Batboard

I recently installed a 100-w Spectra A7 high-band VHF in my truck for vol fire dept use. One evening with the truck turned off, I keyed the mic, and heard a strange hum under the driver's door, which I traced to the antilock brake module. I previously had a 45w radio, so I put it back in, keyed up, and no problem. When transmitting on the 100w while driving, I occasionally get an ABS warning light after 3-4 secs on air... release the mic, it goes away. Not having brakes scares the hell out of me, so I'm not using the radio to transmit until I get it fixed...

Power for the radio and the head come from a factory fuse block under the hood - there are a pair of fused auxilliary lugs in the block for after-market accessories, which I used for my radio/warning equipment (lights/siren/GPS/control head power on one 30A lug, the other 30A lug is dedicated to the radio only) Truck is a 2005 Chevrolet 2500HD, radio ground is to a factory lug on the body under the hood.

My initial thought was that the power draw from the auxillary lugs put enough of a load on the system that it messed with the pump. I put all electronics directly to the battery, and the problem persists.

Control head is mounted in the console, radio is behind the back driver's side seat (crew cab truck), and antenna is mounted to the bedrail just behind the back seat on the driver's side. Planning to move the antenna to the top of the headache rack next week... meantime, has anyone experienced anything like this? Suggestions or solutions?

This is most likely an RFI issue, not a current draw issue. You'll have to experiment with various antenna mounting locations to find one that doesn't cause interference.

I can't bring up the document for some reason, but:

For vehicles equipped with electronic braking systems,
see “ABS (Anti-Lock Braking System) and
Anti-Skid Braking Precautions” publication, Motorola
part number 68P81109E34.

for information purposes, if its interfereing with your ABS and the light comes on, at worst, the ABS system wont function during that time but you hydraulic system will still operate as it would traditionally. just try to avoid talking on the radio in heavy rain or ice/snow and slamming on the brakes when the ABS system would normally come to life..

wazz

I would try moving the antenna to a higher location, possibly the other side of the cab. Also have you checked for reflected power? High reflected can cause interference as well as poor operation. Try re-routing you antenna cable, maybe its letting some rf bleed into a parallel run cable.

What kind of antenna? Gain or 1/4 wave? You really should have the antenna roof mounted or at least at the elevation of the roof and not below it, at that power level.

RF is an issue with high powered radios today. The vehicle mfg companies don't even try to worry
about RF causing problems today. It costs too much to add a few capacitors and maybe even a
couple of ferrite beads.

I guess you could contact the dealer and see if they have a service bulletin on the issue. If not,
then your on your own to solve the issue. If it was me, I would start with the normal RF bypassing
using some capacitors in the order of 0.001 mf on the wires at the module. If you can find a
wiring diagram, start with the +12 volt input and the sensor wires first.

Not that it is the case here, but several years back, I had a low band radio causing all sorts of
problems. The fuel gauge, speedometer, tach and such would do all sorts of strange things. I
was able to tame most of it down by placing an aluminum strap across the rear window in 2 places
vertically. It seemed the rear window was acting like a tuned waveguide port and allowing the
RF to get into the cab. The 2 straps were screwed into the metal around the window and changed
the frequency of the window opening that most of the problems went away. A few more bypass
caps added to the gauges and the rest of the major problems went away.

You could have as simple of a problem as a poor ground on the ABS computer.

Make sure the SWR is as low as you can get it. If you have reflected power, your wasting your time
on anything else you try. By any chance is the coax cable anywhere near the ABS wiring? Most of
the coax used today on the antenna mounts is not very well shielded. It might even take changing
out the coax to a better shielded cable.

Jim

I'm going to guess either a trunk lip / mag mount on the hood or trunk butted up against the glass, or an on-glass antenna causing this.

Like it was said, drill a hole on the roof... it's the best way, esp for high power.

tpg

Your not going to run a glass mount antenna with a 100 watt radio for very long. Maybe the 2nd PTT
and you get the cloud of smoke from the coupling loop on the window. They are only made to work
with radios under 50 watts.

