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Maco v58 and IMAX 2k
- Thread starter Bearcat
- Start date
You did have that in the quote, apologies.
Fair enough.
A lot of people see just the vertical element of an antenna as the only part that is important, that is not true. A 5/8 wavelength ground plane antenna is more than just that vertical element. The radials are of a length that attract most of the currents on that part of the antenna, effectively containing the currents on that part of the antenna system. The antenna then acts very similar to a dipole, more specifically an off center fed dipole. You have essentially a 5/8 wavelength electrically on one side of the feed point and a 1/4 wavelength section on the other side of the feed point, essentially giving you 7/8 wavelengths electrically. When it comes to the antennas impedance, and the matching requited to tune said antenna, the radial system is very stable, keeping the antenna close to the same tune no matter where it ends up.
Now, to look at the same antenna without radials, you still have the 5/8 wavelength part of the antenna, that has not changed. However, the other side of the feed point, where in the prior example you had radials controlling most of the antenna currents, you now have some length of mast and some length of coax. Both of these will have an effect on the antennas impedance (and matching system), and the effect will vary by length. You no longer have a stable system like you do with the radial system attached. Think about it this way, you change an antennas tune when you change its length, on an antenna with no ground plane installed changing the coax and mast lengths will effectively change the tune of the antenna just like changing the length of the antenna itself. There is also no guarantee that tuning the antenna on a 6 foot pole with no radials will give you the same impedance (matching) as when the antenna has radials installed. You can try, but if your antenna ends up with close to the same tune after adding the radials it is little more than luck.
Just because someone was able to tune this antenna without radials does not mean it would have tuned the same with radials attached. It is entirely possible that they just happened to get lucky, and the coax/mast was of a favorable length and the antenna cooperated. It is also possible that there could be something else in play that we don't see. It is definitely not something we see very often, although this antenna above a yagi antenna is far more likely. I would call them both compromise installations at best.
If you can't have the radials attached while it is on the 6 foot pole, then even though it will be far more of a hassle, get the antenna mounted where it is going to be and tune it there. This antenna isn't like the Imax, it has a very narrow natural bandwidth, and slighter changes than you likely realize can affect it.
The DB
What about the matching network, doesn't it add the missing 1/8 wave to complete a full 360°?
Contrary to what many people will tell you, a matching network, does not actually work that way, although there is a familiar form of matching that does...
You can add a load to the antenna, just like a loading coil used to shorten a 1/4 wavelength mobile antenna. That will electrically do that you are talking about, essentially make the one side electrically 3/4 wavelengths long, combine that with a 1/4 wavelength radial section and you can tune it for a reasonable SWR. However, based on my modeling experiences, you need to keep it in the physical bottom 1/8 wavelength of the antenna, and for best results the load needs to be right at the base of the antenna. This has to do with the fact that part of the 5/8 wavelength antenna is out of phase with the rest of the antenna, if you put said load in the in phase portion of the antenna, you shorten that part of the antenna and increase the length the out of phase portion of the antenna, which on a 5/8 wavelength antenna will be detrimental to the radiated signal.
Think of this type of matching as modifying the antenna itself to achieve resonance. This is creating something that I call "self resonance" because the antenna itself if resonant, and is what most people think of when they think resonance.
In the case above, the antenna itself is considered resonant.
When it comes to matching networks, they work a bit differently. Instead of modifying the antenna, they modify the input signal instead. They change it to be equal and opposite of feed point reactance, effectively creating a balance between the two. This is what I call an "induced resonance", because you are inducing a resonant situation as opposed to finding that self resonant spot as mentioned above. It is essentially attacking the problem from the other direction. These matching networks are also created in such a way as to modify the R portion of the signal to be more desirable...
In this case, the antenna itself is not resonant, but the antenna system is.
The end effects between the two methods are the same, only one tends to work better with shortened 1/4 wavelength antennas, and the other works better with the longer antennas. The effective use of one method or the other is really more situational than anything...
