4-bay VHF dipole array project

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Skypilot007

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Exactly what im looking for! I am going to build this as a omnidirectional antenna for 444.500MHz

Do I attach all of the cables from each of the 4 parts together at the bottem and solder to a connecter, also can i use 50 ohm cable on this?
the duplexer and radios are 50

Great design....!!!
If you follow the design exactly for the frequency you need you will be pleased with the results. I have built a UHF version of this antenna and it is the best UHF antenna I have or ever had. I have mine configured as omni directional. It fits perfectly on a 10ft mast. My SWR is below 1.8:1 from 440-470MHz. It really pulls in the signals and I get excellent signal reports on 440.
 

W2NJS

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I don't understand your quarter-wave measurements. I would think that a quarter-wave (half of the dipole) cut for 153 mHz would come out to 234/253*12 which is 18.35 inches.

Your construction details are great and very useful.
 

prcguy

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18.35 inches would be about right for a thin wire 153MHz dipole in free space.

The 16.25 inch copper pipe elements also have 1.25 inch feeder wires fanned off the coax, which makes them 17.5 inches. Then comes the effects of the large pipe diameter shortening the dipole and finally the dipoles are sitting about 1/8 wavelength away from the mast causing the impedance to be lowered well below 50ohms.

Tuning them to resonance to 50ohm coax under these conditions causes the dipoles to be even shorter since removing some length also reduces capacitance to the mast and raises impedance slightly.

Thanks for the comments on the construction details. I took an idea of using copper pipe stuck in PVC Tees and using a cut off Tee as a mast clamp, then threw one together and documented everything in one shot. I wish other writing projects went that easy.
prcguy
 
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TeRayCodA

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I made one of these back in 2003,for my MURS set-up.I only used the 2 dipole method,and my VSWR was NOTHING! I made Gamma matches for both dipoles,fed with RG6,Used a 10ft support pipe.And,type F connectors for the gammas.
My downfall was only using 3/8"diameter solid aluminum rod for the dipoles(not much BW to play with,but,only used 151.820) VSWR was 1.2:1.Mounted it on top of my 40ft Rohn tower,and the range was extremely great on just 2 watts.
Thank You,for putting this on a PDF,Been there,done it,They are GREAT antennas for MURS!!!!!!
Will build another one this fall,after the heat leaves us,this time with the 3/4" dipoles.
Regards;
Teray
 

WA4HHG

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Feeding Individual Elements

Extremely well done ! Am wondering if you might comment on the actual connections to the copper elements. Is the center conductor of the co-ax connected to the ascending elements and braid to the descending elements in the array or ...... element?

Shooting from the hip, I imagine they would have to be all configured the same way but would be interested in your thoughts.
 

prcguy

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Generally the polarity of a dipole doesn't matter but in a phased array you should keep them all fed the same way like coax center to top element and braid to bottom element.
prcguy

Extremely well done ! Am wondering if you might comment on the actual connections to the copper elements. Is the center conductor of the co-ax connected to the ascending elements and braid to the descending elements in the array or ...... element?

Shooting from the hip, I imagine they would have to be all configured the same way but would be interested in your thoughts.
 

krazybob

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Keep in mind that this design is tall and has a beamwidth of about 16 degrees. From a low altitude it will hear at a distance but will have a doughnut hole above it. I personally love the design and use the DEB224 on two of my repeaters very effectively. But they are at 4,300 feet and 6,400 feet. Nonetheless, this is an excellent project!
 

prcguy

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Aircraft at high altitude are usually line of sight for quite a distance and can be picked up with a very marginal antenna. For aircraft at extreme distance you want the most gain at the horizon where even at high altitude the aircraft is at or near the horizon.

An antenna having a 20dB or more null straight up is usually not a problem and an aircraft at 35,000ft above you is only about 6.5mi line of sight and even at a 30deg look angle an aircraft at 35,000ft is only about 13mi away, not counting curvature of the earth, etc.
prcguy







Keep in mind that this design is tall and has a beamwidth of about 16 degrees. From a low altitude it will hear at a distance but will have a doughnut hole above it. I personally love the design and use the DEB224 on two of my repeaters very effectively. But they are at 4,300 feet and 6,400 feet. Nonetheless, this is an excellent project!
 

alabamarailfan

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End caps on dipoles

Hello folks,
Got any thoughts about adding end caps to the dipoles? I have added two to this dipole and put a small drain hole in the bottom one. Do you think these would reduce moisture or actually make it worse compared to being open?

Thanks for the design, prcguy! Enjoyed building this one and have already made one tuned to 160 MHz.
 

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prcguy

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My opinion is you don't need to cap the tubing for the dipoles. Copper will get a thin layer of patina that will protect it in the long term and open tubing will dry out faster than capped tubing. You can also paint the elements but I would put more effort into waterproofing the coax phasing harness and connections to the dipoles.

I still get all choked up when I see people making some of the project I posted on RR, looks like your doing a great job.
prcguy
 

alabamarailfan

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Thanks, Prcguy! I really enjoy the design and hope to use it on future live audio feed installs. I have built 4 dipoles so far for different projects and plan to build the full-fledged 4-dipole array soon for myself as well as a couple of 2 dipole arrays for live audio feed projects.

