Jump to content

Recommended Posts

My fire radios are VHF, i have a Motorola CDM 1250 in the utility and Motorola CM300 in the pickup. I am going to be putting another in my jeep,  the CDM 1250 gets markedly better reception than the CM300, both have magnetic bases but slightly different antennas supplied through the company that programmed them, so both should be compatible with our frequency, we operate at a range of 143-159 MHZ.

questions:

As both trucks are of similar size and mass can i reasonably assume that the reception differences are in the radio? 
as the new radio will go in the jeep, i want to mount the antenna to the bumper meaning i need a 3-4 ish foot whip, presumably these things are a bit like CB where they are available in various lengths but fine tune by being trimmed or length fine tuned? If so could I expect better reception from a longer antenna? Or considering it would need the 4’ to clear the windshield would the effective length only be the amount it clears the windshield? 
in order to improve the reception on the trucks could I expect better reception with a fixed mount vs a magnetic? Do i need to do anything special to ground it somehow? 
any other wisdom? 

Link to post
Share on other sites

I'm by no means a expert. It appears that both radios have the same wattage output, so I'd assume it may be the difference in antennas. I know that the more metal mass under the antenna the better and that they clip the antenna to length depending on the frequency range. 

Link to post
Share on other sites
9 minutes ago, mjcolemn said:

I'm by no means a expert. It appears that both radios have the same wattage output, so I'd assume it may be the difference in antennas. I know that the more metal mass under the antenna the better and that they clip the antenna to length depending on the frequency range. 

I presume the concept is not wildly different from a CB?

Link to post
Share on other sites

Probably not, I'm just familiar with our fire radios. I have a CM300 in my pickup with a fender mount and a CM300d with a permanent mount on the roof in my utility truck. I don't notice any difference in reception between the 2.

Link to post
Share on other sites

How the antenna is mounted can be a big factor. A good ground plane is important. A good bond to the chassis is important, and an antenna in the middle of a roof in theory will do better than one mounted on a wing mirror.

A longer antenna will not necessarily better than a shorter one. A tuned short will can do better than a misaligned long one..... there are many many variables. Antennas should be tuned and checked with a SWR meter... or bought tuned for a specific frequency

With today's radios a layman is likely to get things running just fine, however after doing years of electronic design and some radio work, I have half a clue to know that some skills are needed to to get peak performance.. .and to know enough that a local expert will do better than advice from a distance.

  • Like 1
Link to post
Share on other sites
15 hours ago, vtfireman85 said:

My fire radios are VHF, i have a Motorola CDM 1250 in the utility and Motorola CM300 in the pickup. I am going to be putting another in my jeep,  the CDM 1250 gets markedly better reception than the CM300, both have magnetic bases but slightly different antennas supplied through the company that programmed them, so both should be compatible with our frequency, we operate at a range of 143-159 MHZ.

questions:

As both trucks are of similar size and mass can i reasonably assume that the reception differences are in the radio? 
as the new radio will go in the jeep, i want to mount the antenna to the bumper meaning i need a 3-4 ish foot whip, presumably these things are a bit like CB where they are available in various lengths but fine tune by being trimmed or length fine tuned? If so could I expect better reception from a longer antenna? Or considering it would need the 4’ to clear the windshield would the effective length only be the amount it clears the windshield? 
in order to improve the reception on the trucks could I expect better reception with a fixed mount vs a magnetic? Do i need to do anything special to ground it somehow? 
any other wisdom? 

I have a military jeep antenna if that will work for ya About 8 feet long

  • Thanks 1
Link to post
Share on other sites

Hard mount (drill a hole) as close to the center of your roof as possible. 

Have done side by side comparisons and magnetic mount can not compete. 

In the Sandhills where I live, the ground absorbs radio waves better than a sound booth wall. 

We need the best performance possible. 

We have Motorola XTL 1500 and XTL 2500 radios in our fire trucks. 

I also have Radius GM 300 and Maxtrac radios. 

Love my Motorola radios but.............we have started switching to ICOM F5021  45w mobile radios. They are a lot cheaper than Motorola, perform just as well and the icing on the cake is the software and patch cable to program them can be purchased online with the radios. Motorola's software is proprietary and radios have to be taken to authorized service to be programmed. 

Really hate to give up on my Motorola's but ICOM is winning me over. 

1828721087_ICOMF5021.thumb.jpg.d3aeed04b623f7212e76afe122ffdf28.jpg

Just reread your post Seth, on my 79 CJ5 I mounted the antennae to the center of the hood. May not be your cup of tea but works for me. 

  • Like 1
Link to post
Share on other sites

@sandhiller i think ill take a pass at the center of the hood, however, what is the advantage in a center of vehicle mount vs off center or bumper mount?  I am not a fan of center of roof mounts because of ladder rack interference. 

