Batboard

I have no way to test this as I do not have a field strength meter.

I know some about no ground plain antennas but here's my question.

I have mounted a 3 db gain Motorola Spectrum antenna on top of a combine. There is a flat metal strip around the hopper about 2 3/4 inches wide on which it is mounted. I never thought much about this as when I tuned it I had 85 watts FWD with only 1.5 watts reflected. So this all seems good but I am not getting any more distance than 25 watt radios with a 3 db gain antenna with sufficient ground plane.

Could the antenna really not be producing a 3 db gain, but still show a good SWR on the wattmeter??

I'm thinking of replacing the spectrum with a 2.4 gain no ground plane maxrad hoping to get more actual radiated power out. Need more distance.

I have some 3 db antennas on fiberglass roof Peterbilt trucks, they seem to work fine. Never gave them a second thought but this combine seems a little weak.

I know the knowledge is with someone on this forum so please help. Any and all comments, ideas, or recomendations greatly appreciated.

Thanks to all!! BJ

BigJim wrote:I have mounted a 3 db gain Motorola Spectrum antenna on top of a combine. There is a flat metal strip around the hopper about 2 3/4 inches wide on which it is mounted.

Antenna radiation problems can only be a combination of poor placement, tuning, or cable/connector problems. Suggest you put a 1/4 wave antenna on a mag mount and move it around the roof looking for the lowest reflected power. Should equal more power out there, or just do as you say and use a no-ground plane antenna. I presume you are using quality cables and connectors and checked for intermittent short/open antenna cable with an Ohmmeter (especially solid-center cables)?

Hi Jim,

I have gone through this with a couple combines. Just wondering if you could let me know the make and model of your combine as I might be able to send some useful info your way.

Jamie

There are many, many factors here which can be affecting performance

First, a "3 db gain" figure may be immaterial. A properly mounted 1/4 wave might be as effective as a "3 db gain" antenna that has problems.

Poor mounting position and nearby objects may be effecting the pattern.

3 dB gain is not very much "over a ground plane", only the so called "1/2 an S unit."

A "no ground plane" antenna such as a marine antenna, may NOT provide much improvement, either, because it will be affected by nearby metal, as well.

Not knowing the present, or expected range, we really don't know if your expectations are reasonable

Also, so far as your receiver performance, a large amount of generated noise from your combine, due to engine ignition, onboard processors, the charging system, and static from belts, chains, etc, can be generating a high local noise floor which causes an apparrent receiver degradation

This is on a JD 9860STS and the first one I've had a problem with.

As for my expectations I just thought a high power Spectra (performing better than factory spec) should work better than a 25 watt Maxar 80 with no rx preamp that has never been touched. The old Maxar 80 beats the spectra on both tx and rx, doesn't make sense.

The cables are maxrad and test well. They are new.

I'm going to do some more testing today.

I'll let you know and thanks..

You said that you have your antenna mounted on the edge of your hopper which has metal on it. Instead of mounting an angle bracket to this, make your own. If you can get a circular piece of steel that is 8" in diameter and then bend it in half. Screw that to your hopper then drill a 3/4" hole in the new braket for your antenna cable. This will give you a better ground plane. I did this on a Case 2388 and it's been working great.


Jamie

No one has mentioned the fact that you are not getting much futher than a 25 Watt radio.
How much futher are you trying to get.

10 times the power = twice the distance.

Bruce,

It's not getting any farther, in fact it doesn't get as much distance as the 25 watt unit and rx is not as well either.

I know about how much distance I can expect as I have been doing combine radios since 1985. We even noticed quite a difference over the 25 watt units when we added 55 watt maxar 80s back in the late 80s.

This is the first one on a 9860 that has acted this way. I'll find the reason.

Well part of this makes sense, part doesn't.

Took the antenna off the top of the hopper as it was picking up electronic noise from the mass flow sensor and the moisture tester.

Switched from the 3db gain Spectrum to a 3 db gain wideband Maxrad mounted on top of the cab. (the old one with the black plastic tube that looks like an elevated feed antenna)

RX greatly improved can't give an exact figure because combine and trucks, pickups are always moving.. IE: Sometimes you're in the clear sometimes by trees or other obstruction. Doing quite a bit better than the 25 watters with antennas on the cab also, both TX and RX.

TX is improved also. Here's the kicker 70 watts out 3 reflected is TXing way better than the other antenna with a better match. Can the transmitted signal send the noise it is picking up from the combine and degrade the transmitted signal?

Anyway it's doing the job now and we only have 5200 acres of soybeans left to thrash so it's good enough for this year!

THANKS TO ALL WHO VIEWED AND THOSE WHO OFFERED HELP!!!

BigJim

TX is improved also. Here's the kicker 70 watts out 3 reflected is TXing way better than the other antenna with a better match. Can the transmitted signal send the noise it is picking up from the combine and degrade the transmitted signal?

Poor antenna match does not directly translate to poor performance, there are just too many variables.

I'd suspect, simply, that one problem is that the irregular "ground plane" is causing both pattern distortion AND matching problems

Local noise pickup in the receiver, caused by the machine, will most certainly degrade RX performance