High power RF fields and W124 300D - PeachParts Mercedes-Benz Forum

2.5Turbo · #1 03-21-2006, 12:57 AM

Well I finally got my ham radio installed in my '90 300D 2.5T, and have noticed something curious when transmitting on 144-148MHz at 50 watts. Every once in a while, the little yellow burned out bulb light will go on when I transmit on the radio. It doesn't do it all the time, but when it does, it lights up when I start transmitting, and goes out when I stop. No bulbs are out (that I know of, at least

). I don't think it's related to voltage drop since it will do it when cruising down the freeway at 75mph at 2900 RPM, and I'll be willing to bet money the alternator is putting out enough power at that speed to give twelve extra amps to a radio. How does that burned out bulb detector circuit work anyways?

Wes Bender · #2 03-21-2006, 09:45 AM

I'm not sure how that system works, but there are other circuits that don't take too kindly to nearby RF energy. I vaguely recall seeing my rear window defogger light glowing while I was transmitting. I suspected that the wire elements in the rear window were acting as an antenna and carrying the current back up to the indicator lamp.

Jim H · #3 03-21-2006, 12:58 PM

This circuit works by sensing current balance (a bridge circuit) between left and right lamps. If one is burned out, there is a difference in current draw.

Using lamps of different maunufacturers, with a slight variation in wattage, has been known to illuminate the lamp.

Where did you connect the power lead to the radio?

I'm going out on a limb to guess that your lamp circuit is sensing a voltage drop from the heavier current of the transmitter at (10A or so at 50W) and this is what is causing the lamp to glow.

Of course, the bridge circuit could be sensing RF...

SD Blue · #4 03-21-2006, 01:05 PM

Another suspect would be antenna grounding, especially cranking out 50 watts. Where is your antenna located and what are you using for grounding?

In the past, I have worked with HF radios and long-wire antenna systems in aviation and grounding problems can give you all kinds of wierd problems.

richard u · #5 03-21-2006, 06:26 PM

I had some RF in my HF Mobile set up (200W) First I tried clip on rf chokes at the battery and a few other spots...Then the moon roof and the power windows stopped working! Apparently some RF signal controls them. So now I have Chokes at he antenna lead as it enters the trunk, on the am/fm antenna in the rear window, and on the power and various wires of the trunk mounted xcever, plus the wires leading to the control head. So far so good. Getting up enough nerve to put in the 600W amp (G)

K2KOQ

Palangi · #6 03-21-2006, 09:11 PM

I wonder what would happen if you changed the coax to a premium (double shielded) type like 9913?

WC5P