Hello all and thanks for this BB. I work for a small AG company as the IT Manager, and they have also decided to task me with maintaining all the 2 Way radios we have installed. Apparently here in the US, they are really starting to crack down on cell phone usage in commercial autos. Anyway, we have a complete LOW Band system in place running at 43MHz. The entire system was set up and configured by a real radio outfit many years ago, and I know very little about the entire set up, but am slowly learning. We have pretty much standardized on CDM750's and purchase the radios pre-programmed for our use. I use only Belden RF195 RG-58 Cable lowl oss solid core 50 Ohm cable, and RFI RFU-600-1 (Mini UHF M) cable ends. For all the installations, we try to use a 102" whip and ball cut to about 66" as per the instructions for our frequency. In places where a whip just will not work, we have been using TRAM base loaded antennas with an NMO mount.
I try and keep all the cables (regardless of Whip, or base loaded) at 18 feet, and if there is any excess, I try and bind it up in a 12 - 18" loose loom (avoiding any tight coils or bunches and hard bends - trying to avoid Chokes etc...). I have a Bird 43 Thruline Watt meter that I am trying to use to test the radios during install and calculate the SWR etc... which brings me to my questions that I cannot seem to find answers for by reading or searching:
1) Can the transmit power of the CDM750 exceed 60watts through programming? - I have tested some with my Bird and they are running 80 watts forward, and 8 reverse. Others I see at 40 - 60 which seems reasonable, but some of the newer ones are definitely hitting at or above 80 Watts.
2) How can the antenna system (cable, mount and antenna) change the Watts being transmitted? If I have a CDM750 connected with a mini UHF --> 1foot cable PL295 --> Bird 43 --> pl295 to Mini UHF female --> Antenna system. If I connect a whip and ball with 18foot coax - it indicates about 20 watts on the bird. Hook the same thing up to a roof top base loaded and I get 86 watts forward. What causes the TX to drop? Sholdnt the radio output be consistent?
3) Is there a way to test the radio out put on its own? Like with a dummy load - so I can get a starting point before determining what is getting lost in the Antenna System?
Any assistance or guidance you folks can provide is greatly appreciated, I am very new to this and would like to get a better understanding. If there are any books specifically about Low Band that any of you can recommend that would also be appreciated.
New Radio Installer Questions and Confusion
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Re: New Radio Installer Questions and Confusion
Low band is a funny bird. Just because the chart shows cut the antenna to "X" length for "Y" frequency isnt necessarily true. Ground plane, proximity to other objects, coax length, antenna grounding all have an effect on SWR and radiated power. IIRC most modern radios start cutting back power if the SWR is high. In effect a radio which is rated to transmit 100 watts sees excessive SWR on the antenna line it may shut down to 25 watts out with 8 watts reflected. Proportionally that would have been 100 watts out with 35 reflected if the transmitter wouldnt have reduced power to preserve the finals.
Bad radio power connections or low batteries can also effect power out.
If you place the same radio on a tuned base antenna it may show 80 watts forward and 2 watts reflected because of a better match on the coax and antenna and a clean power supply. The 80 watts forward may be due to slight mismatch on the antenna or some SWR on the feed coax.
If the radio is showing more power out than it is rated, its time to do a current check to see how much the radio is drawing on TX. Excessive current draw will only shorten the life of the finals.
Hope that helps some
Bad radio power connections or low batteries can also effect power out.
If you place the same radio on a tuned base antenna it may show 80 watts forward and 2 watts reflected because of a better match on the coax and antenna and a clean power supply. The 80 watts forward may be due to slight mismatch on the antenna or some SWR on the feed coax.
If the radio is showing more power out than it is rated, its time to do a current check to see how much the radio is drawing on TX. Excessive current draw will only shorten the life of the finals.
Hope that helps some
Q: Does this unit come with a speaker?