The DB
You can add a load to the antenna, just like a loading coil used to shorten a 1/4 wavelength mobile antenna. That will electrically do that you are talking about, essentially make the one side electrically 3/4 wavelengths long, combine that with a 1/4 wavelength radial section and you can tune it for a reasonable SWR. However, based on my modeling experiences, you need to keep it in the physical bottom 1/8 wavelength of the antenna, and for best results the load needs to be right at the base of the antenna. This has to do with the fact that part of the 5/8 wavelength antenna is out of phase with the rest of the antenna, if you put said load in the in phase portion of the antenna, you shorten that part of the antenna and increase the length the out of phase portion of the antenna, which on a 5/8 wavelength antenna will be detrimental to the radiated signal.
Think of this type of matching as modifying the antenna itself to achieve resonance. This is creating something that I call "self resonance" because the antenna itself if resonant, and is what most people think of when they think resonance.
In the case above, the antenna itself is considered resonant.
When it comes to matching networks, they work a bit differently. Instead of modifying the antenna, they modify the input signal instead. They change it to be equal and opposite of feed point reactance, effectively creating a balance between the two. This is what I call an "induced resonance", because you are inducing a resonant situation as opposed to finding that self resonant spot as mentioned above. It is essentially attacking the problem from the other direction. These matching networks are also created in such a way as to modify the R portion of the signal to be more desirable...
In this case, the antenna itself is not resonant, but the antenna system is.
The end effects between the two methods are the same, only one tends to work better with shortened 1/4 wavelength antennas, and the other works better with the longer antennas. The effective use of one method or the other is really more situational than anything...
The DB
Bearcat, until I got my Sirio GainMaster, a fiberglass antenna, I would have agreed with you...it was the FG that made the A99/Imax seem to have more noise...at least for me when conditions were quiet...I could make that distinction.
I also think we have heard plenty of owners tell us their GM antenna was very quiet in operations.
So, in light of this real world experience, how do we explain one FG antenna being the cause of noise, like the A99/Imax, and the another FG antenna, my GM being among the quiets CB antennas I have ever owned?
Consider that the other differences between these antennas might help us explain this issue. IMO, the FG probably has little to do with the noise we acknowledge for the Solarcon antennas, and for sure what we hear all the time is true, they can be noise.
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DB can you describe these matching networks without getting too technical? Are you referring to a coil or a gamma?
Most of what you said above seemed technical and went over my heard, but I have pondered the issue regarding CMC on the Imax for sure, and using modeling it looks pretty clear to me that Solarcon uses the CMC on the feed line and/or mast to enhance the performance of their antenna. This assumes the primary CM currents on the mast are 1/2 wave and in-phase with the antenna above. I suppose these current can vary in unknown ways and the models below do not cover different heights and the resulting differences in feel line length.
The models are described as ISO, being the mast is physicaly isolated from the antenna. The other models described without ISO are not isolated, the mast connects to the antenna.
I compared the Imax with 4 x 72" inch horizontal radials vs. my model of a Maco V58 with both models isolated and the other two not isolated. They are all at 36' feet to the feed point over Eznec's idea for Average Earth. I also tested these models in Free Space and ran an Average Gain Test...with both FS models showing better than .999 = 0.10 db before I made the real Earth model.
I also added an overlay of all 4 models for easy comparison of these pattern, gain, and current effects. I added the Antenna View for each model to give some idea of what Eznec predicts the currents on the mast might look like for the 1/2 wave radiator above as noted for 5/8 wave type antennas.
I note the isolation of these currents does not manifest into added gain that is otherwise wasted in the lower 1/8 wave of the 5/8 wave design, and it does not look to increase the gain if we prevent the currents somehow. In one model, where the CMC on the mast is noted to be in-phase we see more gain. In another model that shows the CMC out of phase, we see eliminating the currents improves the gain a little bit. So, CMC can be constructive or destructive...they are not always categorically bad.
Carefully take note of the red lines for currents and when the lines are on the same side of the antenna...they are said to be in-phase.
IMO, Solarcon is probably right when they claim the RF on the mast of an antenna can be beneficial if the primary currents are in-phase with the 1/2 wave radiator above...stating these mast currents can add to radiation showing increased gain. Other prominent antenna gurus will sometimes say we can get lucky with these currents and IMO that suggest a good thing. I guess the big question should be...how do we ever know if CMC are good for our system or are they hurting our performance. Maybe this helps us explain a little bit why we hear all these different reports from good folks installing these CB antennas.
