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Build Your Own Antenna - Discuss topics for building your own antenna.

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Old 12-06-2018, 9:47 AM
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Default Dipole measurement guidelines

I plan on trimming the rods of a dipole and needed a clarification on measuring before cutting. Is the measurement for "overall" to include the bakelite insulator between the 2 radials, OR is it each rods length ONLY and not the distance of the "rod, bakelite, rod" measurement? ALSO, When trimming each rod is the rods measurement from the rivet to tip, OR from where the rod begins protruding from the bakelite to the tip?THANKS to all respondants.
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Old 12-06-2018, 10:31 AM
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Quote:
Originally Posted by colbrz1319 View Post
I plan on trimming the rods of a dipole and needed a clarification on measuring before cutting. Is the measurement for "overall" to include the bakelite insulator between the 2 radials, OR is it each rods length ONLY and not the distance of the "rod, bakelite, rod" measurement? ALSO, When trimming each rod is the rods measurement from the rivet to tip, OR from where the rod begins protruding from the bakelite to the tip?THANKS to all respondants.
1. Rod length only. One would not include the length of the center insulator in the calculation; you only want to figure the actual conductive part of the antenna.

2. Depends on where the feed line is connected to the dipole; usually it's right at the inner end of each element (rod).

Assuming a single band antenna, if your SWR is 2:1 or less across the band of interest I wouldn't even worry about trimming it, as the closer you get to 1:1 you could easily lop off too much and end up going the other way.

However, if you have to trim the best bet is to trim as little as possible with each try. This gets very tricky the higher in frequency you go as a small amount has a much greater effect at higher frequencies (particularly VHF and above).

Useful info here: Building and Tuning A Dipole The Easier Way - A Faster Way To Tune A Dipole Antenna Project
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Old 12-06-2018, 10:46 AM
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You would measure from the tip of the rod to the feed point. The portion of the rod that is past the feed point will only a slight effect on the resonate frequency of the rod. But that effect will increase as that portion gets longer. The insulator isn't part of the measurement.
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Old 12-06-2018, 11:03 AM
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Default OK, GREAT.

SO, Rod Tip to Rivet Location is the best measurement? The reason for asking/verifying is the rod travels an additional 1 and 1/2 inches blocked/obstructed by the bakelite before it reaches the rivet location so I was concerned about a mismatch with that "unknown zone" of 1 and 1/2 inches of rod/bakelite before I make the final cut. THANKS.
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Old 12-07-2018, 5:34 AM
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The radiowaves are a wavelenght in the air and the distance between the dipoles tips should equal the 1/2 wavelenght minus the shortening factor of the material of the dipoles 3-5%. The radiowaves doesn't care what distance you have for the insulation between the elements.

The same goes for an antenna on a car roof or a ground plane antenna. The distance are measured from the car roof, or from the ground plane elements, to the tip of the element, if it is a 1/4 wave or other equal type of antenna.

/Ubbe
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Old 12-07-2018, 7:42 AM
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You haven't clearly defined just what use this dipole is going to be used for. All of us have been assuming and I hate to use that word, that you will be using this antenna for transmitting. If that is the case, your better off cutting it a little on the long side and slowly walking it in to the frequency you want to use it at.

This walking it in, may take a number of attempts to get it to where you want it. I always cut my antennas long and then go through the effort of putting the antenna in place and transmitting to measure the SWR. Write down what you have for both length, frequency and SWR. This way it's on paper and can be used in the future as a guide. But remember that the antenna must be measured in the location where it will be used.

If you are stringing the antenna between 2 trees, raise it up to where it will be used and then measure the SWR. Lower it, make the length change and raise it back up.

I always use a pulley system on my dipole antennas. Have a pulley at each end so you can move the antenna around from one end of the tree support to the other so you can center it. Once you have the centering where you want it, then you only need to lower one end to make changes to the antenna.

I use 2 ropes at each end for my tree supported dipoles. I make a loop to control the pulley and the second rope to pull the dipole up using the pulley. I have an air type of potato gun that shoots a round lead weight with a fish line attached. This way I can just about pick where in the tree I can shoot the line to pull up the black Dacron rope to contain the pulley.

A little suggestion is not to use regular nylon rope or poly rope. They will not last long out in the sun. Also some trees have a bad habit of trying to grow bark over anything that is putting pressure on the bark. So you need to move the rope every month or so to insure that it will be able to move. This is just for the pulley rope loop. When I say loop, that is exactly what it is. There is a continuous run of the rope from the pulley, over the tree branch you use, then down to the ground with a little slack to allow you to tie it off and then back up to the pulley. This way you have a way of lowering the pulley and keep it near the tree. Helps in measuring both sides of the dipole in the space you have.

If your placing this antenna up in the attic of a house, pay attention to see if you have a metal roof ridge air vent running along the peak of the roof. If so, you want to stay as far away from it as you can. The metal will screw with your antenna performance and SWR. The same thing goes for just trying to lay the dipole on the ceiling joist of the attic. You have electrical wiring to contend with.

Placing it against the roof and ceiling joists is not a good place either. Now you have 3 things that will cause you problems. Most houses have metal gutters. This is where you will run into the most electrical wiring. Plus it's very hard to even get to this location. You have to contend with the nails from the roof shingles that will do a fine job of opening up some serious puncture wounds and scrapes from them.

I generally cut my antennas about 6 to 8 inches longer than they should be. This gives me the chance to walk the frequency in doing several attempts at cutting maybe an inch off and then measuring the SWR. It may take a while, but you won't get mad because you cut too much off and now have to splice a piece of wire back on.

Have fun with your new antenna and the experience of getting it on frequency with a low SWR.
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