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Need help with antenna choice VHF/UHF

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#1
I am new to building a radio system. I am familiar with the theory and use but I'm not with accepted practices.

My goal: Setup my truck with a VHF/UHF radio to operated on all/most of the frequencies available on my radio

My radio: Motorola CM300

My reason: I am an emergency management nut. I am a member of Search and Rescue, Volunteer Fire Fighter, EMT and I want to be able to communicate on mission while POVing with all agencies we potentially will communicate with (Navy, Sheriff, State Patrol, ourselves....)

My radio operates on 136-162 MHz, 146-174 MHz and 438-470 MHz. I've been having a hard time sourcing an antenna that works well on a span with these ranges. Does that mean I will end up with 4 antennas and a quadplexer?
 
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#2
?

Are you saying you have 3 radios or 1 radio? The CM300 is a single band 32 channel radio, not a multi-band radio. The VHF is available in a choice of one of two VHF range splits, and the UHF in one range split, and that's apparently the 438-470 MHz split. Didn't see a 450-512 MHz split, but you didn't mention that one anyway. You are going to need at least two radios - a VHF and a UHF - to do what you want to do, and 32 channels in each probably ain't gonna cover the myriad of frequencies you'll probably see a need to have, even if only for receive purposes on most of them.

To begin with, there is no single band or dual band transmit antenna that would deliver an acceptable SWR over the spread of 136-174 MHz, and same for single or dual band in 438-470 MHZ configuration. The laws of physics shall prevail. Receiving on the other hand is quite a bit more forgiving, and some dual band antennas will yield acceptable but probably not sparkling receive results over the whole range of both VHF and UHF.

An antenna di/tri/quad~plexer doesn't split up within VHF or within UHF, so that's no solution. You could use a multi-antenna switch to choose your antenna from however many are needed to get decent SWR, but I'm not sure you want the trouble of switching antennas when you switch channels. Properly spaced 1/4 wave antennas on the roof will usually have the most acceptable frequency spreads, but not likely 136-174 or 438-470. Gain type antennas increase the range somewhat, but as you add gain, you subtract frequency spread. Again, the laws of physics shall prevail.
 
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#3
Thank you for your reply, you are right, the radio is a single band radio. I misunderstood the specs when I looked them up as them being all in the operation range of this radio. My particular model will provide me 25 watts on 146-174 MHz. Thankfully this is the range that I would use the most of the time with SAR. I mentioned wanting to interface with the other agencies because I thought I had hardware to do so, but I will continue with what I have to interface with my agency.

In that case, what would be the best antenna setup for both transmitting and receiving across that range? I've seen the 1/4 wave antennas but I don't know what that means, would I get a 1/4 for a section and a 1/4 for another and combine them?
 
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#4
The nice thing about quarter wave antennas is that have a broad bandwidth with acceptable SWR.
Unless you have an amateur radio license, you'll have no need to use your radio below 148MHz. Truth is, most public safety VHF is going to be in the 150-160 range. Some federal stuff is up in the 160 range, but then it gets kind of sparse above 170 or so. So while your radio will cover a wide range, you realistically only be using a portion of that.
A quarter wave antenna is likely going to be your best bet. It'll work well over a wide range. Going to a higher gain antenna usually lowers there acceptable bandwidth. If you look at what the agencies you are planning to work with are using, you'll get an idea of what works in your area. Going to a high gain antenna just because it increases your ERP isn't necessarily a good design practice. If your local agencies are all using quarter wave antennas with good results, likely that's all you need.

If you install a permanent mount (highly recommended!) and use the NMO type, you'll have a wide range of antennas and antenna manufacturers to select from. Antennas are relatively cheap, and with a standard mount, you can easily swap out antennas as you need to. It's cheap, easy and can really teach you a lot.
 
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#5
A 1/4 wave VHF antenna is about 19 inches long and will definitely give you a broader TX range with decent SWR and great coverage if it's permanently mounted (NMO preferred) in the center of the roof. In addition, the relatively short length gives you better access to parking garages and more protection from low hanging tree branches, not to mention that in black, they are much more discreet visibility-wise, if that's important.

