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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|