John Deere Combine Antenna Selection

[Trm70c]

I'm trying to decide whether to put a gain antenna or a quarter wave on the combine. I have recently been putting quarter waves on the tractors because they seem to survive the trees that we run into at the field edges. I wish I would have taken a picture, but to describe the set up, John Deere sets the cab up with a NMO mount already installed and runs the coax into the cab. You just remove the chrome nut and screw on your antenna. Really its a pretty sweet deal. My concern is the quarter wave will be below the grain tank extensions, they are about 2 feet higher than the cab and only about 2 feet behind where the antenna mounts. Will this obstruction so close to a quarter wave cause problems or is it no big deal? A gain antenna would put part of the mast above the extensions, but will take a pretty good beating ( it is 14' from the ground to the hopper extensions). Another reason I'm leaning towards the quarter wave is our repeater tower is pretty high, and we get pretty good coverage in most areas. Handhelds can access the repeater from probably 75% of the ground we need to cover, so I figure the 40 watt CDM I plan on installing should do fine. I really think a lot of the users of this site and thought I would get your opinion on which style antenna we should use.
A little other info that might be helpfull:
UHF System 451.***/456.*** through a MICOR 70 Watt Repeater and it feeds an antenna on top of a 200' tower that sits on a pretty high hill, I'd say the antenna is around 400' higher than most of the ground we cover. The MICOR is getting tired and my father in law says it doesn't cover near like it used too, but I think it works good enough. He said last time the techs were out they said the "cans" where getting bad, I assume they meant the duplexors? There are just two, not 4 like you see in a lot of the ebay auctions.
The farthest farm they need to cover is about 20 miles away, with not a lot of elevation change in between.
On a little side note, my father in-law has been using gain antennas for 30 years and he gives me funny looks when I put the little antenna on his tractors, it's kind of funny, but hey, they work. And they don't rip the NMO mount through the plastic roof when they hit a tree.

Thanks in advance,
Tyler

[Tom in D.C.]

I'd stick with the quarter wave cab antennas if they work for you. However, what's this about a plastic roof on the cab? Does the NMO have a decent metal groundplane as part of the
antenna's electrical base? If you're operating with just the NMO and no metal groundplane you could improve the antenna's performance immensely by putting something like a six-inch
diameter pie pan on the roof, or under it on the inside.

[Trm70c]

My brother in law pulled one of the NMOs through the roof of one of the tractors so I had to take out one of the lights and fix it, they have the braiding on the antenna grounded to a large piece of sheet metal that sits under the antenna under the plastic roof top, I assumed this was their form of a ground plane. The piece of sheet metal reminds me of HVAC duct.
Tyler

[Tom in D.C.]

Grounding the braid to that metal beam is certainly better than nothing but it also probably skews the directional pattern of the antenna to some degree, which would be more noticeable at greater distances. Also, when I said "six-inch" diameter above I misspoke; the average diameter of a pie plate type ground plane on UHF would be twelve inches.

[tvsjr]

On your combine:

I had to do a similar ground plane on a Kenworth T300 chassis tanker/pumper. It's not the greatest way to go, but it works. We used a piece of metal that was previously a kick panel for a door, along with copious silicone adhesive. 5 years later, still going strong.

Personally, I wouldn't go with a whip antenna. Go under something, the whip catches, and bye-bye mount.

Assuming you can make the RF path workable, I'd look at a RadiallLarsen LP450NMO - this is a low-pro NMO-mount antenna, 4.5" diameter and 1.5" tall. A ground plane is required for this antenna, but it sounds like that's covered. Assuming the diameter is OK, since it's so short, if you hit something hard enough to destroy the antenna, you've probably wrecked the cab.

In most mobile installations, you're making a compromise. Unless you put one quarter-wave in the center of a roof with nothing else (no lightbar, no extra antennas, nothing), it's not electrically "right". The big question is, how much compromise can you live with? This is a short antenna, but you aren't looking straight across the plain - you're looking up at a repeater with a significant height advantage.

On your repeater:

Two cans doesn't sound very good - assuming that's all of the duplexer and you're really looking at two cavities, you're likely dealing with significant desense as TX-RX isolation will be quite low. I'd suggest looking at replacing the duplexer first - EMR isn't cheap (though they are cheaper than RFS, TXRX, etc.) but they'll build a proper duplexer to your specs. I'd recommend the EMR 65546/SNCI-2 6-can pass/notch duplexer with dual-stage isolator. Expect to pay about $2500. You can get away with no isolator if there are no other transmitters on the site, but I prefer always using an isolator.

