Homebrew Yagi

[Nashua Area Radio Society]

I am oddly and consistently able to hear conversations on, and key the VHF repeater on Killington in VT (W1ABI, 146.880 MHz/PL 110.9) some 90 miles away from me here in Milford, NH (confirmed via an email exchange with a QRZ member local to that repeater).  Conversely, no one is able to hear me (50W at the radio down to about 20 at the antenna after line losses)  I would like to experiment with an simple and inexpensive homebrewed yagi.  I see a number of calculators & designs on the web, but any thoughts/experience on a good place to start?

[Hamilton]

Hello Luke,

It sounds like a extra 10dB of gain would make the difference on Tx.

A 3 or 4 element 2m Yagi is an easy antenna to make.

There is a “Tape Measure” version on this web site.

I have attached the charts for the 3el 2m yagi I demonstrate during the NARS license classes. It used ezNEC2 for analysis.
The analysis tool I prefer and typically use is 4NEC2. Unlike ezNEC2 the free version of 4NEC2 does not have a restriction on the total number of element segments and it permits optimization. Otherwise they are very similar to each other.

My demo antenna uses telescoping elements from Amazon which are great for a demo but would not do well outside. For actual operational 2m and 70cm antennas I used 1/4″ diameter aluminum tubing (McMaster Carr 7237K28 $7 for 6′) and a non-conductive fiberglass boom.

Another approach is to use aluminum arrow shafts (size 1816) and .24 diameter inserts. The inserts accept a 8-32 threaded rod. It is easy to drill a hole through the boom for the 8-32 metal rod for the reflector and directors and screw an arrow/insert onto both sides. Assuming a non-conductive boom the driven elements uses a 8-32 threaded nylon or PBS rod with the coax attached to the two isolated elements of the driven element.

I hope I have touched on a few of the areas you are interested in. I also hope this generates a least a few question on NEC2 and various construction techniques. Many of us are here and ready to help with NEC2 and assembly approaches.

If it helps we can do a Zoom session on NEC2.

Hamilton   (K1HMS)

 

 

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[Dave Michaels]

Hamilton, Thanks for the pptx on combining the demo 2m yagi construct with 4nec2 software.  I’ll add it to my lecture as well.  BTW, I now feed the receiver antenna direct to an air-cool dummy load, convert signal to dc and display with a 1 in LCD display 5v meter.  I would appreciate your configuration.

73,  N1RF

Dave M.

[Hamilton]

Hello Dave,

For the class demo I quickly go through the NEC2 design process and then have the students (pre-covid) extend the element lengths per the design and selecting which end is the front based on element lengths. The process clarifies how to use the NEC2 XYZ data.

The Yagi is driven with a HT (146,5xx MHz) with 1 watt of RF power using coax directly to the driven elements. The SWR is <1.5:1 across the full band.

The receiver is a wooden wand with a dipole that is extended to 468/MHz tip to tip with a incandescent light bulb between the dipole elements.

The students perform the following tests;

Forward Gain: When the Yagi is approach from the front with the wand/diploe the light illuminates at about 20′.

Polarization: As the wand/dipole is rotated towards vertical the light dims. At beyond about 45 degrees it goes completely dark.

Elevation: The NEC2 analysis shows a main beam elevation of about 15 degrees. When the wand/dipole is raised the light gets brighter and is at maximum brightness at about 15 degrees.

Directivity and front to back: When the Yagi is approached from the back the wand/dipole can be brought up to nearly touching the reflector and the light remains dark.

Each test is directly related to a possible question on the Extra test and may help with understanding antenna pattern plots. It is also a fair introduction to NEC2.

If time permits a final test is to set the director to the same length as the reflector. Now the light will not illuminate at the front or rear of the Yagi…but it shines bright when the wand/dipole is held over the Yagi. If measured the SWR will remain good at below 1.5:1 across 2m. This demonstrates the conservation of energy and the fact a good SWR does not always mean a good antenna.

I bought the telescoping antennas with F connectors and bulkhead F connectors on Amazon. The connector mounts are simply pieces of 3/4″ square Al  tubing cut 3/4″ long and drill 3/8″dia for the F connectors and 1/8″ dia for a screw into the boom. The total material cost was less than $20.

I would like to hear more about your test.

73

Hamilton K1HMS

 

[Dave Michaels]

Hi Hamilton:

Thanks for the details.  I sent you a response email to the “.gmail.com” . address.  Did you get that material?  In short to your question, I am still using the “Lego Yagi” construction as described and demonstrated in 3 articles for the club’s website.  I have made minor changes that I described in the email.  My goal is to make quantitative measurements versus the lightbulb bridged receiver antenna, which is a good visual.

BTW, on Amazon I see telescoping antenna with F connectors…they are 75 ohm connectors.  Is that an issue?  I cobbled bnc connectors to my driven antenna elements, but it’s quite kludgy.  F connectors are definitely a better choice.

The christmas tree lights 1/4 wave antenna that I use to show the current profile along the elements is a good visual demo.  However, I have tried to tweak the Rogowski “pickup” coil to give a quantitative profile of the current without success.  I understand this circuit’s output drops significantly at high frequency.  So, is there a low-cost circuit that can achieve my goal?

Thanks & 73,  Dave  (N1RF)

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