Build a 3-element Yagi-Uda antenna for 2 meter and 70 cm Amateur Radio bands with low-cost common building materials from your local hardware supply store. This antenna requires no extraneous impedance-matching components or complex tuning procedure. Just follow the construction outline here and in a couple of hours you’ll be on-the-air with a high-performance antenna.

Design

As an FCC licensed radio amateur I take great pride and satisfaction in designing and building my own antennas (antennae). This is my first Yagi antenna project. I was intrigued by the idea of passive elements creating directional gain and the wide variety of possible performance optimizations. This antenna was designed with the following goals in mind.

• Intrinsic 50 ohm feed point impedance for maximum efficiency and to avoid the necessity of adding an impedance-matching network or using an antenna tuner.
• Quick breakdown of the antenna for easy storage and portability.
• Durable for low maintenance long term outdoor deployment.

EZNEC application program was used to simulate the antenna design. 4NEC2 could also be used but EZNEC is EZ. Some Yagis are built with the element centers electrically shorted to a conductive Boom (and each other). But here the Boom is made of PVC so the elements are insulated from the Boom. Following my intuition, I iterated element length and spacing values to optimize for a 50 ohm feedpoint impedance. These dimensions are shown in the table and diagram below.

Simulation Results

Azimuth plot indicates a forward gain of 12 dBi with Front/Back gain of 21.5 dB and 67 degrees Beamwidth at 146MHz.

Horizontal elevation at maximum gain is 4.5 degrees. This certainly seems strongly directional to me. In the SWR plot below, the impedance vector Z is 45+i0.84 ohms at 146 MHz which is close to 50 Real ohms. SWR is about 1.1:1 throughout the entire 2m band (144-146MHz). More than 98% of power is radiated.

Testing and Validation

Using a NanoVNA Vector Network Analyzer, a Smith Chart and SWR plot were captured for both the 2meter and 70cm bands. Colored markers 1,2,3 show the min, mid, max extent of each band. The optimal 2m SWR frequency corresponded well with the simulation even though an SWR of 1.1 was not achieved. Even so the measured SWR is better than 1.5:1 throughout the band which is 97% efficient and “good enough”. The broad SWR bandwidth is attributed to the relatively thick (1/2″) Copper pipe elements.

I connected the antenna to my Handy Talky (HT) transceiver and contacted several repeaters and received good signal reports. Even in the Yagi horizontal orientation I was using less than 2 Watts to engage repeaters. By rotating the Yagi from side-to-side horizontally I could hear static noise rise and fall, thus demonstrating some directional gain variation.

Materials

The table below details a shopping list with estimated prices to build one antenna. Everything but the SO-239 UHF coax connector can be found at your local hardware store. At a cost of just $36 you can’t go wrong.

Build Process

• Separate the 2 driven element pipe pieces by 1″. Drill 2 holes 1.5″ inches about the center (3/4″ either side) of the PVC Cross fitting for 2 #8 sheet metal screws which connect the signal wires to the Copper pipe.
• I left the PVC pipe fittings unglued for easy take down purposes. The natural friction of assembly seems strong enough for permanent deployment.
• The SO-239 UHF connector is mounted on Plexiglass sheet (2″x 4″) and held in place by a pair of 1/4″-20 x 2.5″ screws. A pair of nuts serve as spacers on each end.

Discussion

My first attempt to design and build a Yagi-Uda antenna turned out well. I was surprised to discover the 70cm band workable, while this makes sense since 70cm is the 3rd harmonic of the 2m band, the simulation did not hold promise. But sometimes you get lucky if you try. You too can build this 3-element Yagi antenna without fancy test equipment by using accurate dimensional measurements and good construction technique.

References

Renegade Home

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.

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.

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.

• 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.

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.

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

A procedure for tuning Standing Wave Ratio (SWR) of a simple dipole antenna, by trimming wire length, is presented here with empirical data. This is a follow-up to a previous post about design and construction of an off-center fed dipole antenna for the HF Amateur Ham radio bands.  ocf-hf-dipole-antenna-diy

Antenna Analyzer

To aid the tuning process I acquired a RigExpert AA-54 Antenna Analyzer. This instrument can easily log Freq, R, X  data, over a USB cable, using the AntScope product application software. Then CSV formatted data is imported into an Excel spreadsheet for VSWR calculation and plotting.

