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Multi-Bay Folded Dipole for 70cm
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Greetings everyone,
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I am new to the group and looking for help on constructing a multi-bay 70cm antenna for our repeater.?
I am thinking of using copper pipe for durability.
I have looked in all of my antenna books but they are not helping.
Does anyone have plans or information for such a project that would get me started.
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Thanks in advance,
Dale?
N3DJO |
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I'm assuming this is an amateur repeater ? There are some used 408s, 404s, and if you want to go top shelf, some 420s online?for sale.? You just have to hunt for them ...? ?I paid $250 for our DB-420B.? Though it's cut for 450-470, It has plenty of sweep into the amateur 440 band.? ?Ours is on 444.000+ .... 1.5 to 1 SWR.? It really "screams" !! 40 mile range depending on topography. On Tue, May 6, 2025 at 11:07?AM Dale O'Neill via <djoneill63=[email protected]> wrote:
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¿ªÔÆÌåÓýI agree with this assessment of the 420. We have a DB 420, 16 Bay UHF on 440.950 and like Dave says, it really screams! Tower is 60 feet with a 20 foot antenna and people think the tower is at least 150 feet.Chad Nelson WI9HF/WRPL979 Janesville Wisconsin 608-754-8671 Send from my spectrum iPhone 16. On May 6, 2025, at 12:53?PM, Dave Bates via groups.io <Kf0xq1@...> wrote:
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¿ªÔÆÌåÓýSave your self the headache and time. Buy a commercialy made antenna. I like the Telewave ANT-450 or the Sinclair. Sinclair all the cable if is internal. Telewave it is external. If you deal with ice the black telewave hold up better. Both can be order with downtilt. I wouldn¡¯t go more than 6 degrees DT. I have the Telewave two bay version on my mountain top sites I. The Sierra Nevada mountains for 15 years no issues.?They work well for combined systems 420-470 MHz. I have amatuer, GMRS and commercial repeaters all on two antennas. ?Saves money and time when adding more repeaters later.? My .02 Jim? On May 6, 2025, at 11:05?AM, Chad Nelson <chad.nelson71@...> wrote:
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Hi Dale,
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Building say a 4 bay UHf array is not going to be as easy as purchasing one, (say DB products) used, and remaking the harness for your frequency, as was already stated. It may not be cheap either, but it is something that can be done. I have built a number of 6M vertical antennas for fun. If that is what you really wish to do.
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There are 2 basic dipole element designs. The simplest is a fed (usually upper) loop with lower loop grounded on both sides to the center mast. The other is a true folded dipole that is balance fed to both upper and lower folded diploe halves. The first type requires the dipole to be close in to the tower leg or mast, and the second requires the folded dipole out almost 1/4 wave from the tower leg or mast. There is much greater SWR bandwidth with the balanced feed, but it is also more directional, so it would require 2 elements on opposite sides at the same level to give more of an omni pattern. The second type also requires a series 1/4 wavelength of 125 ohm coax inside the element tubing to match the element to 50 ohm feedpoint.
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This is roughly what would be involved for the simpler type. I recommend aluminum rod instead of copper tubing say 1/4-3/8" OD rod. Commercially they use tubing, but a rod is easier to place the end bends without needing a bending device to prevent kinking. Ideally find someone that does TIG or Heliarc welding to join the loops to say a 1"-1 1/4" OD square horizontal aluminum tube in the center of the 1/2 wave folded dipole element that will fasten to a tower leg or mast. The distance between the loop and tower leg or mast is critical so that would have to be trial and error for 50 ohm feedpoint match. Your element would be an electrical 1/4 wave usually fed between one side of the? upper 1/4 wave loop and the horizontal center? square tubing, where the coax cable passes from element to tower leg or mast inside the s to the bottom tip.quare tubing.?
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For 445 MHz, 1/2 wavelength is 13 inches, so quessing at the dimensions,? the upper loop needs to be this long from where the coax shield ends to where the return loop joins the square tubing. If the coax comes out the far end of the square tubing where the shield is grounded will then need to be 13 inches, so about 5 1/2 inches from the shield ground point to the upper loop tip top, if the loop space is 1 3/4 inches C-C between element sides..? ?The lower loop may need to be slightly shorter. Minimum of 3/4 wavelength vertical spacing C-C between dipoles vertically.?
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I am sure others may add or actually disagree with my comments, but this is at least a starter.
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John
? ? ? On 05/06/2025 12:03 PM EDT Dale O'Neill via groups.io <djoneill63@...> wrote:
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