Considering you're still using a Micor, I'd give serious consideration to a new repeater and PA, if necessary. A Kenwood TKR-850 and a Crescend P10 100-watt amp (if that power level is allowed by license) would be a great combination.

[Trm70c]

$2,500 for a new duplexor, wow, i was thinking it might be 750-1000 bucks when ours gave out, but sheesh, thats a ton of money. Ours is the only transmitter on the tower, until we put the RTK transmitter on it. I appreciate the comments, and I think I'll go with the quarter wave.

[motorola_otaku]

PCTEL Maxrad makes a metal plate (click) and a metal tape kit (click) to do exactly what you need to do, but personally I'd just fab something from a pizza pan or whatever sheet metal I had handy.

Being on UHF instead of VHF-Hi like a lot of farm ops puts you at an advantage antenna-wise because you can get away with using shorter antennas without compromising on performance. I'd look into using these (click) since they'll give you a sturdier mount versus a 3/4'' NMO.

[Wes]

For UHF applications check out the Phantom series from Laird (formerly Antenex). We use these on all of our farm vehicle cab installs. Combines, Tractors, Sprayers, Dump Trucks, Etc. They perform well, are 3db gain and are only 3 1/2" tall. It screws right onto the NMO mount. We have had very low failure rate and have never had one damaged or torn off. Avaliabe in black or white.

http://www.lairdtech.com/WorkArea/linki ... temID=2496

Just a side note, the Phantom 800 MHz work very well also. VHF do not work near as well as the UHF and 800.

Wes

[Andy Corbin]

I'm trying to decide whether to put a gain antenna or a quarter wave on the combine. I have recently been putting quarter waves on the tractors because they seem to survive the trees that we run into at the field edges. I wish I would have taken a picture, but to describe the set up, John Deere sets the cab up with a NMO mount already installed and runs the coax into the cab. You just remove the chrome nut and screw on your antenna. Really its a pretty sweet deal. My concern is the quarter wave will be below the grain tank extensions, they are about 2 feet higher than the cab and only about 2 feet behind where the antenna mounts. Will this obstruction so close to a quarter wave cause problems or is it no big deal? A gain antenna would put part of the mast above the extensions, but will take a pretty good beating ( it is 14' from the ground to the hopper extensions). Another reason I'm leaning towards the quarter wave is our repeater tower is pretty high, and we get pretty good coverage in most areas. Handhelds can access the repeater from probably 75% of the ground we need to cover, so I figure the 40 watt CDM I plan on installing should do fine. I really think a lot of the users of this site and thought I would get your opinion on which style antenna we should use.
A little other info that might be helpfull:
UHF System 451.***/456.*** through a MICOR 70 Watt Repeater and it feeds an antenna on top of a 200' tower that sits on a pretty high hill, I'd say the antenna is around 400' higher than most of the ground we cover. The MICOR is getting tired and my father in law says it doesn't cover near like it used too, but I think it works good enough. He said last time the techs were out they said the "cans" where getting bad, I assume they meant the duplexors? There are just two, not 4 like you see in a lot of the ebay auctions.
The farthest farm they need to cover is about 20 miles away, with not a lot of elevation change in between.
On a little side note, my father in-law has been using gain antennas for 30 years and he gives me funny looks when I put the little antenna on his tractors, it's kind of funny, but hey, they work. And they don't rip the NMO mount through the plastic roof when they hit a tree.

Thanks in advance,
Tyler

Based on what you are telling me about the coverage area and the repeater system, I would say go with as low a profile antenna as possible.

A taller gain type antenna, even without the trees, is going to take a beating from the constant flexing of being on a piece of agricultural equipment.

As for the repeater, I would take a very close look at the antenna system. You may very well have some duplexer issues, but I would look at the antenna and feedline. I have seen antenna systems degrade slowly over time. Water can get into feedline, water can get into small cracks in the radome of an antenna (like the old stationmasters). If your antenna system was installed when the repeater was new, then I would say if the repeater checks out OK, the antenna system probably needs to be replaced. The Micors are 1970's technology and is probably 35-40 years old.

I recently replaced an antenna on a UHF site that actually checked out OK on the test meter but we were getting terrible coverage. Once the antenna was replaced, the coverage returned to normal.