Trim Length

SWR tuning is performed by snipping short lengths of antenna wire, at the farthest ends, in order to raise the resonant frequency to within a desired frequency range. Fortunately there is a simple, accurate, formula to calculate the trim lengths. Consider:

c = L1 * F1 = L0 * F0

Where c is the speed of light, F is Frequency and L is Wavelength,  0 subscript is the initial and 1 is the target case. Using some algebra we derive Trim_Length:

Trim_Length = L0 – L1 = L0 * ( F1 – F0) / F1

For example, raising the resonant frequency of a 65 ft. dipole from 7.0 to 7.3 MHz:

Trim_Length = 65 ft. * (7.3 – 7.0) / 7.3 = 2.67 ft.

For this case, where the antenna is fed in a 1/3, 2/3 off center ratio, the trim lengths at each end are respectively:

short_side_trim = 2.67 ft. / 3 = 0.89 ft.

long_side_trim = 2 * 2.67 ft. / 3 = 1.78 ft.

Trim Results

I’m somewhat risk averse because shortening wires is easier than lengthening them. Optimizing performance on several bands can be tricky so I trim the wires in small increments, and measure the results across the relevant bands at each step.

The plot below shows the SWR performance across the entire frequency range for the AA-54 Analyzer (i.e. 0-54MHz). Note the original length “1st” and trim lengths of -6, -15 and -17 inches

Local SWR minimums are noted at the 40m, 20m, 15m and 10m bands.  Local minimums are observed around 43 and 50 MHz as well. 43 MHz is not usable for Amateur operators and 50MHz (6m) is somewhat marginal and not interesting to me now.

Focusing on the 40m band, the effect of trimming can be measured at each step. Each incremental trim raised the resonant frequency and after trimming 17 inches, SWR ranges from 1.4:1 to 1.8:1 across the 40m band (7.0 – 7.3 MHz) (yellow).

Similar results are shown for the 20m band. After 21 inches trimming VSWR is a nearly ideal (1.4:1) across the entire band. Considering the effect of trimming on other bands I stopped trimming here. 21 inches is somewhat less than the predicted 2.67 ft. (32″) but close enough for practical purposes.

A Z-normalized Smith Chart below shows the impedance across a range of frequencies in the 40m and 20m bands. Black markers are at mid band. The lowest SWR, nearest the center, for both bands, is found at about twelve-o’clock in the chart.

A circle of constant 2:1 SWR is shown in red.

SWR vs. Reflected Power

The motivation for trimming dipole wires is to match the antenna’s impedance to the transmitter output impedance, in this case 50 ohms real resistance. When matched, the reflected wave is minimized and power radiated by the antenna is maximized. So how low a SWR is good enough? Below is a plot of SWR versus reflected power.

As shown, a 3:1 SWR reflects 25% power, 2:1 SWR reflect about 11% and 1.5:1 reflects just 4%. Therefore a 2:1 SWR or better is generally regarded as “tuned”.

Conclusion

1. An Antenna Analyzer was a useful tool for measuring complex impedance across a wide range of frequencies (outside amateur bands). With this data, VSWR was calculated  and a strategy formulated to improve performance.
2. I discovered a new usable amateur band on 15m. This bonus band was not anticipated in the original design. In fact I have made several QSOs throughout the USA using this band.
3. As initially constructed, the length-measured antenna wires were close to the design goal but SWR improved substantially after trimming.

As a ham radio enthusiast, I take great pride in designing, building and testing my own antennas. I’m not an expert but I’ve had success at VHF, UHF frequencies and I wanted to try longer range communications possible on HF bands. A full blown HF tower 60 ft. in the air is not practical at my humble suburban abode. Nevertheless, a row of trees in the backyard provided space to explore the capabilities of a homemade antenna.

The antenna described here is commonly known as a Half-Wave Off Center Fed (OCF) Dipole or it’s cousin the Windom antenna. It is designed to resonate on a fundamental frequency and its even harmonics, in this case the 40, 20 and 10m amateur radio HF bands. The form is similar to other dipole antennas i.e. long wires suspended horizontally with a balanced electrical feed. Except for the ferrite toroid cores, all the components are commonly available at your local hardware store.

Design

Referring to my ARRL Amateur Radio Handbook, for #14 insulated Cu wire, a half-wave length for 40m (7MHz) was 68 feet according to the formula below.