A: hello yes its in the radio thanks
A: hello yes its in the radio thanks
Re: New Radio Installer Questions and Confusion
So it is doubtful that the radio supplier is somehow "boosting" these stock radios to 80 with programming - it is more likely that power needs to be checked? The CDM750 for low band states that it should be 40 to 60 watts (which is what I see on most radios). When I tested the antennas (one whip and one base loaded) the power supply remains the same. On the Whip (that knowingly had a bad ground plan) - was run with 18' of coax and everything ohmed out well. The base loaded turns out to have a great ground plane - and a short antenna cable. Power supply did not change.
Whip 23 forward 6 reverse
Base loaded 86 forward 8 reverse
Whip 23 forward 6 reverse
Base loaded 86 forward 8 reverse
Re: New Radio Installer Questions and Confusion
Welcome DAXD. Yes, you can buy a 100w dummy load to check the radios. It is not uncommon for mobiles to have higher than rated output into a good load. It also common to have a radio fold back power when it's looking into a bad load. Low band ball mounts come with their own qwerks, though they are my favorite. Hands down, installed correctly, they outperform base loads. OTOH, base loads tend to match better, and are easier to install, but are less efficient. However, if they serve your purpose, they may be the better choice.
You should have the local radio shop give you few lessons in how to use and read the wattmeter.
You should have the local radio shop give you few lessons in how to use and read the wattmeter.
Re: New Radio Installer Questions and Confusion
Due to the relatively long wavelengths compared to higher frequencies, low band VHF is much more easily affected by metal structures within a short distance of the antenna, and the effect is that the proximity will especially skew low band radiation patterns. Couple that with the shape of the vehicle, the mounting location, the shadowing by portions of the vehicle, and several other factors and you can see why these things will critically affect the antenna's ability to radiate with efficiency. Also, you should know that the higher the reflected reading, the higher the forward power reading will usually be skewed, which makes obtaining an accurate SWR reading difficult with low band. It is a good idea to move the vehicle outside a building and away from any metal structures when using an SWR meter or analyzer of any kind. That at least removes the proximity factor from the vehicle's antenna radiation pattern. It's just the nature of the low band RF beast.
curmudgeon.....and I like it.
Re: New Radio Installer Questions and Confusion
Boy this stuff is really a pain! coming from computers, I normally deal with a much more specific type things with this plus that equals this kind of world =) Thanks for all the information and things to try. I think I would definitely like to try and get my hands on a dummy load, just to have a good starting point (so I can say - well this radio is doing blat! with the dummy load and go from there - otherwise, I am just starting blind.
I really like the Whips, and if you can find nice old metal trucks to put them things are typically great. But with AG equipment, they are trying to replace everything with non-metalic type of stuff to get the equipment lighter. Then once that part of it is done, they add on a bunch of GPS, Touch Screen monitors, and loads of other electronic interference. Then add in that much of the equipment, the roof is 13feet off the gound - so adding a 102 whip to that can be troublesome to drive down the road.
When testing the autos (like PickUps) - I always drive them out away from the surrounding clutter and try to get them into the open, not always possible with the larger AG equipment. I just had no idea that the antenna would effect so much what is coming out as far as power. I would expect a 50Watt antenna to send 50 watts no matter what - lot more detecting going on than I thought there would be.
The toughest part is dealing with the end user - can only say well these low band things are just plain picky and anything can set them off making to identical installs completely different in the way they act. At least when I was a meteorologist, there was a plethora of excuses you could throw at them to make it sound like you knew why it was doing what ever it was doing.
I really like the Whips, and if you can find nice old metal trucks to put them things are typically great. But with AG equipment, they are trying to replace everything with non-metalic type of stuff to get the equipment lighter. Then once that part of it is done, they add on a bunch of GPS, Touch Screen monitors, and loads of other electronic interference. Then add in that much of the equipment, the roof is 13feet off the gound - so adding a 102 whip to that can be troublesome to drive down the road.
When testing the autos (like PickUps) - I always drive them out away from the surrounding clutter and try to get them into the open, not always possible with the larger AG equipment. I just had no idea that the antenna would effect so much what is coming out as far as power. I would expect a 50Watt antenna to send 50 watts no matter what - lot more detecting going on than I thought there would be.
The toughest part is dealing with the end user - can only say well these low band things are just plain picky and anything can set them off making to identical installs completely different in the way they act. At least when I was a meteorologist, there was a plethora of excuses you could throw at them to make it sound like you knew why it was doing what ever it was doing.
