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Author Topic: Wire dipole - simple questions  (Read 1064 times)
N4ONT
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Posts: 2




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« on: January 25, 2001, 08:38:43 PM »

I am about to make an 18 mHz wire antenna and need to know whether the length of each side should have extra inches added to wrap around the insulators and be twisted back on itself.  Also in my QTH it would be easier for me to erect the antenna if I could put the wires inside plastic water pipe with a T junction in the center and a long upright piece to get it up in the air.  (I have no garden area)  My question is, will the pipe surrounding the wire degrade its performance very much if at all?  Thanks in advance.  Barry
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K3UOD
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Posts: 145




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« Reply #1 on: February 02, 2001, 04:18:31 PM »

Yes, you have to add 3-6 inches, depending on what type insulator.  Measure the overall length after the insulators are installed.

However, cutting by formula is only approximate.  Always cut the antenna long, then trim for minimum VSWR.  However, at higher frequencies, dipoles can be very forgiving.

Don't solder the wire until you've determined the final length.  If there is not a lot of strain on the antenna, you don't need to solder at all (but I always do.)

If the antenna will be in a pipe you don't need end insulators.  Just use solid #12  or #14 wire and let it lay in the pipe.  Some will argue that PVC pipe is a poor dielectric and will hurt your performance.  I doubt that it will be significant at 18 MHz.  If you can mount the pipe so it can swivel, you can use it as a "rotatable dipole".  Sort of a one element beam.  Not very directive, but turning the antenna broadside to the signal path will give you some gain.
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KB3CDF
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Posts: 27


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« Reply #2 on: February 06, 2001, 03:12:33 PM »

I agree with K3UOD.  Cut longer and trim back in order to tune to the desired frequency.

'Something to consider. Dipoles are great at any elevation.  I have read in many sources and observed in practical application it's best to have the dipole at least 1/2 wave above the ground. This allows grasping all the benefit and directivity possible presented by dipole propagation.  Not necessary but it will be notably better,(approx 26 feet for 17m).

Until then on 17m (best kept secret),
KB3CDF/M
John
73's


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WB2WIK
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Posts: 20595




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« Reply #3 on: February 06, 2001, 05:20:42 PM »

Great replies.  One comment re PVC: Most of it (lawn & garden variety stuff) seems to be a fine dielectric for amateur HF work, although it will reduce the velocity factor of propagation of the wire placed within it, from 95% for standard wire, to something somewhat lower (~90% - 92%), which will make the wire want to be somewhat shorter than the "formula" length.  But this will vary with height above ground and other influences as well, so the "cut it longer than needed, and trim it to resonance" suggestion is a fine one.  If you want to check the PVC you intend to use, to see if it is a good dielectric for RF work, a quick & easy test is to cut a small section of it (a few inches long) and place it in a microwave oven, along with a glass of water (this is important to "load" the microwave oven).  Turn the oven on "high" and run for a minute or two.  The water should boil and the PVC tubing should remain cool to the touch (touch it immediately after turning off the oven).  If so, the PVC is fine.  If the PVC is hot to the touch or began to melt in the oven, this is not a good sign...
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