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twinsen
Joined: 27 Dec 2001
Posts: 104
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 15 Jul 2002 20:57 A question for the hams and antenna specialists |
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While searching for a suitable antenna to build for my antenna challenged home I came across this...
hxxp://www.antennex.com/preview/archive4/Apr601/microvert.htm
hxxp://home.t-online.de/home/dl7pe/ant.htm
Unfortunately the author gives only enough info to get your interest...the rest is on some CD he sells... Does anyone know anything about this antenna or has maybe built one?
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CADIX
Joined: 24 Mar 2002
Posts: 21
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| 15 Jul 2002 21:13 |
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There's a link to an interesting site about HAM, antennas...
http://personal3.iddeo.es/ea3qp/soft1.html
I hope it will help you.
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klug
Joined: 04 Jul 2002
Posts: 757
Helped: 5
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| 25 Jul 2002 23:55 |
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Hi!
Do not pay much expectation to this short antenna. As writen by author there are difference about 10 dB to 1/2 dipol (really you can wait much more figures). This antenna is example of shorten ground plane. But it is more worse than GP - GP has good diagram in vertical plane, but this antenna has sphere diagram.
For my 25 years experience in HAM radio, I have never seen good and efficient shorten antenna. In some conditions (depends on metallic constructions of house) a short simple wire can be very efficient, but it is a magic property of metalic constructions of a house, but not of this wire. In other house this wire in the same configuration will be a "piece of wood".
So, look to wide used antennas, do not wait a miracle in this field.
If you have some difficult conditions in your house write details, I'll try to help you to find suitable antenna construction.
73!
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twinsen
Joined: 27 Dec 2001
Posts: 104
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| 26 Jul 2002 11:05 |
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Thanks klug
I hear you.. I was wondering why there was very little info on the web about it...
Antenna restrictions ****! I tried to tune up my aluminium gutters but since the house is single story I am not getting good radiation - probably just warming the sky or something. Right now I have a dipole on 40M but it has to lie flat on the roof tiles. It doesnt work too badly I guess.
I thought of building a magnetic loop which I believe works very well, except for the fact that the bandwidth is only a few khz, so you have to keep tuning the damn thing as you move around the band....
<sigh>
One day I will own a farm and stick up a bloody log-p
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klug
Joined: 04 Jul 2002
Posts: 757
Helped: 5
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| 26 Jul 2002 13:51 |
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Hi, Twinsen!
You are living in single story house. If you have 40 meters half wave dipole in your roof, than you have about 20 meters of roof in one direction - it is a big field (almost enough to play football :)! Please, tell me:
What is real size and form of your roof, is it flat or high?
Could you maintain vertical mast in center of your roof?
Is your house made of reinforced concrete?
Have you some points to fix wire outside of your house?
I think, dipole lying flat in a roof can not work well. At least it can not transmit well. I know a curios fact that polar stations in Antarctica simply stretch a dipoles on the ground (They have an ice under feet and this deep ice is a nice dielectric). I think you are living not in antartic and your roof is not perfect dielectric. And all antennas with open end (dipoles) need good height (at least 1/4 of a wave length) . Antennas with closed wire (quads, loops) do not need big height.
About magnetic loop. I've made magnetic loop at 1983. It was aluminium circle (hula hoop). I made a coil inside a pipe with a number of turns with fluoroplastic isolated wire. It was tuned to resonance by air isolated capacitor. The main purpose of this antenna was to clean QRMs from jammer located few kilometers near by me. This hula hoop antenna allows me to listen a lot of propaganda from abroad of USSR :). This antenna has near round diagram with very deep hole in side direction. The hole was near -25 -40 dB to main direction signal. I tried to use it in HAM radio band in 40 meters to clean from "Radio Tirana". But it was not enough efficient to listen weak amateur signals from DXs.
73!
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