3
$\begingroup$

I've been poking around the internet on how to make a Yagi for my HT. I understand all the math for cutting directors and reflectors to length and placing them at the correct spacing for 146.52 mHz. However, I keep finding different descriptions on how to make the driven element, or worse, no description at all.

How do I make the driven element? More importantly, how do I make sure that the antenna is 50 Ohms? Preferably, I'd like to not solder.

$\endgroup$
1
$\begingroup$

Pulling it off without soldering will be tricky, your only option will probably be to get a chunk of coax with the connectors already on it, cut off a connector, then use bolts and crimp on lugs to connect the coax to the antenna element.

You're finding multiple descriptions of how to do it because there are multiple ways of doing it depending on the intended use of the antenna.

  • You can feed it directly like a dipole, splitting the driven element in two and connecting one side to the coax shield and one to the center conductor of the coax. It will not be possible to get maximum bandwidth and maximum gain with this method, though you should be able to get a reasonable compromise that works on all of 2 meters. 70cm would be much trickier.

  • You can feed it with a gamma match. This gives a wide match and has the advantage of also acting as a balun. They're finicky to make, and would be very difficult to tune without an SWR meter or antenna analyzer

  • You can feed it with a hairpin match. Similar problems to the gamma match, but similar advantages.

Personally, I'd recommend finding a design that specifically calls for a directly driven element. Google for NT1K Tape Measure Yagi for a good project write-up.

Another option if you don't need portability would be to ditch the yagi and build a cubical quad instead. There are many calculators available online that give you all the required dimensions, and they tend to be a bit more forgiving on the SWR front. They also provide more gain per element than a yagi, at least until you get out to about 7 elements. They can be made very inexpensively out of PVC or wooden dowel and 1"x1" hardwood lumber for the boom and wooden dowels for the spreaders.

$\endgroup$
  • $\begingroup$ Cubics are pretty nice for gain & forgiving SWR curves, but they do absolutely require the use of a balun, and IMO the only 'good' way to make the needed 4:1 balun is with an air-cored coil & a bit of soldering. ;) $\endgroup$ – Robherc KV5ROB Feb 19 '16 at 21:35
  • $\begingroup$ Multi element Quads will present pretty much a perfect 50 ohm impedance. Only single element vertical loops and horizontal loops require impedance matching. On V/UHF I've had good luck getting a perfect match and no common mode with a half a dozen or so turns of coax about 4" in diameter right at the feed point. $\endgroup$ – Hamsterdave Feb 20 '16 at 3:34
  • $\begingroup$ Interesting. I built my hybrid (loop D.E. & loop reflector + 2 dipole directors) around maintaining the 200Ohm characteristic impedance of the D.E. loop; never even thought to try to push the arrangement all the way down to a 50ohm impedance (let me use identical 4:1 air-core baluns on that one & my single loops) $\endgroup$ – Robherc KV5ROB Feb 20 '16 at 3:41
  • $\begingroup$ You can get better gain that way, but aiming for ~1.5:1 only costs you about 1dBi max on VHF according to the models I ran when I designed my V/UHF quads. Most online calculators are optimized for 50 ohms. Just about all passive directional arrays are similar, a standard yagi can range from almost 100 ohms all the way down to about 25 ohms depending on element spacing. $\endgroup$ – Hamsterdave Feb 20 '16 at 4:05
1
$\begingroup$

I made my 5 element with the driven split element like a diapole. I used cu tubing flatening one end and screwing them to a wolmonized square boom about 1/4" apart on boom direct feeding coax with terminal lugs. Of course you must drill hole in flattened end for wood screw hole.Cut them a little shorter than wanted length,about 1/4 inch. I used ss cotter keys at both ends of driven sliding them equal distances in or out to match. Worked great,flat match for 5 element beam.

$\endgroup$
0
$\begingroup$

The easiest way to make a driven element for a hand-made Yagi, without having to solder is to get out a knife+some elec. tape or heat-shrink tubing and make a 'bazooka dipole' antenna from your feedline coax & use that for your driven element.

  • Advantages:
    • Quick & easy to make, with absolutely no soldering necessary.
    • Forms its own balun, so you don't need an impedance-matching network added to your feedline, or your yagi.
    • If damaged, a 'new' D.E. can be made simply by re-cutting & inverting the same feedine (until the feedline gets too short to be practical, at least).

  • Disadvantages:
    • Feedpoint impedance is not easily adjusted, so you'll have to be careful to place your elements in an arrangement that closely matches your feedpoint impedance to your coax's characteristic impedance (Antenna simulation programs are available for free download on most platforms to help with this).
    • Your D.E. won't hold its own shape well, and will most likely need to be supported in order to maintain proper element alignment/spacing.
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.