Build a high-performance Amateur Radio UHF Slim Jim antenna for little or no cost using scrap materials and a few common hand tools. This portable antenna’s VSWR is less than 2:1 across the entire UHF band from 420-450 MHz.

Using a convenient design calculator available on the Internet: Slim Jim Calculator. The length of the antenna is about 18″.

For the purpose of HAM Repeater frequency coordination, the UHF band plan skews towards higher frequencies. A 450Mhz Frequency Slim Jim input value resulted in a minimum VSWR centered at 435Mhz. My VSWR bandwidth is <2:1 over the entire UHF band, from 420 to 450 MHz.

I chose to input a velocity factor of Vf=0.90 for Slim Jim calculation. Some 450 ohm Ladder Line specs. say Vf=0.91 but (through repeated attempts) the value of Vf=0.90 came closer to the design goal: minimum VSWR across the entire UHF band.

Using a NanoVNA, the performance data is shown above. Note that VSWR approaches 1.2:1 at 435MHz and is less than 2:1 up to 450MHz (red arrow).

Assembly and Test

A 3 ft. length of 471 gauge coaxial was manually stripped at one end, and soldered to the Ladder Line. Make sure the coax center conductor connects to the 1/2 wave radiator element.

Not shown are a 3/16″ hole at the top for a Zip-tie mounting hook and, at the bottom, two 1/8″ holes for a smaller Zip-tie to secure the coax cable feedline.

For outdoor weather durability, the exposed metal at both top and bottom of the antenna were wrapped in Nashua “Stretch and Seal” tape.

I easily made many distant (10+ miles) Repeater contacts in the UHF band. Signal reports were consistently good and correlated with distance and terrain.

Conclusion

Building this UHF antenna was a very gratifying DIY home project. The resulting UHF antenna performance approaches commercial products that I’ve purchased and tested before. Build it with surplus components from your Ham Shack, for little or no cost.

Borrowing from my Woodworking skills, I created a “Story Stick” for rapid and precise duplication of the UHF antenna. You have to line-up the Slim Jim gap on a solid length of Ladder Line insulator

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The 6 Meter Ham band covers 50-51 MHz. Using an online Slim Jim calculator to get the proper dimensions, assume the Velocity Factor (Vf) = 0.9 for 450 Ohm Ladder. At 50 MHz this makes for an antenna about 20 feet tall.

The diagram below shows the various elements (A-F) calculated above.

According to the diagram above the E Gap (6cm, 2.4″) is longer than the ladder insulated window 2″. This means that the bottom portion of the antenna would be supported by a single 18AWG wire and would bend awkwardly if mounted vertically. To support both sides of the Ladder, I cut out a 2″ plastic section of Ladder line and Bonded over the insulation void 1.25″ to fill the Gap and support both Right and Left sides of the antenna.

By Bonding I mean using a particular Locktite Plastics Bonding Glue, generally available in your local Building Supply store. This plastic bonding joint has held up outdoors for more than a decade.

The Choke Balun shown below is constructed of Snap-on Ferrite beads and Shireen RFC100A coaxial cable, 6 feet length. NanoVNA measurements indicated this Balun had no adverse effect on VSWR.

The finished antenna standing tall in the backyard. I was able to make SSB QSOs between Portland Oregon and the Mississippi river.

Bill of Materials (BOM)

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The “front panel” is formed from a rectangle of printed circuit board (PCB) material and a SO-239 UHF radio connector.

I purchased this BeO resistor for $6 at my local surplus electronics recycler. It is mounted on a salvaged heatsink. In practice, power dissipation is only limited to heatsinking ability. I’ve tested 100W, at least intermittently, for 15 sec.

My poor soldering technique lead to an abrupt impedance discontinuity at the UHF connector. Nevertheless, S11 performance for frequencies up to 300 MHz is tolerable.

From 1MHz to 300Mhz, S11 insertion loss (reflection) is minimal (~1.2)

Conclusion

50-ohm Beryllium Oxide resistors are rated for up to 30GHz frequency bandwidth. Using a different resistor form factor (surface mount) and better soldering technique you can build a high performance 50-ohm dummy load for almost any radio frequency situation.

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Design and Simulation

Given the constraints of my backyard, the antenna’s highest point is only 20ft. so most of the gain is straight up into space. The off-center feed ratio is 2:1 i.e. 46:23 ft.

Construction

Except for the two size 240 ferrite toroids (type 61), all other components were sourced from my local home improvement center for a total cost of about $100. The wire elements are insulated copper, #14 gauge. Other components are made from Stainless Steel.

Testing and Tuning

The VSWR empirical plot above indicates the OCF antenna is below 2:1 on 40, 20, 15, 10-meter radio bands i.e. (7, 14, 20, 28 MHz). Using external Antenna Tuner equipment is unnecessary. The measured VSWR plot is much more optimistic than the EZNEC simulation.

Tuning was performed by clipping small lengths of wire from both ends of the dipole antenna elements in a 2:1 ratio according the OCF ratio, up to 21 total inches, then measuring SWR after each iteration. At some point I decided that SWR was “good enough” across both 40 and 20 meter bands and quit tuning.

Conclusion

Finally, I’m on-the-air from my backyard in North America. Despite the low height compromise, I’ve made more than 200 FT8 QSO contacts in 6 continents. SSB contacts range to about 1,000 miles. My antenna has been up and down several times due to yard maintenance issues (including tree removals) and refurbished and upgraded a few times but remains fully operational for over 6 years (since 2018).

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This pair of stools was inspired by a video I watched about making Stanton Stools with hand tools only. These two stools were made from a single 2×12 by 6′ board of local construction lumber (Douglas Fir) board. This was a gratifying woodworking project and the total cost was ~$12. The stools were finished with a Cherry stain and several coats of Polyurethane.

