DB Products 4-bay dipole question

[brushfire21]

I have a 4-bay dipole setup that I am thinking about breaking apart into (4) single dipoles as my application I do not need the gain. Can any other tell me if I cut the phasing harness to each dipole, will each still have the correct impedance to run as a single antenna into a 50ohm radio?

 

[prcguy]

Each dipole is around 50ohms so you would connect a new piece of 50ohm coax to each individual dipole and not use the harness.
prcguy

 

[talkpair]

That should work fine.
Although I've never seen one in the real world, DB/Andrew/CommScope makes a DB220, which is a single folded dipole on a mast.
The specs show it having a 3 db gain.

It's my understanding that DB's single folded dipole has an impedance of about 110 ohms.
An odd quarter wavelength of 75 ohm coax is used to make the impedance transformation from 110 ohms at the dipole, down to 50 ohms at the connector.
DB's phasing harnesses are made up of combinations of 35 ohm, 52 ohm, and 75 ohm coaxial cables

The cable you currently have connected to the dipole should be coded VB11, which if 75 ohms and has a velocity factor of 66%.

If you do the math for 155 MHz, this works out to about 37.75" which is a 3/4 wavelength. If you measure the cable from the dipole lug to the tee-junction, it should be around this dimension. You need to try to maintain this length, as it is critical if you're going to use it to transmit.

I just built a couple of these a couple months ago for my feeds and put them directly on the tower leg. I couldn't be more pleased with the performance.
They really outperform the simple textbook wire dipoles that I was using previously.
In addition, they are side mounted, and DC grounded, which should offer a little more lightning and static protection than open ended dipoles.

I found them to be pretty deaf to any signals on the back side of my test mast, but that is to be expected.

The real beauty of these antennas is that they don't have to be on the top of your mast or tower. By design, they work in conjunction with the (metallic) mast, so you need to make sure the mast you attach it to extends above and below the ends of the dipole about 18".

Good luck.

 

[prcguy]

The single dipoles from a DB Products array have an excellent match to 50ohms, where did the 110ohm info come from?

I've cut apart the phasing harnesses on older versions of these and it all appears to be silicone grease flooded 75ohm RG-11, where did the 35, 50 and 110ohm cable info come from?
prcguy

That should work fine.
Although I've never seen one in the real world, DB/Andrew/CommScope makes a DB220, which is a single folded dipole on a mast.
The specs show it having a 3 db gain.

It's my understanding that DB's single folded dipole has an impedance of about 110 ohms.
An odd quarter wavelength of 75 ohm coax is used to make the impedance transformation from 110 ohms at the dipole, down to 50 ohms at the connector.
DB's phasing harnesses are made up of combinations of 35 ohm, 52 ohm, and 75 ohm coaxial cables

The cable you currently have connected to the dipole should be coded VB11, which if 75 ohms and has a velocity factor of 66%.

If you do the math for 155 MHz, this works out to about 37.75" which is a 3/4 wavelength. If you measure the cable from the dipole lug to the tee-junction, it should be around this dimension. You need to try to maintain this length, as it is critical if you're going to use it to transmit.

I just built a couple of these a couple months ago for my feeds and put them directly on the tower leg. I couldn't be more pleased with the performance.
They really outperform the simple textbook wire dipoles that I was using previously.
In addition, they are side mounted, and DC grounded, which should offer a little more lightning and static protection than open ended dipoles.

I found them to be pretty deaf to any signals on the back side of my test mast, but that is to be expected.

The real beauty of these antennas is that they don't have to be on the top of your mast or tower. By design, they work in conjunction with the (metallic) mast, so you need to make sure the mast you attach it to extends above and below the ends of the dipole about 18".

Good luck.

 

[talkpair]

I've cut apart the phasing harnesses on older versions of these and it all appears to be silicone grease flooded 75ohm RG-11, where did the 35, 50 and 110ohm cable info come from?

Here's a hand drawn diagram I found on the repeater builder site.
http://www.repeater-builder.com/db/pdfs/db-224-af5u.pdf
Notice the fax header, the handwritten name, and the name of the engineer on the technical drawing.