I would have used Coax-seal but I am out right now so I used a combination of Scotch mastic tape, heat-shrink tubing and Scotch Super 88 electrical tape to seal the connection to the dipole. Do you think this looks okay?

This single dipole is going to be for my Public Safety feed since all the entities I am monitoring are close and you can practically monitor with a desktop scanner antenna (the end caps are already off and the antenna is going up tonight or tomorrow!). Next will be a couple of multi-dipole arrays tuned for 161 MHz for railroad monitoring. I don't have an antenna analyzer right now so I am just using a radio and SWR meter to tune each dipole with. Am I correct in thinking that the larger diameter elements make them "appear" to be a little longer than a thin-wire dipole would be?

Thank you again very much and I will share more as I get them built/installed! :)
 

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prcguy

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You seal job looks fine. I usually seal the point where the coax splits and all the braid with black RTV sealant.

Its best to tune the elements individually using a length of 50 ohm coax with the exact same length pig tails that will be used on the phasing harness. The pig tails are part of the radiating elements and their length will change the tuning.

The fat elements will make the overall length of the dipoles shorter than calculated.
prcguy


Thanks, Prcguy! I really enjoy the design and hope to use it on future live audio feed installs. I have built 4 dipoles so far for different projects and plan to build the full-fledged 4-dipole array soon for myself as well as a couple of 2 dipole arrays for live audio feed projects.

I would have used Coax-seal but I am out right now so I used a combination of Scotch mastic tape, heat-shrink tubing and Scotch Super 88 electrical tape to seal the connection to the dipole. Do you think this looks okay?

This single dipole is going to be for my Public Safety feed since all the entities I am monitoring are close and you can practically monitor with a desktop scanner antenna (the end caps are already off and the antenna is going up tonight or tomorrow!). Next will be a couple of multi-dipole arrays tuned for 161 MHz for railroad monitoring. I don't have an antenna analyzer right now so I am just using a radio and SWR meter to tune each dipole with. Am I correct in thinking that the larger diameter elements make them "appear" to be a little longer than a thin-wire dipole would be?

Thank you again very much and I will share more as I get them built/installed! :)
 

alabamarailfan

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Working great so far! Gonna get some more copper pipe this payday and build some more dipoles. For now, this one is providing signal for my public safety feed and a single dipole is more than enough gain since you can practically monitor on a coat hanger since everything is close.

Once I build a couple more, I will post some photos. Thanks again for the help!
 

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doublescan

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dipole array

Alabama railfan looking forward to some pictures, when you do your array project. I have a single dipole, like the one in the photo, that I cut for aircraft. Works better than anything I've stuck on the tower yet. With some good help here I was able to assemble it on the first try, haha. Phasing harnesses may be a bit complicated for me, but I may decide to try this dipole array project yet. As for the end caps, I stuck them on mine just for appearance' sake.
 

prcguy

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That is an unusual arrangement and I believe you need 8 dipoles in that version to get 6dB omni gain. Sinclair seems to use that config on their models with downtilt and without downtilt they seem to used a conventional 4 dipole array for 6dB omni gain.

Downtilt is usually done by changing the length of the phasing harness to delay propagation to parts of the antenna. Not sure why they double up the number of dipoles on that model.
prcguy


PRCGUY or anyone else that has any input:
Any thoughts on mounting four dipoles in this type of configuration? Pros, cons?
This is from Sinclair's website. Thanks a bunch!
 

kayn1n32008

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That is an unusual arrangement and I believe you need 8 dipoles in that version to get 6dB omni gain. Sinclair seems to use that config on their models with downtilt and without downtilt they seem to used a conventional 4 dipole array for 6dB omni gain.

Downtilt is usually done by changing the length of the phasing harness to delay propagation to parts of the antenna. Not sure why they double up the number of dipoles on that model.
prcguy

I believe that configuration will give 3db gain in an omni pattern, and prcguy is is correct, 8 are needed for 6db gain. If you have 4 dipoles, 1/2wave(roughly 1m) off of the mast, all facing the same direction, will give you 6db. and although not a true omni pattern, it will give decent 360 degree coverage IF mounted above the top of the tower. If you side mount it, you will have some shielding from 'behind' the tower.


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kayn1n32008

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That is an unusual arrangement and I believe you need 8 dipoles in that version to get 6dB omni gain. Sinclair seems to use that config on their models with downtilt and without downtilt they seem to used a conventional 4 dipole array for 6dB omni gain.

Downtilt is usually done by changing the length of the phasing harness to delay propagation to parts of the antenna. Not sure why they double up the number of dipoles on that model.
prcguy

I believe they are doubled up to give 3db gain 90 degrees in a bi-directional pattern, and when 2 more are added at 90 degrees it gives 3db in an omni pattern,

There was an old pre-cellular system in Alberta on UHF that used Sinclair antennas in this configuration on majority of the sites. A ham club I belong to has a repeater on one such antenna, it has 4dipoles, and is at 300', fed with 350-ish feet of 7/8" hard line, with 12.5w into the duplexer, we see a 80+km radius of usable coverage.


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