Help with a yagi

[mm]

As I remember that original ARRL/QST article appears to be from one of the NBS yagi designs but I don't remember if the NBS designs accounted for element diameter, it appears that for your antenna and those spacing in that article, an element diameter around .2 inches is the best overall for low VSWR and good gain.

Unfortunately with that online calculator the element diameter is not specified and without a better modeling of those dimensions then the performance is questionable.


With the same lengths that you are using and if you went to .5 inch diameter elements the best VSWR is 2.1: 1 and the element lengths and element spacing would need major changes to achieve even a reasonable lower VSWR.


If you were to use .125 inch diameter elements you would achieve a very good match across 144-148 with the articles element lengths but with slightly lower gain than if using .25 inch or .2 inch diameter elements whereas .5 inch diameter elements need numerous changes to most lengths to achieve even a reasonable match.

With .125 inch diameter elements the best VSWR is 1.29:1 at 146 MHz and 1.53:1 at 144MHz and 1.55:1 at 148 MHz.

The .125 inch diameter element appears to be the best option for low VSWR with the dimensions from that article and the gain at 146 MHz is still 8.21 dBi with -15.39 dB F/R
At 144 MHz the numbers are 8.09 dBi and -14.64 dB F/R and at 148 Mhz they are 8.36 dBi and -13.73 dB F/R.

The driven element length either side of center is 18.9 inches for a centered match at 146 MHz when using .125 inch diameter elements.



With those dimensions that you are using right now and the .25 diameter copper you use, if you set the driven element length each side to 18.6 inches, this is the length from the pigtails of your coax outward on each side of center, the match is centered at ~146 MHz at 32 -J.1827 ohms or essentially a 1.55 :1 VSWR.

At the band edges the VSWR is 1.74:1 at 144 MHz and 1.8:1 at 148 MHz.

The gain at 146 MHz is 8.62 dBi with a F/R ratio, worst case, of -14dB.

At 144 MHz the forward gain is 8.44 dBi and the F/R ratio is -15.29 dB.

And at 148 MHz the forward gain is 8.78dBi and the F/R ratio is -11.72 dB.

The -3 dB beam width is 60.6 degrees and these are pretty typical numbers for a short boom 3 element yagi.


To get a lower VSWR across the operating band along with higher gain, from 144-148MHz, you would need to change element diameter and spacing along the boom along with changing the DE, director and reflector lengths but you would not gain much forward gain unless you then extended the boom length and added another element.

8.6 dBi is not bad with a 1.8 :1 worst case VSWR, I would leave it as is and operate.

 

[temchik]

Thank you!

Also, that means that my cheapo SWR meter actually works!

 

[AK9R]

Along with a larger soldering gun you might try removing the corrosion from the copper tubing. Using an external flux might help as well.

I agree. The copper tube is acting as a big heat sink which draws the heat away from the point where you are soldering. You'll need to apply a lot of heat to counteract the heat sink.

Before soldering, thoroughly clean the copper with steel wool so that it's shiny bright.

Get some rosin-type soldering flux and liberally coat the area before applying heat. The flux will chemically clean the copper so the solder can bond to the copper.

 

[prcguy]

What you described is not an ideal way to feed the driven element of a Yagi. Its like a gamma match but without the series C to tune out reactance in the wire that taps the element. Feeding a split element directly with coax is generally acceptable because the close proximity of the reflector and director will cause the driven element impedance at resonance to be well under the theoretical 70ish ohms and probably somewhere between 25 and 50 ohms and not the 50 to 100 ohm range. Close proximity of a vertical VHF Yagi to the ground has very little effect if any on impedance.

Trimming the split driven element will usually allow for a good match and the only thing to worry about is potential common mode RF current on the coax, which is best suppressed with several inches of 43 mix ferrite beads or maybe two turns of the coax through a 43 mix toroid. Unless you know the exact size to make a coil of coax it would not be very effective.

Its also a good idea to end mount this particular Yagi so the coax runs along the boom and behind the reflector before heading down the mast. The way the Yagi is pictured with the coax running parallel to the driven element will probably affect tuning and the radiation pattern.
prcguy

The split design you have is non-adjustable and could vary between 50 and 100 ohms, depending on height above ground and other factors. If feeding with 50 ohm coax, you're pretty much guaranteed an impedance mismatch between the coax and antenna, and thus non-optimal SWR. The matching setup I describe will allow you to get an exact between coax and antenna, and will also prevent static buildup on your antenna, especially if you connect the coax center wire to the driven element below the center of the antenna instead of above.



Yes, coiling the coax around the vertical PVC will create an inductor coil that will block RF from traveling down the coax. But it's better to avoid RF then trying to brute-force it into submission after the fact.