As a practical matter however, I have never been able to detect such a difference, added gain just operating my radio. Maybe the differences are just too insignificant. That said however, my models tend to show differences to a small degree, and within reason...this may be all there is to these discussions on CMC.
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Technical? I though I took the technical out of it in the post you quoted from... Lets try it this way...
Loading, such as the coil you mentioned, will change where on an antenna the resonant point physically appears. This is how a mobile CB antenna can be five feet long and still be resonant. The signal from the feed line is not modified, this is an important point. This maintains a "self resonant" length.
Matching networks do not change where the resonant point on an antenna physically appears, instead, they take the signal from the feed line and modify it to match the feed point impedance of the antenna, in essence creating resonance instead of finding the resonant point. A gamma match is one example of this, as is the ring match on the Maco vertical antennas, and the trombone match on the I-10k and Shockwave antennas. This is not maintaining a "self resonant" length on the antenna, but is "inducing resonance" by changing the input signal to force a balance at the feed point of the antenna.
The difference is in what is modified. One of these methods modifies the antenna itself, and the other modifies the signal going to the antenna. I don't think I can get any simpler than this.
And why are we on CMC's here? How is that relevant to what you quoted, or really anything in the post you quoted from? I'm confused...
The DB
Bearcat, or whomever it was told us why he had the idea for his no radial question. Did you miss that?
DB, here are a couple of posts where you raised the issue on CMC, but I didn't question you as to why you said what you said. We both know that whatever Bearcat or anybody else does per this idea...it is likely we will have some CMC issues at the very least. And you would probably be the first to tell anybody...that radials help prevent this problem, and how bad it is if we don't prevent CMC from working. I think you also add that when we stop CMC on the feed line that those currents are then made available to radiate in the real antenna, not the mast...producing added gain into the far field at a distance there-to-fore unattainable with such a setup that does nothing about CMC.
With my models, I was trying to show the guys how these two antennas might perform, and how they compared. In doing that, I also thought it important to mention the currents on the mast, and their effects on the patterns, gain, phase, and sometimes even on the angle of maximum radiation.
Sorry, I should have asked your permission first I suppose. When you get over your upset with me...maybe you could tell us if these models I posted are in error, and if that is why you object to my approach. If you can do this...I would be thankful...for I've learned something new in the process.
This said I will go back and check my words and work to see if I made any gross errors. If I did...I will post a correction, OK?
If any of you guys find the radiators are too long or too short...then let me know.
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In the post you quoted I was simply answering a question posted by Needle Bender. I didn't go back and read prior posts in this thread, posts that were posted several months ago. CMC's had nothing to do with the post you quoted. That is why I was confused.
Give me some time, I'll come up with something for you on CMC's if that is really what you want to talk about...
The DB
Do you know what experience Bearcat based his question or curiosity on?
Yea DB, that was sorta what happened to me too. This is why I just go with the flow sometimes and don't try to finger-out' how all these conversations dovetail to make any real sense to anybody in particular.
Do you find no merit in the idea I posted here? It is alright if you disagree.
Take your time...the only way I learn anything new is considering new ideas. My old ideas just don't always cut it.
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I just the IMAX. It will replace the A99. I will put it up on my "super mobile base mountain top antenna support system." I drive to the third highest mountain in Orange County CA and put it atop a 30 foot mast. I will let you know the results as soon as I can get it up (and at my age that aint easy
). Here are some pics.
). Here are some pics.Attachments
Are you familiar with the Hy-gain Penetrator matching system?
Looking at it I'm not sure if it's a modified T-match, Linear Loaded, or both, but it's spot-on a .75 wavelength measurement, when adding together the 283" radiator, 40" center-conductor matching network & 2" or so of unshielded wire & thru-bolt in the bottom insulator connecting between the center of the S0-239 connector & the 40" portion of the 2-piece matching network which then connects to the base of the radiator.