| Author |
Message |
    
Znut
| | Posted on Wednesday, September 11, 2002 - 6:18 pm: | 
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I'm building a 2 element quad with full wave loops. I'm using 1/2 inch CATV hardline for the elements. Somebody reminded me on that thread that the diameter effects the length. All the calculations I've found are for wire.
Do you techs know any formulas or guidelines to correct for the diameter?
Thanks,
Znut |
Marconi
| | Posted on Thursday, September 12, 2002 - 8:46 am: |
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Hey Znut, I thought you already had your loop working. There is a chart showing the K factor curves for the difference in physical and electrical that is used after you determine your length to diameter ratio.
Formula is: Physical 1/2 wave length/element diameter. 18/.5 = 36
The value 36 on the chart indicates something a little less than .95
If you add this K factor value to the formula that figures the length of the loop, you should be very close. In your case, if you used .94 - .95, you should be real close.
You can check all this out by using the information you already might have. Use your formula to get the physical length you started with. Measure the loop after you tuned it. Divide the smaller (electrical) length by the longer (physical) length and the number is the K factor for that setup. However, when using this approach you must assume that your loop antenna is already resonant and reflecting no reactance at resonance.
As I have stated, the K factor you should be using in your formula should be about .94 with your 1/2" diameter element in 11 meters. This is a 1/2 wave chart but the K factor should be the same with your full wave formula.
Marconi |
Znut
| | Posted on Thursday, September 12, 2002 - 12:29 pm: |
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Marconi,
Yes, I did have the loop up. It is cut at 37'. I decided to make a 2 element quad with it. I cut the reflector
37' 10". When I had the loop I had OK SWR over a wide range. However, I want to try to make sure I have all of the dimensions as accurate as possible to realize the potential gain.
Using the "K" factor the dimensions are 34' 9 3/8" for the driven and 35' 7 3/8" for the reflector versus 37' and 37' 10". So I guess I'll have to do some trimming. The elements will be fairly easy to adjust but I'll have to get some sig reports to fine tune for maximum directivity. The VSWR will likely be good over a wide range even when the gain and front/back ratio suffer.
I really appreciate your help! You'll hear more soon(hopefully on the air!) and I plan on having pictures.
Take Care,
Znut |
Marconi
| | Posted on Friday, September 13, 2002 - 8:45 am: |
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Hey Znut, don't go cutting anything, not quite yet. According to what you have told us, which is not full of details, you were already near resonance with the loop. I mentioned this already.
What I gave you was a method of checking the length/diameter (K factor) affect on the data you should already have. If the data and the new information don't jive with your real world results, then you have to reconsider what is correct. Without trying to guess how you are figuring your measurements, I would say, hold up a bit. You should be saying to yourself, "Something just doesn't make sense, why aren't the measurements closer. I know I was near resonance the way the loop was trimmed when I had it up." Look at what is physically in front of you and not what is theoretically behind you.
Personally I would not even consider this topic (L/D ratio) in my antenna work. The process of actually tuning an element takes this principal, plus all reflective and dielectric affects into account.
BTW, I think your concern about balance with the new beam idea has some merit. Arguably, for the beam pattern to be truely best, you should make this happen. I am not sure how you will know what happens with/without a balun, however. I am also not too sure how well it will help in matching the impedance you are sure to get when you add the reflector.
I do not know what you want to build here, but a proper coax coil choke attached directly to the element will probably serve you best. Unless you make some sort of matching device, you may be OK but you will just have to live with some reactance in this two element loop. |
Marconi
| | Posted on Friday, September 13, 2002 - 9:55 am: |
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Znut, for you guys that want some good on line reading about antennas, here's something that may be of interest to anyone wanting to learn more about antennas and how they work.
http://www.firecommunications.com/school/reading37.pdf
and another one on transmission lines.
http://www.firecommunications.com/school/reading38.pdf
there is a collection of 41 files altogether.
http://www.firecommunications.com/school
Note: acrobat reader 3 or higher is required to view them. check out the files.txt file for the complete listing.
Thanks to Jack for this information.
Marconi |
Znut
| | Posted on Friday, September 13, 2002 - 8:56 pm: |
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Wow!
Marconi, as soon as I get the antenna constructed it will definitely be fed with RG8X wound into a choke right at the feed point, about 6 feet of coax wound 8 or 9 times and pigtailed right to the element. No chopping right away. It's easier to take the length away than to add it. Also, I can live with some reactance if it will help the bandwith can't I?
When Smiley brings the analyzer over we'll know the true resonant frequency, impedance and the reactance and go frome there. I have some work to do in the meantime.
Thanks again techs for all your help.
Take care,
Znut |
Marconi
| | Posted on Saturday, September 14, 2002 - 7:33 am: |
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Znut, I do the very opposite, I use the analyzer to try to get a tune that has absoutely no reactance at the frequency I work most. I just don't have time to talk on a lot of frequencies anyway. I usually have about a 1000 -2000 kHz's to work with and for 11 meters that is enough.
For some ideas go look at Jay's beam antenna: Choke Installed the Correct Way |
Znut
| | Posted on Sunday, September 15, 2002 - 1:08 pm: |
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Yes, I will hook the analyzer right to the antenna. Until I get that opportunity I'm gonna do like I said and try her out. You know how it is, ya just have to try em out.
Still ugly weather. I don't have a two car garage to bring it inside and work on it.
Znut |
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