[Amps] Retert: Best Coil Material for 160 Meters Mobile Ant.

[Paul Marbourg]

Hello Richard.  I am in the midst of doing the same thing here.  I have spent a lot of time trying to find the optimum combination of factors to give the best overall efficiency for the antenna system.  One thing was clear from the get-go: you must do what you can to minimize Rc (coil loss resistance) above all else.  I initially approached my design with a simple center loaded monopole.  My target frequency was up around 1.992 MHz, as I a group I like to hang with is there.  I made a wire frame model of my Chevy Tahoe truck chassis and ran ELNEC analysis over medium ground.  With this frequency and the required Xl was around 5000 ohms for an 12 foot center-loaded radiator.  Doing the math, this worked out to a coil length of 24" for #10 AWG Teflon insulated wire on a four inch polycarbonate form!!!  An ideal ratio for high Q is a coil diameter to coil length ratio of 2:1.  To even get close to this ratio, the diameter of the coil approached 10".  However, using the uH and # of turns equations, this (10") was a very good diameter (d/l = 1.67).  Also the length of wire required to achieve the desired inductance in the coil drops to a minimum at the best coil diameter.  When using a 12" diameter form, the required wire length began to increase again and the d/l ratio exceeded the optimum 2:1.  However, finding polycarbonate tubing of that diameter at a reasonable price was a real challenge.  Probably the best supplier of plastic I have found is McMaster-Carr Company.  They will sell 1 foot lengths of larger diameter polycarbonate tubing.  

Upon further brainstorming, I decided to model various positions of the coil above the lower (1 5/8" diameter copper) mast section.  In order to achieve the best current distribution for the antenna, I tossed out bottom loading and large capacitance hats above the coil approaches.  Calculations showed no significant improvement in efficiency by raising the coil above the center position, as the required inductive reactance (Xl) went through the roof, so to speak.  This meant more wire to make a bigger coil with greater Rc loss.  

The best solution I came up with is to run two or more wires off the tip of the 5.5' long upper 1/4" copper-braid covered fiberglass whip section.  With two 14 foot wires running to poles on each corner of my Tahoe's front brush guard (with the ends at about 12 feet above ground), the required loading coil reactance dropped down to around 1580 ohms!!  By running an optimization for best coil diameter, I came up with a diameter of 6" and a coil length of right around 4".  The fiberglass upper section is bent forward by the top wires to lower the overall vertical height of the antenna to manageable proportions.  The "vee" spread of the top wires also serves to stabilize the antenna.  This is the best I could come up with for a mobile 160 meter antenna.

Polycarbonate was chosen as the coil form material for it's high impact resistance and uv stabilized property (as well as ease of manufacture).  This plastic can absorb up to 0.14% moisture (at atmospheric saturation) and this has a derogatory influence upon the plastic's RF dissipation and dielectric constants, but since I am using Teflon insulated 10 AWG wire for the coil, I think this is a very insignificant tradeoff.  I would like to cover the coil with PTFE heat shrink tubing, (or at least a good polyolefin heat shrink tubing) but this material is not manufactured at diameters greater than about 4.5" expanded, and is VERY expensive.  Once I have the coil resonated to the desired point, I may wrap the solenoid in PTFE tape and then apply a polymer overcoat of some type to be determined.

The base impedance of the antenna will be quite low.  ELNEC shows an impedance of around 4 ohms resistive when I include my truck's wire frame model and medium ground with copper inductor losses and an Rc for a Q = 300 of about 5.3 ohms & design Freq. = 1.975 MHz.  I may use a remote controlled L-network at the base of the antenna to match up to rig and amplifier.  You could use the old trick of making the antenna have a capacitive reactance at the desired operating frequency so as to have a virtual capacitance for the L-network.

I will include some scans of my ELNEC analysis for you here.

Remember, even the very best mobile 160 Meter antenna will be lucky to achieve a 3% efficiency!!

Good luck with your project.  I hope this has been of some help.

73, Paul WN7T