I understood the whole thing better by beginning at the transmission line connector, then tracing the branches outward to the dipoles.
For the DB224, they have used sections of 52 ohm cable where both ends are at 52 ohms impedance.
They use the quarter wavelength 35 ohm cable section to transform a 25 ohm impedance to a 52 ohm impedance, and use the three quarter wavelength 75 ohm cable section to transform the 110 ohm impedance to the 52 ohm impedance.

The idea that the dipole could be 110 ohms was put into my head when I stumbled onto the phasing harness for the DB420 (the UHF version).
http://www.repeater-builder.com/db/pdfs/db-420-b-phasingharness.pdf
In the handwritten note on this page, you'll see the 110 ohm reference. Unfortunately this sheet is lacking the credentials the other sheets have, and is for the UHF version of the DB dipoles.

To be quite honest, I have found several different impedance figures tossed around for these dipoles. Numbers ranging anywhere from 50 ohms to 300 ohms can be found in various forums all over the net. This 110 ohm figure I find only on this one handwritten note.

I'm no expert by any means, but when I sat down with the DB224 phasing harness diagram and a impedance transformation calculator, the only way it made sense to me was when I used the 110 ohm figure.

 

[nd5y]

A folded dipole's impedance in free space is supposed to be 300 ohms.

When you increase the spacing of the loop (or the area enclosed by the loop) the impedance decreases to something like 90 ohms for a square or round loop, like what is used in a quad.

Mounting a folded dipole a few inches from a metal mast decreases the impedance. It would be interesting to know how the engineers calulated the dipole dimensions and impedance to be able to figure the lengths of the harness sections.

There are some manufacturers like Sinclair that used fatter loops mounted further away from the mast and Cushcraft that used regular non-folded dipoles fed with a gamma match.

 

[prcguy]

These are not folded dipoles, there is a grounded center support and the hot side is a folded grounded 1/4 wave element and the ground side has both connections grounded to the support. The impedance is about the same as a conventional dipole and the close proximity to the mast does lower the impedance some.

If you remove the grounded support in the middle so the elements are a true folded 1/2 wave dipole then the impedance will be in the several hundred ohm range.

BTW, I have several DB Products 4-bay arrays, a Sinclair, a Maxrad and a Cushcraft and all individual elements match well to 50ohm and all phasing harnesses appear to be 75ohm cable.

I also designed and documented this seemingly popular VHF 4-bay dipole array project which uses a novel method to hold the dipoles from the mast: http://forums.radioreference.com/build-your-own-antenna/109144-4-bay-vhf-dipole-array-project.html
prcguy

A folded dipole's impedance in free space is supposed to be 300 ohms.

When you increase the spacing of the loop (or the area enclosed by the loop) the impedance decreases to something like 90 ohms for a square or round loop, like what is used in a quad.

Mounting a folded dipole a few inches from a metal mast decreases the impedance. It would be interesting to know how the engineers calulated the dipole dimensions and impedance to be able to figure the lengths of the harness sections.

There are some manufacturers like Sinclair that used fatter loops mounted further away from the mast and Cushcraft that used regular non-folded dipoles fed with a gamma match.

 

[brushfire21]

Thankyou to all that responded. The phasing harness is made of of RG-11a cable, so it appears that it's 75ohm cable. I still have decided what I am going to do, whether to keep it as is and maybe trade the antenna for a lower gain or maybe tweak it into (2) 2-bay dipole setups.

Does anybody have any configuration or spec's on the radiation pattern for different configurations of 4 and 2-bay dipole setups?

 

[talkpair]

I was going to suggest this anyway.......

If you think you might have regrets later about butchering up a good phasing harness, I would suggest just removing the whole harness intact, and use new line to the individual elements. If you change your mind later and want to restore it to it's original condition, you simply put the old harness back on. Before turning a single screw, nut or bolt, take notes, measurements, and pictures so that 5 years from now you know what the antenna should look like if you want to restore it.

Does anybody have any configuration or spec's on the radiation pattern for different configurations of 4 and 2-bay dipole setups?

http://www.repeater-builder.com/db/pdfs/db-224-data-sheet-(andrew).pdf

 

[brushfire21]

I really do not want to break into the phasing harness on this beauty. I like the idea of disconnecting the harness from the dipole itself and will look into that more seriously. Much appreciated on the spec's!

 

[ka5lqj]

Hmmmmm!

