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1/4 wave ground plane antenna design question

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cristisphoto

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It may...

kokomo1 said:
On this type of design...

http://www.pbase.com/dickh/image/27104369

Does the length include the "hooks" on the ends?
I duuno LOL
I do know that ast night I tried a coaxial
Cabe and it works wonder on Low band and 460 range.....
I stratted to pick up more CHPops and San Jose PD just fine
wheras with my stock duckie bearly if at all :p
SOO I reckon something is better than nuthin..
Crista
 

ka3jjz

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Yes, the length of the 'hook' is included, I'm sure. Actually, since this antenna is supposed to be tuned, the Hamuniverse's website has the formula (and this link comes from our Scanner Antennas wiki - which you should really look at more closely :.>>)

http://www.hamuniverse.com/2metergp.html

Altho the antenna shown is designed for 2 meters, the formula makes it a snap to tune it for another band. And another tip - it's possible to add a 2nd vertical element - cut to the same formula - to give the ground plane a somewhat wider frequency response. The wiki has a link describing this, too. 73s Mike
 

VintageJon

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The "hook" actually is a capacitive element, thus it shortens the efffective length.

SWR match is not that critical for receive-only but the general quality of workmanship in this example disturbs me. BUT, if it works it's good.

See my avatar for my version. A large photo is available if you email me. It works very well here in Central Austin for 866 MHz APCO-25. (I even hear Georgetown on it!). Feed is 10 ft. of high quality coax.

73's,
Jon
 

Chris0381

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The workmanship of this antennae is what impresses me because it shows you dont need some fancy moma's boy looking antenna to get good results. What I would like to know is the purpose of the loops and would it work just as well without them and would soldered connections give better performance.

The loops may be so it can sit on a dest without scratching it ?
 

kb2vxa

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OK folks here we go,

That's for 800MHz which is why copper is acceptable and the same for UHF only double the length. Make one for VHF and it increases to 19" so 1/16" diameter brazing rod is a must, copper ends up like a pretzel after the first wind storm. I argue the angle on the radials, down like that you have a dipole with a 75 ohm feedpoint impedance. For 52 ohms they should be at 90 degrees in true groundplane fashion. Forget the hooks except atop the vertical element if you want to hang it from the ceiling, they have negligable capacitance and negligable electrical effect. (They'd have to be considerably larger.)

I made lots of them over the years BUT the hardare proved problematical so I soldered the radials into the holes after putting a slight hook in one end (leave a bit of extra length for this) and it works much better in the long run. Note the gob of silicone bathtub caulk on the insulater to keep rain water out and the connector must be taped from the flange to about two inches down the coax.

The beauty of this beast is that after filing off the excess solder and ends of the radials under the flange it fits nicely atop a standard TV mast without any special mounting considerations. Just shove the coax up through it, tape, and set it in and the weight of the cable holds it firmly in place. So what if it's a bit off center? Never mind, so am I.

"The loops may be so it can sit on a dest without scratching it ?"

"It is possible."
Antonia

Not likely though, note the fact there is no coax attached. If it were the antenna would be lying on it's side and if the radials were angled properly, well, it was for photographic purposes.

One final note, with 19" elements it resonates on VHF, UHF and 800MHz and has a feedpoint impedance of 52 ohms on those bands. 1/4 wave and odd multiples thereof exhibit those characteristics and gain rises slightly as the frequency goes up. Don't count on it elsewhere, it's NOT one of those DC to daylight snake oil antennas.
 

jhooten

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I put loops on the ends for a couple reason. They do the same thing the little ball does on a store bought antenna. Blunt the end so it don't poke out an eye as easily and acts as a corona ball to help disipate static build up.

A PL259 or N connector fit snug in the end of 3/4" pvc pipe, BTW.
 

kb2vxa

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Hi again,

"I put loops on the ends for a couple reason. They do the same thing the little ball does on a store bought antenna. Blunt the end so it don't poke out an eye as easily and acts as a corona ball to help disipate static build up."

I hate to rain on your parade but there are two holes in that logic. First, unless you're playing with it right in front of your face you won't poke an eye out. I had that argument with my mother several times. If you put it on the roof where any antenna really should be it's the birds' problem. I once saw the skeleton of a pigeon impaled on a radial of a CB antenna so maybe it's a good idea after all.

