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Paper765
New member
Username: Paper765
Post Number: 5
Registered: 12-2004
| | Posted on Friday, December 31, 2004 - 11:41 am: | 
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I'm in the process of making a homebrew base antenna. I've done it before, and it worked pretty well, but I wanted to make a modification on my initial creation. On the old antenna, I had a 9 ft steel whip mounted on a base plate, and 4 radial wires coming off, roughly in north/south/east/west directions. These I tied down with 60 lb test fishing line (non conductive) to the four corners give or take of the roof (of my house).
This project is different in a couple ways. First off, I don't live in the old residence, and I am putting this new antenna on top of a tower. Recycled the mounting plate, and I wanted to attach radials to it, shorter but more of them (like 8 three foot radials). Some reading I did tonight in a OLD cb book (The big dummy's guide to CB radio) gave me the impression that those ground plane radials all must be a 1/4 wavelength. That is to say, 8 short radials won't work, instead I should stick with 4 long radials (102 inch each).
Now I'm aware of the Antron and lots of other stick antennas, and they are doing a much shorter ground plane radial length, but they are also doing all sorts of coils and capacitors and nonsense I don't want in my homebrew. I want this to be an antenna based strictly on simple radio physics.
My question is, what do I need to know about the ground plane radial length. Will a number of short radials not work?
Could I use 5 to 6 ft lengths of 6063 Aluminum tubing to make my radials? Just how strong is Aluminum tubing? Would it be strong enough to just out horizontally from a stainless steel plate, without any support in the middle or at the end of the tubing?
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Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 101
Registered: 9-2004
| | Posted on Saturday, January 01, 2005 - 7:59 pm: |
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paper765, you are correct about the antrons and other antennas using shorter ground radial lengths. they can get away with that because the radials are designed to work with only that particular antenna. for your project you need to keep all ground radials (no matter how many you use), to a length of 102". this is 1/4 wavelength at 27mhz.
here's how i see your setup working the best. use a fiberglass 102" whip for the vertical section. (they bend a lot less in the wind and your signal will be more steady.) your mounting plate should be about 6"x6" square. buy a mounting stud from rad. shack that goes from a 3/8" stud to an so-239 connector. drill a hole in the middle of the plate that will accomodate the stud and insulator. mount the 102" whip to the mounting stud through the plate. (make sure you use the heavy duty spring with it or your swr will not be matched. buy 8 u-bolts that will fit around the alum. tubing you wish to use and drill holes in the mounting plate so that each radial will stick out from the corner and two u-bolts will anchor it to the plate. calculate the length of each radial so that from the edge of the center hole to the end of the radial is 102". do not let any of the ground radials touch the spring. the tubing that you use should be .120" (about 1/8") wall thickness or close to that. 3/4" o.d. tubing would be ideal but use what you can find. if you cant buy the aluminum in the leangths that you need; find some lengths and make a telescoping radial. find a size that fits inside the 3/4" tubing. cut a 3" slot in one end of the 3/4" tubing and use a hose clamp to secure the smaller diameter tubing in place. this will also make tuning the radials to size (102") easier. this should make a pretty decent antenna and shouldnt require any tuning.
good luck!
matt |
Tech833
Moderator
Username: Tech833
Post Number: 931
Registered: 8-2002
| | Posted on Sunday, January 02, 2005 - 11:39 am: |
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Hi Paper.
What you describe is a 1/4 wave ground plane antenna. The many shorter radials will not substitute for the four 1/4 wave ground planes. In order to make that antenna work, you do need the 1/4 wave long ground planes under the whip.
You CAN use loading coils in the ground planes if you like, although with slightly reduced efficiency. The SWR will still be about the same. Use mobile whips or make your own. Just get the loading coils as far away from the vertical whip as possible (top loaded mobile whips would be better than center or base loaded).
Avanti used to make a 5/8 wave antenna that used loaded ground planes and a diamond shaped top cap arrangement. |
Paper765
New member
Username: Paper765
Post Number: 6
Registered: 12-2004
| | Posted on Wednesday, January 05, 2005 - 1:08 pm: |
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I talked to an electronics friend of mine. He said that perhaps four 4.5 ft long Aluminum rods would work, giving the diameter of the circle (if you drew a circle around the four rods, with the end of each rod being a point on the circle) a total diameter of 9ft. Or in other words, four 4.5 ft long radials. Any thoughts on this?
