Problem with Antenna on School Bus

Mikey · Post by **Mikey** » Tue Oct 26, 2004 10:39 am

I just installed a new radio in a School bus and am having nothing but fits with it. Here is the basics. This is a 50 Watt VHF Mobile that is mounted above the drivers head just above the Sun Visor; It has about a foot to a foot and a half of coax that goes straight out the bus just above the Drivers side window to an "L" Bracket that is screwed to the side of the bus (thats how they had it before and thats what they wanted this time) it then has a MaxRad 5/8 Wave antenna with Spring on it. Everytime you key the radio and watch the Bird WattMeter, it goes up to about 15 watts and then starts falling till it gets to about 9 watts. i've tried new antenna's, new Mounts, a different Radio, Checked Power and Ground and still the same Thing. If i switch it to a Magnet Mount and stick it on the bus it works fine Full 50 watts. But they want these "L" brackets on the side above the Drivers Side Window because thats what they have always had. The Old radio's were only 25 watts so we have basically doubled what they were putting out before. I'm about ready to pull my hair out on this problem, anybody got any ideas?

Mikey

kmoose · Post by **kmoose** » Tue Oct 26, 2004 10:45 am

I'm not real familiar with the Maxrad antennae, but are you sure you have a good bond to the chassis of the bus, with the L bracket? What kind of wattmeter are you using? If you can, check to see how much reflected power you have. This could be an antenna mounting problem. It working with a mag mount cerainly points in that direction. I will bet that the low power that you are seeing is the result of a lot of power being reflected back down the line toward the radio.

Mikey · Post by **Mikey** » Tue Oct 26, 2004 10:59 am

Yes KMoose, i am getting about 15 Watts of Reverse power, but i cannot figure out why, this is a brand new Mount and i have tried another brand new mount with the same Results. I was thinking that maybe the short run of coax and the fact that the radio was just on the other side of the cab from the antenna might have played a part in it, but we have 8 more Buses with the radio's in the same spot that work Fine. I am looking at the roundness of the buss roof. The "L" Brackets are screwed to the flat side panel of the bus above the Drivers Side Window, then the top of the bus Starts rounding up to the roof, it puts the load coil of the antenna about 3 inches from the metal side panel of the bus. But that doesn't explain why it works on all the rest.

Mikey

Garyf629 · Post by **Garyf629** » Tue Oct 26, 2004 12:33 pm

Did you tune the antennas? Are all your connections good and tight with no shorts?

Mikey · Post by **Mikey** » Tue Oct 26, 2004 12:41 pm

Antenna is tuned, i even tried a 1/4 wave to see if that made a difference, same thing. All connections are tight and no shorts in coax.

JD · Post by JD » Tue Oct 26, 2004 1:04 pm

Have you tried placing a metal disk about 6" in diameter between the antenna and L brackett ? I had the same type of problem , this fixed it. It must of been a ground plane issue. I did notice though a 1/4 wave did not experience as much of a problem as a gain antenna did.

fire-medic8104 · Post by **fire-medic8104** » Tue Oct 26, 2004 1:15 pm

Most of the time it is a grounding problem. I saw this on a bus one time with the same type of mounting situation as yours. The problem is they use an adhesive between each layer of sheetmetal. This causes a ground to be lost between the antenna mount and the vehicle ground.

I would take a DVM on continuity selection and see if you have continuity between the mount and the vehicle ground, preferably at the batttery. Then you will know for sure. You will probably have to find a different way to ground the antenna or use a no ground plane antenna.

abbylind · Post by **abbylind** » Tue Oct 26, 2004 1:37 pm

My friends shop uses the "brow" (shade) thats over the front flashing lights to mount the antenna. Drill a 3/4" hole in it and use a NMO mount
Remove the flashing light and route the coax through a hole drilled above the light into the cab area.
Works slick! The "brow acts as a good ground plane for the antenna

Hope that helps

Fowler

abbylind · Post by **abbylind** » Tue Oct 26, 2004 1:44 pm

I also noticed that you only a have a foot or so of coax to the antenna from the radio. We used to keep the total length of coax coiled inside enclosed area of cab. Sometimes the radio needs to see at least a half to full wavelength of coax to function correctly.
Had the same problem with CVCs and VHF amps years ago. If it didnt see at least 6' of coax the amp wouldnt operate at full power.