Jim

The Pager Geek wrote:I'm going to guess either a trunk lip / mag mount on the hood or trunk butted up against the glass, or an on-glass antenna causing this.

Like it was said, drill a hole on the roof... it's the best way, esp for high power.

tpg

Exactly my point...

Jim202 wrote:Your not going to run a glass mount antenna with a 100 watt radio for very long. Maybe the 2nd PTT
and you get the cloud of smoke from the coupling loop on the window. They are only made to work
with radios under 50 watts.

Jim

The Pager Geek wrote:I'm going to guess either a trunk lip / mag mount on the hood or trunk butted up against the glass, or an on-glass antenna causing this.

Like it was said, drill a hole on the roof... it's the best way, esp for high power.

tpg

OK, I've been off the board for a few days, thanks for the replies... To the questions asked:

Wazzzzzzzzup: don't have to worry about snow/ice too much in south Texas, esp this time of year, but unfortunately the nature of the business (fire dept) has me out in the rain frequently runnig calls, and when I am out, I am usually having to talk on the radio... regardless, I will use make sure to use extra care until I get to the bottom of it

Batwings21: The antenna is going to the top of the headache rack (once I get it built) on the passenger side of the cab

WB6NVH: 1/4 wave antenna

Jim202: The radio drawer is behind the seat on a crew cab pickup; antenna goes out the OEM cab vent behind the back seat on the driver's side, under the bed, and comes up into the front stake hole on the driver's side. At the closest, the coax is 3-4 feet from the ABS module under the truck, almost directly below the driver's seat

Another thought: the control cable from the head to the radio is routed out the back of the factory console, under the carpet behind the gas/brake pedal, into the factory channel/under the moulding at the edge of the floor next to the driver's door, and comes up from behind the moulding behind the back seat... now THAT cable is very close to the ABS, maybe a foot, through the floor pan of the truck. I hadn't thought about that before... is there enough power on the control cable between the head and the drawer upon transmitting to cause RFI from that distance through the floor of the truck?

I know just enough about radios to be dangerous... I can do installs, but the technical aspects of what's inside them are way over my head... I know mounting the antenna in the middle of a conductive surface (roof, trunk, etc), will boost the range with the reflectivity, but will it do the same mounting it with the middle of the antenna even with the roof 6" or so back, or does it HAVE to be ON the roof?

I know it will vary with weather, but what kind of range can I expect with a 100-w high-band VHF, 1/4-gain with the above setup?

do you have a magnet mount to try temp? mount it on the center of the roof and just run the cable strait down and in the back side window and directly to the radio and keep it as far away from the cars electronics and see if it goes away

Don't forget that the ABS system has rotation sensors at all four wheels, with lots of wire taking the tiny signals from the sensors back to the ABS computer. The sensor levels are in the millivolt range.

Technically, an antenna system in match will not radiate from the coax to any significant degree, but an antenna mounted on a pipe rack or similar poor horizontal ground plane may have some mismatch which allows the cable to radiate.

When I was in the business years ago I had some installations similar to yours, where the customer had plumbed coax out of the cab down on the frame, then back up between the cab and the bed to the roof of a camper-shell sort of housing in the bed. The heat from the exhaust and catalytic converters was melting the RG-58, not to mention road debris chewing it up. I had to buy teflon cable from DB products to stop the melting, sleeve the cable in an armored jacket, and the antenna VSWR and radiation was so goofy that I replaced their 1/4 wave with a Larsen 1/2 wave ground-independent antenna, solving the problem.

UPDATE: New headache rack installed today, antenna moved to the top of the rack on the opposite side of the truck, and problem solved.

Thanks for the feedback and tips from everyone!

If moving the antenna doesn't solve your problem, you may be able to get some relief by adding a few ferrite beads to the outside of the coax.