Because the stools are of different heights (leg lengths), the sanded pieces are labeled with Painter’s Tape to avoid errors in final assembly.

This simple fixture was fabricated from the remainder my 6′ Douglas Fir stock. The purpose is to drill 9-degree angled holes in the seat and legs using my Drill Press.

Being a wood turning neophyte, I had trouble measuring and sizing tenons. I created this measuring tool from a 4/4 (15/16″) scrap Oak piece. Along both long sides I marked a pencil line to pass through the diameter of each hole drilled with my Forstner drill bit set. The holes were arranged to fit a variety of hole sizes on my scrap piece. After drilling, I accurately sawed (ripped) through the diameter of all the holes, leaving a “perfect” half circle. Due to the relative thick height (15/16″) of this gauge I can identify which parts of the tenon need further trimming. Once my tenon fits into the half circle the fit is perfect. My turned tenons went from being sloppy to a snug fit every time.

Finally, to get a proper angle on the feet for flat resting, I temporarily glued a sheet of sandpaper to small Melamine rectangle. Then, standing the stool upright on the Melamine, I manually sanded each foot flat to the horizontal plane.

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Build this high-performance DIY Dual Band VHF/UHF Yagi antenna using the simple hand tools pictured above. Low-cost PVC pipe fixtures and 1/2″ copper pipe materials are available from your local home improvement store. This was a very gratifying Ham Radio project. Actual performance exceeded simulated expectations.

EZNEC simulation shows a forward gain of 12 dBi and beamwidth of 67 degrees. Not bad for a 3-element Yagi. Its light weight enables handheld manipulation, and I was easily able to receive satellite communications with a common handy-talky (HT) radio set.

A construction detail shows a SO-239 UHF adaptor mounted on a thin sheet of plexiglass. The center driven element (1/2″ Cu pipe) is connected to the PL-259 adaptor through a short length of coaxial cable with solder lugs and sheet metal screws.

A NanoVNA frequency scan shows VSWR to be about 1.5:1 over the entire 2 Meter VHF Ham Band.

Likewise, VSWR is nearly ideal in the lower half of the 70 cm UHF Ham Band and never exceeds 1.67:1 across the entire band.

Detailed antenna dimensions, in units of Meters, are shown above. Note the 0.0254 Meter (1.0″) gap in the driven center element.

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Introduction

Pictured above is a Raspberry Pi Model 4b (L) connected to an SIM800C module (R) to send and receive Short Messaging Service (SMS) Cellular phone messages commonly referred to as “Texting”. This device has possible applications for Emergency Management organizations where Response Teams can be alerted or updated with timely information and instructions.

Hardware Description

The Raspberry Pi Model 4b is a computing platform featuring a high-performance 64-bit quad-core processor, dual-display support at resolutions up to 4K via a pair of micro HDMI ports, hardware video decode at up to 4Kp60, up to 8GB of RAM, dual-band 2.4/5.0 GHz wireless LAN, Bluetooth 5.0, Gigabit Ethernet, USB 3.0. Only a small portion of the features is required to rapidly send lists of names, phone numbers and text messages to the SIM800C module. Cost is about $80, including heatsink as shown.

The SIM800C module from SIMCom is a Quad-band GSM/GPRS Module that operates in the 850/900/1800/1900 MHz bands for voice, SMS, and data information. It delivers a data rate of 85.6 kbps and supports CS 1, 2, 3, and 4 coding schemes. This module also supports point-to-point MO & MT, SMS cell broadcast, and text & PDU modes for SMS. Total cost is about $10 on eBay.

Software Description

A #! /bin/bash shell script takes command line arguments for a text message (*.txt file) and a contact list (Name, Phone #) in CSV format. Iterating through the contact list, commands are sent through the system’s minicom modem program to the SIM800C module. The result of each call is logged to a file. This bash shell script was mainly written by ChatGPT.

Note that a single SMS message is defined as alphanumeric characters only with a maximum length of 160 characters (including spaces). Smartphone designers add features to handle longer messages, perhaps containing emojis and graphic files.

SIM Card

A cellular SIM card is required to connect to the cellular network. I chose UltraMobile who offer low cost, pay-as-you-go plans for Data, Text, Voice access. Monthly Rate is $3, Unlimited 1-day data Pass (including SMS) is $5.

Reference Links

Raspberry Pi 4B product description: raspberry-pi-4-product-brief.pdf (raspberrypi.com)

SIM800C Module Datasheet: SIM800C (simcom.com)

UltraMobile SIM Card: https://ultramobile.com

SMS message guide: https://www.androidpolice.com/sms-guide/

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I made this Gate Leg Drop Leaf Table in my garage using only a table saw and Router. When unfolded, the tabletop is flat and smooth, about 6 ft. wide, strong and sturdy. Finished with one coat of Shellac, Oak Stain and 4 coats of Polyurethane. This table was used for many years at my niece’s home as their main kitchen table.

Dropping the leaves reveals the signature Rule Joint. One or both leaves can be deployed as necessary. When the leaves are folded, the table stores compactly and conveniently out of the way.

The gate legs fold snugly inside the dropped leaves.

Download the full, original woodworking plans below. Makes a handsome, sturdy table that will last for years to come.

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After gluing all 64 squares together, in strips of 4, I rubber-cemented sandpaper (rough side up) to a Melamine Particleboard Panel and sanded the board perfectly flat manually using random circular and rotating motions. It didn’t take long.

Note the center Oak strip to ensure proper Chessboard playing orientation. I was pleased at the tight square corners and mitered border.

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These nested wooden bowls are made of Figured Maple, about 10″ in diameter and 3.25″ tall. Finished in Tried and True Original (Beeswax and Linseed oil).

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