I just bought 4 6 ft long .58 diameter aluminum rods, and hope that these will work. I'm planning on leaving them in the 6 ft length. That was the longest I could find.
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Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 108
Registered: 9-2004
| | Posted on Wednesday, January 05, 2005 - 3:27 pm: |
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paper765, i imagine that 6 ft. radials will work. the reasoning behind using 102" radials is that it provides the correct electrical counterpoint to the quarter wave radiator that you are using. go ahead and try the 6 ft radials and see what the swr is. nothing worse than a project that never gets finished. if the swr is over, say, 2.5, then try to find some hollow aluminum tubing that will fit over the rod youve got now. that way you can make a telescoping radial like i described above. if youre having trouble finding the aluminum tubing look in the phone book and find your local steel supply shop. they usually carry aluminum too. most of the time they have old stock or cut offs that they will sell pretty cheap. good luck and be sure to post your results.
BTW, you might also try getting creative at the home depot. just wander around looking at shower curtain rods and the like and maybe youll strike aluminum!
matt |
Bruce
Senior Member
Username: Bruce
Post Number: 2001
Registered: 9-2003
| | Posted on Thursday, January 06, 2005 - 7:14 am: |
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765 they need to be 4.5 foot but try what you have and if that works leave well enough alone .... |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 112
Registered: 9-2004
| | Posted on Friday, January 07, 2005 - 12:28 am: |
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i think the actual measurement would be 51". that would be 1/8 wavelength.
matt |
Bruce
Senior Member
Username: Bruce
Post Number: 2011
Registered: 9-2003
| | Posted on Friday, January 07, 2005 - 7:12 am: |
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you need a 1/4 wave |
Allagator
Advanced Member
Username: Allagator
Post Number: 528
Registered: 9-2002
| | Posted on Friday, January 07, 2005 - 6:12 pm: |
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Paper i have the same book and tried the sme antenna with the 8 shorter radials and it didnt work and i had higher swrs ! but with the 8 radials i could hear better ! i havent tired the 8 radials at 102 long now that could improve the recive but im not shure how much !!!!
Hope this helps !!
Allagator ! |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 114
Registered: 9-2004
| | Posted on Saturday, January 08, 2005 - 7:12 pm: |
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more ground radials = less ground loss.
matt |
Allagator
Advanced Member
Username: Allagator
Post Number: 533
Registered: 9-2002
| | Posted on Saturday, January 08, 2005 - 8:02 pm: |
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correct ! but only if they are the right length ! |
Kj7gs
New member
Username: Kj7gs
Post Number: 9
Registered: 12-2004
| | Posted on Wednesday, January 26, 2005 - 9:08 am: |
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A question on that -- if radials have to be the right length, then how does a car body work? |
Tech808
Moderator
Username: Tech808
Post Number: 4641
Registered: 8-2002
| | Posted on Wednesday, January 26, 2005 - 4:12 pm: |
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Kj7gs,
On a car the antenna is using the WHOLE COMPLETE car as a ground plane.
The Bigger the car & the more metal the better.
Lon
Tech808
CEF808
N9OSN |
Tech833
Moderator
Username: Tech833
Post Number: 969
Registered: 8-2002
| | Posted on Wednesday, January 26, 2005 - 7:20 pm: |
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A car is not made of wires. A solid sheet of metal will change everything, but makes for an ugly base antenna, not to mention windload. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 331
Registered: 11-2001
| | Posted on Friday, January 28, 2005 - 1:32 am: |
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Well 765, you said that you did it before and it worked fine, so why change what you know works?
In my experience with just a 1/4 SS whip as the radiator, I found that I could only use one 1/4 wave element as the ground plane. This element was attached directly to the mount. Anything much shorter or longer tended to cause the SWR to go up. Adding another element also made the SWR go up. Adding three shorter elements made the SWR go up.
I attached this element directly to the L-bracket mirror mount using vice-grips to hold it in place. I found that only 1 ground plane element could be attached with my project. It worked fine, but was probably a little directional producing a lopsided pattern.
I do not know how to reduce the size of my image to the forum limits or I would include a picture.
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Marconi
Intermediate Member
Username: Marconi
Post Number: 333
Registered: 11-2001
| | Posted on Friday, January 28, 2005 - 10:12 am: |
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In addition to my remarks above, Kj7gs makes and excellent point about this topic when he states the following:
"A question on that -- if radials have to be the right length, then how does a car body work?"
