A 6 Meter SlimJim Ladder Line Antenna Designed with NanoVNA

This blogpost describes the design, construction and performance of a home-made DIY 6 meter (50MHz) SlimJim type amateur radio antenna made from 450 ohm Ladder Line (aka Window Line, Balanced Line, Open Wire, Twin Lead ). This Ladder Line material is typically purposed to connect a radio transmitter or receiver to an antenna (as a transmission line) but here it is made into the actual antenna. A NanoVNA Vector Network Analyzer measurement instrument aids in design and evaluation of antenna characteristics. The Slim Jim is a variant of the J-pole antenna. J-poles are very  popular among radio amateurs because they are easy to make from common, low-cost hardware store materials and because they perform well. This antenna can be used in a permanent or stealthy portable installation.

Motivation

As an amateur radio operator I take great pride in designing and  building my own antennas (antennae?). Amateur radio is the only class of broadcast license that the FCC privileges in this way. Currently my amateur radio station transmits on the 40-20-15-10 meter HF bands as well as VHF (2 meter) and UHF (70 cm) frequencies.  Through my HF antenna tuner I’m able to switch in this additional antenna for 6 meters. Also, because this antenna is lightweight, it can be hung from a tree to fit nicely in my backyard.

Antenna Design

The website M0UKD offers a handy J-pole/SlimJim calculator. This page reveals formulas for calculating dimensions. The only required user inputs are frequency and velocity factor. Velocity factor relates to the antenna dielectric material, in this case JSC #1318 450 ohm ladder line sold in my local Ham Radio Outlet at $0.70 per foot. The velocity factor (Vf) for this cable is specified as 0.91 (91%) here. However I decided to measure it myself using my NanoVNA.  This method gave me a value of Vf=0.90 (90%). Entering the lowest band frequency(50MHz) and slowest Vf (0.90) gave me some margin for element fine-trimming later. The resulting element dimensions are shown below.

SlimJim vs. J-Pole Diagram image
SlimJim vs. J-pole Configuration Diagram
M0UKD Antenna Calculator
6 Meter Antenna Dimensions from M0UKD Calculator Webpage

Choke Balun

A choke balun is commonly recommended for j-pole type antennas and it makes sense to me to stifle any common mode current. The transmission line is not intended to be a radiating element. I made this choke balun from Shireen rfc100a 50 ohm coax. About 5ft of coax is coiled 4 times through 4 TDK ZCAT 2035-0930 snap-on ferrite core inductors. I was proud of myself for combining 2 types of choke balun into one (coiled transmission line and ferrite core). A photo is below.

Choke Balun
Choke Balun made from coiled coax cable and snap-on ferrite cores.

Antenna Construction

I tried to position the ladder line to avoid cutting insulting dielectric which forms the mechanical support for the parallel wires but the required 6 cm Gap is longer than any “closed window”. This left a good part of the antenna hanging by only one #18 steel wire. To provide more structural support I cut out a length of insulation from excess material and glued it in place of the missing webbing. Since the dielectric material is polyethylene a special bonding adhesive (Loctite Plastics Bonding System with an activator pen and glue tube) was required. This proved very effective.

Ladder Line Patch
Gap Patch Glued on Ladder Line
  • I measured and cut the different wire lengths to within 1/16 inch of the metric equivalent.
  • The steel wire is brittle. When forming the ends (shorting the 2 wire conductors), bend with a large radius (1/4″) using needle-nose pliers.
  • Plastic zip ties were used to hold up Choke Balun and provide strain relief for transmission line coax.
  • The outer shielded part of the unbalanced coax transmission line connects to the longest element.

I was much gratified to see my new antenna resonating close to the design goal on the very 1st try, proving M0UKD’s calculator to be very accurate.

To find the optimum 50 ohm feed point I connected my NanoVNA to the antenna via a 6″ length of SMA coax with the inner and outer conductors separated. Then using a hand clamp to press the wire connection, I moved the connection point up and down the feedpoint area while measuring VSWR.  I targeted a VSWR minimum for the lower end of the 6m band (between 50 and 52 MHz) and determined that 15 instead of 13.5 cm (design target) was best. My target VSWR and the final field-installed (with balun, 50ft RG-8X coax) VSWR is shown in NanoVNA graphic below.

VSWR target vs. acutual
Target VSWR (blue) versus finished, installed antenna VSWR (brown)

I was not terribly disappointed that VSWR shifted higher but still maintained a ratio < 2:1 throughout the 6m band (50-54MHz).

NanoVNA

I used a VNA in my work and often fantasized about owning one for my home shop/lab. Recently I found this new (to me) device that’s great for the RF builder. Some featured points:

  • Cost: ~$45. Complete kit includes calibration standards, much cheaper than a commercial Antenna Analyzer.
  • Wide frequency range: 50KHz – 900MHz (covers HF, VHF, UHF amateur bands and more, like 2.2Km, 630m amateur bands)
  • Has 2 ports for S11 return and S21 through measurements (think filters)
  • The unit comes with a color touch screen but it’s much easier to  use the FREE and very capable NanoVNA-saver application software.
  • Powered through USB cable or internal rechargeable batteries

I used my NanoVNA to measure conductor velocity factor and VSWR through the major stages of construction, balun, transmission line and installation.

Conclusion

My new antenna performs well as I’ve made numerous QSO contacts across several states. I was concerned that the balun coax wouldn’t transmit my transceiver’s full  power (100W)  but I calculate a current of 1.414A ((100W/50ohm)^0.5=1.4A) which is not excessive. I’ve routinely run 80W. Here’s a photo in all it’s glory.

6m antenna photo
6m Ladder Line SlimJim antenna, hung from a tree branch, standing 14ft. tall.

Construction is simple and cheap ($60) and any ham radio operator will get a lot of use in a permanent or portable application.

Bill of Materials

Total cost less than $60