First off math is not my strong fort. That’s why I am asking for help. I got a home made dual band J-Pole antenna for 2 meters/440 band. The antenna works. See the drawing and picture bellow. The lowest swr I can get on 440 is around 1.2 to1 @ about 435.00mhz. with my MFJ-269 analyzer. The 440 side of the antenna is fine. It is the 2 meter side that I don’t know how to fix. The lowest 2m swr reading that I can get on my antenna analyzer is around 1.5 to 1. The problem is that 1.5 to 1 reading is around 143.36mhz. Now according the plains the long rod is spose to be 58 inches long. The long rod is 5/8th of an inch to short. Do I need to make the long rod longer or shorter?
or change the amount of the inductor coil windings on the small small rod? I would Appreciate some help please.
I wouldn’t bother. I could live happily with 1.5:1.
Isn’t the issue really related to the maths. Where we make the assumptions on performance based on UHF ham band being exactly 3 times the frequency - which as the J Pole already has it’s components based on multiples of 3, quarter waves just operate as three quarter waves and the match remains similar, apart from increased losses at UHF. I wonder if you could test the thing with frequencies exactly on a multiple of 3, and see if the figures are more constant? I actually suspect they won’t be and the compromise in the design to allow operation on a multiple of the fundamental design (with the tuning components) is all that’s causing the problem. If you compare to a normal J pole for one band, you’ve got an inductor to help the match? rather than a simple connection to the bottom tubes. This might well be helping the UHF side, but probably what messes up the VHF. I’m not certain a J Pole or other variations like the Slim Jim really perform that well as a dual bander - hence the little design changes to make the matching work a bit better.
If you have an SWR of 1.5, how well does it actually perform. If it’s working pretty well as an antenna on both bands - better than something similar like a simple dipole, then stick with it. If it performs poorly, tinkering to lower the VSWR might be wasted effort!
your radio should handle anything less than 3:1 just fine… and a simple truth is that they will not hear the difference on the other end…if you fool with it, you are just doing it for self satisfaction, which is ok, but you will not improve the performance of your radio. 1.5 to one is gravy.
On 2 meters by the time the freq gets up to around 146.52 mhz the swr is above 3 to 1. I am not to picky as long as it gets the job done. But I was plainning on making a few of them and sell them at the Spokane hamfest in Sept. And as we all know there is some ham’s out there that are very picky as to were the lowest center swr reading for the antenna is on the band. By the way the center freq of the antenna on 2 meters (lowest swr wise) is around 143.36 mhz. which is outside the lower end of the 2 meter band. Everything I have ever learned about making antennas that the center freq of the antenna should be in the middle of the band that the antenna is supposed to be operating in.
I agree with the poster who basically said the design is a compromise antenna.
It is put together differently than I am used to messing with. What does the designer say about the coil and adjustments? Have you tried just moving the coil up and down the rod a bit? For the 2M section I would be looking at a long rod in the neighborhood of 57.12 inches and the short rod at 18.96 inches, but I would be looking at different connecting points.
The coil appears to be used as a tuning circuit… so the number of turns and the actual location would both affect that.
As far as the actual effect of the SWR, you are correct. Lots of hams are **** about that…they read too much in the way of advertising and ignore the practical effect. 3:1 Vs 1:1 will not actually be noticed, it will have less than a 1 Db loss factor, and it takes 3 Db to move the S meter 1 number… so overall effect is less than 1/3 of an S unit… can you really hear that?
It’s not really a slim jim anyway - the inductive coupling is not part of the slim jim design, so it’s just using the elements and the dual band capability seems to simply be because the UHF band is also resonant with those element lengths. What the performance is, I have no idea.
I wonder if a ? vertical on a ground plane would actually out perform it, as the lossy coupling would not be needed. It’s also a **** great thing to stick a mobile label on isn’t it?
I tried moving the inductive coupling up and down on the short rod. There wasn’t much of a change on ether band. I am also thinking of cutting the aluminum bottom in half high wise. After reading what people have posted about this antenna. I think I will build the antenna’s the way there are. One other thing I need to know is what is this antenna worth (fair asking price)
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the value of the antenna is a hard question. I know you are not going to like part of my answer but, until you have experimented with the antenna… know more about how it works, what tunes it effectively, etc…until you are able to answer questions the perspective customer might ask…you should not sell them. It is one thing to tinker with antennas and try your luck for your personal use and satisfaction without really knowing and understanding what you are doing… it it quite another thing to make them and sell them under those circumstances. Simply put, with the questions you are asking you are not qualified to commercially produce antennas. Just my opinion… do what ever your personal ethics allow you to do.
I got the compromise better on the antenna between 2m and 440. By changing the coil spacing (making the coil longer) on the inductive coupling. I brought the 2m center dip (lowest swr) up to about 146.52mhz. More or less the center of the band. But the 440 side of the coin took a bit of a hit. 440 went up from center dip (around 435mhz) at 1.3 to 1 to about 2.7 to 1. Around Tri-cities (Kennewick, Pasco, Richland) 440 is all but dead anyways. There is a few 440 ham repeaters up and in the repeater directory but they aren’t open for public use. All activity (on vhf/uhf ham wise) around here is on 2 meters.