A gain type antenna works great in the flatlands and gently rolling terrain, but shows less return on the investment in hilly and mountainous terrain since the price difference is about 3 to 1. That's based on real-world personal experience over 40+ years in the radio business. I live in the Piedmont of NC and I use quality brand 1/4 waves mounted in the roof, and on frequent travels from the Piedmont to the coastal plain or to the mountains, I regularly see the differences between the antenna types because I'm so anal I carry gain antennas cut for the desired ranges and I sometimes change them out as I travel. Overall, I rarely see where a gain type antenna is worth the added expense, but if I lived in the coastal flatlands or gently rolling terrain, my choice would be the gain antenna. You'll have to weigh the desired frequency spread against the physics of the antenna design and how discreet you want your antenna array to be and decide for yourself.

Another advantage of the 1/4 wave VHF antenna is that it can be satisfactory as a limited dual band VHF/UHF antenna. In the event your VHF frequencies fall where the antenna will also allow a decent SWR on the desired UHF frequencies, it will work with a diplexer or duplexer (your choice on the term used), BUT the angle of radiation on UHF isn't likely to be optimal. If you are just receiving UHF and 800 on a scanner like I do, then it is a great "dual band" solution. Just make sure your di/duplexer is a quality brand with the best attenuation specs between bands.
 
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#8
Right, if you have the one radio in your truck, then one antenna mounted in the center of the cab roof (NO magnetic mounts!) will give you the best performance. Use name brand mounts and antennas. If you are not sure how to do it, or don't have the right tools, then go to a local radio shop and have them do it for you. They'll charge you a bit, but they have the right hole saws, connector crimp tools and should check the SWR for you. Might be $100 or so, but it would be well worth it. If you feel like doing it yourself, it's pretty easy if you have the skills.

You'll be happy in the long run if you do it right the first time.
 
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#9
My suggestion:
Larsen NMOWBQB Wide band, 150-170 MHz Black with spring base. NMO mounting type. The black tends to blend in well and not stand out. If you don't care about that, then the chrome model would be fine.
Larsen NMO150 would be the 5/8th wave antenna of choice. It would have more gain, but less bandwidth. About 4 feet tall.
Larsen NMOKUD, 3/4 inch NMO mount with dual shield RG-58, has a bit less loss than the single shield. Just a personal preference.
You'll need a mini-UHF male connector for the RG-58 cable. This will require crimping.
You should have no trouble finding these parts on line.
 
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#10
Thanks mmckenna! I would like to leave as much of my truck stock as possible, so I would like to avoid putting holes in it. The fact that almost all posts have said NMO leads be to believe that this is a big issue. I have a 2005 dodge Dakota, so mounting on the hood is difficult due to the lip on the hood overlapping. I've been searching for options that don't require drilling any exposed paneling but have come up short. Suggestions?
 
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#11
Something else to remember is that on any public safety/commercial frequencies you will also need an FCC license or a letter of "concurrance" from the actual license holder. In fact you may not be able to get your radio programmed without it. The FCC has started to crack down on radio shops who program frequencies for which the user does not have an FCC license or a concurrance letter from the licensee. It was common practice for us to program a public safety radio with frequencies from adjacent public safety agencies. Since the FCC provided us with a personal letter informing us of the violation we now check every frequency we program to be properly licensed by the user. We also remove any frequencies for which the user has no license because it would be hard to prove we didn't program it in the first place.

BB
 
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#12
If you do any antenna mount on your truck that is NOT a hole in the roof the performance of the system will be compromised to some extent. This includes putting the antenna on a mast mounted to the truck's bed. Drill the hole in the roof and be done with it. When and if you get rid of the truck you put a 3/4 inch rubber hole plug in the hole which is hard to notice for most people. The radio shops and others sell these plugs.

