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Tuning a beam by sitting it on its reflector
- Thread starter Zman
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I've done it on numerous occasions. It isn't perfect, but seems to work better for me than having it flatside near the ground. My result is there is much less tendency for the antenna tune to slip up or down the band when raised to height using this method.
Of course, the best method is to tune it at height of use. . .
Others may have a different experience than I've had.
Of course, the best method is to tune it at height of use. . .
Others may have a different experience than I've had.
It's the best way to do it other than at the operating height. The idea is that the reflector acts as a ....well.......a reflector, and so does the ground so at least the RF is still trying to go in the same direction. This method causes minimal detuning and really works best if the antenna is not actually sitting directly on the ground but at least a couple feet above it.
Yep; it works. That is how tuned my 4 element Yagi. Resting the beam on its reflector with the beam pointed into the sky. Then I used my antenna analyzer and adjusted the gamma match to the best that it would work. Then I put it up and re-read the analyzer. The antenna got the same readings as it did when the gamma was being adjusted. Go for it.
This idea for tuning a beam antenna, pointing up, may work if you're just tuning the SWR and match to make the radio happy, but there might be a lot more to tuning a beam to get the element phasing working correctly than just considering and measuring the match. I think however a properly phased antenna may also produce a very good match, and maybe even a better match and resonance as a result of being properly phased. I wish I knew an easy way to accurately do that, so the following is all hypothetical and is intended for consideration only.
I use to have a sense of this happening while setting up several of my beams, and building a homemade model, but I didn't understand the importance of phasing, and I didn't know how to measure it either at the time. However, what I sensed in the tuning process was that when I got real close the adjustments tended to get real small and I could detect that the beam oscillated with noticeable difference is the directional properties and rejection. Sometimes the beam even appeared to be omni directional is how bad it could get.
Since I started modeling however, I can see that setting a beam to resonance and best SWR is important, but ofter times the phasing is not correct, with the driven element fully in phase all the way across the element, and the reflector being out of phase all the way across the element.
I still don't know how to effectively measure phase in elements and I probably don't have the equipment to do so, but I think I do know how to watch for the effects as I tune and being aware of phasing does seem to be very important to consider. You would really be surprised to know that the effects of correct vs. in correct phasing can be due to very small changes in element length and spacing to be sure. Tuning a beam is really like tuning a guitar, with very fine changes, if the phasing for the elements is expected to be close to correct.
My conclusions:
I don't think there is any way to tune a beam and ever be sure that the tune does not change if you have to move it after tuning. So, IMO the only way to tune a beam is at installed height and location. Just making the SWR good will work, of course, but that is not all that should be considered to be accurate.
If the very small adjustments, regarding phasing I see in modeling where not so small...then I might suggest that getting close is good enough, but that is not the case with the models.
I am trying to model this to a presentable conclusion, and if successful, I might post the results and try to explain the idea about how important I think proper phasing can be.
NB, I just can't explain this idea using one-liner' comments like I promised, but I still hear you.
I use to have a sense of this happening while setting up several of my beams, and building a homemade model, but I didn't understand the importance of phasing, and I didn't know how to measure it either at the time. However, what I sensed in the tuning process was that when I got real close the adjustments tended to get real small and I could detect that the beam oscillated with noticeable difference is the directional properties and rejection. Sometimes the beam even appeared to be omni directional is how bad it could get.
Since I started modeling however, I can see that setting a beam to resonance and best SWR is important, but ofter times the phasing is not correct, with the driven element fully in phase all the way across the element, and the reflector being out of phase all the way across the element.
I still don't know how to effectively measure phase in elements and I probably don't have the equipment to do so, but I think I do know how to watch for the effects as I tune and being aware of phasing does seem to be very important to consider. You would really be surprised to know that the effects of correct vs. in correct phasing can be due to very small changes in element length and spacing to be sure. Tuning a beam is really like tuning a guitar, with very fine changes, if the phasing for the elements is expected to be close to correct.
