I hope someone can help. I need to install a second DB 224 on a tower and they both will need to face the same direction. How close can they be to each other vertically? Space is limited and they will be on the same leg of the tower, one over the other.
Two side mounted DB 224 antennas
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Are you using these on a repeater? If so, anything closer than one length of DB224, i.e. ~20', would probably be too close, IMO. This would mean one mounted at the top of a 100' tower could have the other mounted at 60'. On the other hand, I drove past a site yesterday that had three of these, all mounted at the top of a Rohn 25 on a triangular stand-off mount. I'm guessing these are not being used to feed a repeater but I have no info about the site.
Need a bit more information:
Simplex or repeater, one or both?
Frequencies?
Feet of vertical separation gives you an order of magnitude more isolation that feet of horizontal separation.
Vertical is the way to go. I have diagnosed problem sites where the contractor put three DB224's in a triangle at the top, all facing into each other. They are so closely coupled that you might as well T all three transmitters and receivers into one antenna. Any active repeater would desense the others into near deafness. Don't even get me started on the weird propagation pattern. Not a good plan.
Rule #1 in infrastructure design: Forget your mobile simplex experience. Receive paths and transmit paths should always be handed independently from each other, including antennas, coax, and designing receivers independent from transmitters. Step back and look at the overall picture.
A better solution often uses the top antenna for all receive, and the lower for all transmit. The top antenna feeds a receiver multicoupler through filtering, and the lower antenna is connected to a transmitter combiner. You CAN have four 100W VHF transmitters active at one time. I do that at many sites and it works very well. I loose half of the PA output in the TX filtering, but wattage is cheap; I just make more. If they are both unrepeated base stations, you can also use a single multi-channel transmitter direct to the lower antenna- that is often easier and cheaper. The only down side is that only one channel can be in TX at any one time.
Mobile-to-base uplink is always the weak link, so it gets the better location. The higher 224 will have better coverage than the lower one - give me height over horsepower any day - and the coverage is identical for all receivers. Base-to-mobile coverage from a lower antenna can be increased by upping the output (within the license limits of the ERP). A lower TX antenna 'should' have a higher ERP anyway - that's something your coordinator can fix.
You will need some filtering and/or antenna switching to ground during TX. All of the filter manufacturers will engineer that for you.
One other nice feature of combined receive: need another receiver? Order the receiver multicoupler with extra ports (50ohm terminators on the unused ones!). All you have to do is plug the new receiver into the multicoupler. No tower work required.
The DB228 is two 224's on a single pole. Technically one antenna if you are paying by the foot for tower space. A beast to hang, but a real high gain performer. 228's are often split with the top half RX and the bottom TX. It is an order mod that the manufacturer can do. Consider electrical downtilt in your design.
Simplex or repeater, one or both?
Frequencies?
Feet of vertical separation gives you an order of magnitude more isolation that feet of horizontal separation.
Vertical is the way to go. I have diagnosed problem sites where the contractor put three DB224's in a triangle at the top, all facing into each other. They are so closely coupled that you might as well T all three transmitters and receivers into one antenna. Any active repeater would desense the others into near deafness. Don't even get me started on the weird propagation pattern. Not a good plan.
Rule #1 in infrastructure design: Forget your mobile simplex experience. Receive paths and transmit paths should always be handed independently from each other, including antennas, coax, and designing receivers independent from transmitters. Step back and look at the overall picture.
A better solution often uses the top antenna for all receive, and the lower for all transmit. The top antenna feeds a receiver multicoupler through filtering, and the lower antenna is connected to a transmitter combiner. You CAN have four 100W VHF transmitters active at one time. I do that at many sites and it works very well. I loose half of the PA output in the TX filtering, but wattage is cheap; I just make more. If they are both unrepeated base stations, you can also use a single multi-channel transmitter direct to the lower antenna- that is often easier and cheaper. The only down side is that only one channel can be in TX at any one time.
Mobile-to-base uplink is always the weak link, so it gets the better location. The higher 224 will have better coverage than the lower one - give me height over horsepower any day - and the coverage is identical for all receivers. Base-to-mobile coverage from a lower antenna can be increased by upping the output (within the license limits of the ERP). A lower TX antenna 'should' have a higher ERP anyway - that's something your coordinator can fix.
You will need some filtering and/or antenna switching to ground during TX. All of the filter manufacturers will engineer that for you.
One other nice feature of combined receive: need another receiver? Order the receiver multicoupler with extra ports (50ohm terminators on the unused ones!). All you have to do is plug the new receiver into the multicoupler. No tower work required.
