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Commercial Radio Antennas - Please keep discussion related to professional, commercially used antennas and antenna systems for the two-way radio industry. Topics for the use of these antennas on amateur bands are accepted here.

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Old 09-07-2017, 7:54 AM
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Default Help with antenna selection

Ok, we are putting our new system online in a few weeks and are ordering the antennas. Currently, we are slated to use 2 Commscope AVA5-50FX one for the Sheriffs department and one for the fire department. Both systems will be VHF 110watt running sinclair duplexers through 7/8 Heliax total run of 330f, thats from duplexer to antenna for each system. Concerns are being raised about the antennas being sufficient and if there is something better out there. What are your suggestions?
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Old 09-07-2017, 8:14 AM
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What ERP does the license allow?

The FCC license will show a maximum ERP. That needs to be considered in your design.

As for "best" antenna, that can mean a lot of things depending on exact location, terrain, what the license allows, etc.
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Old 09-07-2017, 8:24 AM
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we are replacing ours right now until the other site is completed. and it calls for 110 ERP for both.. The new systems are 150 ERP. We are in a vary mountainous region and the tower and 2 antennas are at 1410 elevation, the new site will be 3.5 miles further away and at antenna 1551 ft.
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Old 09-07-2017, 8:38 AM
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The elevation that I mentioned are elevation at antenna.
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Old 09-07-2017, 8:55 AM
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Default Help with antenna selection

If top mounting on a tower, I generally stick with the folded dipole designs. Quick and easy, a DB-222 but I will use engineered arrays from Telewave and Sinclair if needed.

Your looking at about 2.6 dB of loss through the filters and feedline so if choosing higher gain solutions you will have to back the power down on the repeater.

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Old 09-07-2017, 9:24 AM
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Another major point here that hasn't been mentioned is what the average elevation is where you expect the radio system coverage to be?

I ask this question because if your antenna is mounted up on a high mountain and your use is down on the level ground below the mountain, then a normal antenna will be a poor choice. It will put all the transmit and receive ability out on the horizon. You might be better off with an antenna with some down tilt.

You really haven't given enough information to even make a guess here as to what antenna would be the better choice. This is an area where many agencies fall off the technical curve and use the wrong antenna.

Some homework is in order here to make sure your installing the antenna that fits your tower location and provides the coverage you expect. Walk into this with an open mind and get some engineering help if needed.
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Old 09-07-2017, 9:54 AM
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Quote:
Originally Posted by jim202 View Post
Some homework is in order here to make sure your installing the antenna that fits your tower location and provides the coverage you expect. Walk into this with an open mind and get some engineering help if needed.
I agree.

There are software packages that will allow you to figure estimated coverage based on exact antenna location, ERP, HAAT and a bunch of other factors. Knowing which antennas will provide the coverage down in the low spots, canyons, valleys, etc. is really important. It takes more than just picking an antenna design or gain number. There's a lot involved.

Considering a decent antenna is going to cost $1500 or more, spending a bit of money on getting a coverage study done might be a good investment. At least it will give you something to look at, and especially point at when/if the coverage complaints start coming in.
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Old 09-07-2017, 12:21 PM
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I don't want to use the wrong antenna... That's what I'm getting at. We have virtually no help with decisions on the system or what we can and cannot use. Our county has no regulations on that other than they have to be able to communicate with us. Our current is WQEZ310.. AMSL 372.0m HAAT 128.0 HGT to Tip 60.3m. The new site is located on KNKN848 site number 6 AMSL 377.0m HGT to our antenna 96m. The new license hasn't been update on the database yet. Even though I have received confirmation. I hope this helps.

Also, what software is available?
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Old 09-07-2017, 2:45 PM
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You could try this site: https://www.towercoverage.com//

However, it's only as good as the data you put in.
I'm a bit confused, are these two systems for the Sheriff and Fire department? If this is, then it's a "life safety" type thing, and you'd need to be very careful about this. Not saying you should't try, but be aware of the liability if things don't work.

