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NEW: Mobile Antenna Testing Results Part 1

Updates:

Oct 2004: finishing up the mobile antenna shootout

July 2004: Added antenna testing and more photos

June 2004:
Laptop crash so I decided to completely redo the site..

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What do I want? I want an antenna that covers 3.0 to 60Mhtz that can fit under trees and bridges, is safe at highway speeds and radiates wonderfully. Dream On! This is mobile and given short antenna lengths, there are tradeoffs. Here are some testing results from my working with the Tarheel. Now I am not writing a technical paper here for QST, so some of the testing is a bit sloppy, and I would have included more cases I was doing this fro “real”.. hey this is a hobby!!! Lets have some fun!

Configuration Column Label is a link to a detailed description below

Word of caution: Remember on the screwdriver, I am moving the coil to achieve the lowest SWR.. This is not a one number view of how “good” the antenna is (afterall a dummy load has an SWR of 1.0!) But it is atleast a place to start! Remember I am not using laboratory grade equipment either, so dont look at the least significant digits too closely. Testing was done at the same location in all configurations, with the vehicle located in the same spot, pointed in the same direction.

Configuration A: This is the bare antenna, no whip, no cap hat, loading coil installed. The results are pretty good (I have all the counts and impedances recorded as well, but that is too much for the web pages) There is no operation on 80m though... and I wonder if my radiation efficiency is any good? At 28Mhtz I got strange results, SWR skyrocketed! Do I have RF on the line? (I will install some chokes later in the testing) Bad spot on the coil? Weird!

Configuration B: I wanted to check if the truck was acting as a good ground plane. Do I have things well tied together? So I put out a couple of radials from the truck. This is just for testing. I put out 8 redials. From my reading it did not seem that the length would make much of a difference and 8 was more than enough. There was some slight differences from A, but not enough to be concerned that the truck needed more work. I suspect that cars driving next to me would probably have more influence than anything I could do in additional grounding.

Configuration C: I added a whip, left on the loading coil,  and took off the radials. I added the choke to the feedline.Hey pretty good results and probably what I would expect. I get 80M at the sacrifice of 6M. (My radio does have a built in tuner so I can turn that on to make sure I dont harm the finals, at the expense of some higher loss in the coax.) I wont operate 6m this way (a bit too high!) Of course, the real problem is that where I live in the Northeast, I cannot be driving around with a whip ontop! ... Time to play with another idea!

Capactive Top Hat: I threw together my own top hat just to play with.. This was made out of the steel wire that holds those roadside political signs. (There should be alot of them around after every election!) The wire is not good enough to hold up at highway speeds, but the performance was good enough for me to go buy a commercial one. I finally decided upon the DX Engineering HotRodZ... But the Top Hat can be configured in a number of ways, an experimenters dream... so lets play with this:

Configuration D: This is the DX engineering unit with as many long “radials” on it as possible. (Lets go for it!) Makes for a pretty strange looking antenna and everyone asks about it! In this case, since the whip worked so well, I disconnected the loading coil at the bottom of the antenna. I have no idea what the losses due to this coil are, but I assume there is some. (I will come back to this later) The results were mixed. Certainly not as good as with the whip and the SWR at the frequencies I want to work with has gone up. What I dont show is that the length of the antenna has been reduced from a count of 334 to 297 at 3 Mhtz. in configuration C. Wow that is about 30% less coil used: more efficient! This looks pretty good and maybe by playing around I can get a configuration that works! (I am thrilled... obviously does not take much!)

Configuration E: What I did here is put the loading coil at the bottom of the antenna back in. The results are as expected, which is good as this suggests my antenna system is working as it should be (good grounding on the truck) There are alot of radials on the top hat and now 80M is in the ball game, but no 6M. I used this configuration for a couple of weeks while driving and had a great time mostly on 40M. Wonderful contacts with alot of people in the mornings. (Thanks all!) I really should have stopped playing at this point.. but ... I went on...

Configuration F: I left the coil in, and took off some of the radial wires (4 long ones installed) I was suprised there was a difference. It seems playing around with the top hat made a difference. The coil length to achieve resonance increased about to about halfway of using it with a whip, makes some sense: less capacitance of the top hat. Everything looks really good, but I dont have 6m operating just yet..

Another problem: At some highway speeds in the presence of wind, the radials on the Hot RodZ started to bend. The result is pretty yucky looking and after each trip I would bend them back. Not fun! So while I was pleased with the results of the 4 radials installed, I thought I would take them all out and use the shorter radials and start some experiments with all of them installed. The idea is that the shorter ones might not bend. (Darn: and I was so close to a perfect solution) If I can play with it to get the swr in a range I am confortable with the TS-B2000 tuning out, then I can operate 6m tool

Then it happened!

The antenna broke. The original AMAC controller has a few software bugs. One of the most serious is that even after setting “top”, the unit allows you to move the antenna past the top setting off the rail. There are no limit switches, so the Tarheel goes off the end of the rail and spins. Usually it is pretty easy to fix. I have to reglue the end of the rail to the antenna (it gets torn off when this happens) and I move the antenna back down onto the rail. Annoying but I can live with it. Of course, I am showing only some of the tests here. I did many more trying different ideas (and a whole series that ended up finding my coax being torn apart by the truck body, but that is another story!) Well, this time the tarheel is stuck.. nothing I can do to fix it. (Lots of swear words)

Now I want to be clear. The final breaking of the Tarheel is a combination of problems: Operator error for sure. The AMAC controller problem of being able to move past the top of the antenna and to a much lesser degree the Tarheel being able to break itself.  It just seems that while I like the Tarheel, I have run into too many problems with it. It is well made and as reported in my eham review I was delighted with the antenna and the great customer service. But now I have to send the thing back for repair. I really am disappointed especially since I was so close. (All this testing was over a series on months!) So I am done with the antenna. I am going to start all over (sigh) and look for another screwdriver.. My choices narrowed to the Kj7U which looks like a great antenna and the High Sierra. I received quick replies to my inquiries (as well as Tarheel by the way) over a holiday weekend. In the end I decided on the HighSierra. in almost a toss up between the two.

Testing the HighSierra HS1800PRO!

SWR DOESNT MATTER!!! (MUCH!)

really. it doesnt.. high swr means line losses but the line is short in mobile settings.. but I do know a high SWR is a good thing but not sufficient... but all that I had to measure is SWR and I am glad I have something!

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Copyright 2004 by Philip P. Thompson, All rights reserved.