More noob radio questions

[DAXQ]

i just finished two night mare installs of cdm750 radios into ag combines. apparently these ginormus vehicals do not like lowband (43mhz) radios. we tried moving the radios, rewiring the power, moving the antennas to limited places available on these large square things and are still getting garbage swr. just wondered if anyone had any suggestions for things to try.

as i said, they are cdm750 radios with 43mhz low band antennas.
One install is brand new, the combine already had an NMO mount in the roof with about a 8foot cable and power going to a mounting location in the cab. This one is aweful with about 28 watts going out, and 18 watts reflected back on the bird.

The other has been completely redone as it had no roof mount, so we ran 18feet of cable relocated the thing (as we suspect the onboard GPS and computer guidance systems are whats causing the problems for both these vehicals) as far away as we could get. THis one has a respectable 38wats, with a reflected of 7 watts on the bird.

Aside from the aweful readings, this is what they do - non stop noise on the radio as soon as it powers up. If I key the mic, the noise stops while I talk, and I can hear people comming in. If I put the mic in the mic holder the noise stop (well the noise is still there i see the red light blinking, but it is not driving the user out of the cab).

any suggestions are greatly appriciated - if i can provide any more information to help, please let me know.

[KitN1MCC]

are the antennas cut properly.

may want to try a real low band antenna get a ball whip mount and maybe you will have to mount it away from the cab like they do on the front unload cement trucks.

larsen may does make a wideband lowband antenna that is tunable and has a less coil and more whip


http://www.tessco.com/products/displayP ... ventPage=6

[tvsjr]

Find a real antenna analyzer and have them swept. I bet the antennas are improperly cut.

[DAXQ]

What would be a good antenna analyzer? We use whips and balls where we can, but these combines are just too high. We get real good action out of max rad whips and base loaded on some semi's and our other ag equipment. I think these guys are either not grounding the body very well (it's literally a big hollow metal box with no decent horizontal plain) or there is so much rf interference from the onboard computers and gps that they. Was wondering if there is anyway to shield it better. I will also check on the length these come precut for 43mhz and I don't think are cut and tuned like the whips.

[Jim202]

Did you ever think that there is a computer in the vehicle that is generating a signal on the frequency your trying to use. Try killing everything and running the radio off the battery with the ignition shut off and no other devices turned on. No GPS, no other radio, no vehicle ignition turned on. See if your noise is still there. If there is a clock, make sure you kill the power to it. Some AM/FM radios have a clock built into them that has a power feed that needs to be hot all the time to keep the clock alive. Kill this power feed also.

Low band antennas want to have a ground plane to work against. If there is any metal other than the roof where your antenna is mounted, it will cause the SWR to go up. No matter how much you play with the antenna length, it might not come down. Another trick you can try is either shorten the length of the coax or make it longer to see if that effects the SWR. Have seen some real strange problems over the years with low band antennas.

[kb4mdz]

'Nother question: When you do SWR checks, are you inside any sort of barn or structure, or out in the yard? Esp. with lowband, you need to be 'in the clear', or any metal (ESP. sheet metal) will reflect power back into your antenna. I'd suggest at least a couple wavelengths; that works out to over 45 ft or so, doesn't it (sorry, no coffee yet). Double that would be better.

Second the motion on adequate ground plane. Any likely hood the roof is made of fiberglas or other non-metallic?

[Bill_G]

What would be a good antenna analyzer? We use whips and balls where we can, but these combines are just too high. We get real good action out of max rad whips and base loaded on some semi's and our other ag equipment. I think these guys are either not grounding the body very well (it's literally a big hollow metal box with no decent horizontal plain) or there is so much rf interference from the onboard computers and gps that they. Was wondering if there is anyway to shield it better. I will also check on the length these come precut for 43mhz and I don't think are cut and tuned like the whips.

I have never seen a NMO mount base loaded whip come precut for any band. There should be a cutting chart either on the bag or in it. Pay attention to the difference in lengths between WITH and WITHOUT springs. Cut an inch longer since we already know the ground planes are inadequate. That's a given on any vehicle at lowband. Lowband + truck = hardly nothin there. Lowband + heavy equipment = even less.

But, your main problem is all the squaking the radio make as soon as they turn on. Jim202 already touched on it, but it bares repeating - there is something in the combines that is generating on channel noise. I have a feeling there is not a lot you are going to be able to do about it except verify the noise exists. It's raining. You're going to get wet. Period. 43Mhz is right in the heart of manmade noise. It was bad enough when thrashing pieces of metal, ignition systems, and safety lights could turn a lowband receiver into stone. Now we pile on with processor clocks, LCD displays, vehicle area networks, and precision farming controls. It's like a giant cloud of crap around the cabin.

You might get lucky by turning the squelch control up as far as it can so the operator doesn't have to listen. Or you could modify the mic plugs so the hangup is applied all the time. I dunno. The high vswr is your problem. You get to fix that as best you can. 35/7 might be as good as it gets. But, the invisible field of noise around the combines is something your boss is going to have to negotiate with the customer. Maybe they will be willing to pay a tech to look into it, and possibly mitigate it.

Me personally I would escape to UHF though that is no guarantee that the crap cloud doesn't reach that high. Part 15 goes out the window when all the equipment is within a few feet of each other.

[DAXQ]

Most of these big behemoths are fiberglass everywhere! On the John Deer tractors that come with a factory installed NMO antenna mount, they actually work very well. I don't know what they are doing under that fiberglass roof that makes them work, but they work about as good as anything else we try and hook up - if not better. That is until they get ripped off the cab because they are so high. Then the guys usually just seal up the hole, and hang the antenna off the side of the cab and get terrible radio transmit with pretty good reception (no ground plane). This is a Case IH combine, and I really expected/hoped the factory mount to work as well as the John Deer's I have done in the past but its not even close.