The formula for figuring the length of the quarter-wave antenna is:

234 divided by the frequency in mHz (say, 155) X 12, which gives the length in inches.
In this example that comes out to 18.2 inches. Whatever your operating range in mHz is you want to cut the antenna for the middle of that range. When you do, cut it slightly longer than the answer you get from the formula so you can trim it for best performance.
 
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#13
If you do any antenna mount on your truck that is NOT a hole in the roof the performance of the system will be compromised to some extent. This includes putting the antenna on a mast mounted to the truck's bed. Drill the hole in the roof and be done with it. When and if you get rid of the truck you put a 3/4 inch rubber hole plug in the hole which is hard to notice for most people. The radio shops and others sell these plugs..

Thank you, I think I will... I think I am more concerned with the holes in my dash I might make I guess.
 
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#14
Something else to remember is that on any public safety/commercial frequencies you will also need an FCC license or a letter of "concurrance" from the actual license holder. In fact you may not be able to get your radio programmed without it. The FCC has started to crack down on radio shops who program frequencies for which the user does not have an FCC license or a concurrance letter from the licensee. It was common practice for us to program a public safety radio with frequencies from adjacent public safety agencies. Since the FCC provided us with a personal letter informing us of the violation we now check every frequency we program to be properly licensed by the user. We also remove any frequencies for which the user has no license because it would be hard to prove we didn't program it in the first place.

BB
Thank you for the advice, but I wouldn't have an issue supplying a letter but that is made unnecessary by the fact that we (SAR) own a copy the Motorola programming software for my radio.
 
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#15
???

Thank you for the advice, but I wouldn't have an issue supplying a letter but that is made unnecessary by the fact that we (SAR) own a copy the Motorola programming software for my radio.
How does that statement relate in any way to having a valid FCC license? It sounds as if you are equating the access to a Motorola piece of software to having an FCC license, which is certainly not true.
 
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#16
Please keep this post to my original question and not straying off on other tangents. I have sufficient license to broadcast, I only said that since I have the software, I would not need to prove that to anyone in order to program my radio.

Back to the topic:

I have a maxrad antenna that is listed as working for 144-174, but is much longer than the 18 inches required for a 1/4 wave, can I cut this down or would it be a different antenna type?
 
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#17
Yes, it needs to be cut to the center frequency. The model is just designed for that frequency range. There should be a cutting chart included with the antenna. What you need to do is decide how you want to cut it. Usually if you are only working on one frequency, you trim the antenna for that one frequency. If you are going to be working over a wide range of frequencies, then you need to find the center of that range and trim it for that. That would give you the best overall performance. The cutting charts are usually pretty close, but they are designed for a near perfect location. Having it down on the fender will change that a bit. Ideally you want to cut it a bit long and then use an SWR meter to do the final trimming.
 
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#19
No. If it's a half wave antenna, that means the coil at the base is designed for a half wave antenna. If you cut it for 1/4 wave, it MAY not work correctly.

As for the roof or the hood, the term "performance" can mean a number of different things. A half wave antenna with a ground plane should outperform a quarter wave. The issue that I see is that when you mount them next to the hood, on the fender, you do NOT have a good ground plane. A proper ground plane extends in all directions. When you mount it on the fender, it's lopsided. The pattern from a fender mount antenna is going to be directional, favoring the side of the vehicle the antenna is on.
If you mount the antenna in the center of the roof, you will get a more even ground plane under the antenna, and a more equal radiation pattern.
So, performance can depend on what you are trying to do. If you are trying to talk to someone that is always on the right side of your car (if the antenna is mounted on the right fender), then the half wave on the fender might work better when compared to the quarter wave. If you are just looking for good all around performance, go with the quarter wave mounted in the center of the roof.

While drilling a hole in the roof of your truck may seem unappealing now, you should remember that there is a reason that police cars, fire trucks, anyone that really depends on a radio does this. It works, and it works well. Anything else is a compromise. People will argue with you, but they are wrong. Any location other than the center of the roof is going to have slightly less performance.

It's your choice, either will "work", as in tune up, likely, and spit signals out, but if you want it to work consistently, do it right.
 
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