I can't answer this question. I can't even come up with some believable words to explain. I was surprised to find what I found in my project, that only one (1) 1/4 wave horizontal radial would work to allow a tune for the SS 1/4 whip I was using. Again, only one element seemed effective. I did not attempt to angle the element or elements down as you noted above however.
There is also a possibility that I might try it again someday and find something else entirely different to be true. If you are just plunkin' around with stuff, what I found is probably not and unuasual result. Surprise!
765, you will just have to experiment a bit. Antenna design is obviously technical, following some rigid scientific rules, but there is also some Art involved, so things are not always so cut and dry. Just ask yourself how does a little 1/4 wave whip, near the ground and on a mobile, hold its head up against a monster antenna 40' high at some base station? Because of that possibility, to me, all of these antennas mentioned are more similar than they are different.
Hang in there and keep us posted.
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Tech833
Moderator
Username: Tech833
Post Number: 974
Registered: 8-2002
| | Posted on Friday, January 28, 2005 - 6:04 pm: |
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Marconi,
What you had with only one ground plane was an inverted V dipole on its side. An inverted V has a nice 50 ohms feedpoint but is a bit of a 'cloudwarmer'. Tilt the V on its side, as you did, and you can make that lobe go wherever you want instead of against the ground then straight up.
Adding more ground planes (let's say 3 total) will get your feedpoint resistance down to about 30-35 ohms R and about +13j. If you tilt the ground planes downward (like the Skylab antenna Copper sells), then the feedpoint R goes up and the j goes down.
Of course, you could do the same thing with a hairpin shunt on the antenna and leave the ground planes at 90 degrees. |
Kj7gs
Junior Member
Username: Kj7gs
Post Number: 11
Registered: 12-2004
| | Posted on Saturday, January 29, 2005 - 12:43 pm: |
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Does that mean I can take a 19" length of wire and attach it to the bottom of my HT antenna, hold it almost upside down and have one of these? |
Marconi
Intermediate Member
Username: Marconi
Post Number: 336
Registered: 11-2001
| | Posted on Saturday, January 29, 2005 - 1:28 pm: |
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833, I was expecting to find just what you suggest about the Skylab or something similar. As I said I never slanted the element down, because I never did get anywhere near a good match except when using only one radial. I was surprised that two and maybe three horizontally did not work for me. I did not see this as a dipole, but that is probably what I had with one element bent making an L shaped dipole.
I did not work it very much for affect, but you may be right about it mostly going straight up.
I had a buddy that built a mounting plate with 4 horizontal radials and one vertical radiator to mount in his attic. He used a short loaded mobile stick of some kind for the radiator and wire for the radials. I don't know the lenght of the elements, but he said he got a good match and I know it worked well for him. That is more or less what I was attempting to do using a 1/4 wave SS whip. Do you think that being a 1/4 wave instead of a loaded and shortened antenna had anything to do with the results I saw in my project? |
Ferd1605
New member
Username: Ferd1605
Post Number: 3
Registered: 1-2005
| | Posted on Sunday, January 30, 2005 - 3:10 am: |
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Hey . tilt the raidials down at a 45 deg. angle to get the feedpoint impedance to 50 ohms ... NOT straight out .. and you might try a hairpin match also for DC grounding for static/lightning protection. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 338
Registered: 11-2001
| | Posted on Sunday, January 30, 2005 - 9:10 am: |
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765 if you wish to read something, learn some more, or maybe just get more confused about this issue, then get Walter Maxwell's book on Reflections II. It is a hard read, but it touches on some of your concerns in his section 5.5. Be aware that, for the most part, he is discussing ground mounted 1/4 wave vertical radiators mounted to the earth and generally this type of system requires physical grounding for operations. He comments on physical grounding as it relates to both safety and operations. Herein lies the likely source for some of CB's misinformation the issues about grounding CB antennas. He touches on raised Ground Planes also.
Walter Fair, W5ALT has a web site that discusses this issue also. He gives tables and is much easier to read. His detail work here is in fact more about what you wanted to know.
My best advise to you is don't stop experimenting with your interest, but in this case, going on a tower, get yourself a good vertical up there. One that doesn't cost you much. You will probably want to do something else soon afterwards, and this business can get expensive. Plus going up that tower isn't something you will want to do very often.
Good luck, |