Link to post
Share on other sites
47 minutes ago, vtfireman85 said:

@sandhiller i think ill take a pass at the center of the hood, however, what is the advantage in a center of vehicle mount vs off center or bumper mount?  I am not a fan of center of roof mounts because of ladder rack interference. 

Ground plane is bigger as you are using the whole roof. 

1316137748_antennaemount.jpg.bc6bd9c158430cfa2bc70ea9ba299d21.jpg

  • Thanks 1
Link to post
Share on other sites

We have had good luck with the fender mount no ground plane antennas for the same setup as you.  Simple L bracket that mounts to the fender where the hood and fender meet. We run both business band and fire band vhf.

  • Thanks 1
Link to post
Share on other sites
4 hours ago, m.c.farmerboy said:

I have a military jeep antenna if that will work for ya About 8 feet long

Thanks, the FD usually buys me that kind of thing, that would be a cool item for a restoration though 

  • Like 1
Link to post
Share on other sites

Just a question.  We use radios like the one pictured above at work.     They use a repeater.      Meaning they don't talk radio to radio. It has to go through the repeater (kinda range booster)     would that be vhf.  Hf. Or something totally different 

Link to post
Share on other sites

Hm, you listed a frequency range in your post.  Is 8t just the band you operate in or do you have multiple frequencies?

 

antennas are matched to the wavelength of the frequency they are using for best results. So a higher frequency has a shorter antenna.  And band shave a range of different frequencies.  The best antenna length is set to match wave length.  Add to this that the line to the antenna can affect the whole assembly and it gets a bit complicated.

So longer is not always better, there are variables and like mentioned try an SWR (signal wave ratio) meter.  It will tell you how much power is going out verse how much you make.  Look around for a local HAM radio guy, and they should be able to help.

 

ground planes and antenna are a huge topic, some antennas need them, others are engineered to work without them, and finally some have additional planes added at the base.  Again, try some ham radio stuff, it will help

 

magnetic mounts are a temporary and mobile set up that do everything poorly, move towards other mounts when you can, NMO style seem to be the most practical and have the largest selections of antennas.

yes, it works like a CB, but since your not just on 40 set channels the pre-built stuff is not as easy to get already tuned 

 

good luck, and let us know how it goes

Link to post
Share on other sites
On 3/9/2021 at 8:12 PM, dannyredfan said:

Just a question.  We use radios like the one pictured above at work.     They use a repeater.      Meaning they don't talk radio to radio. It has to go through the repeater (kinda range booster)     would that be vhf.  Hf. Or something totally different 

we have a repeater, it is a pain, we have 2 channels for ourselves, repeater and straight to and from. biggest issues are 1 people hearing a call on scan over repeater and trying to answer on standard channel or vice versa. another is that the repeater has a significant time lag that some cannot seem to figure out, it leads to a lot of unnecessary back and forth which can be incredibly frustrating. 

 

On 3/10/2021 at 10:26 AM, AKwelder said:

Hm, you listed a frequency range in your post.  Is 8t just the band you operate in or do you have multiple frequencies?

 

antennas are matched to the wavelength of the frequency they are using for best results. So a higher frequency has a shorter antenna.  And band shave a range of different frequencies.  The best antenna length is set to match wave length.  Add to this that the line to the antenna can affect the whole assembly and it gets a bit complicated.

So longer is not always better, there are variables and like mentioned try an SWR (signal wave ratio) meter.  It will tell you how much power is going out verse how much you make.  Look around for a local HAM radio guy, and they should be able to help.

 

ground planes and antenna are a huge topic, some antennas need them, others are engineered to work without them, and finally some have additional planes added at the base.  Again, try some ham radio stuff, it will help

 

magnetic mounts are a temporary and mobile set up that do everything poorly, move towards other mounts when you can, NMO style seem to be the most practical and have the largest selections of antennas.

yes, it works like a CB, but since your not just on 40 set channels the pre-built stuff is not as easy to get already tuned 

 

good luck, and let us know how it goes

we are either 159 or 154 MHZ and state dispatch is the other, I don't now remember which is which. other departments and dispatch organizations use other frequencies in that range, I have perhaps 40 different departments programmed into my radio. my intent was to put a fixed mount antenna on the headrack of each truck. so I am  confused by the ground plane issue, it seems logical to me that if the antenna is attached to a large metal object that entire object becomes the ground plane, the diagram posted before leaves me puzzled. we have a radio man but for some reason he is impossible to get hold of, so to ask these questions is difficult. I have immense respect for my chief but he is frustrating with this kind of stuff, when I ask who it is I get a vague answer, and the phone number is written somewhere but he cant remember where. for a man who seems so scattered he seems to get things done that need doing and do them efficiently...but he cant remember what the radio mans name is. 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...