While it is a big expense, hiring a consultant to figure out the myriad of details can be very valuable. That can save a LOT of headaches and a lot of money.
Putting antennas at 330 feet is going to take some work. If you don't get this right, you'll spend more money on new antennas and install. That can add up quickly. There's a lot involved in getting this right, and having an experienced person assist is a real good idea. While I can appreciate your desire to save the county money, this really isn't something to design using input from bunch of strangers on an internet hobby radio site. I'd strongly recommend hiring a professional 2 way radio consultant to assist you. They'll have the tools and experience to start this off on the right track. Trying to do this on the cheap using the internet as a source of information might turn into a bigger issue. Especially if the radio coverage puts officers/fire fighters lives at risk. That can quickly turn into a nightmare with unions, lawyers, etc. Just don't want to see you get hoisted for this.
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Old 09-07-2017, 3:38 PM
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The above is very good advice.
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Old 12-13-2017, 7:58 AM
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Ok. Update.. We have our system up and running. We have lost some portable coverage because we are 4miles further outside our area. Our radio techs were out the other day and mentioned putting a receive antenna at 400ft and leaving the transmit antenna where it is and that would greatly increase our portable coverage. We are limited on our antenna selection due to tower space is leased. We are considering the 220-3BN like we use currently. Is there any other suggestions on antennas that we should strongly look at? And we will use AVA5-50FX 7/8 coax again.
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Old 12-13-2017, 12:03 PM
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Also consider a tower top amplifier.
I've got one on my 800MHz system and it solved some portable coverage issues. Of course, coaxial loss (even on 1 5/8 Heliax) is a thing with 150 feet of cable, and probably not as big a deal on your VHF system.

With coverage software, you should be able to model the coverage impact of moving your RX antenna up the tower.
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Old 12-13-2017, 12:52 PM
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If you have access to a spectrum analyzer, you might see what the noise floor is at the current location. Also, instead of a TTA like mmckenna suggested you could try a pre-amp there in-front of the receiver and see what kind of change it makes. That will really tell you whether or not there is a need to invest in the antenna run up the tower. Pre-amps are finicky things to get right with some repeaters while others they are as easy as install and power on.
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Old 12-13-2017, 1:54 PM
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Let me add a few more comments regarding antenna selection and pre amps.

I tend to try and use folded dipole antennas on mountain tops for several reasons. The first is they are grounded and take much better to lightning strikes than do the fiberglass sticks. The fiberglass sticks tend to whip around in the wind and this tends to fracture internal connections. when the internal connection does fracture from all the movement, this causes noise in the received signal and probably also looses gain. How much will depend on where the fracture occurs in the electrical connection of the antenna.

Most folded dipole antennas can be ordered with different electrical down tilt. This is done by how the phasing between the different pieces of the antenna are set in the connections between the different antenna sections.

Installing an antenna pre amp needs to be done with care. If the device is mounted on the tower, it is more prone to lightning damage. If it is mounted at the repeater, you can change it out easy. It can also have additional surge protection placed before it easily. Up on the tower, your amp only has the surge protection installed internally if at all. Different companies do surge protection in different ways.

It would be a good idea to talk technical details before you purchase any pre amp unit. Also remember that if it's up on the tower, you will need to feed DC power up the coax transmission line. This will require a device to be inserted into the run of the coax transmission cable inside the equipment shelter that injects the DC power. Remember anything you put in series with the coax transmission line will inject loss. The idea is to keep this injected loss to a minimum.

It's not much, but most people don't think about the issues I have mentioned above.

As others have mentioned, it would b e in your best interest to get an RF consultant involved with the changes being talked about There are some good consultants and then there are some that will suck you dry and produce poor results. Make sure you ask for references from them for customers they have done work for. If you sense any hesitation to provide a list, walk away and go to the next one I like to do a job once and have it work that same first time.



Good luck with your efforts.
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Old 12-15-2017, 1:54 PM
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Dear EmtUnderwood,

It is difficult to advise when the complete objective is not offered. All of the suggestions offered to date are valid. However, you might be better served by starting at the beginning, by creating colored maps, showing: [a] must-have indoor HT coverage, [b] must-have outdoor coverage, and [c] like to have outdoor coverage.

You can also create similar colored maps showing what coverage you currently have, and listing all of the equipment and sites now providing that service. Add to this, a complete description as to how the current system does not meet your needs. Be sure to include a description as to how you currently maintain your fleet, to ensure it is still working at full performance.

From this data, any experienced communications engineer with good RF modeling software can offer solutions. It is possible you may need additional receiver sites to enhance HT talk-in, or diversity receive at your main repeater site. But a regular monthly program to remotely test the repeater input sensitivity, with a calibrated signal is essential for long-term success.
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Old 12-19-2017, 1:03 AM
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I was on a job where the county put in new TTAs that required a monitor antenna. The county was renting space on 2 of the sites and had to pay for an engineering study to determine the extra wind loading.
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