At the beginning of summer 2012, I designed and built a 3 band fan dipole. I was so happy with the results that I decided to try the same building design on a larger 4 bands parallel fan dipole covering 80,40,20 and 10 meter to replace an Alpha-Delta DX LB Plus which was setup as an inverted-V at the far end of my backyard.
I already had a leftover spool of PVC covered solid copper wire from my previous project, hence, went out and bought a dozen half inch by 4 feet round pine dowels, a can of black spray paint and an Alpha-Delta C kit ($39). With the necessary hardware in hand for this project, I began calculating and cutting the wires length for each band. 80m band wires are a bit challenging to deal with because of their overall length which require plenty of manoeuvrability space. The longest wires will also be the support for the whole antenna, therefor the used of 12 gauge copper wire is not an overkill and is recommend for strength and durability.
For this project, I used the usual formula 468 divided by the frequency to get the length of each dipoles, divided the result by two to find each legs, then added some extra length on each sides to allow for tuning and threading.
Keeping track of how long the wires you trim on each sides is very important. Each legs should be as close to exact length as possible to one another. Also, I could not have done, or agreed to undertake this project without a good antenna tuner. The AA54 antenna analyzer served me well indeed!
I built the antenna at ground level to help with the layout of the wires, threading them, make adjustments, then tuned the antenna to the frequency I wanted. Stretching a 140 feet wire to tension is not an easy task when you have limited space. Fortunately, I had just enough space to do so. For this project, I calculated 6 inches of spacing between each dipole elements and I will admit, I based this calculation of pure instinct and a few articles on multi-band fan dipoles. I am not sure how much changing this spacing will affect the performance of each elements, but I didn’t want to take too much chances and decided to leave 6 inches between the wires. I also noticed that the Alpha-Delta DX LB Plus I previously used only left 2 inches between each elements.
Threading all of these wires was not too much trouble. Just make sure you drill holes big enough to allow the wires to slide freely or you will quickly notice how hard it can be to move the dowels around. Dowel spacing does not affect the antenna’s performance. Place them 3-4 feet apart, or just enough to prevent wire sagging. The 12 gauge wire is still enough to allow for several feet between each dowels.
Changing the antenna’s position, arc angle and elevation and especially the inverted-v configuration will obviously change all my tuning and measurements. That’s why I left plenty of spare wires when I first tune it. Cutting a wire too short is not dramatic and can easily be fixed.
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.
The first time I elevated this fan-dipole at its 35 feet resting place, SWR was all bent out of shape, but that was expected. It is VERY important that you begin tuning each elements at the lowest frequency and work your way up.
The 80m band wire was the first elements began tuning. After raising the antenna to it’s final resting place, I used the antenna analyzer to measure the resonance frequency of the 80m wires, did a bit of math to calculate the different between where it resonated and where I wanted it to resonate, figured out the length difference and proceeded with trimming the 80m elements.
As an example, if your antenna is resonating on on 3.5 MHz, (468/3.5= 133.7 feet), then if you want it to resonate on 3.75 MHz, (468/3.75= 124.8 feet). This said, 133.7 feet minus (-) 124.8 feet = 8.9 feet. With that information, I would then consider trimming each elements by 4.45 feet (8.9 divided by 2), but actually trimmed them to about 80% of that length, so approximately 3.75 feet on each sides, then raised the antenna and re-measured. I prefer to remove a few inches at a time then have to add wire to this antenna, especially on the longest wire which holds the entire weight of the antenna.
When trimming, you cut the excess length of wire at both ends, and leave enough to fold fold back and twist the wire around itself for at least 3 turns. Leave enough spare wire to be able to increase the length of the wire just in case your calculation was off a bit and your SWR reading are a bit on the hight side. Increasing the length of your wire will lower its resonant point. Trimming it will raise its resonant point.
Here’s an antenna construction tip, when you remove excess wire from one side of your dipole, use the removed wire to measure the excess wire you will remove from the other side. This will assure very accurate trimming without having to measure the other side with a measuring tape. Once the 80m elements are tuned, you begin the process all over again with the 40m elements and so on for 20m, then 10m. Every time I finished tuning a band, I re-measured all elements for each band and took note of the SWR variations. This helped me re-tune some elements while I had the antenna down. Properly tuning this 4 band inverted-v dipole took the better part of the day.