Re: New Radio Installer Questions and Confusion
Oh - I also meant to ask, with the 102" whips, should I leave them at 102 and start the tuning process there rather than cutting them as the MFG sheet suggests.
Re: New Radio Installer Questions and Confusion
The antenna is the termination of the feedline and is the object that shapes and distributes the electromagnetic field created at that termination. The antenna power rating refers to the amount of power the antenna can safely handle under normal conditions and is not an indicator of power output. An important fact that becomes apparent with an antenna analyzer that also shows you the impedance is that the SWR can actually be acceptable with impedances that are NOT 50-52 ohms. This has a great effect on the antenna's radiating efficiency. I have had several hams come to me with an antenna that shows an SWR of 1.5 to 1, but the impedance was nearly 70, and while the 50 watt radio seemed to receive well, it wouldn't talk across the block. The MFJ analyzer is a worthwhile investment on a budget to make sure the impedance is satisfied also.
Where you start cutting the antenna is a personal thing, I suppose, but foremost is the fact that the factory "suggestion" is only a reference for nearly ideal conditions, followed by the fact that there are no such things as antenna stretchers, so make sure that wherever you start cutting the whip, it needs to be somewhat LONGER than is required for the application. I always start 1/4 wave 42-43 meg whips at 80 inches, and start the base loaded whips at the full factory length. You can start with small cutting increments on the first ones until you get a feel for how much cut yields how much results.
I highly recommend a dummy load so that you start off knowing what you should see forward when the antenna is connected.
Where you start cutting the antenna is a personal thing, I suppose, but foremost is the fact that the factory "suggestion" is only a reference for nearly ideal conditions, followed by the fact that there are no such things as antenna stretchers, so make sure that wherever you start cutting the whip, it needs to be somewhat LONGER than is required for the application. I always start 1/4 wave 42-43 meg whips at 80 inches, and start the base loaded whips at the full factory length. You can start with small cutting increments on the first ones until you get a feel for how much cut yields how much results.
I highly recommend a dummy load so that you start off knowing what you should see forward when the antenna is connected.
Last edited by wx4cbh on Sat Feb 04, 2012 6:46 pm, edited 1 time in total.
curmudgeon.....and I like it.
Re: New Radio Installer Questions and Confusion
Yes sir! Always easier to start long, cut and measure SWR than to cut too short in the beginning (doh!)
Q: Does this unit come with a speaker?
A: hello yes its in the radio thanks
A: hello yes its in the radio thanks
Re: New Radio Installer Questions and Confusion
And of course, I've NEVER done that.........
Hey! My pants are on fire!
Hey! My pants are on fire!
curmudgeon.....and I like it.
Re: New Radio Installer Questions and Confusion
I guess a couple questions are in order. It's tough coming into a trade cold. Why doesn't your company hire the local radio shop to maintain the radios? Low band is not that common any more. Since it was set up years ago, is the license still current, or has it lapsed? Is there a reason they use low band as opposed to UHF which has much more product available, and is easier to work with?
Re: New Radio Installer Questions and Confusion
Always felt it was strange to cut the whip prior to testing anything, but am a bit of a stickler when it comes to reading manuals and instruction guides. Cannot believe I have never actually just tried the 102 out of the box on any of the installs so far - been planning to do so on a totally fiberglass semi that is being a real pain for its install. So that will be my new install plan - start tuning longer if not the entire length of the whip then at least 80 or so!
They normally had the drivers of the equipment take care of it themselves - and in the first few I worked on, I found quite a few with Ground and Hot all wired together at the whip (they were wondering why they kept loosing radios and got crappy action out of it). So I think they wanted at the very least to have one guy kind of dedicated to it.
We did use a local radio shop for some time, but the cost was pretty high, and being rural - local was something of an impossibility - we would loose the equipment sometimes for a entire day or two - and time is definitely money - the equipment looses all the way around when its not in the fields or hauling product plus the cost of the multiple driver(s) to have time to take it and get back then go and pick it up again.
