>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.
Yea, Verily
> 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.
>
I would try 4" ABS. It's not expensive and it's D-factor is low.
>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.
>
Thanks, Paul
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