A 3/8 wave vertical for 20m

 

Building this antenna came about after I wanted to put something up at home specifically targeting 20M FT8 on 14.074 Mhz.

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The 3/8 wave vertical for 20m in my front yard…

I’m fairly time poor so I needed something quick, simple and cheap to get going with asap. Whilst doing a bit of research I came across the winners of the 2018 QST Antenna Design Competition, and interestingly, third place was a 3/8 wave vertical for 20M by Joe Reiart, W1JR. As I’m not an ARRL member, I asked if anyone in my HAM community  would be kind enough to let me have a copy of the article. Several people kindly sent it to me.

Surprisingly, there is virtually nothing on the internet about a practical example about this antenna. I did find an old reference to the antenna, but no solid build details.

In a nutshell, according to the article, the 3/8 wave vertical has the advantage of requiring less radials than your standard 1/4 wave, but is about 50% taller for the same band. A 1/4 wave vertical requires at least 16 1/4 wave radials for good performance due to the low feed point impedance of around 35 ohms, whilst the 3/8 wave requires just 4, having a feed impedance of around 200 ohms. It also sports a lower take off angle of radiation of 23°,  vs 26° for a 1/4 wave (better for DX), and as the radiating current maximum point is 1/8 wave up the vertical instead of at ground level, it is ideal for ground mounting in situations of nearby clutter. It is easy to match to 50 ohms Coax via a simple 4:1 Unun and a series capacitor.

The materials

I decided to go with a squid pole for the vertical radiator, as they are cheap and easy to keep in the air, so I purchased a 6M pole to which I added 2 base sections from older broken squid poles to bring me up to 7.5m in length. This size slides neatly over 38mm PVC pipe, which is the diameter of my portable stand mount that I use for field work. I also had to purchase the 2, FT240-61 Ferrite cores for the Unun and choke, ( I got mine from Minikits in Adelaide) and a box for the Unun (Jaycar) at the feed point. So for less than $100 I was ready to go.

The Unun

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The 4:1 Unun…

This turned out to be very straight forward, and is explained in the article. There is quite a bit online about 4:1 Ununs, I’m sure you’ll have no drama constructing your own. I used Jaycar red and white power cable cat. No. WH3057, I stripped off the black outer PVC sheath and lightly twisted the wires in a drill, then wound on the FT240-61 ferrite core to get the result shown in the above pic…

The In-line choke

The article is fairly vague on this describing it as 10- 12 turns of RG-303 wound “W1JR style” A quick Google returned lots of hits for this, and you can see my resultant choke in the pic…I used RG-316, as it was what I had lying around.

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The W1JR choke…

Mounting it

By now, I had gathered all the materials I needed so it was time to put the antenna together and see if it did what it was supposed to do.

I initially mounted the antenna on one of my ALDI bike stands as that’s what I normally use for testing and portable operating but my initial tests showed there was a LOT of interaction with the stands metal base/legs and support and the antenna. Readings on the antenna analyser were nothing like I was expecting and confirmed when I moved the Unun away from the base, the readings began to move towards something more like I was expecting.

So I then decided to shift the whole shebang to my PVC vertical antenna mount in my front yard…

This is simply a length of PVC pipe driven into the ground and about 600mm protruding out to mount antennas on, with a short extension that the squid pole just slips over…simple! The pictures below show how…

I set the whole thing up, with the vertical wire length calculated to 8.010 metres in length and the matching unit lying on the ground with the 4 radials connected. This showed the 50 ohm point to be around 13.8Mhz, so I shortened the antenna to raise the 50 ohm point to 14.075 MHz. I had a variable capacitor in between the unun and vertical, so I adjusted this capacitor to bring the X=12 reading on the analyser to zero. This measured at 7pf, close to the 10pf mentioned in the article…

Gotta be happy with that!

 

 

 

 

 

 

The squid pole ended up being about 7.5 m in length, so the vertical wire and match box at about 7.8m hung just nicely just above the ground on the mount. You can see the match box with its coax capacitor, how the choke ( to keep RF of the coax outer) connects to the match box and where the 4 radials attach…

I only had a brief opportunity to have a listen with the FT817 on the antenna, and the receive was very lively, especially on the frequency of interest. SWR was flat, with the radio developing full power on TX.

I’ll update this article when I’ve had a chance to evaluate the antenna on the air…

Andy, VK5LA

 

Reference: The 3/8-Wavelength Vertical – A Hidden Gem. Joe Reisert, W1JR P44 QST April 2019.

 

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