Second, a sharp point is best for dissipating static charge, take the lightning rod and the StatiCat (TM) atop the broadcast tower. A gently curved surface such as a sphere provides far less leakage, take the corona protection shields on high voltage transmission equipment. In any case VHF/UHF FM is practically noise immune so it really doesn't matter, take the antennas mounted atop high voltage transmission towers and in the transformer yard at the substation.

"A PL259 or N connector fit snug in the end of 3/4" pvc pipe, BTW."

Thanks for the heads up, I never really thought of that with a Rat Shack right up the street. Considering grounding requirements I would check the fit in 3/4" EMT. Then since it rusts there are two other related possibilities. One is use the next larger diameter, wrap the '59 with tape as a shim to keep water out (it should be waterproofed in any case) and spray the tubing with a good weatherproof paint. The other is to use aluminium but still wrap the connector to waterproof and insulate it and prevent galvanic corrosion. It's not as important to ground the antenna itself as it is the mast since it'll take the brunt of a lightning strike anyway whether the antenna is connected to it or not. Oh, just an afterthought, don't allow the flange to rest on aluminium or it will corrode at that point and could cause spurious signal problems for reasons beyond the scope of this discussion.
 

jhooten

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Balcony rail at the hotel. Can't always get to the roof when traveling. and the loop lets you tie a non conductive cord to the antenna to hang it from the curtain rod if their is no balcony.

More surface area allows the electrons to bleed off slowly. Sharp points concentrate electrons to the point where the surrounding air can ionize and the static then has a low impeadence path to rapidly discharge, potentially damaging the attached equipment. Or so the tech manuals said in signal school (many years ago).
 

n8chb

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jhooten said:
I put loops on the ends for a couple reason. They do the same thing the little ball does on a store bought antenna. Blunt the end so it don't poke out an eye as easily and acts as a corona ball to help disipate static build up.

A PL259 or N connector fit snug in the end of 3/4" pvc pipe, BTW.
This is an old joke that still folks want to make up things it's used for.

Putting a protective ball, hook or what ever is to keep one from
putting there eye out. It does nothing else.

73,

Roger
 

jhooten

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from:
http://www.repeater-builder.com/rbtip/buster.html

repeater-builder.com said:
Corona Discharge Noise

The other type of noise called corona is due to the flow of electrons through the antenna (tower, boom and antenna elements) into the atmosphere when a charged cloud is near the antenna (fig.2). Corona or brush discharge occurs when a charge is built up and electrostatic lines of force are developed. More lines of force per square inch appear at the sharpest points and the more likely it is that a strong field will pull free electrons from the point (note 3). Electrons pulled out from a sharp point form a corona or brush discharge. This noise can also be generated by a mobile in motion and is the reason that mobile antennas have a ball of some sort at the tip. The ball eliminates the sharp point and tries to minimize the effect. If enough electrons leave in such quantities that the air is heated and becomes ionized, a spark of electronically heated air will be visible, also known as St. Elmos Fire. Corona is a major noise source but not the only one that should be in this category. There are also streamer currents, arcing and others. When a charge is built up on the antenna and tower or other supporting structures, it is possible for arcing to occur. The arcing will occur between any insulated parts or poorly connected ones if the charge potential raises to a sufficient level which it does frequently.
If it is a joke it is still put out as fact, as it was 30 years ago when I went through tech school.
 
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K9GTJ

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This has been some great discussion and learning.

One last idea...

Instead of 4 radials, what if I made a ciruclar disk instead? It wouldn't be that hard to make a 7 (apx size) inch disk for the ground plane. Is that bad???
 

Al42

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n8chb said:
Putting a protective ball, hook or what ever is to keep one from
putting there eye out. It does nothing else.
Well ... one other, closely related thing, if the antenna is more than about 3" long - it leeps you from accidentally putting it up your nose and into your brain. There's actually at least one recorded case of this happening with an antenna from which the ball had come off. (It was in QST many years ago - antenna in the guy's garage, he was looking for something, tripped - end of story and guy.)

As far as the disk is concerned, kokomo, no, not bad at all - just mount the connector in the center of the disk, at least 1/4 wavelength radius at whatever frequency. That's how mag mounts work.
 
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