Treatment of the -ve DC return path for transceivers in mobile installations

A correspondent wrote with questions on the -ve return connection in a mobile installation of a typical ham transceiver. He was confused by the advice on an online expert who opined…

If instead, you decide to connect the negative lead to the nearest chassis ground point (seat support, trunk brace, etc.), there will be a difference in resistance between any of these points and the battery’s chassis ground. A differential of three to five ohms is not uncommon. Whether this causes a ground loop to occur is moot, the resulting voltage drop under load is not.

A resistance of 3-5Ω from any part of a metal car body to the terminal clamped to the battery -ve terminal is way above anything I have observed, and would seem to be sign of a fault rather than not uncommon.

In my own vehicle, the measured voltage difference between the car body at the opposite end to the battery and the terminal clamped to the battery -ve terminal with a current of 3.7A flowing is 0.005V, giving a -ve return path resistance R=0.005/3.7=0.00135Ω… a long way short of 3-5Ω.

Lets explore an example of a 100W transceiver (eg IC-7000) that needs up to 20A @ 12V remote mounted at the rear of the vehicle (opposite end to the battery).

Lets examine three configurations:

1. radio DC isolated from the car body other that the battery connections;
2. radio DC isolated from the car body other that the battery connections and antenna grounded to car body by 1.5m of RG58 coax;
3. radio chassis bonded to car body at the radio and antenna grounded to car body by 1.5m of RG58 coax;

1. Radio DC isolated from the car body other that the battery connections

The supplied DC cable is #12 (3.3mm^2) has a resistance of 0.00159 Ω/m. If I used 4.8m of that cable to reach the battery, the resistance of the -ve return conductor would be 0.0076Ω, and the voltage drop @ 20A would be 0.152V.

This relatively high DC drop (repeated in the +ve lead for a total of 0.3V) could be improved with thicker cable… but there is a simpler way.

2. Radio DC isolated from the car body other that the battery connections and antenna grounded to car body by 1.5m of RG58 coax

We have already calculated the battery return conductor resistance to be 0.0076Ω, and of the car body return conductor for my car to be 0.00135Ω.

The DC resistance of the shield of RG58 is specified as 0.013..4Ω/m, so 1.5m has R=0.020Ω.

The configuration can be represented by the power cable -ve return resistance in parallel with the coax shield resistance and car body return resistance in series, so R=0.0076 || (0.0013+0.020)=0.0056Ω and voltage drop @ 20A is 0.112V.

The voltage drop in the -ve return is lower than the previous case… but it is because the coax shield is carrying DC current

How much current?

Well, the voltage drop is 0.112V and resistance of the shield + body is 0.0213, so current flowing that path is 0.112/0.0213=5.3A. This is not good, Worse if the direct negative connection to the battery is fused, and even worse if that fuse blows or has a high resistance… the radio will pass high DC current through the coax shield and is likely to damage it.

3. Radio chassis bonded to car body at the radio and antenna grounded to car body by 1.5m of RG58 coax;

The analysis of this scenario is no more complex than the first, but the relevant resistances become more dependent on the actual implementation.

If the -ve return to the body is say 0.5m, and the DC potential at the antenna is the same as at the -ve bonding point, the current in the coax will be of the order of 3% of 20A, less than 1A and quite acceptable. The total -ve return resistance is 0.0021Ω for a voltage drop of 0.042V… again quite acceptable.

In my own installation, the +ve run from the battery is #8 (8.4mm^2) which delivers a total +ve side voltage drop at 20A of about 0.05V.

This latter configuration is the most practical and best performing in most cases.

DC isolation

Note that none of these installations is truly DC return isolated from the car if the coax socket is connected to the radio chassis and antenna is connected to the car body… almost ALL ham transceivers connect the -ve supply to the chassis.

If you truly want to isolate the transceiver from the body for -ve return current, you must use some form a DC-DC inverter which has a host of disadvantages (and of course some advantages).

You will find dissertations on why the -ve return should be fused, but they seem to ignore the path from transceiver chassis to car body via the coax shield. Inserting a fuse with its attendent voltage drop in the nominal negative return conductor increases the DC current flowing in the coax shield.

Conclusions

• There is an abundance of advice online and in other channels of ham myths that do not stand up to rigourous circuit analysis.
• The voltage drop in DC conductors and body return paths is readily measured with low cost accurate high resolution digital multimeters that have been commonly available for decades, yet the BS prevails.
• This is DC, a lot easier to analyse than AC circuits yet analysis is sadly lacking from all the expert advice. Hams have forgotten to ask “Why?”