I wonder if you took the four dipoles pointed North, East, South & West and turned them to a horizontal position, would it be possible to either use it on
a repeater? The mobiles would require to have a "halo" antenna, but you should get an omnidirectional, horizontal pattern. The same should hold true if you use it simplex FM as well.

GOD BLESS,
73,

Don/KA5LQJ

 

[prcguy]

The dipoles are tuned with the mast in the same polarity and in close proximity. If you turn them horizontal each dipole will become horizontally polarized and bi-directional unless you place a reflector behind them. The match may also suffer unless they are retuned.

Overall It may work ok and since the horizontal pattern without a reflector is such that only two dipoles 90deg apart will give nearly 360deg coverage it might be good to place the bottom two dipoles oriented say N-S and the top two E-W. That way the spacing between each pair pointed the same direction will be optimum for stacking two antennas resulting in the most gain.

Since most if not all FM repeaters are vertical polarized a horizontal version would be down quite a bit compared to the same antenna vertically polarized but it might work great for 2m SSB.
prcguy

I wonder if you took the four dipoles pointed North, East, South & West and turned them to a horizontal position, would it be possible to either use it on
a repeater? The mobiles would require to have a "halo" antenna, but you should get an omnidirectional, horizontal pattern. The same should hold true if you use it simplex FM as well.

GOD BLESS,
73,

Don/KA5LQJ

 

[TeRayCodA]

Is there a reason DB put aluminum tape over the wiring harness?

 

[prcguy]

It seems that's their way way of anchoring the harness to the mast and there would be no danger of squeezing or deforming the coax like most people do when using Ty-raps.
prcguy

Is there a reason DB put aluminum tape over the wiring harness?

 

[lu5dfe]

guys, do not complicate things ... look, in free space, these dipoles have 300 ohm. When rode the boom, it acts as a parasite decreasing impedance around 100 ohms (the impedance obtained depend on the separation). Then for 50 ohm we put two parallel dipoles. The simplest way would be to use 100 ohm cable, or the more practice is to use any impedance cable cut to a particular length ... this length is "multiple of half wavelength." Then again raising the 50 ohm to 100, we use 75 ohm cable cut "odd multiple of quarter wavelength." This makes it easier to obtain bays 2, 4 or 8 folded dipoles. You see, there is no need for cables strange arrangements.

 

[prcguy]

For the second time in the same thread, these are not open folded dipoles, they are grounded in the center and the free space impedance is less than 72ohms. See post #7.
prcguy

guys, do not complicate things ... look, in free space, these dipoles have 300 ohm. When rode the boom, it acts as a parasite decreasing impedance around 100 ohms (the impedance obtained depend on the separation). Then for 50 ohm we put two parallel dipoles. The simplest way would be to use 100 ohm cable, or the more practice is to use any impedance cable cut to a particular length ... this length is "multiple of half wavelength." Then again raising the 50 ohm to 100, we use 75 ohm cable cut "odd multiple of quarter wavelength." This makes it easier to obtain bays 2, 4 or 8 folded dipoles. You see, there is no need for cables strange arrangements.

 

[TeRayCodA]

Just to add,I hooked my scanner up to this antenna,and,I was pulling in all kinds of VHF stations that my Scantenna was missing.I even received 5 NOAA weather channels that before,I only got 4.
If anyone else on here happens to get one of these,make SURE you go through and take all the feedlines loose from the dipoles,and,ohm out the wiring harness.If all is good,weather proof those junctions as if they are going to be 'submerged'under water!! the more the better,this is the antenna's weakest point of failing!!That's where I found an "open short" in mine.They're not that hard to repair,just time consuming.This thing is great! I'm looking for volunteers to climb my 40ft Rohn Tower,and,mount it for me??? any takers??

 

[WB5ITT]

For the second time in the same thread, these are not open folded dipoles, they are grounded in the center and the free space impedance is less than 72ohms. See post #7.
prcguy

According to DB Products, the dipoles are closer to 100ohms....the same goes for the DB212 lowband versions as well as the DB420 UHF models....the dipoles are SHORT for the freq (if you take the 468/freq equals length of dipole in feet, and put the length of a single dipole in the formula, the freq will calculate 10MHz higher! The design of the dipoles are unique to DB Products...other folded dipole designs are bigger and spaced farther from the mast)

Been dealing with DB for 40 years.