Correct - (on low band being pretty old and touchy as I have learnt), it is also what was originally invested in many years ago - so we now have 100's of mobile installs with 4 base stations. The original set up was professionally done at each location - so they have pretty much tasked me with maintaining old and any new installs. Have also read that Low band has greater distance than most others, and with all the other folks leaving it behind - it has supposedly become less cluttered. It also does not need to ad-hear to some of the new regulations being put in place.
All licensing has been kept up to date and current - the real push for getting things back up to snuff is the ban on hand held cell phone use in commercial equipment. But the cost to move everything over to UHF (or some other) would be pretty steep. Would be nice though - if we could use much smaller antennas on this large equipment - just barely being able to learn a very little bit about low band - I have spent no time at all trying to learn about an alternative tech due to the large cost involved in switching everything over - it did not seem like a viable direction to go. Is it even possible to get the same kind of range with something else out there with smaller cheaper gear that might justify completely redoing everything?
They normally had the drivers of the equipment take care of it themselves - and in the first few I worked on, I found quite a few with Ground and Hot all wired together at the whip (they were wondering why they kept loosing radios and got crappy action out of it). So I think they wanted at the very least to have one guy kind of dedicated to it.
We did use a local radio shop for some time, but the cost was pretty high, and being rural - local was something of an impossibility - we would loose the equipment sometimes for a entire day or two - and time is definitely money - the equipment looses all the way around when its not in the fields or hauling product plus the cost of the multiple driver(s) to have time to take it and get back then go and pick it up again.
Correct - (on low band being pretty old and touchy as I have learnt), it is also what was originally invested in many years ago - so we now have 100's of mobile installs with 4 base stations. The original set up was professionally done at each location - so they have pretty much tasked me with maintaining old and any new installs. Have also read that Low band has greater distance than most others, and with all the other folks leaving it behind - it has supposedly become less cluttered. It also does not need to ad-hear to some of the new regulations being put in place.
All licensing has been kept up to date and current - the real push for getting things back up to snuff is the ban on hand held cell phone use in commercial equipment. But the cost to move everything over to UHF (or some other) would be pretty steep. Would be nice though - if we could use much smaller antennas on this large equipment - just barely being able to learn a very little bit about low band - I have spent no time at all trying to learn about an alternative tech due to the large cost involved in switching everything over - it did not seem like a viable direction to go. Is it even possible to get the same kind of range with something else out there with smaller cheaper gear that might justify completely redoing everything?
Re: New Radio Installer Questions and Confusion
100's of mobiles and 4 base stations is not a small company unless most of the trucks sit idle a lot of the time. It sounds large enough to make a capital investment in new equipment that can be written down just like the purchase of a new semi or heavy equipment. If these radios are important for the day to day operation, then ease of installation and maintenance has to be a consideration. Modern systems integrate voice and data to support features like text messaging, gps vehicle location, and wide area networks. You can land the coverage where you need it rather than try to depend on brute force to (possibly) make it between two points.
Low band has a wizard factor, and some critical ground plane requirements to work properly. Done right, it can cover huge areas. But, it is subject to man made noise, atmospherics, cropland reflections, vegetative absorption, adjacent structure obstruction, wheel bearing noise, loose engine cowls, fifth wheel chatter, neon signs, power lines, etc, etc, etc. The only reason to choose low band over other services is because you have no other options. Your terrain is too rugged, or you have tens of thousands of square miles to cover with no repeater sites. Even when low band is working right, it is known to do odd things like talk a hundred miles to one location, but not talk five miles to another. When the skip is in, you can talk from Kansas, but not across town.
I lurv the low band, but it does like to be romanced, and it is easily distracted. With a fleet your size, it is worth getting some competitive bids to replace it all with something you can maintain, and will perform the tasks you need done.
Low band has a wizard factor, and some critical ground plane requirements to work properly. Done right, it can cover huge areas. But, it is subject to man made noise, atmospherics, cropland reflections, vegetative absorption, adjacent structure obstruction, wheel bearing noise, loose engine cowls, fifth wheel chatter, neon signs, power lines, etc, etc, etc. The only reason to choose low band over other services is because you have no other options. Your terrain is too rugged, or you have tens of thousands of square miles to cover with no repeater sites. Even when low band is working right, it is known to do odd things like talk a hundred miles to one location, but not talk five miles to another. When the skip is in, you can talk from Kansas, but not across town.