I am suspecting that the particular on-board computers, gps and sensors in use are all operating on the radios freq causing the irritating static - I am also getting the feeling that factory installed mount is not all that and a bag of chips for the Case equipment. When we tried to use the supplied power for the two way, the radio clipped out drawing too much power for the supply. Then when we tested with the combine running, it blew the fuse. So we ended up running new power wires - I suspect they provided power for a second FM radio or something similar, not a two way - which does not really make sense considering it was added with the antenna mount for a two way? Given that - I suspect that not a whole lot of design planning went into the factory install for this radio antenna mount.

[DAXQ]

You are correct! Did a search on the internet and found a cutting chart for the MLBDC4000 we installed and I should have cut it to about 44.25". There was no chart in the bag, and it just said it was for 43MHz - I will check out the length and see if it helps at all!

[DAXQ]

The bird readings are done outside, in the field if at all possible.

[KitN1MCC]

maybe you do a new mount as well. also maybe try running a ground to the rest of the chassis

[tvsjr]

What would be a good antenna analyzer? We use whips and balls where we can, but these combines are just too high.

An antenna analyzer is a piece of test equipment that measures VSWR/return loss across a broad spectrum. Rather than trying to guess where the antenna is tuned, it shows you the big picture. Common models seen are the Anritsu SiteMaster and the Tektronix NetTek. These are expensive devices... you'll need to find someone who has one, perhaps at a local radio shop. However, they are invaluable.

I sweep all of my antennas when they're first installed and terminated. Then, they're stored and, if I start having odd issues, I can re-sweep them and overlay the two on my NetTek's screen. This gives me an instant indication if something is screwy.

[Bill_G]

So, why lowband? This can't be their first time at the rodeo. Stuff had to have gone wrong in the past. All that flying metal - sounds like a rolling fubar generator to me. All the more reason to escape to UHF.

Heavy equipment OEM's tend to underrate their 12V converters. They are sized for accessories like AM/FM tune machines and maybe a cellphone charger. But, we have always had to put in our own converters to get the current - and the isolation - we needed. I have found some gnarly hash on some power systems. Fine for lightbulbs, but not so great for radios. Couple that with positive ground, and/or 24V, and you have to shoe horn in a good 12V converter someplace to run a radio.

[DAXQ]

They have looked at it and had they chosen to switch over about 15 - 20 years ago, would have been possible. Now given the size and cost to convert 7 facilities each with base stations and all the equipment, it's just not practical. With everyone leaving low band there is less cross talk now than ever, and these do work - just kind of sucky and annoying at times for the driver/user as well as the installer to try and get and acceptable install to begin with.

[Tom in D.C.]

To the OP: Do not take the dimensions given on a cutting chart as being accurate, because the actual required antenna length is controlled by the environment where you install the antenna, and will almost always vary from what is on the chart. As was noted above, good antenna measurement equipment is the only real answer to getting the lowest SWR.

[DAXQ]

I always try and cut about 1 to 2 inches long, but honestly every time I do, I usually end up grinding or cutting it down 1/4 to 1/8th at a time to the cut sheet and then fear going any lower (which is what I did for the OEM combine. It did improve the readings on the bird to 32watts out and 12watts reflected, still bad, but better. I think for this one we will really need to try and move the antenna.

[Will]

With low band, 36 mHz on tow trucks, and other vehicles, for more than 40 years, I find:

The old full one quarter wave whips worked well until the trucks started having plastic and discontinuity in the metal parts.
Tow trucks with antenna mounted to the 'A' frame,

I went thru almost all antenna types. Still not good.

I use the Comtelco shunt feed antennas. I also order a longer and heaver 53" whip. Using a old standby MFJ antenna analyzer, change the tap and length of the coil to get a good match, leaving the whip full length. With the MFJ I can 'tune around the frequency and find out how it does at other frequencies, then make changes and look again.

It helps having cut my teeth on two different antenna ranges, and earned my AR badge... The experience is something money and school can NOT buy.

No to get too far off topic, the California Highway Patrol has wide spaced low band frequencies. A real challenge. They are using 1/4 wave whips and antenna tuners on some of their cars and trucks.

[Bill_G]

I did lowband on logging trucks, yarders, cement trucks (front and rear discharge), ag trucks, school buses, and sheriff cars. Thems were the days.

[escomm]

At least it aint low band on a motorcycle

[AL7OC]

In defense of lowband...

Unbelievable, but we somehow managed to get away with mag-mounted Larsen 1/4 waves on 52-53 MHz. We were using 5 watt Ten-Tec transceivers, and the antennas were stuck square in the middle of a van or car roof. Base to mobile range - 20-30 miles in the flats.

If the antenna is in the clear and has a decent ground plane, it should work OK. That can be a problem on some applications.

We also got away with murder with full 1/4 wave whips on the side of pick-up trucks on 29.6. Same deal - 10 watts FM, and we could talk base-mobile for miles. Ditto for fire service on 33.98 and 46.12. No repeaters and we could cover the county.

Low band is more suceptable to noise, but for out in the country, it works quite well. Good old-fashioned 5khz deviation is still OK after 2013.

Nothing wrong with lowband so long as you get a decent antenna and aren't dealing with portables and pagers. You think rubber-duck antenna suck on high band? Try one on 33.98...