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Simple 2 m 1/4 wave groundplane Radial Angles?

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#1
I am constructing a simple 2 m 1/4 wave groundplane which is made of 5 brass rods, one center vertical 19.5 to 20 inches long soldered to the center of an SO-239 connector, and four 19 inch radials bolted to each of the connector holes of the SO-239 connector.

My question is "what angle is best for the radials?" The drawing I have shows them at about 45 degrees but I have seen other antennas with angles of 60 or 80 degrees.

.................Cut four pieces, 19 inches long for the radials. On one end twist the rod to form a loop. Fasten the radials to the chassis connector holes by running the four small bolts through the loops and bolt on the other side. .
Note :If you have access to an SWR meter, you can cut the center element at 20 inches and then prune the element ( about 1/8 " at a time ) for best SWR...................
 

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Wichita Falls, TX
#2
The angle changes the impedance. A 1/4 wave antenna over a flat ground plane or horizontal radials is about 30 ohms. Bending the radials down to the standard 45 degrees raises the impedance to around 50 ohms. Bending the radials 90 degrees down from horizontal would raise the impedance to around 70 ohms or the same as a dipole.
 
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#4
That 45 degree angle is a good starting point. After that, it depends on several other factors and there's no absolute just one 'angle' that will be 'best'. Start with what you've shown and vary things a bit to see if there's any improvement. No improvement at 48 degrees? Put it back like it was. It amounts to what works best in your particular situation.
Have fun.
- 'Doc
 

kb2vxa

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#5
Just to add my 2c worth of experience making these things, first of all use 1/16" dia. brazing rod for a good fit and solder everything. Eliminate the extra hardware by making the radials just about 1/4" longer and bend a small hook in them, stick the hooks through the holes in the flange and solder.

If you're going to use it outdoors waterproofing is a MUST, begin with a generous application of silicone rubber bathtub caulk over the insulator, not the PL-259 side dummy. (;->) Then after connecting the coax and using the cheapest black vinyl electrical tape you can find (premium brands don't stretch well) spiral wrap the tape beginning a few inches below the connector stretching and keeping tension as you go follow upward to the flange and wrap a turn parallel to it over the exposed threads. Then wrap down stretching and keeping tension back to where you started but don't stretch or tension the last turn, just lay it flat over itself making a total of two turns. The tape will shrink somewhat making it tight which is what you want and the slack turns won't come loose which is what you don't want. It works for the military and it'll work for you, I've used this method of waterproofing and fastening coax to masts for years and not one has ever come loose.

So how do you go about mounting this thing? Simpler than you think, no clamps or hardware needed! Just shove the coax through the center of an ordinary 1 3/4" TV antenna mast (before you attach the antenna stupid (;->) ) and the antenna will sit on top quite nicely. How and where you mount the mast is your problem, I can't tell you EVERYTHING or you'll be just as smart as I am. (;->)
 
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#6
A 1/4 wave whip over a perfectly conducting infinite ground plane at a 90degrees to the whip has an impedance around 35ohms. 3 or 4 1/4 wavelength ground radials have less capacitance between the whip and ground plane than the first example and the impedance will be slightly higher and closer to 50ohms.

If you make the 4 radials stick out 90deg from the whip it will tune fine and should get to a perfect match because as you trim the whip it will come out slightly short which will be slightly less capacitance to ground and with a correspondingly slightly higher impedance closer to 50ohms.

So make them stick out straight or bend them down at 45 deg and it can be tuned to a perfect match either way. Usually don't bother bending them down and all my ground plane antennas tune up just fine.
prcguy
 
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#7
I assume that this is just for reception - if you want to use it for transmitting, then you should have an SWR meter - tweak the angle of the radials for best SWR - it will be affected by the mast material and other close by metalwork.
 

kb2vxa

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#8
I don't know why you resurrected a thread over a month old but what the heck. Scroll up and see how I made several and never had a less than perfect match, nothing need be tweaked. Oh and BTW, the mast material is irrelevant, nothing below the radials makes a hill of beans difference and like any antenna you don't mount it next to metal without decent spacing... OOPS!
 
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#9
If the radials stick straight out at 90deg, the whip can be tuned for a perfect match. If you bend the radials down at 45 deg the whip can be tuned for a perfect match. Bottom line is it doesn't matter much because the whip can be trimmed a bit long with slightly more capacitance to the ground plane for 45deg radials or a bit short for less capacitance to 90deg radials.
prcguy




I assume that this is just for reception - if you want to use it for transmitting, then you should have an SWR meter - tweak the angle of the radials for best SWR - it will be affected by the mast material and other close by metalwork.
 
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