Fowler

KG6EAQ · Post by **KG6EAQ** » Tue Oct 26, 2004 1:48 pm

abbylind wrote:My friends shop uses the "brow" (shade) thats over the front flashing lights to mount the antenna. Drill a 3/4" hole in it and use a NMO mount
Remove the flashing light and route the coax through a hole drilled above the light into the cab area.
Works slick! The "brow acts as a good ground plane for the antenna

Hope that helps

Fowler

God help the groundplane if you're not on 800mhz, that shade is barely 6" across, no where close to a 1/4wave radius.
EDIT: on the 1/4" to 1/4 wave

Tom in D.C. · Post by **Tom in D.C.** » Tue Oct 26, 2004 1:52 pm

The foot and a half of feedline sounds suspiciously like you might have a resonant feedline. Put an extension on the present feedline and see if that helps. Are you certain that the installs on the other buses don't have a coiled up feedline to keep this from happening?

Mikey · Post by **Mikey** » Tue Oct 26, 2004 7:37 pm

Thats how i usually mount the antenna is on the overhead light shield. The school just insists that we mount them on the side as thats the way they have been for awhile. I am going to try longer Coax tommorow and see if that fix's the problem. I was always under the impression that the shorter the coax the better. Guess that isn't the case. Thanks for all the help

Mikey

Will · Post by **Will** » Tue Oct 26, 2004 11:29 pm

The bus antenna installation you describe is not to the RF Exposure standards, the guide is in most antenna packages. Especially with a very poor RF ground. At a lower power you might get away with it, but not at 50 watts. The antenna is too close to the radio AND the driver, they both are getting overdosed by RF.

The antenna must be at least 7 feet or more from ANY person at that power level. At least the radio is complaining about the excess RF.

va3wxm · Post by **va3wxm** » Wed Oct 27, 2004 5:35 am

I was going to suggest RF feedback as well. 18" from radio to aerial running 50 watts WILL cause problems. You'll have to move the radio, the antenna, or both.

Having said that if the antenna is matched properly with a good RF ground it shouldn't matter what length of feedline you need. IMHO using coax to impedance match/transform your system or act as a counterpoise is poor installation practice, especially on VHF.

Mikey · Post by **Mikey** » Wed Oct 27, 2004 12:35 pm

I better Weld a 7 foot pole onto the top of the Bus then because no matter where i put it on the entire bus, its going to be within 7 feet of a living person. I get at where your coming from though. I would love to just put an NMO mount on top of the bus that takes care of all this problem, but that is not the way the had it before. They keep telling me "it has worked that way for years" you know the places. I seem to have remied the problem by getting an Antenna that requires no ground plane. Ironically the only buses i was having problems with were mounted on a Chevrolet Chassis, the Ford buses worked like a charm first time. Thanks for all your help.

Mikey

kmoose · Post by **kmoose** » Wed Oct 27, 2004 1:17 pm

Probably has something to do with the construction of the Chevy chassis. We have all kinds of problems bonding antennae on our helicopters. Our sheet metal mechanics just cannot understand the concept of a bonding brush.

SlimBob · Post by **SlimBob** » Wed Oct 27, 2004 9:15 pm

blah blah blah blah

smiple fact: installation technique dictated by group with NO radio knowledge. piss on them.

raymond345 · Post by **raymond345** » Thu Oct 28, 2004 4:02 am

Well to me it sounds like the antenna.
We would trade out the (Bad Radio)
wih a second BUS.
Put a good radio on the bad bus.
WHat Happens?
This is the least amount of work.
We look at three things.
1:Antenna (We use an antenna tester)2 minute
test and no radio needed.
2:Radio test on the service monitor (5 min test).
3:Power supply test.

We luv test GEAR.

Time in and out/15 min.
ps 20 mins if we have a coffee.

Problem corrected.

ASTROMODAT · Post by **ASTROMODAT** » Thu Oct 28, 2004 10:00 am

If your shop can get ahold of a Bird SA-4000, it will analyze your situation, and tell you everything you ever wanted to know about the antenna, all connectors involved, and the coax, all the way back to the SA. It will give you both soft and hard plots of the antenna and transmission line sytems, including accurate fault locate information. We have bought this product, and it has saved us many $'s in saved time over the last year.

Good luck.

Larry

http://www.bird-electronic.com/products ... aspx?id=50

raymond345 · Post by **raymond345** » Thu Oct 28, 2004 2:00 pm

The Bird SA 1700 will do the same job also.

ASTROMODAT · Post by **ASTROMODAT** » Thu Oct 28, 2004 2:46 pm

Correct, as long as it covers your freq, and you don't want to do power measurements. We need the 4000 for the extra freq range for mobile WiFi installs.

Larry

Mikey · Post by **Mikey** » Fri Oct 29, 2004 5:23 am

Well i took care of this problem by finally talking the school into moving there Antenna's to the Pass Side Mirror, Problem solved, Full 50 Watts. I want to thank everybody that replied to this post and helped out in any way, This board has helped me through a lot of problems. Thanks Again

Mikey

Problem with Antenna on School Bus

Problem with Antenna on School Bus

School Bus Antenna

Bus antenna problem...

Have you traded radio's?

There are a number of testers.