20 Meter Moxon experiment and questions about feedline interaction
I think some background on my project may be necessary to understand my question. I have been experimenting with Moxon type antenna designs. My first attempt with building a basic Moxon for the 6 meter, and this initial test worked great. I used one of the many online calculators to calculate the wire antenna element lengths. Used some fiberglass rods as spreaders to hold the wire elements in the correct shape. to feed the antenna I used an LDG RBA-1:1 Current Balun. I wired the driven element of the antenna directly to the binding posts on the Balun, and then attached coax and ran that directly to the radio. I put the antenna up on a 25 foot mast, and everything worked great. No trimming of the elements was necessary. The measurements I took with a VNA show a good SWR well below 2.0. The reactance plot across the 6 meter band matched what I expected to see from the documentation I had been reading about the Moxon designs and the computer modeling I did on my own. Using the antenna on the air showed good obvious directionality. Rotating the antenna easily showed a change in signal strength on received stations.
Next step was to scale this up to the 20 meter band. I opted to work on a vertically polarized Moxon as apposed to the usual horizontal arrangement that most people do. I built a PVC Spreader bar to go across the top of my 25 foot mast to span the almost 10 foot width of the 20 Meter Moxon. I attached the Driven and reflector elements across the top of the spreader bar and let them hang down towards the ground. This puts the feed point on one side of the antenna half way up and about 5 feet way from the center mast support. I had some comer insulators on the bottom corners of the antenna near the ground, and just used some elastic cord to pull the wire elements straight and staked the elastic cord down to the ground.
Since the feed point is out about 5 feet from the mast just hanging in the air I thought I would try to use a short, very light weight piece of typical 450 Ohm ladder line to go from the feed point over to the mast. At this point I attached the same LDG 1:1 current Balun to the mast. Hooked up the 5 foot length of ladder line to the binding posts and then ran about 50 feet of coax down the mast and over to the radio. This is now when I discovered things were really not working correctly. When measuring the antenna at the end of the length of coax with the VNA, I was seeing impedance plots across the 20 meter band that did not resemble anything close to what I expected. 50 feet of coax can really transform things on you. So I got out the ladder and removed the coax and directly connected to VNA to the ladder line with some short test clip leads. The impedance plots looked better, but not close to what the computer modeling showed. The other big problem was the antenna had a measured SWR some where between 20-16:1 across the 20 meter band. There was a dip down to 1.8:1 which was 1.5 Mhz lower, so I decided to start trimming the antenna smaller. From this point I spent a number of evening taking down the antenna. Using the online calculator to recalculate antenna element lengths for higher frequencies, cutting the wire lengths, and putting the antenna back up outside. I was able to get the antenna to give about a 2:1 match at 14.1 Mhz which at least the built-in tuner in the IC 7300 could deal with. I thought I would test things out by setting up the laptop to monitor FT8 signals and then watch how things were showing up on the PSK Reporter web site. After a few evenings of playing with things It became obvious that although I was receiving a lot of stations very well, the antenna did not have much directivity like it should. It did have some directivity, but it was in the opposite direction than it should be.
After about a day of pondering the issue it occurred to me that the short length of ladder line I was using, instead of acting like just a feedline, was really probably just extending the length of my driven element and making it now longer than the parasitic reflector element. Which means I have some sort of driven element antenna with a single director element. The two elements would also not be in any sort of optimized arrangement. This would explain why the antenna did not work with the initial online calculator values, and had very little directivity and what little was there was in the opposite direction than it should have been. So I got the VNA out again, went up on the ladder cut the ladder line completely out of the system and hooked up the VNA directly to the driven element at the feed point. As I am sure you can guess by this point everything looked great on the VNA, everything looked as I expected for the impedance curves for a typical Moxon antenna. I had a great 50 Ohm match, it was just now tuned for 15.3 Mhz from all my cutting and trimming of wire elements.
So now after all of this I now know why the use of that short piece of ladder line was really a big problem. My ultimate question at his point is why? From my experience with the first 6 meter antenna that I built, and all the reading about other people working with the Moxon designs, If I connect 50 Ohm coax directly to the antenna driven element, and run that directly to the radio everything should work just fine. Does this really mean that feeding a Moxon antenna with any length of ladder line would be a huge mistake? Would a 50 Foot length of Ladder Line not given me the same issue? Again if so, Why? Why would it be that the Coax feedline does not alter the affective antenna element lengths and seriously detune the antenna system? Does this really all depend on the characteristic impedance of the feedline matching the antenna feed point impedance? And if so why? I am perplexed at this point and would really like to understand this problem better. But that is one of the reasons some of love this hobby, right.
Any thoughts or explanations would be greatly appreciated.
I got some more time to work on this project this last weekend. I got the wire elements cut shorter to match what the online calculators showed for the 17 Meter band. I then connected a 50 foot piece of RG-213 directly to the feed point with out a Balun. I hooked up the VNA, it could see the antenna working as expected. I had close to a 50 Ohm match that had a little capacitive reactance. I found that depending on how I routed the coax feedline away from the antenna that I could get an SWR between 1.3 - 1.8. I also tried connecting the VNA directly to the antenna feed point to see how things compared. I got pretty much the same results. I am guessing the difference had more to do with the physical proximity of my body and the ladder I was standing on, than the removal of the 50 feet of coax from the system.
I had about an hour to work FT8 with this Antenna Sunday Evening. I had the antenna pointed east and as I mapped all of the stations I received I was receiving almost completely stations that were located east of Colorado. I did have a fairly wide pickup patter that covered most of eastern Canada down through coverage over all of South America. I was not receiving much of anything from Europe. I spent a little time watching the other U.S. stations I was receiving to see if I could find any of them working any European stations. I did not see much on anyone working Europe from the US or South America. PSK reporter did not show to many people uploading reports. I believe that the poor European coverage with this antenna on Sunday, had more to do with propagation and quite possibly at least too me what seems like a lake of European stations using FT8 on 17 Meters.
I hope to get some time soon to cut new wire elements again for 14 Mhz. and get the Moxon working properly on 20 Meters. I will get another update out with details on how that works out when I get that finished. I hope this info helps some of you with any Antenna projects you may working, and avoid some of the mistakes I have made. or just encourage you to get out and start experimenting.