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Drgrant
New member Username: Drgrant
Post Number: 5 Registered: 6-2007
| Posted on Sunday, June 10, 2007 - 12:17 pm: |
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Hi All- Just put up an IMAX 2000 about 10 feet above the back end of my house's roof. What is interesting, is that when I was testing the antenna when it was attached to a test jig about 10 feet off the ground (basically I mounted the 10 ft mast to a clothesline post) the SWR was better, it was almost flat from 26 to 28 mhz, peaking up a bit at the ends and flattening out around 27.305 or so with a 1.1. Now with the antenna up in the air, from 26 to 28 mhz I get about 1.4:1 max, and on most frequencies the antenna is 1.2:1, but what is strange is it never bottoms out any lower than that. The antenna seems to work well enough, so I'm not too concerned, but am curious as to why it changed... maybe height above ground changes it, etc. I also went from like 3 to 7 ferrite snap ons total, so maybe that has something to do with it... although theoretically that should only choke off stray RF... The other weird thing about my house is the back side of it, the distance to ground is a lot higher. So on the side the base of the antenna is maybe 25 feet above ground but on the back the base is about 34 feet above the ground. Another factor might also be that I also put a ground wire on this antenna, but the ground is not connected to anything yet... so there is 50 ft of 8ga wire coming off the antenna... maybe it is causing an interaction of sorts. I will report back if it changes once I "plug it in" to my ground rod "network". This may or may not be applicable, but my house has aluminum siding and there is an aluminum gutter about 10 feet or so below the antenna. Any insights the gurus on the board have are welcome... I am curious to learn about the forces at play here. -Mike |
Doc_holliday
Junior Member Username: Doc_holliday
Post Number: 14 Registered: 6-2007
| Posted on Sunday, June 10, 2007 - 5:38 pm: |
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Well first of all, is the antenna up in the air, mounted on some type of pipe from the ground up, if not then grounding may be your issue? A friend had mounted an imax like that on a chimney mount about 30ft in the air and on the mount was a pipe only about 2ft long and big enough to mount the antenna to and his swr was a little strange to until he grounded the pipe real good then swr dropped.. Just a suggestion, not sure if this is the problem or not. |
Captian_radio
Intermediate Member Username: Captian_radio
Post Number: 224 Registered: 9-2004
| Posted on Sunday, June 10, 2007 - 9:01 pm: |
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Could be a result of less ground reflection, the the closer an antenna is to the earth the more the signal is absorbed by the earth, antennas closer to ground can show a lower swr reading because of this.The fact that you are seeing a higher swr over a smaller bandwidth probably means that your antenna is much more efficient, less of the signal is being absorbed by the earth and being radiated up and out to where it should be. Even a dummy load will show a 1.1 swr over a large frequency range but makes a very poor radiator.Years ago I put up an inverted L antenna for 160 meters and had a great swr to bandwidth ratio due to ground losses, I installed hundreds of feet of ground radials and the swr to bandwidth dropped greatly but my antenna efficiency increased greatly because the ground screen or radials that I put down lowered the ground losses.I hopes this helps to explain it a bit. CEF451/VE1CZ Robert L. Spicer The days of radio are just beginning!
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Marconi
Advanced Member Username: Marconi
Post Number: 731 Registered: 11-2001
| Posted on Sunday, June 10, 2007 - 9:19 pm: |
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I think you are just seeing ground loss affects of being close to the earth. These losses are ohmic losses and tend to added to the feed point impedance. This typically appears as an improvement for the antenna. |
Tech237
Moderator Username: Tech237
Post Number: 807 Registered: 4-2004
| Posted on Monday, June 11, 2007 - 8:52 am: |
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Added to the above antenna resonance can and and most often does change with height above ground. I cannot recall the exact figures, but I think it is around 5KC shift for every 10ft but don't quote me on that. I also seem to recall that the frequency of resonance drops with an increase in height. Tech833 can you correct me on that? Simon Tech237 KD7IEB
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Drgrant
New member Username: Drgrant
Post Number: 6 Registered: 6-2007
| Posted on Monday, June 11, 2007 - 11:44 am: |
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Thanks for the responses, guys... a lot of this sounds pretty reasonable. I guess I'm just kind of freaked out by an antenna that doesn't bottom out at any one frequency. I could see the bandwidth changing, etc, but I haven't had an antenna that didn't bottom out (eg, score a 1.1) on any frequency. ) The previous setup I had was an M400 starduster, which was up in a tree at my old house, and the feedpoint of it was about 35 feet off the ground... that antenna performed in the test jig (which was a tripod on a piece of plywood w/ rocks and 20 feet of mast in it) pretty close to what it did in the trees.. and I got a 1.1 on like channel 2. (26.975) Another guy on the radio said that it might be caused by the presence of the gutter below the antenna, etc. Some other food for thought.... I kind of doubt it, but would coax length have anything to do with it? Currently there is 100 feet of CXP1318 on it (that stuff that is like 9913F). I have at least 20-30 feet of excess just laying in the yard right now, but will probably end up trimming it off. Not sure if that will change anything, or if there is a magic length I should use, etc. (eg, leave it at 75 instead of 65 ft, etc. ) -Mike |
Tech237
Moderator Username: Tech237
Post Number: 810 Registered: 4-2004
| Posted on Monday, June 11, 2007 - 1:24 pm: |
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DRGrant - reasd the article on Coax in the Subcribers section. It may answer that question and others for you. Simon Tech237 KD7IEB
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