Properties of Hustler whips on the roof rack of Subaru Outback Wagons

  A standard "mirror mount" can be used to support the antenna on the roof rack, however, the short (1.5") bolts will have to be changed for longer ones. I used 2.5 inch stainless steel bolts of the same diameter and thread as the original bolts. Figure 1 shows a picture of this mirror mount, spring and added grounding wires. Attaching the mirror mount to the rack does not provide an electrical ground, so you should remove the three screws that attach to rack to the roof of the car. The protective coating on the underside of the screw head should be removed with a file. Three wires should be formed from coaxial braid and prepared with appropriate lug connectors. Actually, only one ground line may be required, but I found that one can lower the ground resistance slightly by using all three. Attach one end of each wire to a mirror mount bolt and the other end should be screwed down to the roof rack mountings.

 

  Roof Rack Mount and Grounding

  Detail of Roof Rack Mounting and Grounding Connections

 

  I have operated the mobile of the following bands 10, 12, 15, 17, 20, 40 and 75 meters with standard Hustler whips supported above the spring. Here are the approximate settings for the resonators as measured from the bottom of the loading coil to the top of the stinger.

 

 Resonator Band

  Length (inches)

 Ctr. Freq. (MHz)

  R (ohms)

  SWR

 Bandwidth

  RM 10

  16.125

  29.500

  85

  RM 12

  24.250

  24.930

  100

  RM 15

  19.125

  21.250

  80

  1.8:1

  RM 17

  29.375

  18.110

  77

  1.9:1

  RM 20 S

  26.000

  14.225

  55

  1.3:1

  RM 40

  41.000

  7.225

  30

  RM 75

  46.250

  3.850

  26

Table 1

Properties of Hustler whips mounted on the roof rack on a Subaru Outback Wagon

 

The table also contains other information that can be of use such as the base impedance and the bandwidth for a 2.0 : 1 SWR. Note that the impedance for 40 and 75 meters is nearly one half of the desired 50 ohm line impedance, and the bandwidth for both of these bands is narrow enough to be a problem (at least it confines you to a small segment of the band). This low base impedance would normally indicate an SWR of 2.0 to 1, however, if the stinger is adjusted for lowest SWR at the desired frequency, the SWR is never this bad because the whip will not be exactly at resonance but will be long (inductive) and the impedance increases. It is possible to improve the performance slightly by using a 2 to 1 unun. Such ununs are described by Jerry Sevick, W2FMI in his book " Building and Using Baluns and Ununs." This book has a wealth of information on properties of loading coils and techniques for improving the efficiency of mobile antennas.

We had some good weather in mid-January, so I decided to determine the best SWRs for each band by selecting the frequency that minimizes the SWR. Note, in this case there was 6' of RG-8 X between the antenna and the SWR meter. I left the stingers in the same position, however, the 20 stinger had to be re-adjusted to bring it in the band. (Since then, I've purchased the standard RM 20, and it has a nearly perfect match). The table below shows that we can normally get a better match slightly off resonance:

 

 Band (meters)

  Frequency MHz

  SWR

  10

  very broad

  1.75:1

  12

  very broad

  1.70:1

  15

  broad

  1.50:1

  17

  whole band

  1.60:1

  20

  14.220

  1.80:1

  40

  7.225

  1.50:1

  75

  3.870

  1.70:1

  Table 2

  Best SWR by Tuning Off Resonance With 6 Feet or RG-8X

 

  I made the impedance measurements using a Palomar Engineering R-X noise bridge with selected calibrating resistances and shorts. These measurements are probably no more accurate than about 10%. The SWR measurements were made with a Daiwa CN-410 SWR & Power Meter. The accuracy of the SWR measurements is difficult to specify, as the accuracy of the cross-needle meters depends upon power level and SWR. However, I made the final measurements with the meter near full scale, so the accuracy may be about 10 to 15% in the range of SWRs characteristic of these antennas near resonance.

*** After Note ***

I did build up a 2:1 Unun that works quite well. This corrected the impedance at the base of the whip to a value proportionally closer to 50 Ohms but bit higher. As a result, the bandwidth on both bands nearly doubled and so did the power output. I get a lot of reports saying "You sure have a good signal for a mobile!" or "How much power did you say you were running?" I think having the antenna on the roof rack makes a substantial difference; getting most of the power to the antenna helps too. The Ununs are quite efficient. I connected two of them back to back and compared the power to a dummy load going direct with two feet of coax or through the dual Ununs and could see little difference.