Also, If you have to raise and lower your antenna several times, you might consider installing pulley systems at the feed point and at both ends. This will make things much easier on the tuning process of your antenna, help it raise it exactly at the same position each time you raise it, and finally, make it easier for any quick maintenance in the future.
To increase the obtuse angle of my inverted-v above the recommended 90 degrees minimum, I had to raise its extremities by using a pair of 18 foot telescopic aluminium poles anchored at both extremities of my yard. Guy wires help counteract the tension on the telescopic poles. This will help keep the wires well above the ground and help increase the obtuse angle of your antenna. The obtuse angle I was able to achieve with this antenna was approximately 115 degrees.
Once I have completed the tuning of this antenna, I reviewed the whole structure for strength. Since I previously had a lighter antenna, I replaced the pulley cable with stronger gauge marine cord, I replaced the coax connect seals, double check the shield grounding connections and winterized all critical points. I do not want to troubleshoot this antenna in the middle of a canadian winter!
After operating this newly built antenna for a few days, I discovered that the static-discharge fuse on the Alpha Delta C kit failed, giving me 10:1 SWR on all bands. If you ever buy and use the Alpha Delta C kit, I recommend removing this fuse as it might give you problems when trying to tune your antenna over 2:1 SWR or if you operate with more than 100 watts of Peak Envelop Power.
This fuse can help protect your rig from static discharge if you live is a high static environment. Since this is not my case, I removed the fuse all together to avoid blown fuse in the middle of winter. It’s an extra protection feature, but Alpha Delta sells all of its commercial antennas with this fuse installed.
Grounding the shield of the coax is very important when operating an Alpha Delta C kit. If you don’t ground the shield, your antenna will not see the ground, loose its counterpoise and you will notice huge amounts of RFI being retransmitted back to your radio-shack. Follow the manufacturer’s recommendation, ground your shield. If you plan on using a balun instead, use a current balun.
The advantages of building a multi-band dipole over using a multi-band antenna such as the Alpha-Delta DX LB Plus, was the performance gain, both in reception and transmission. I immediately noticed sizeable improvement on bands of interest over my previous Alpha Delta antenna and this, in so many ways that it made me immediately regret having bought the Alpha-Delta antenna and not having built my own sooner. My 4 band antenna is really ugly, but it works really well. Also, construction cost (Not accounting for labour) was about half the cost of purchasing the Alpha Delta antenna.
The inverted-v installation of this antenna at the far end of my yard made this antenna surprisingly stealthy to the neighbours or anyone walking around in my yard. I virtually can’t see the antenna since it is camouflaged within the the vegetation.
In conclusion, I built this second multi-band fan dipole because building the first one was too much fun. I think that is the only reason why I undertook this project. For all I know, I could very well end-up giving away my multi-band antennas and decide to build new ones because I already have new ideas for improvements.
My 4 band (80-40-20-10m) antenna analysis using a RigExpert AA54 antenna Analyzer:
Once tuned to my specifications, I did a complete SWR analysis of the antenna.
Here are the SWR graphs: First, between 0 MHz and 54 MHz:
And just for the fun of it, I checked SWR on bands it wasn’t designed to operate on.
160m Meter: Awful SWR but workable with an antenna tuner.
This 4 band antenna is between 98.6% to 99.3% efficient depending on the band used. Project’s hardware cost was around $100, 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.
This new antenna replaced my Alpha-Delta DX LB Plus, and covers 80-40-20-10 meter bands, plus 6m as a bonus. The downside is the lost of 160m which leaves no sad regrets as I spent no more than 15 minutes of 160m over the past year. 80m saw a huge improvements over the Alpha-Delta DX LB Plus, and am now getting signal reports of 20 dB over 9 instead of 57. An important consideration when building a multi-band fan dipole is that each band you consider adding to your design, will slightly decrease performance on all other bands. If that wasn’t true, I’d build a 12 band fan dipole tomorrow!
Cheers and 73′s..
A few other sites I used for inspiration: