The 20-17-15 meter fan-dipole project

For over a year, I’ve been using commercially bought antennas, one of which is a 160m-10m multi-band Alpha-Delta DX LC Plus. During that period of time, I had the chance to study this multi-band antenna and began figuring how it is built, how they work and how I can make better ones, at least, I can try…

One important thing you have to know, I love multi-band antennas! I enjoy working most HF amateur bands and have little room to do it in. My real-estate barely allows me for a 80m dipole in the far end of my back yard and a smaller dipole in the middle of it, perpendicular to the 80m dipole, therefor, might as well make them multi-band dipoles to maximize the bands I can play in.

This project idea began soon after my full size G5RV antenna broke at one end following a severe storm. That G5RV was a “Radiowavz” and the end-isolator they are shipping it with is just awful. It’s designed to break the wire over time. Rather than fixing the G5RV, I decided to take advantage of the nice weather to build my own multi-band, multi-dipole antennas. In other words, a fan-dipoles. I looked at several designs; caged, wide angles, parallel and trapped. The parallel fan-dipole seems to fit my requirements the best.

Having explored most amateur HF bands last winter, some bands became favourites, therefor, I would build a small antenna in the DX bands I enjoy the most; 20, 17 and 15 meters. I originally thought of building a 40,20,17 meters fan-dipole and use 15m on the 3rd harmonic of the 40m dipole, but tuning a 40 meter dipole and having it resonate where you want on 15 meter is near impossible. Since I really wanted to have control of where I wanted resonance on 15 meter, I decided to stick to the 20-17-15 meter fan dipole design. I already have a 40m antenna and my next project will also include one, so I didn’t need a second 40 meter antenna.

First thing first, getting the hardware. I bought 1000 feet of PVC insulated 12 gauge solid copper wire (Plenty of left over for other project!), some 48 inches by 1/2 inch pine dowels, and polymer black spray pain for the dowels. Instead of cutting the feed point out of plastic boards, I decided to use an Alpha-Delta C kit ($39), which comes with a pair of end-insulators, has built-in static discharge protection and is very rugged. This antenna doesn’t need a Balum. It’s feed with a 50 ohms LMR-400 and simply requires that you ground the coax shield.

I cut about a dozen dowels at 14″, drilled 3 holes along the dowel, spaced 6 inches apart. This will allow me to thread the 12 gauge wire and the dowels will lock the wires in place. I used the formula 468/frequency to get the “total” length of the dipole, then divided the result by two to find each leg’s length, then added 1 extra foot per sides to allow for tuning.

Keeping track of how long the wires you trim on each sides is important. Each legs should be as close to exact length as possible. Also, I could not have done, or agreed to undertake this project without a good antenna tuner. The AA54 served me well indeed!

I build the antenna at shoulder hight to make assembly easy, then tuned the antenna to the frequency I wanted. Changing the antenna’s position, arc angle and elevation would change certainly change all of my measurements, but that’s part of the game. I knew upfront I would have to bring down the antenna a few times to make final adjustments.

One thing I discovered is that if you tune a fan-dipole at ground level, the frequency at which you’ve tuned your antenna will go down on low frequencies (160-80-40 meter) once you elevate the antenna, and at the opposite end of things, higher frequencies (20-17-15-10 meter) will tend to go up. So, plan accordingly. I knew SWR would change once I had this antenna up in its final resting place, and I left plenty of wire to tune it later on. It’s not always fun to have to add wires to your dipole, but not all that difficult.

The first time I elevated this fan-dipole at its 38 feet resting place, I had between 90 Hz and 290 Hz offset on all 3 bands. Not bad! The transceiver’s automatic antenna took care of this little discrepancy for my initial tests. This fan-dipole was strung east to west, my sweet spot lobes pointing north-south. I noticed excellent RX and received great signal reports from stations located north (Russia) and south (Brazil) of my QTH. Experiment until you find the sweet spot. I discovered that the less tension I put on the ropes, the more omnidirectional the antenna becomes. Just like an inverted inverted-v. However, when I first built this antenna, we were in the middle of a severe drought spell. I thought I had tuned the antenna perfectly but when the rainy season came along , and since back yard soil is clayey, the resonance point of the antenna dropped dramatically.

Come Autumn, I had to re-tune this antenna. Shortening each elements did the trick and the AA54 antenna analyzer really came handy once again. Don’t event think about beginning multi-band dipole projects without having a good antenna analyzer. The AA54 RigExpert is a sweet tool to have around.

Once tuned to my requirements, I did a SWR analysis of the antenna.
Here are the SWR graphs: First, between 0 MHz and 54 MHz:

20 Meter: below 1:3 SWR on the whole band.

17 Meter: below 1:2 SWR on the whole band.

15 Meter: below 1:3 on the whole band.

It took a few attempts and some math to land the resonance exactly where I wanted it but the reward was worth the efforts. Between 1:1 and 1:3 SWR on all three band isn’t bad at all. This antenna is therefor between 99.3% and 99.8% efficient! This really helps eliminate RF backdraft. I also notice significant RX improvements on all three bands, when compared with the Alpha Delta DX LB Plus 10-160m. Signals improvements are between +3 and +9 dB, in certain circumstances, can reach over +20dB when RX signals are hitting the sweet spot.

There is a bonus: My 3 band fan-dipole also tunes on 12, 10 and 6 meter bands, but I will admit, its about as efficiently as a G5RV outside the bands it was designed for. While building this antenna, I knew I would be able to tune it on 6m and be able to hit a few local 6m repeaters with 5-10 watts, but that’s where my expectation ends. Just wanted to build the best 20-17-15 I could build with what I had.

Project’s hardware cost is around $90, but I cheated a bit by buying parts to simplify the assembly. I can easily imagine someone using scraps to build a similar antennas with equally good results. What I have learned from this project, I will apply to a new antenna project which will replace the Alpha-Delta DX LB Plus, and cover 80-40-20-10 meter bands. I have already received some comments and critics about this 3 band fan-dipole antenna design, but frankly with the results I’ve enjoyed so far, I wouldn’t change a single thing about it.

Comparing both antennas on 20 meter was almost a draw. The Alpha-Delta has a good 20m element and gave no significant advantages on most signals, except on long path north-south DX, which my fan dipole noticeably outperformed the Alpha-Delta 9 times out of 10.

This project was so much fun that I had to do another one. A few weeks later, I decided to build a 4 band fan-dipole and have it replace the Alpha-Delta. You can read about this other project on this website.

Cheers & 73.

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