160m mobile coil

Summary
When I designed the 160m coil, I was primarily interested in obtaining maximum output at 1900khz within a confined space and with a minimal ground plane (car). Physics tells us a 1/4 radiator at 1900khz is approximately 123ft. and such an antenna configured as a vertical would require a fairly large ground plane consisting of equivalent length radials surrounding the center vertical element. Obviously this would be quite difficult to use while mobile. With this in mind, a compact, compromised antenna would fill the physical void, although antenna radiation efficiency would be hindered greatly at below 7Mhz.

Design Requirements
My High Sierra HS1500MKII screwdriver was already an excellent antenna on 10-80m, but operating on 160m had to be and addition and not a replacement of any other band. I still wanted to work 10-20m during the day, but flip to 160m in the evening all with minimum operator intervention. The coil had to be lightweight, stable, secure as well as quickly interchangeable at a moments notice. The coil also had to be small enough to allow tuning within the typical 13ft 6in height restrictions here in the United States. The photo below provides typical height when mounted to a vehicle.

Operational Design
Keeping the design requirements in mind and after much tweaking, on-air testing and optimization, I ended up with the following.

    Coil Length: 10.5 inches (excluding quick disconnects)
    Coil Diameter: 3.5 inches
    Coil Weight: 2lbs 10oz.

The coil is built on 1/4 inch thick PVC with internal electrical connections sealed inside the form body. The body is assembled using 4 machine screws on each cap at 90 degree angles, making for an overall higher strength assembly, allowing for material expansion and contraction while being less susceptible to cracking. I’ve finished it in a black plastic coat paint which adheres better to the PVC coil form while providing better performance in rain.

N3QO 160m Mobile Coil_7065302111_l.jpg

On-Air tests
Signal reports from operating mobile have proven promising and consistent during reasonable band conditions (less then +15db lighting crashes). My mobile rig consists of an Icom IC-706MkII at 100watts driven by the stock microphone. The map below indicates a typical night’s signal reports operating in and around my QTH of Pottstown, PA. The Monroeville, PA station is 220 miles to the West, while the Pittsville, MD station is approximately 150 miles to my South.

Tuning
First off, in order to tune to 160m, the shunt coil at the base of the High Sierra HS1500 and HS1800 antenna needs to be disconnected. With this out of the circuit, the minimum SWR I experienced on 40m and 80m was 1.3:1. This can be remedied with an alligator clip jumper between the end of the shunt coil and the bottom of the screwdriver antenna. This should bring you back to 1:1.

SWR findings:

    2Mhz – 1.815Mhz at 1:1 SWR
    1.8Mhz at 1.6:1 SWR

Better SWR can be obtained below 1.815Mhz by replacing the stock 72in whip with a 102in whip, although this will increase overall antenna height by 2.5ft, extending total antenna length to 14ft 3in, not including how it’s mounted to your vehicle, typically 20″ above the road surface.

How does it look?