The "Mobile"

This is NOT the way to have a strong mobile signal. Why? We'll get to that but first some history.
Standard 102" mobile whip on a ball mount. The only thing slightly different here is the SGC SG-237 Smartuner used to match the whip. The whole assembly slides into the trailer hitch. I can remove it quickly if I (or my wife) don't want the "ham radio look" for a while.

It looks good but it just didn't put out much of a signal. Other hams told me that the other fellow who drove to Dayton at the same time who had a Hustler was always stronger than me (Don, W0DM). Therefore, I have since removed the Smartuner and whip and replaced them with a Hustler. I am having better luck with the Hustlers. Of course, I didn't just want better luck. I wanted to beat the Hustlers.

I wanted to know how so I read everything I could find. One of my best sources has been Jerry Sevick's Building and Using Baluns and Ununs. The title is misleading because in the appendixes Dr. Sevick included his work on short verticals. Dr. Sevick has since published that work separately in a small book, The Short Vertical Antenna and Ground Radial, Sevick, CQ Communications 2003. Dr. Sevick makes it clear base loading is very inefficient.

According to Bill Rinker, W6OAV, there is a 2db gain just by moving the loading coil from the bottom to the center. But I suspect the real killer is not that 2db, but what happens to the radiation resistance when the coil is at the bottom. It drops to such a low value that it is practically impossible to get a good enough ground system on a car to prevent tremendous losses. Maybe ground losses in a mobile installation could be 10 ohms. Radiation resistance with this antenna could be 1 ohm. I am convinced that this low impedance is most of what did me in.

2008 update: I sold the SGC tuner at a hamfest and the fellow who bought it determined the unit is defective, with the help of SGC tech support. This new fact taints everything I have written above. Was the tuner bad when I was using it or did that happen later? I probably should revisit this whole installation with a new tuner but the research bears out it's not a good installation no matter how good the tuner is.

I wanted to know more about short verticals so I dug up a lot more reading. Turns out the radiation resistance of a 102" whip is even lower than I said above. It's more like .87 ohm on 40 meters. Ground resistance is maybe 15 ohms. You can see the efficiency is going to be VERY low, using the formula for efficiency of actual radiation resistance divided by ideal resistance (.87 / 36 = .02). That 2 per cent efficiency means if my transmitter puts out 100 watts, this antenna is radiating 2 watts of it. Ouch. The rest is heating up the ground and the loading coil. Pretty obvious now why a Hustler beat out my whip on the way to Dayton. The Hustler might have some design issues of it's own but it nevertheless put out a lot better signal than my contraption did. But how can I beat the Hustlers?

A top hat has the lowest loss of the various loading techniques per Sevick. Putting a top hat on the mobile will help another 2.5 db, in addition to gaining the 2 db from raising the loading coil from the base to the center. Another change I have made from the original is to remove the SGC tuner from the base of the whip. Trying to match the .87 ohms of radiation resistance and 15 ohms of ground loss, and maybe even cost the tuner it's life trying. The problem lies in the low efficiency of the design.

Upon studying Walt Maxwell's Reflections II, I understand that there is no gain by putting the tuner at the base of the antenna. As long as I establish a conjugate match, it doesn't matter where the match is physically located within reason. Coax loss with high SWR could be a concern. I'll use low loss coax and in a mobile installation the length is short. A tuner inside the vehicle is easier and cheaper. Ideally I can get the antenna impedance within an acceptable range for the internal tuner in the transceiver.

My next effort is to build a loading coil better than the Hustlers. I will shoot for a Q of 300 with very low distributed capacitance. I plan to do this with wide space 14 guage coils with a diameter twice the length. Another upgrade is to add a top hat. Although I have built top hats, physically they are not too great. I like the designs of a top hat sold for screwdriver antennas made by DX Engineering.

Finally, I plan to use large diameter tubing to reduce loss and increase bandwidth. Wind load is the tradeoff. I figure the perfect diameter is about 2 inches, like the lower part of a screwdriver. Above the coil I plan to use a Hustler mast. That will mate with the DX Engineering top hat.

A big development came about in 2014 in a magazine article in QEX, January/February 2014. Barry Boothe, W9UCW has measured performance for short loaded antennas. W9UCW can demonstrate a small loading coil works almost as well as a huge one. We don't need a huge coil after all.

My final design improvement involves placement. When the whip is near the ground like it is now, the ground loss is higher. Getting the antenna as far away from the lossy ground as possible will help. A car is a poor mobile antenna vehicle. A bread truck would be just about right, or even an 18-wheeler if you could put the whip on the roof. Too bad there are bridges to clear.

I will attempt to get the antenna as high as possible on the car, maybe on the roof, keeping in mind that I have to clear overhead obstructions. I could use a magnetic mount on the roof IF I could find a good ground connection. Since I have a moon roof there might be a screw somewhere that I could use for an electrical contact to the sheet metal. I will probably need a nylon fish line guy system to keep the wind from blowing it all away.

Update: I did part of what I was discussing above on the next trip to Dayton as you can see in the pictures. It worked much better. Unfortunately, the gentleman who drove out last year did not go this year so we don't have a comparison to his Hustler. No one I worked complained of a weak signal this time, though.
I found a good ground connection by removing the dome light and connecting directly to the metal of the car. The entire ground run is less than a foot. I ran the braid through the moon roof.
It got a few stares on the road. Too bad I had to take it all down when I got back home. It really worked well. To review, all I have done is move the antenna from it's near ground location to the roof and eliminate the tuner, replacing it with a center loading coil (Hustler). It makes an amazing improvement.
Thank you for reading this far. Now you know the journey I have taken "Making a Mobile Installation Work".



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