It looks as though I have some work to do in preparation for a 5+ hr road-trip Julie and I will be making in the next couple of days. Last night I reinstalled my FT-857D in the FJ Cruiser and tried out my ATAS-120A antenna. Well, the darn thing couldn’t tune at all! I had a high SWR across all of 20m, suggesting that I have a problem to look into.
SO-239 to PL-259 90-degree elbow
One difference that I can test right away is the use of a 90-degree elbow fitting. I doubt that the elbow is the root of the problem, however, it may be worth testing since it is really the only new addition to the installation. I know that at UHF, these adapters can essentially present a short-circuit to the signal source, however, at HF frequencies I would expect this to behave a bit better.
The next item which I have been meaning to address is the addition of a current choke. In a mobile installation, it is common to have current flowing on the outer portion of the coax due to much less than ideal impedance matching conditions, and the lack of a direct earth ground. In a well engineered mobile HF setup, a screw-driver antenna such as mine should have a coil coupling the antenna to the vehicle ground to alleviate some of the common mode current. In my case, this is neither practical, nor possible. The next best solution would be to add a current choke (not really a balun per-se).
Air-wound choke courtesy of VE7AVV
I do not have any ferrite beads available to me in the next couple of days, so adding at least 25 of these pricey transformers will not be possible for me. The next possibility is to make an air-wound choke using 6m (~20′) of coax. Now this I can do since my feed-line for the current installation is fourty feet long (to accommodate portable operation while at a campsite. The method is dead-simple; coil at least 6m (~20′) of coax around a non-conductive form, keeping the wraps tight next to each other and not allowing the first and last coils to ever meet. It is necessary to use a solid dielectric coax in this case to prevent conductor migration (and therefore an impedance change, possibly leading to a short-circuit).
My hope is that I can find an ABS pipe with a diameter of ~15cm (6″) which would give me a circumference (π x D) of nearly 50cm (half a meter). Given the aforementioned circumference, I would need (6.5m / 0.5m) = 13 turns of coax. Assuming the diameter of my coax is 0.635cm (1/4″), I could theoretically make these 13 turns (13 * 0.635cm) using a pipe section which is only 8.25cm (3.25″) long.
Graph showing diameter vs required length
Using imperial values (since most pipes are in ancient units), a quick bit of math yields the following values for different diameter pipes using 21′ of coax to make the choke:
- 2″ diameter = 40 turns = 10″ long
- 3″ diameter = 27 turns = 6.75″ long
- 4″ diameter = 20 turns = 5″ long
- 6″ diameter = 13 turns = 3.25″ long
- 8″ diameter = 10 turns = 2.5″ long
When I graphed the values for diameters from 1.5″ to 12″ it quickly became obvious that the relationship was an inverse function (1/x). In this case, the formula would best be approximated as: y= 20/x which makes shopping a lot easier. All I need to do is divide 20 by whatever pipe diameter I choose (adding an inch or two for cutting, etc)! The inverse relationship also means that there is no easy min/max value (or balance point). What is obvious however, is that once you get to a diameter of at least 4″, there is not much additional (space savings) benefit in increasing diameter any further.