1
$\begingroup$

A few months ago, I built a 6 element yagi: 3 driven elements, and 3 reflectors. For 10-15-20m. I was able to tune it to SWR 1:1 for the FT8 frequencies in each band. This antenna is only 6m above ground, since that's what I can manage without guy wires (and the fact that steel tube is sold in 6m lengths too).

The 20 and 15m elements are coil loaded, as I didn't want the antenna to exceed my property limits, and I wanted to use whole aluminium tubes (which are sold in 6 meter lengths). I think this is the main issue here.

I've been happy with this setup for a few months, as I thought it was working properly.

Since my mom is also a ham, my dad installed a trapped 20-40m dipole. It's about 9m off the ground. It's only a couple of meters away from my yagi. I noticed her radio was getting FAR more FT8 decodes per cycle: while my computer only showed 3 or 4, hers would show 12 to 15.

So I've been trying to figure out what the problem with my yagi was. Here's what I tried:

  • Cut the 20m element in half and feed it like a dipole
  • Remove the rest of the driven elements and leave only the 20m one
  • Remove all the reflectors, turning the yagi into just a rigid dipole
  • Changed the coax to the antenna to RG-213
  • Removed the coax switch, connecting the antenna directly to the radio

I even tried taking the dipole down to rule out interactions between the two antennas, and lack of height in my yagi. And even laying on the roof, lower than the yagi, it works better in RX than my yagi, so I think it's neither height, nor interaction between the two.

I designed this yagi based on other designs online, and simulated it with 4NEC2. I built it down to the millimeter and it was very easy to tune: I didn't have to trim any tubes once I put it together, it just resonated at the calculated frequencies. The simulations did show lobes to the front (which I measured with a function generator and an antenna 1km away. Only a few milliwatts CW, while i rotated the antenna, and indeed, pointing the antenna towards the source gave me a higher reading, by 2 points in the s-meter: S9 vs S7), so the directionality was also spot on.

What's the problem here? Could it be that being coil loaded makes it perform significantly worse? It's only 7 meters long instead of the 10m required for a true half-wave dipole. Or is this a height problem and I should take it much higher than it is now? Lifting it is not an easy task. It would be expensive, too, so I would prefer to avoid it if possible at all.

Photo

Improve this question
$\endgroup$
4
  • 1
    $\begingroup$ Can you tell us a lot more about the yagi? A diagram of the elements, and describe the feed arrangement(s). $\endgroup$
    – tomnexus
    Commented Aug 9, 2020 at 16:47
  • $\begingroup$ @tomnexus if a picture is worth more than a thousand words, maybe this will help: i.imgur.com/ZVaJxM9.jpg $\endgroup$
    – hjf
    Commented Aug 9, 2020 at 16:50
  • $\begingroup$ Have you compared the antennas with the same receiver? $\endgroup$
    – Phil Frost - W8II
    Commented Aug 9, 2020 at 22:07
  • 1
    $\begingroup$ @PhilFrost-W8II yes, I put both receivers together, sharing the same PSU even, and swapped antennas. But I gave up - sunday evening, needed to clean up. Put the antenna back together like it was originally. Now the yagi "beats" the dipole by a long margin. And yes, I checked - the right antennas are plugged. I have no idea what happened. I'll do more testing tomorrow and see if anything changes. $\endgroup$
    – hjf
    Commented Aug 9, 2020 at 23:44

1 Answer 1

Reset to default
1
$\begingroup$

It doesn't makes sense that a 6 element yagi doesn't work a good as a half wave dipole. Even a 6 element yagi with the worst design should still work better than a half wave dipole.

I don't think the antenna is the problem.

It sounds to me like one side of the feed line is open circuit at the antenna, if you are using coax the shield might not be connected.

This would mean you are effectively running a long wire antenna with a lump of 6 element yagi connected at the end.

The low SWR could just be a coincidence.

Improve this answer
$\endgroup$
1
  • 2
    $\begingroup$ I did try replacing the feedline but nothing changed. I even removed the connector and soldered the feedline directly. Remember, though, that this is not a 6 element yagi, but 3 2-element yagis sharing the same boom. $\endgroup$
    – hjf
    Commented Aug 10, 2020 at 1:35

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .