| Author |
Message |
    
Gremlin
| | Posted on Tuesday, September 24, 2002 - 7:51 pm: | 
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My A-99 is now 50' in the on top of a push up poll I'm wanting to get it taller by using a crank up tower but i don't know were to find one. Can anyone tell me where I could find one? |
DeadlyEyes
| | Posted on Tuesday, September 24, 2002 - 10:02 pm: |
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The antenna which you stated is an omni directional. It has a "Utility Gain" NOT the gain of an good beam antenna. If you want the gain of a Raker 4 then buy one. If you want the gain of a Lazer 500 then buy one. BUT a vertical, even a phased vertical, will never equal the forward gain of a well designed beam.
Reference our good host at coppers under the base anenna selection. Check out the beam antennas that they offer.
Height is of prime importance only in the VHF and UHF frequencies.
DE |
Taz
| | Posted on Wednesday, September 25, 2002 - 1:15 am: |
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Yes DE has good points.
It your going to put it up higher then your going to want to hit the next(72ft) or the one after the next(108ft) wavelength which are the 2nd and 3rd wavelength.
Antenna height is measured to the bottom of the antenna because thats where it starts radiating.
But it you really want to get out, put up a beam. |
DeadlyEyes
| | Posted on Wednesday, September 25, 2002 - 8:16 am: |
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When and why height makes a difference....
When height makes a difference...
Re taz post of 09/25 is a prime example of one of the phiosophies concerning height. The one wave length above the ground theory. AND it is in fact true in that coupling a beam with the ground does play hell with the intended radiation pattern of the antenna. The ground acts as a reflector to cause more of the signal to perhaps go in unexpected directions. Example 1: If you put up a slanted dipole close to the ground will act as a reflector sending the signal in the direction of the slant. Example 2: If you put up a horizontal dipole the proximity of the ground causes changes the the feed point rsisitance and thus changes the SWR. One of the ways you adjust SWR in a Horizontal Dipole is to change the height of the antenna above ground. Height can also change the angle of radiation--the angle at which he signal leaves the antenna. This angle can effect the distance of your signal as well as the number of stations that are exposed to your signal.
BUT most importantly, height above ground is critical in line of sight communications aka UHF/VHF frequencies and line of sight aka local HF communication. When you have to essentially be able to see the antenna you are working then being low to the ground is a liability. Being higher in CB Frequencies means that you can talk to more local people but not necessarily more DX people. Distance communication is dependant heavily on ionization factors and the angle of the signal hitting your antenna.
When height does not make any difference...
There is an antenna called a ground mounted vertical. This antenna is terrible for local communications but great for Distance. The technical reasons I will not go into here. You see being low to the ground you eliminate many local interfering signals WHILE at the same time receive the distance/skip signals that are falling directly in your area. For these DX signals you do not need height. All you need is to be in the proper location. Think of it in this way as it may help you. If you are talking to your friend 15 miles away it helps a great deal if your two antennas can "See Themselves" signal wise. However if the DX signals are falling on your antenna like rain a low antenna will receive just as well as a beam much higher up. And in fact a ground mounted vertical has a low angle of radiation compared to that of the angle of radiation for a beam meaning your skip distance will be longer than the antenna with a high angle of radiation. A good DX station has a beam anenna at least one full wave length above ground, a vertical antenna on say a 35 to 40 foot pole and a ground mounted vertical. Whichever antenna does the best overall job of both receive and transmit that is the one you use. Such stations are called "Antenna Farms" with good reason. The antennas are everywhere.
Before you go forward with your anenna project you should acqire or borrow the ARRL Handbook or the ARRL Antenna Book and read up on Propogation of Radio Waves. You would be surprised as to the complexity of the subject.
DE |
bruce
| | Posted on Wednesday, September 25, 2002 - 2:06 pm: |
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If a verital is decoupled from the mounting mast and at least 36 foot up as long as it is clear of things not much will be gained in going much higher. Now 2 things come into going above 50 foot
1) guying the mast most citys have a building code.
2) lighting protection if you dont first hit will be a charmer.
Playing with the MOSLEY DI-6's and running them up and down showed little change in signal once you got clear of the trees even 20 foot had very little effect.
On my repeater system we used 2 MOSLEY antennas 60 foot apart and 60 foot up thats 4 waves at 53 mhz both had polyphasers and 30 foot ground rods both got hit at least once. The code here at that time exsempted " push up masts " it does not any more if you go over 35 foot you got to pull a permitt. DE is right the ARRL books cover this quite well as does the bill Orr books my antennas right now are only 30 foot up but the power wires are only 25 so even at 30 im clear of everything except one tree for a block. Free and clear is the key.
Bruce |
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