Building a 5/8 wave antenna

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BoogieBertil

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I am new at this, but i would like to understand the theory behind the antenna before building it.
First, if the antenna is tuned, both radiator and radials, why do i need a matching coil?
There is a lot of homemade projects on the Internet, and most of them suggests a coil.
Somebody who can advice me?
Thanks.
Bertil - Norway.
 

nd5y

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Most antennas are designed to have an impedance of 50 ohms.
If the vertical radiating element is 1/4 wavelength and the radials are 1/4 wavelength or you have a solid ground plane with a radius of around 1/4 wavelength or longer then the impedance will be close to 50 ohms.

If you make the radiating element 5/8 wavelength the antenna will have slightly higher gain but capacitive reactance increases the impedance to something like 1000 ohms if I remember correctly.

You need some type of matching circuit that changes the impedance back to 50 ohms. Adding a coil (inductor) adds inductive reactance which cancels out the capacitive reactance.

Here are some pages with links to antenna theory web sites
Antenna - Antennas: Theory
Amateur Radio Antenna Projects
 

nanZor

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I would choke it near the feedpoint. RS 273-105's that clamp around RG-6 would be suitable, although expensive, as you'd need at least 4, preferably 6 or so to really do the job.
 

prcguy

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A 1/4 wave monopole over infinite ground is around 39 ohms at the feedpoint. Lessen that to 3 or 4 radials and the impedance goes closer to 50 ohms, so no real need to droop the radials downward.
prcguy

It should not make any difference for scanning. The reason that most ground plane antennas have radials sloping down is because it changes the impedance. A 1/4 wave vertical over a perpendicular ground plane has an impedance of about 30 ohms. when you bend down the radials to about 45 deg. the impedance increases to about 50 ohms. It also lowers the radiation angle a little. If you continue to bend them down 180 deg parallel with the feed line (or use a sleeve instead of radials) the impedance is 70 ohms, which is the same as a dipole.

Tom ND5Y
 

BoogieBertil

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Jun 29, 2014
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Norway
Most antennas are designed to have an impedance of 50 ohms.
If the vertical radiating element is 1/4 wavelength and the radials are 1/4 wavelength or you have a solid ground plane with a radius of around 1/4 wavelength or longer then the impedance will be close to 50 ohms.

If you make the radiating element 5/8 wavelength the antenna will have slightly higher gain but capacitive reactance increases the impedance to something like 1000 ohms if I remember correctly.

You need some type of matching circuit that changes the impedance back to 50 ohms. Adding a coil (inductor) adds inductive reactance which cancels out the capacitive reactance.

Here are some pages with links to antenna theory web sites
Antenna - Antennas: Theory
Amateur Radio Antenna Projects
Thank you.
I am more related to impedances in a more physical mode. (capacitors/resistors/sound)
I know some basic rules in a coil running currents.
Is there a template showing changes in impedances
when using different effects, coil/copper diameter, frequency, lenght.....
Maybe far fetched, but this is interesting to me.
Thanks again.
Bertil - Norway.
 

BoogieBertil

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Jun 29, 2014
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Norway
Absolute science

Is it optimal to have as low SWR as possible with a 5/8 wave GP antenna?
I fine tuned the matching coil, found the sweet spot, tuned the antenna, and managed to get the
SWR down to 1.2.
But the output showed higher reading with higher SWR, and the distance range seemed a bit better too.
Is this relation between output and SWR-numbers an absolute science?

Thanks - Bertil - Norway.
 
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