| Author |
Message |
    
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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