Hi guys. I would like to hear opinions regarding this. I’ve read that antennas like IMAX and A99 use coax as a counterpoise. That can make interference issues. That can be fixed by placing a choke at the feed point. That’s in case that this antenna is alone in a tower or mast.
Many years ago I built a 4 element 11 meter yagi from some parts I had from old beam antennas I took down in the neighborhood. Building the beam process took a long time because it was my first time building a beam. I finally got it to work well, as best I could tell. For years I used a couple of Stardusters up 50' as my main working antenna. They worked great too. This was in the beginning of the hay-days of CB.
At that time I had no concept in mind for Common Mode Currents (CMC) or a counterpoise.
At some point I got my first A99 from a buddy that said it didn't work. He mentioned about getting water in the antenna when he took it down after about 3 years. When I got it home the water had drained out and the antenna worked fine. Today it still works fine, but over this time period I only used it for comparison testing.
I once mounted it about 1' directly above my beam and I never saw any effects on it or the beam and they both worked fine. Later I put my SD'r above my beam on a 10' x 1' fence rail above my beam. The bottom of the SD'r radials were about even with the boom, right in the middle of its elements and close.
Again, I saw very little difference as best I could tell. I still didn't know anything about CMC or a counterpoise. That is some of my history on the topic up until years later when I took-up modeling using Eznec 5.
I’ve also read that a yagi antenna can be beneficial to it because the yagi will act as a countepoise.
I've also read lots of similar stuff about these topics. However. I was never fully convinced about these vague concepts, that I could not be see or measured.
This is why I took up modeling...thinking maybe I could begin to see things more convincing and in more depth, and thus gain some understanding.
Here is the questions: Should I install a line isolator at the feedpoint ? The line isolator will prevent the IMAX to use the yagi as a counterpoise?
Have you tested and proved to yourself that the isolator worked as intended when used on the Imax and mounted alone without the beam close by?
I think your Mosley TA-33 is multi-banded. If you use it on all of its design frequencies, then I would be more concerned with how the presence of the Imax effected the beam isolator or not.
Consider that the Imax is vertical and the beam is horizontal...and this idea is suppose to mean there will likey be a notable polarity difference...like 20+ db difference, suggesting to me very little effects between the two.
Will the use of a line isolator defeat the purpose of using the yagi ( Mosley TA-33 WARC ) as a counterpoise for the IMAX?
i would like to hear from MarconI about this very technical questions. I know he is has vast experience modeling antennas.
Alexis, thanks for the nice comment. Your question here is the $24,000 dollars question.
With my modeling I can only
simulate your isolaror device. What I do is use an Eznec feature at the location where I think you would place the isolator. This feature provides me a way to add a simulated capacitor between to wires,
the mast and the antenna. A capacitor will not allow DC currents to pass, and thus the model shows the mast is physically isolated from the boom. More advance skills in modeling could possibly create a physical choke of sorts...that would work as intended. I wish I knew how to do that.
How do you know if your isolator is working as intended?
By the way, I forgot, in my former QTH, I had the IMAX choked with a line isolator. Both antennas worked fine. Distance between antennas was around 3 feet. SWR fine on the IMAX as if it were alone, as well as in the yagi.
Have you compared your setup with and without the isolator, where you were able to know for sure you were seeing positive results or NOT? I see many sites on the internet with ads and designs for such devices and I see a world of differences too. I also read some warning that if a choke (isolator) is not right then the problem of CMC can be made worse.
Who are we to believe?
For short, what I would like to know if I can improve performance on the IMAX with the yagi as a counterpoise, the use of a line isolator would prevent that from happening or not.
This is what I read on other forum:
That is a good question. I don't know if I have an Imax model that might show improved performance when using an effectively working isolator, but I have a 1/2 wave dipole model that shows a feed line that is transparent (no CMC effects) where the model can use a feed line of any length with little ill-effects on match, gain or angle.
This is what I describe as a transparent feed line. See the link below.
“ Essentially, a choke balun is designed to "divorce" your antenna from the feed line. If your feed line is coaxial cable then you don't want it to be part of your antenna. You want to be able to deliver all your power to the radiator itself, i.e. "the antenna". A choke balun does this admirably.”
Is that bad news for antennas like A99 and
IMAX’s ?
I agree with your conclusions here. My concern is how do we know if everything is working as intended.
Sorry to be so long, but maybe it will give you some ideas.
BTW, if you want to see my recent modeling demonstration of how I see CMC can ill-effect antenna performance, here is a link with models. I have more detail, but thus far I think this work is a bit complicated for most to understand, so I didn't add the extra exhibits that show models with different feed line lengths.
Correct Place to Test SWR?
I hope I made sense here. I might find some errors later and I might have to do some editing, but I'll try to make it clear when there are edits.
Sorry I was late in responding Alexis.
Good luck. I encourage you to continue your ideas, questions, and experimentations.