Andy

[Trm70c]

The concrete at the base of the tower has 1982 scrawled in it, so we are getting close to 30 years old. A tornado wiped out the tower about fifteen years ago, a new tower was put up, but I'm pretty sure the feedline and antenna were resused. Within the last five years high winds bent the antenna about 10 degrees off center, so the connection could be damaged, I don't know. I really don't want to climb up there to find out. But maybe it needs to be done. It isn't a fiberglass antenna but a metal one with little loops on the sides, I don't really know the proper name for it. Really the coverage is pretty sufficent for our needs. There a couple of farms where a handheld won't key up the tower, but the mobiles pretty much work everywhere. And with good quality as well.

[Andy Corbin]

The concrete at the base of the tower has 1982 scrawled in it, so we are getting close to 30 years old. A tornado wiped out the tower about fifteen years ago, a new tower was put up, but I'm pretty sure the feedline and antenna were resused. Within the last five years high winds bent the antenna about 10 degrees off center, so the connection could be damaged, I don't know. I really don't want to climb up there to find out. But maybe it needs to be done. It isn't a fiberglass antenna but a metal one with little loops on the sides, I don't really know the proper name for it. Really the coverage is pretty sufficent for our needs. There a couple of farms where a handheld won't key up the tower, but the mobiles pretty much work everywhere. And with good quality as well.

Sounds like a folded dipole antenna perhaps similar to these-
http://www.tessco.com/products/displayP ... =38&page=2

These are pretty rugged antennas but a 200 ft tower that was toppled by a tornado is some major damage. I would be surprised if there was anything salvagable. Even if the antenna was salvageable, it could have weakened some of the hardware or compenents to where it is not making a good connection. I have knowledge of a 50mhz antenna similar to that that broke a place in the dipole loop. I think they took some metal vent tape up and patched it and it is working OK now but still needs a permanent fix.

Based on what you told me, I would give very close scrutiny to that antenna system. If you father-in-law is saying the performance appears to be degrading over time, eventually it will cease to work at all.

I would be rather surprised if an antenna system (feedline and antenna) that were on a 200 ft tower that was toppled by a tornado would not be damaged to the point performance was degraded. Just my $.02 worth.

Andy

[Motrac]

On a little side note, my father in-law has been using gain antennas for 30 years and he gives me funny looks when I put the little antenna on his tractors, it's kind of funny, but hey, they work. And they don't rip the NMO mount through the plastic roof when they hit a tree.

Had to laugh when I read this. 30+ years ago we were installing the first EMS UHF MEDCOM system in a MI county ambulance service's fleet. The Mot engineers told us to use the supplied gain antennas with an artificial ground plane in the fibreglass roof for best service. We installed the ground planes per the manual, attached the antennas and sent a rig all over creation to run tests for county-wide coverage. They didn't work worth a poo. Called the rig back, stuck the quarter wave toothpick on there, sent it back out and Presto! County-wide coverage. There's a lot of theory to antenna engineering but practice makes perfect most of the time.

[Bill_G]

Necro-threads always bring up interesting stuff. I was around 25 years ago in Detroit when Oakland Co put in their UHF med system, and Suburban ambulance tried to use gain antennas on their fiberglass roofs. Not stellar. Never tried to put one on a combine, but have plenty of experience dealing with damaged roofs from vandal trees. Trees, however, tend to be non-denominational in taking out marker lights, exhaust stacks, as well as antennas. Natures height restriction enforcers. They seem to have a keen appreciation for a clean rooftop.

[Motrac]

I was around 25 years ago in Detroit when Oakland Co put in their UHF med system,...

Heh heh...I sat on the SEMCEMS communications committee that wrote the specs for that system. We started work in the old Book Building in downtown Detroit and later moved to Southfield to finish things up. Pretty radical goings-on for a state that was about 80% 30-50 MHz at the time.

[Bill_G]

Heh heh...I sat on the SEMCEMS communications committee that wrote the specs for that system. We started work in the old Book Building in downtown Detroit and later moved to Southfield to finish things up. Pretty radical goings-on for a state that was about 80% 30-50 MHz at the time.

I seem to recall it was in Beaumont. It's been a while fer sure.

[Trm70c]

Well this is way late but some after pictures of the quarted wave I wound up going with. We were really satisfied, it spent the whole harvest under a pile of corn/soybeans that is always spilling from the grain tank and transmitted/recieved crystal clear everytime. The new cabs are so quiet the external speaker was probably overkill, but I think it adds to the sound quality as well. The other pictures just show some of the other electronics in the cab. To top it all off this machine was traded last week, so now it all has to come out to go in a new 9770. (The combine in the photo is a 9760)