Try a bug-catcher 6160 (or at least use the pattern for your design). It
seems to win all the challenges/ shoot-outs... I worked 27 countries in under
an hour from my truck with it yesterday in the contest.
http://www.texasbugcatcher.com/
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
"Collector of Edison Wind-up Phonographs... Do you have one for me?"
Email: bill@wjschmidt.com
Alternate Email: wmschmidt@charter.net
WebPage: www.wjschmidt.com
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On Behalf
Of Paul Marbourg
Sent: Saturday, March 27, 2004 7:22 PM
To: Amp Mailing List
Subject: [Amps] Retert: Best Coil Material for 160 Meters Mobile Ant.
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
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