The DB228 is two 224's on a single pole. Technically one antenna if you are paying by the foot for tower space. A beast to hang, but a real high gain performer. 228's are often split with the top half RX and the bottom TX. It is an order mod that the manufacturer can do. Consider electrical downtilt in your design.
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What are the Freq's? 15x.y is close enough. 3/3 MHz sounds like you have a tx on top of a rx. That won't fly unless one is neg and the other pos.
If you are hooking them up to two different repeaters, then I'd go with Jeatocks suggestion and use a combiner for both repeater transmitters on the lower antenna and connect both receivers to the top antenna via a multi-coupler and filtering.
This is exactly what I'm doing with my setup. I have three UHF repeaters and am using two 4-bay dipoles. One is at 290 feet and the other is at 250 feet. Each antenna is about 6 feet long, so from the top of lower antenna to bottom of upper, there is 34 feet of separation. I am having zero problems with my setup.
Just for info, my frequencies are:
441.8375/446.8375
444.6750/449.6750
462.6250/467.6750
Depending on the combiner, you can have 250 KHz separation, but they will cost a lot more than one that requires more separation.
This is exactly what I'm doing with my setup. I have three UHF repeaters and am using two 4-bay dipoles. One is at 290 feet and the other is at 250 feet. Each antenna is about 6 feet long, so from the top of lower antenna to bottom of upper, there is 34 feet of separation. I am having zero problems with my setup.
Just for info, my frequencies are:
441.8375/446.8375
444.6750/449.6750
462.6250/467.6750
Depending on the combiner, you can have 250 KHz separation, but they will cost a lot more than one that requires more separation.
Here in VHF-Land where the offsets are are free-form, in my perfect world I put all of the mobile uplinks as close as I can get them, and the repeater Tx/downlinks with at least a half-meg separation.
My regional system uplinks are 158.8 to 159.2 and downlinks are 155.1, 154.7, 154.1 and 151.1. All four can TX at the same time with a 200w ERP on each.
The real magic is in the EMR cans.
Works like a champ.
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Perfect combination.
Use the top antenna for receive, feed it through a notch filter into to an active amplified receiver multicoupler. (Buy a four-port and terminate the unused ones with a 50ohm dummy load.) The preamp will let you drive the receiver as hard as you need to.
Because the RX freq's are so close you can probably use one filter for both, installed before the amplifier. All the filter needs to do is pass the 159's and reject anything lower, including the nasty-noisy 158.1 spark-gap paging transmitter that is always nearby.
Mount the new antenna directly below the old one. See this for the distance: Isolation between the repeater receiver and RF sources
Isolation between the TX and RX should total 90db (more is better). The isolation distance between the antennas is a function of the filtering; the better the filters the closer the antennas can be.
Use a combiner on the transmitters and feed both to the lower antenna. You will probably find that the losses from the receiver side filter have gone away and that offsets the losses through the TX combiner. Check your ERP.
The wizards at the filter builders can give you fine detailed engineering.
LMR600 is not the sharpest knife in the drawer for repeater use, and has a bad habit of drawing water through the jacket and into the shields. If the original antenna has been in the air more then seven years, replace all of the coax with LDF4.5 or LDF5. That will save you countless headaches down the road. Secure the coax every three feet coming down the tower. Ground the coax at the top, every 50 feet, at the bottom before it turns off the tower, and where it enters the shelter/building. R-56 is your friend!
Use the top antenna for receive, feed it through a notch filter into to an active amplified receiver multicoupler. (Buy a four-port and terminate the unused ones with a 50ohm dummy load.) The preamp will let you drive the receiver as hard as you need to.
Because the RX freq's are so close you can probably use one filter for both, installed before the amplifier. All the filter needs to do is pass the 159's and reject anything lower, including the nasty-noisy 158.1 spark-gap paging transmitter that is always nearby.
Mount the new antenna directly below the old one. See this for the distance: Isolation between the repeater receiver and RF sources
Isolation between the TX and RX should total 90db (more is better). The isolation distance between the antennas is a function of the filtering; the better the filters the closer the antennas can be.
Use a combiner on the transmitters and feed both to the lower antenna. You will probably find that the losses from the receiver side filter have gone away and that offsets the losses through the TX combiner. Check your ERP.
The wizards at the filter builders can give you fine detailed engineering.
LMR600 is not the sharpest knife in the drawer for repeater use, and has a bad habit of drawing water through the jacket and into the shields. If the original antenna has been in the air more then seven years, replace all of the coax with LDF4.5 or LDF5. That will save you countless headaches down the road. Secure the coax every three feet coming down the tower. Ground the coax at the top, every 50 feet, at the bottom before it turns off the tower, and where it enters the shelter/building. R-56 is your friend!
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