundplane and a whole lot more on a discone but that's because they're emulating a true discone which has a solid disc and cone structure. There's no point in using more material than you need to construct an antenna, there is always that point of diminishing returns.

That's it, short, sweet and simple but watch how they muddy the water in the posts sure to follow. (;->)
 

LarrySC

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Short answer: They are needed only for transmitting to simulate an earthen ground. They complete the wave pattern. For receiving they only reflect signal to the [+] element. Scanner antennas like the Channel Master 5094A and the Scantenna dont even have a ground because the antenna is a 300 ohm Vertical Diapole using a 4:1 balun to match 75 ohm coax. Hope this helps.
 

kb2vxa

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Larry, you've got a LOT to learn. Meanwhile please stop disseminating misinformation and above all stop contradicting me and others with nonsense. The "short answer" for you is you can get an education by watching me sleep. Until and unless you have spent 30+ years studying RF communications the best you can do for yourself and others is keep your eyes on the monitor and your hands off the keyboard.

Now go back and READ what I wrote, then compare it with what YOU wrote. Then compare BOTH with the expert engineers' writings in the ARRL Antenna Handbook, the graphics will help your understanding. Which is closer to what's in the book? Eyes are for studying, brain is for understanding, mouth is for feeding eyes and brain. Misuse of body parts results in confusion.

Just to set you straight on a point of misinformation you "educated" us with, a vertical or horizontal (the only difference is polarization) dipole most certainly HAS a counterpoise "ground". Take that groundplane with the radials at 90 degrees (52 ohms) and bend them down to 180 degrees and you have a dipole with a feedpoint impedance of 75 ohms, NOT 300 ohms! FYI, a dipole doesn't need a balun to operate, it's function is providing an unbalanced feed from a balanced one, it's a BALanced-UNbalanced device which decouples the transmission line from the antenna reducing or eliminating feedline radiation. We can discuss turns ratios, current, voltage and choke baluns when you learn enough basic electrical theory to have a grasp of the more advanced subject material.

Good luck with your studies, "a mind is a terrible thing to waste".
 
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