In a nutshell: The link dipole is a dipole cut for the highest frequency desired with extensions added on for the subsequently lower frequency bands. These add sections are separated from the other sections with short insulators which are then either "jumped" or left isolated by using alligator clips. A little trimming of the length of each segment may be required to bring it to the desired resonance point within each band. Keep in mind that each segment closer to the center insulator impacts the overall length so there may be some back and forth required to get it exactly where you want it. It's two main advantages over other options such as a trapped or fan dipole are better bandwidth on each band and each antenna is a full half wave antenna, not impacted by either traps or other radiating elements. It's main disadvantage is that to change bands requires lowering the antenna and physically clipping or unclipping the alligator clips as needed.
2018-09-27 As intended, I strung up the antenna this morning and tested the SWR on it. The results are in the shot of the x5105 SWR screen below for all three bands. I also verified them on my Icom IC7000 by checking the SWR at the extreme end of each band. By any measure they are impressive readings.
The pictures below show the details of the center insulator, the antenna segment links and the end termination. This is the setup used to take the SWR measurements above.
Conclusion? I think I'm finished fiddling with it. It seems like a very good three band portable antenna. It's advantages are good SWR bandwidth on each band and it provides a full length dipole for each band. Its one disadvantage is that it must be lowered to change bands. I found that much easier using the Shakespeare TSP20 Six-Piece Wonderpole than the 5.4 meter Bamboo Pattern 11 Section Telescoping Fishing Rod, though it only takes a minute or two with either one. Of course it could also be hung from a tree or other support as well, either as a dipole or an inverted vee.
2018-09-26 In case you hadn't noticed, I can't stop fiddling with my antennas! I did a bit more trimming of the 20 and 30 meter segments today to move the resonant frequency just a bit higher. Then I decided to replace the very fragile #24 RCA speaker wire with SotaBeams light weight antenna wire. I'll test it out tomorrow to make sure the lengths are correct with this new wire.
As the photos below show I have modified SotaBeams center insulator considerably. First, I drilled four small holes which in pairs form a strain relief system for the antenna wires. Second, I used screws, washers and nuts to connect the antenna to the coax. Third, I added a bracket I fabricated from scrap aluminum so I could mount a female BNC connector.
I checked the SWR after making those mods and nothing changed. It looks a bit less scrappy than my previous version which had the BNC connector dangling on a coax tale. it also hangs better on my fiberglass mast. And it is easy to attach and detach the 25 ft. coax feedline.
2018-09-25 10:45am Just checked into ECARS on 7.255mhz (SSB) using my X5105 and the just-erected link dipole. He was near Pittsburgh, PA, about 450 miles south east and running 900 watts. He was a S6 in here and gave me an S7! We both were amazed! So, I think I can say the antenna works!
The SWR was flickering around 2:1 but the radio didn't seem to care. I may trim all three elements a bit as they all tend to be toward the low end. I do need to play with the installation to see how that impacts it. Currently one end is about three feet above ground and the other is only about a foot off the ground. The center is around 17 feet.
2018-09-25 It's about 9:00am and I just finished hanging and testing my Linked Dipole Kit from Sotabeams, in spite of the hoards of mosquitoes! SWR results are shown in the photos below. I'm sure it could be tweaked a bit but it more than covers all three bands so I'm not messing with it further, especially with the mosquitoes like they are today!
Initial observations? Great antenna! So simple it is almost embarrassing. On my Shakespeare TSP20 Six-Piece Wonderpole it is really easy to put up. But more importantly, it is quick and easy to change bands. Each time I only had to lower the pole by one or two sections to reach the clips and add or remove a segment. But the broad bandwidth, very low SWR and hopefully full-sized effectiveness make it well worth the small trouble of lowering and clipping/unclipping. It is as easy to erect as my 40/20 trap dipole and much easier than the 40/20 fan dipole which has two separate elements that must be staked out. I'll use it for a few days and then may have additional observations. But for now, it's a winner!
Below are some photos of the installation and antenna details. I like shock cord and cord locks. To tie the center insulator off to the mast I put a small grommet on the mast near the top and then using a cord lock made a tight loop in the shock cord that will not slip down over the grommet. It makes it simple and quick to hang and the shock cord provides some flexibility.
2018-09-24 A week or so ago I ordered a Linked Dipole Kit from Sotabeams. The kit contains pretty much everything you need except the wire and I have lots of that. So I built it using the small #24 RCA speaker wire I had on hand from other antennas I've been playing with. I began by winding the choke using some RG174 I have had for along time. It was pretty time consuming as it was really stiff and slick and hard to hold in place. Then came attaching a BNC connector. What I had was a crimp on one but it became obvious quickly that I don't have the tools to use it. But I did manage to solder it onto the coax and it appears to work.
I began with the 20 meter element. A link in the documentation for the kit provides a calculator so I went with its recommended lengths for the legs. Stringing it outside, SWR measurements verified that was pretty close. Tonight I cut and added the 30 meter and 40 meter extensions. I'll put it up tomorrow and test it out.
The concept of the linked dipole is really simple. It is essentially a dipole cut for the lowest frequency. That then is subsequently cut at appropriate lengths for each higher frequency band. A spacer is then inserted at each cut and alligator clips are used to add or remove each section so the remaining length is correct for the band chosen. The result is a resonant dipole for each band. The negative is that the band change is manual, requiring lowering the antenna enough to reach the alligator clips to connect or disconnect. But the positive is that you have a full length dipole for each band so efficiency is high.