Greetings from a fairly long blog posting hiatus. A few cool things have been in works around N3QO, on or near the top of the list is going 160m mobile. That’s right, 160m.
Barry, KU3X and I discussed the capabilities of 160m mobile in the summer. He had a successful coil design for his mast and whip combination with good signal reports into our nightly 160m net on 1900khz.
Fast forward to this last weekend where I acquired a quick disconnect (QDC) for my screwdriver antenna and a spool of 16ga enameled wire from the LVARC table, all at the Cherryville Hamfest in NJ. Now that I had the wire and QDC, what better time to build the coil and attempt to go mobile on 160m with my High Sierra HS-1500MKII screwdriver antenna.
At first I just winged it, and used a 3 inch PVC as the coil form, with 7 inches of exposed PVC between the end caps. I ended up with a 100 turn inductor that I would later discover calculates to 390uH of inductance. After reviewing someone elses blog about an inductor for this purpose, I ended up being about 110uH of too much inductance. After a little experimenting, I removed about 20 turns and tested again. I had found my resonance was around 5850khz.
This is where I started utilizing my MFJ-259B analyzer beyond the typical SWR readings. I had found a relationship to inductance at the resonant frequency to be 0uH, and 25uH at 1900khz, our target frequency.
With this new information I removed another 5 turns and now found I was around 4:1 SWR at 1900khz with resonance still up and around 5Mhz. It was then I remembered that the 40m shunt coil at the base of my antenna was still in the circuit. Once I removed the matching coil, SWR came down to 2:1. All I had to do now was to raise or lower to the desired resonance, 1:1 at 1900khz.
After I had the test case completed successfully, I re-evaluated the connection points, I decided to drill an entry point to wire the coil from inside the coil, which makes for a cleaner appearance. I also decided to anchor the caps with 4 screws each and not sealing the form in the event of internal coil failure. Because of this, a drain hole exists at the base, only millimeters away from the bottom connection point.
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Here are the specs on the coil:
Length not including quick disconnects: 11 inches
Turns: 75 (i think)
Calculated Inductance: estimated 270 uH
Tuning: Allows tuning down to 1815khz at 1:1 with screwdriver fully extended.
On air reports were surprisingly good. I have an Icom 706Mk2 running 100w.
I was S9+20 into Pittsville, MD, S7 into Western PA all from the Pottstown, PA area. This is typical of what I expected, but never that great of a signal.
I’m planning on building another coil with a few less turns with hopes of bringing resonance down to 1800khz even. Then this will truly be a 160m coil.
–jeff
N3QO
