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DG1JAN XOTA antenna

This page was last updated : 21-Oct-24 22:29:30

 

Content 

• Introduction
• Building the XOTA
• Testing
• Building a second one and starting to experiment.
• Out in the fields (kind of)
• Preliminary conclusion
• 100W edition build and testing
• 100W edition 10m vertical test

Introduction

For more then a year now I have been using the DG1JAN UniBalun, a very nice small universal PCB on which you can create a 1:1, 1:4, 1:9, 1:49 Balun for QRP operation, for quite a few fieldradio experiments. It's quite popular and there a few video's about it.

But when you look at DG1JAN's github page, there is another interesting project the xOTA-Antenna. A PCB design which includes a EFHW and a wire winder. 

Building the XOTA

When I needed to order a few new prototype pcb's, I also ordered a small batch of the xOTA-Antenna PCB's for me and a few soldersmoke loving fellow hams. And after collecting the parts, the BNC Molex 731385033 was a bit harder to get so it took more time, it was time to meld some solder.

Note: I used a FT114-43 core because I wanted to have a xOTA which could handle the maximum of my Elecraft K2 without any restrictions.

Bending back the wire for aprx 20cm, and clamping that end (not the loop) and the main wire with my Knipex Combination Pliers, I used my battery power drill to twist the wire at a low speed. 

Because I wanted to have a EFHW 1:49 configuration, I used the 3:21 winding ratio (3 turns twisted plus 18 turns single wire) because it also works good on the DG1JAN UniBalun.

Checking the wire spreading to match the PCB.  

I removed the enamel from the wire by scraping it off with a scapel knife and applied tin to the wires.

After some pre-bending the wires match the holes in the PCB and could be soldered into place. Using 3 tiewraps the core is hold into place preventing transport and usage vibrations the put stress on the wire and joints. 

Adding the 100pF / 3Kv capacitor for the high end part of the band (I will add a few VNA screenshots why that is needed).

Then it was time to install the BNC chassis part. 

And it was time for some inspection. 

For this antenna I also wanted to install 2 times the 4 mm Gold Bullet connectors (normally used for RC Lipo-batteries) to allow easy changing of different length EFHW wires 

But with these connectors there is one thing to keep in mind. On one side there is a two holes in it. And the smallest one you don't want to have sucking up solder (this can prevent the male counter part to fit correctly).

So holding the female connector, I pre-tinned the opposite side.

I also pre-tinned the PCB area's where the female connector should be installed.

 

After applying heat to the female connector and PCB area, a nice connection was made. 

Using the trick on the other connector . . . . 

The construction was almost finished. 

Testing

Using my NanoVNA H4 and the DG1JAN Balun_Test_Tool (which I highly recommend for testing your baluns) I tested the matching of the xOTA antenna.

SWR value at the minum point are very acceptable.

 

But the SWR value a the endof the HF band is to high. But this can also part because we are testing with a pure resistive load of 2450 Ohm. In practice this never will be so the might be some inductance and capacitive effects also which may temper the SWR.

For now I'm happy with the result, I'm going to test it in the field with a various endfed wire lengths including one for 15, 12 and 10m. 

Building a second one and starting to experiment.

For my friend PA1MAR I assembled another xOTA and as a experiment I removed 2 windings, so instead of a 3:21 I now have a 3:19.

The results are shown below. It's squeezing the dB out of it, but now the lowest SWR is 1:1.092. Which IMHO is good enough. 

 

As you see, there is always room for improvement.

 

Out in the fields (kind of)

Last weekend during a family outing in the HuttenHeugte, I was able to subject the DG1JAN xOTA antenna to a field test. Given the location of our cottage, it was not really possible to properly tension the antenna wire, but with the help of 2 parasols and a birch by the water, a horizontal V could be made and I was QRV. Despite the low suspension (about 2.5m above the ground at the lowest point and aprx 4m above the ground at the birch side) and the low power (10W from my K2), I was still able to make a nice number of QSOs, including a number of IOTA stations and a handful of special event stations. 

Because the xOTA isn't build for leaving outside and some morning mist is to be expected at this time of year, I put a seal bag round it in the evening to prevent it from getting (to) wet. 

 

Normally I always use old German field telephone wire for wire antennas because it is . . .

• really cheap,
• super strong,
•  relatively thin (so the neighbors won't see the wire).

I have also been using it for years for /P, but it always takes a bit of work because you always have to wind it in some large turns (or on a large spool) and because of the stiffness, it is difficult to wind it on a "wire winder". ,and therefore also the xOTA as shown below, virtually impossible.

That is why I want to use a thinner and more flexible wire / antenna litz for my /P work in the future.

Preliminary conclusion

I love it, its a nice piece of /P equipment and love it. Once I have found a more /P antenna wire I think it's going to love it even more.

Note: the antenna wire was trimmed for vertical usage, so I needed to use the K2 ATU for a good match. But I'm going to make new wire sets in the near future. 

 

 

100W edition

When building the100W version EFHW with FT140-43 core and 1mm wire, you will find out that the twisted wire doesn't fit into the GND hole.
I fixed it by using a "hair pin" wire (a loop made of 0.6mm zilver wire pulled through the hole pulling the twisted wire on the the GND pad).

Detailed description of the winding of the 1 to 49 transformer can be found on this page. 

But it would be great if there was a bigger hole, or 2 holes of the current size next to each other.

For the winding I used one turn less (so 3+17) to get the lowest SWR possible, but the SWR on the 30MHz is a bit high.

SWR is 1 and a bit higher then with the FT114-43 and squeezing the second part of the winding (the 17 turn) improves it.
So I going to test this out in the field to see what it does on 10m. 

100W 10m vertical test

I used my fiber pole and the 100W XOTA to make a 10m vertical endfed. As you can see, the SWR turns out nicely. 

 

But I'm going to do some tests using WSPR so I can compare it to my 10/20/40m horizontal endfed. 

 

To be continued.

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