[SECC] GQP - Mobile Tips

[ku8e at bellsouth.net]

  Here are some tips for those of you going mobile. This has worked well for K4BAI and myself.

  1) Most mobile signals are weak.  If condx are good you might be loud in some places but you
       signal is probably still weaker that someone running a dipole. Don't get discouraged. Think of
       it this way : It's like you are a weak and rare DX station and everyone is looking for you. When you 
       get spotted on the DX Cluster your rate will really take off.

      Operating mostly CW will be you best bet for the best rate.  Mobile antennas are not very efficient 
      antennas. See the following from The DX Engineering webpage :

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    Some Basics:

When an antenna is shortened from it's naturally resonant length at the frequency of interest, the feedpoint becomes capacitive and it becomes necessary to add offsetting inductance in the form of a loading coil. This restores the resonance of the antenna. Unfortunately, the additional inductor also has resistance that adds loss to the antenna. The resistance in the coil eats up your power and you are not going to be as loud as you would be if the coil loss were not present. 

The more that you shorten the antenna, the larger the coil that you need to use. The larger coil has more resistance and eats up more of the power. Mobile antennas are usually pretty short, require large coils and hence have large losses. 

This loss resistance is also seen at the feedpoint and is partly responsible for making it easy to match this shortened antenna to the 50-ohm feedline and radio. 

The impedance seen at the feedpoint is made up of the radiation resistance, coil loss and ground loss. The radiation resistance of a full size vertical antenna is about 35 ohms. As the antenna is shortened the radiation resistance goes down quickly but the other losses are still there or even increase. 

The table below shows the Real Radiation Resistance of your 6-foot mobile whip. 

Radiation Resistance
of shortened Mobile Whips 
Height (in.)   Freq. (MHz)    Radiation Resistance
                                                     (Ohms) 
72                      1.8                       0.05 
72                      3.5                       0.17 
72                      7.0                       0.69 
72                      14                        2.77 
72                       21                       6.24 
72                       28                      11.10 


So, for instance, if you have a 72-inch, 20m-whip antenna and you are measuring an SWR of 1:1 that means that you have a feedpoint impedance of 50-ohms. The true radiation resistance of your antenna is somewhere near 3-ohms. That means that you have a loss in your system of 47-ohms (50-ohms minus 3-ohms) and a radiation efficiency of about 6 percent (3-ohms / 50-ohms). So, if you have a 100-watt transmitter in the car, 94-watts are being used to heat the air and only 6-watts are being used to communicate! The other 94-watts are used up in the loading coil and ground system. 

If we were able to suddenly remove all the loss from the loading coil that we were using then we would see the base impedance drop by the amount of the loss in the coil. 

How do we reduce the loss in the Loading Coil? 

Basically, we try to use the one with least inductance and resistance that we can still get to resonate the antenna. 

One way to reduce the size of the required coil is to install what is known as a capacity hat. A capacity hat can be thought of as an extension of the Marconi antenna design that adds horizontal elements to the design of an otherwise vertical antenna to add length. If the capacity hat is designed correctly the currents in the horizontal sections of the hat offset one another and preserve the vertical polarization of the wave radiated by the antenna. 
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I have bought some DX Engineering Hot Rodz™ for our Hustler whips to raise the efficency. I am going to try them out this weekend.


2)  Operate some SSB.  You signal might be good if condx are good. If the rate is not there just 
      try tuning around and call a few people and then go back to CW.  SSB is mostly for picking up 
      multipliers.


3)  Make sure you operate both 40 and 20 meters during the day. There are guys in the close in states
    that can't hear you on 20 meters that want to work you. There are some clubs like TCG that 
    sponsor plaques for the GQP. Thank them by working them on 40 meters. You should be able to 
    work some east coast and midwest stations during the day on 40 meters as well.


4) Check 15 meters every once in awhile. I remember one year K4BAI and I made a couple hundred contacts on 15 meters in the GQP. This is the time of the year we start getting some E-Skip.


5) Be SAFE. If you don't have a driver don't operate when driving. Look on a map or something like Google Maps to find the shortest route between the counties you plan to operate.


6) Have FUN. Everyone from out-of-state will be looking for you. The mobile activity keeps up the 
    interest in the GQP. There is only a finite number of fixed stations for them to work and you are 
    giving them a QSO  and maybe a new multiplier from every county you operate from.


 73, Jeff  KU8E