I lurv the low band, but it does like to be romanced, and it is easily distracted. With a fleet your size, it is worth getting some competitive bids to replace it all with something you can maintain, and will perform the tasks you need done.
Re: New Radio Installer Questions and Confusion
Bill sort of glance over the issue of needing a good ground plane for any antenna to perform well. On low band this is even more important.
I caught that you had a vehicle with fiberglass only to do an install on. Before you even start on that one stop. The ball mount and whip won't have any ground plane to work against. This install won't work. Another problem child install is on bucket trucks and cement mixers. You always end up with a bunch of metal close to the whip. All that metal around the whip antenna effects the tuning. You probably won't be able to get the SWR to go down very much.
Another point to keep in mind as you do the ball mount installs. The grounding of the mount and the shield of the coax is very important. If you look close at the mount, there is an Allen screw that is threaded on the metal backer plate. The purpose of this screw is to make sure that you have a good ground connection to the vehicle sheet metal.
Also make sure that the hole you punch for the center stud has clearance in the sheet metal. This is the center conductor of the antenna mount and needs to be isolated from the body sheet metal with clearance. Years ago in an old International Scout, I had a ball mount on the rear of the vehicle. In a heavy rainstorm I was driving under a high voltage transmission line and I started to hear snaps comping from the ball mount. I looked in my rear view mirror and could see sparks coming from the gap between the center stud and the body sheet metal. As soon as I moved away from the high voltage towers, the sparking stopped. The rain was picking up enough static from the transmission lines and it was being deposited into my low band whip.
Keep the patch cable between the SWR meter and the radio as short as possible. Otherwise the length of the jumper coax will possibly effect your readings.
Jim
I caught that you had a vehicle with fiberglass only to do an install on. Before you even start on that one stop. The ball mount and whip won't have any ground plane to work against. This install won't work. Another problem child install is on bucket trucks and cement mixers. You always end up with a bunch of metal close to the whip. All that metal around the whip antenna effects the tuning. You probably won't be able to get the SWR to go down very much.
Another point to keep in mind as you do the ball mount installs. The grounding of the mount and the shield of the coax is very important. If you look close at the mount, there is an Allen screw that is threaded on the metal backer plate. The purpose of this screw is to make sure that you have a good ground connection to the vehicle sheet metal.
Also make sure that the hole you punch for the center stud has clearance in the sheet metal. This is the center conductor of the antenna mount and needs to be isolated from the body sheet metal with clearance. Years ago in an old International Scout, I had a ball mount on the rear of the vehicle. In a heavy rainstorm I was driving under a high voltage transmission line and I started to hear snaps comping from the ball mount. I looked in my rear view mirror and could see sparks coming from the gap between the center stud and the body sheet metal. As soon as I moved away from the high voltage towers, the sparking stopped. The rain was picking up enough static from the transmission lines and it was being deposited into my low band whip.
Keep the patch cable between the SWR meter and the radio as short as possible. Otherwise the length of the jumper coax will possibly effect your readings.
Jim
Re: New Radio Installer Questions and Confusion
Being extremely new to this arena, I would not really know where to begin on replacing things. Based on the information you guys have provided though, I will certainly start looking into it in my spare time (for a pro setup to consult). I have definitely seen some of the wizardry you are speaking about for sure! I have been trying really hard to find decent ground planes on this equipment, but the plain fact is that most of the time it just doesn't exist. They might have been there 20 some years ago when everything was steel, but those are pretty few and far between any more.
For the fiberglass truck, I even found a No GP antenna from a guy in Florida to try. We mounted it to the rear hand rail of the semi just behind the drivers seat. It looks like a small 5' stick ball bat (we call it the ugly stick) - it is sticking up the same height as the stacks with an 18' cable and still sucks donky water. Works about as well as the Whip on a three foot pogo stick they were using before I found the NGP antenna. Its just been kind of frustrating.
