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Author Topic: Rotatable dipole  (Read 1731 times)
SP5QIP
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Posts: 84




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« on: July 04, 2003, 06:14:50 AM »

I`m looking for help and advice. I want to build rotatable multiband dipole like Cushcraft D4. The problem is how to design it. I need dipole working on 40 and 20 meters but not as long as full dimensional dipole. Anyone can help me? Tia, Mike
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AC5E
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Posts: 3585




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« Reply #1 on: July 04, 2003, 08:08:33 AM »

Mike, check out any standard trap dipole calculator. You will have to build your traps on something like a solid bar of fiberglass and weatherproof them, but each pair of traps will substantially shorten your dipole.

One observation - each trap should resonate just below the band in use. That is, a 10 Meter trap should resonate around 27.5, a 15 meter trap about 20.5, a 20 meter trap around 13.6, and so on - and your traps should be resonant at the same frequency if the finished antenna is to be balanced.

After you get your design built comes the fun part. Tuning! Start by bringing the inside section, the highest frequency section, to resonance. Then the next section, which means you will have to retune the center section slightly. Then the outside section - retuning the center and middle sections as you go.

I hope you have a lot of patience! But when you are done you will have a resonant rotatable dipole that is shorter than a simple dipole, and that works.

Good luck with the project!

73  Pete Allen  AC5E
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KG6AMW
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Posts: 616




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« Reply #2 on: July 04, 2003, 11:05:21 AM »

Just to build some background knowledge, refer to June 2001 QST magazine. There's an article in it entitled, "The Arkansas Catfish Dipole" on page 67. In it the author builds what looks like a simple rotatable dipole from a fishing pole, enamel wire and 16 foot pool cleaning pole. Although nothing like a commercial rotatable dipole, it costs him $43 to do it. Quite interesting.

KG6AMW
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KA5N
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Posts: 4380




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« Reply #3 on: July 04, 2003, 11:06:21 AM »

Another approach is to built a dipole for 20 meters total length 33-34 feet (about 10 meters).  Feed this antenna with open wire feeder using an antenna tuner (transmatch) with balun at the tuner end.  This is great for 20 meters and will probably work better than a "trapped" dipole on 40 meters.  Antennas do not have to be resonant to radiate efficiently.  Granted the open wire lead makes rotation a bit harder, but a dipole only has to be rotated 180 degrees to cover all directions.  Plus is that the tuner allows the transmitter to work into a near perfect SWR at all frequencies.  Try it!
Allen KA5N
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KA5N
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« Reply #4 on: July 04, 2003, 11:10:15 AM »

Forgot to mention.  20 meter antenna will work even better on 10, 12, 15, and 17 meters and will do well on 30 meters.  It will load  on 75 and 80 but radiation resistance is a bit low and efficiency drops off.
Allen KA5N
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AC5E
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« Reply #5 on: July 04, 2003, 12:13:26 PM »

Those Old Hams Tales about power lost in traps is true - provided the traps are resonant inside the ham band. Traps resonant inside a band get very hot, often hot enough to soften PVC. Hi Q traps resonant below the band run very cool, "waste" very little RF, and perform very acceptably.

My actual Field Strength tests of trap VS full length dipoles shows very little difference between them as long as the trap is fairly high Q and resonant below the amateur band. I can afford to lose a little in the traps as long as the total system losses including trap, antenna tuner, and feedline losses are less than 1 dB.

My FS tests also show a sharp reduction in far field radiation, radio waves, when trying to use a dipole at a submultiple of the cut for frequency. Combine that antenna inefficency with much higer tuner and feedline losses and the overall excess loss has been as much as 8 dB. Not at all good - but if you don't have a good antenna to compare it with you might never notice.

73  Pete Allen  AC5E
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WB4QNG
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Posts: 362




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« Reply #6 on: July 04, 2003, 02:08:06 PM »

You could use mobile antennas such as hamsticks. Then it would be only 14 ft or so long.  
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« Reply #7 on: July 04, 2003, 03:51:12 PM »

Why don't you consider just buying the Cushcraft D4.  Years ago I used a D3, and it worked very well.  I am all for building stuff but a multiband self supporting dipole isn't going to be an easy project, especially finding the right materials and making traps.
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N6AJR
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« Reply #8 on: July 04, 2003, 09:15:58 PM »

I am using a d4 currently, paid $150 on ebay delivered. and it works well, I have a 5 band hustler verticle, ground mounted,  and the rotatable dipole at 20 feet, and I am quite happy with them both.  You will need a reasonable sized rotor to swing it, I live in a windy area and the dipole took out my tv rotor in about a month.  hope this helps, to build one will probably cost more than to buy one.. good luck  tom N6AJR
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