Length[ft.] = 468/(Vf * Freq) = 468/(0.98*7) = 68 ft.

where Vf is velocity factor (0.98 here) and Freq = Frequency in MHz

I determined that erecting a horizontal 68 ft. antenna was feasible in my backyard.  There is much debate about feedpoint location but I chose 2 nominal segment lengths of 23  and 46 feet for a 1/3 to 2/3 split. A major compromise to performance would be that the maximum height of the antenna is limited to about 20ft. as shown in diagram below,

Feed point impedance is a function of elevation above ground so I wanted to simulate my site’s configuration using EZNEC demo, a free EM field simulator program. The SWR results below indicate a minimum at 7.21 MHz (middle of 40m band).  Here, Z = 61 ohms @ 0.61 deg. (close to ideal).

Indeed 3 resonant frequencies are shown, the fundamental, 2nd and 4th harmonics. While SWR is not perfectly 1:1 at all 3 frequencies I took the EZNEC results as a 1st order guide. The real physical antenna would be optimized on-site by manual pruning. This situation illustrates the purpose of an Antenna Tuner but alas, not in my budget.

More EZNEC results are shown below. Max. field angle is about 19 degrees, pretty close to 15 degrees, considered ideal.

The Azimuth radiation pattern is not perfectly circular but much more omni-directional than a dipole in free space. I suspect part of the omni-directionality is due to the vertical vector components of the inverted-V configuration.

Guanella 4:1 Dual Core Current Balun

The balun transforms the 50 Ohm coax feedline impedance to 200 Ohm at the dipole offset feed point. It also transforms the unbalanced coax to a balanced feed. This 4:1 current balun is implemented using 2 pcs. 1.4×2.4″ ferrite cores of type 61 material.  This is the most expensive component of the antenna. Using these large size ferrite cores and #14 Cu wire give the antenna a transmit power capability of 1.5KW I estimate.  This is the legal limit for any Amateur broadcaster. The circuit diagram is shown below.

Note that the copper wire windings are in opposite directions for transformer 1-2-3-4 versus 5-6-7-8. Fourteen windings were used for each. Using different colored insulated wire helps avoid winding mistakes.

Assembly

Below is a brief Bill of Materials:

1. ferrite toroid core, 2 pcs. P/N FT-240-61,  ebay.com $20 each
2. SO-239 bulkhead mount female UHF connector, 1 piece, ebay.com, $2
3. 100ft. #14 stranded THHN insulated copper wire, Home Depot, $22
4. Plastic electrical utility box, 1 piece, Home Depot, $12
5. Strain relief hardware, 2 sets, Home Deport, $4
6. Miscellaneous 8-32 eye-hook hardware, $5, Home Depot

Total cost: $85

Assembly is very straight forward. I used a long table and tape measure to measure wire lengths. A magic marker was used to mark-off each foot with longer marks for each 10 ft. and a double mark for the theoretical ideal length. An extra 1 ft. was added for strain relief and 1 or 2  ft. for length margin. Two wire ends are soldered to tabs and connect to separate through-hole assemblies.

Testing/Results

After erecting the antenna and connecting to my transmitter, I characterized SWR values using the built-in meter by  sweeping amateur frequencies across 3 bands. Typical values are shown below. To my  delight SWR range was reasonably low across all three bands.

Again, SWR being slightly high and variable is a good reason to use an Antenna Tuner. But this minor mismatch is good enough for my transmitter. I wish I could characterize SWR over a wider frequency range, but something like a VNA or Antenna Analyzer is way beyond my means.

I am  able to hear SSB voice stations from Alaska to Costa Rica to Hawaii to Pennsylvania and make voice contacts about 2,000 miles away. Low-power digital modes (e.g.FT8, JT65) using just 30W of power yielded QSOs as far as 5,000 miles distance.

Conclusion

Due to physical area constraints at my home I was trapped in VHF/UHF world for too long. But I found a way to construct this HF antenna and have enjoyed continent-wide communications ever since.

Although this antenna’s performance is highly compromised at my site, it was still one of the most successful and gratifying amateur radio projects I’ve completed. With no antenna analysis capability, I had faith that employing good design and construction technique would get me close and that would be good enough. If you’ve been holding off getting on the HF bands because building a grand ideal antenna is not feasible or too expensive, I encourage you to try such a low cost solution .

References

Dipole Introduction

How and why an OCF dipole antenna works

Dual core 4:1 Guanella current balun

by KF7GAX, 73