As I look at the number in my post - 100's - that is definitely stretching it - we do have four base stations, and between the field equipment (sprayers, spreaders, tractors etc...), trucks, semi's service trucks, there cold be 100 to 120 units deployed at any given time. Covering maybe a total area of 100 - to - 150 mile diameter circle (the majority of the radios for each location are within 40 - 60 miles of each base station but everyone is on the same freq and channel and everyone hears pretty much everything from the base stations). Geographically we are spread out enough that would could definitely have some kind of grid relay thing going on.
As for the Whips, I am pretty careful during the install, use a nice steel punch and get things grounded through the body every time. I try to get the users to let me put the antennas in the best location, but as with any users - I am allowed put it right there and make it work. Have actually had pretty good luck with the Whips in pick ups, and base loads if they let me put them on the roofs. The patch cable to the Bird 43 is about 10 inches and then the antenna connects directly to it.
For the fiberglass truck, I even found a No GP antenna from a guy in Florida to try. We mounted it to the rear hand rail of the semi just behind the drivers seat. It looks like a small 5' stick ball bat (we call it the ugly stick) - it is sticking up the same height as the stacks with an 18' cable and still sucks donky water. Works about as well as the Whip on a three foot pogo stick they were using before I found the NGP antenna. Its just been kind of frustrating.
As I look at the number in my post - 100's - that is definitely stretching it - we do have four base stations, and between the field equipment (sprayers, spreaders, tractors etc...), trucks, semi's service trucks, there cold be 100 to 120 units deployed at any given time. Covering maybe a total area of 100 - to - 150 mile diameter circle (the majority of the radios for each location are within 40 - 60 miles of each base station but everyone is on the same freq and channel and everyone hears pretty much everything from the base stations). Geographically we are spread out enough that would could definitely have some kind of grid relay thing going on.
As for the Whips, I am pretty careful during the install, use a nice steel punch and get things grounded through the body every time. I try to get the users to let me put the antennas in the best location, but as with any users - I am allowed put it right there and make it work. Have actually had pretty good luck with the Whips in pick ups, and base loads if they let me put them on the roofs. The patch cable to the Bird 43 is about 10 inches and then the antenna connects directly to it.
Re: New Radio Installer Questions and Confusion
The only "no ground plane" antenna you're going to find is an "end-fed half-wave".
468/frequency = half-wave feet
feet * 12 = inches
43MHz, that's 131 inches for a 1/2-wave "no ground" whip.
Just make sure there's roughly a quarter-wave of metal in at least one direction under the antenna. If need be, attach it to the underside of the mounting platform and lead it away.
You may also have RF riding on the coax.
There's no reason to keep the cables long. Cut them to length and terminate. Actually, making a circular bundle of the wire can act as an RF choke. You just don't want to have it jump across the coils and onto another coil of wire nearby.
468/frequency = half-wave feet
feet * 12 = inches
43MHz, that's 131 inches for a 1/2-wave "no ground" whip.
Just make sure there's roughly a quarter-wave of metal in at least one direction under the antenna. If need be, attach it to the underside of the mounting platform and lead it away.
You may also have RF riding on the coax.
There's no reason to keep the cables long. Cut them to length and terminate. Actually, making a circular bundle of the wire can act as an RF choke. You just don't want to have it jump across the coils and onto another coil of wire nearby.
Re: New Radio Installer Questions and Confusion
SlimBob wrote:The only "no ground plane" antenna you're going to find is an "end-fed half-wave".
468/frequency = half-wave feet
feet * 12 = inches
43MHz, that's 131 inches for a 1/2-wave "no ground" whip.
Just make sure there's roughly a quarter-wave of metal in at least one direction under the antenna. If need be, attach it to the underside of the mounting platform and lead it away.
You may also have RF riding on the coax.
There's no reason to keep the cables long. Cut them to length and terminate. Actually, making a circular bundle of the wire can act as an RF choke. You just don't want to have it jump across the coils and onto another coil of wire nearby.
If you get an arc between coils, it is because of a very high reflected power. This is why it is important to get the SWR way down so the power is going to the antenna and not back into the coax and radio.
Jim