Using a magnetic mount for HF portable communication

 Mag mount connection to vehicle

You can measure your magnetic mount’s coupling to your car. Since there is no direct dc connection the coupling is through the capacitive reactance. The capacitor consists of two parallel plates, one being the bottom surface of the magnetic and the other the metal surface of the vehicle directly under the magnet. You can calculate the capacitance using the formula in the ARRL handbook. However that will not be very accurate as you need to know not only the spacing but the value of the dielectric between the two plates. That dielectric is of the vehicle’s thin layer of paint and whatever the covering is over the magnetic to keep from scratching the paint. I find it easier to measure with an inexpensive meter that will read capacitance.

It is easy if you have two identical mag mounts as no connection to the vehicle is needed. With that setup you have what amounts to two identical capacitors in series. The capacitance of one becomes simply twice the value you read on the meter. Another way is to connect one test lead to the vehicle and read the capacitance. Sometimes it’s not so easy to find a convenient place to connect the lead. 

Another good way is to make two identical capacitors out of aluminum foil and place them on the vehicle. This is a method I use. You can then measure the series capacitance by connecting the meter to both pieces of aluminum foil. Again the capacitance of one is simply one half of the meter reading. Now armed with the value of this capacitor you measure the series capacitance of your mag mount and the aluminum foil. You then solve for the unknown mag mount capacitor. The formula for that is in the ARRL handbook. These formulas are usually in the study material for basic amateur radio licenses. Certainly they should be known for the Extra Class.

One of my simple capacitors is a loaf pan used for baking a standard loaf of bread. It can be sat directly on the roof or hood of my vehicle and gives me close to 500 pf of capacitance. Because my mag mount is not large enough to give me enough capacitance on most of the HF bands( it does on two meters) I add this extra capacitance in parallel to my mag mount to increase the capacity. Remember capacitance of parallel capacitors simply adds like resistors in series. Capacitance in series is like resistors in parallel and is not quite as simple unless the values are the same and then the total value of the two series capacitors is simply one half of the total.

Once you have the value of capacitance you can obtain the reactance in ohms at any desired frequency. Xc = 1/ (6.28 x f C). Remember a reactance of 10 ohms at 40 meters is only 5 ohms at 20 meters and 2.5 ohms at 10 meters. 

From my calculations a value of 50 ohms will give you about a 3 dB loss (50 % power loss) whereas 5 ohms will be approximately 10 % loss. These are based on a 50 ohm radiating resistance for the antenna. The actual antenna you have will determine the actual numbers. Typically a quarter wave vertical has a 35 ohm radiation resistance and shorter verticals even less. So a low impedance ground is much more important with a short vertical. 

In any case good results can be had on 40 meters and higher frequencies with 1000 pf of capacitance. 

Approximately that calculates out to be as follows:

22.2 ohms at 7 MHz

11.1 ohms 14 MHz

5.6 ohms at 28 MHz

I my case using a Hustler mobile antenna on the hood of my Ford Transit Van I get good results with my small mag mount and a bread pan used as a parallel capacitor.

I measured the capacitance of the mag mount as about 45 to 50 pf. I measured the bread pan as about 550 pf. So the total capacity is close to 600 pf. That calculates to be an impedance of 37 ohms, 18.9 ohms and 9.4 ohms on 40, 20 and 10 respectively. I find that works quite well for me to do Parks on the Air with 50 watts. I am not sure if the difference would be noticeable with a lower impedance, obviously the losses would be less but in most cases the law of diminishing returns governs how far you should go. A 50 % improvement is worthwhile while a 5 % improvement may not. 

For 40 meter work you may want to go with a couple bread pans or maybe a cookie sheet as losses are always going to be higher on that band than on 20 meters and above. I use a small magnet in my bread pan to keep the pan from slipping. 

Some mag mounts have a way to connect to the ground side of the mount and some don’t. On mine I modified the mag mount to hold another 3/8-24 antenna mount which made it easy to connect parallel capacitors ( bread pans, cookie sheets, aluminum foil, whatever). There is a commercial version of auxiliary capacitor called Diamond Mat-50 that costs about $42.00 and has about 1000 pf of capacitance. It connects the same way as my bread pan so some mag mounts will need modification.