My conclusions:
I don't think there is any way to tune a beam and ever be sure that the tune does not change if you have to move it after tuning. So, IMO the only way to tune a beam is at installed height and location. Just making the SWR good will work, of course, but that is not all that should be considered to be accurate.
If the very small adjustments, regarding phasing I see in modeling where not so small...then I might suggest that getting close is good enough, but that is not the case with the models.
I am trying to model this to a presentable conclusion, and if successful, I might post the results and try to explain the idea about how important I think proper phasing can be.
NB, I just can't explain this idea using one-liner' comments like I promised, but I still hear you.
If you are dealing with a commercial product and not a home made antenna then the spacing etc is already done and you are simply tuning for the best match. That is probably what 99.9% of the people do as very few people home brew a beam antenna. In any event you are not concerned with "phasing" of the parasitic elements as they are not driven. All you simply do is space them for maximum forward gain or best rear rejection whatever you want. That is simple with a field strength meter. After the gain/FB is peaked then you need to tune the feedpoint for best SWR. That is when you decide what type of matching system you will used based partly on what the unmatched feedpoint impedance is. Standing an antenna on it's reflector will not upset the FB or gain characteristics and allows a fairly accurate way to tune an antenna on the ground. A lot of beams are impossible to tune once in the air and MUST be tuned on the ground. Tribanders are a good example of that. It is impossible to reach the ends of the elements to adjust the lengths of tubing when the ends are 15 or 20 feet from the tower. Spending time mounting and unmounting and remounting the antenna a dozen times to do so is out of the question. On all my ham antennas I have gone with the manufacturers suggested dimensions and have always had thiongs work out. Who cares if the SWR is 1.5 or 1.6:1 at the band edges on 20m?
Nope my antenna is commercially made. I just wanted to know if other people have tuned their beams this way pointed skywards. I know a few locals have done it this way because i have heard them talking about it on the radio. As foir the tribander beam i have a Mosley TA 33 that i set up on the ground. The Mosley beams are cut a dry as they have set points (screw holes)for the CW and SSB portions of the bands. I just set mine for SSB and rolled with it. Yes its high in the CW portions but that's what an antenna tuner is for, LOL.
As for home made antennas i have a 4 element beam for 6 meters that i made from some old diagrams i found in an old ARRL handbook. I just set the elements as per the diagram and went with it... I'm NO TECH or antenna engineer by any means or have any interest in techie stuff.
As for home made antennas i have a 4 element beam for 6 meters that i made from some old diagrams i found in an old ARRL handbook. I just set the elements as per the diagram and went with it... I'm NO TECH or antenna engineer by any means or have any interest in techie stuff.
CK is absolutely right about the adjusting business up high, it is not practical for the reasons he noted even though I noted the idea of tuning at installed height was best.
I went back and checked again, and the practical difference when standing the beam up close to the ground vs. a 36' for example made very little difference in the match as, so he's right on that too. I used a model that came with the Eznec software and it was tuned for resonance only. But, it was not showing the proper phase, and that is what got me off into talking about phase.
CK, this model had no directors, it was a simple two element yagi with reflector and driver elements only. I didn't intend to leave the thought that this idea about phasing was anything but hypothetical.
When I adjusted the model for the best resonance, spacing, and element length I also saw improvement in the match and the reflector showed to be out of phase, which I think it should show, and the driven element then showed to be in phase, also like I would expect. You might agree with this much regarding phase?
I said in the beginning of my earlier remarks this was for consideration only, regarding the importance of phasing. But, in looking back the difference indicated very little difference after all was said and done. So you are correct again.
On second thought guys, I would not overly worry about this idea on phasing, even though a multi-element beam is a phased device.
My original idea was to see what Eznec might show when placing a beam on its reflector and the front end straight up in the air. I did that and the results supports what CK said about the idea of tuning with the beam sitting on or near the ground.
I jumped the gun guys, not to worry about phasing as long as your antenna is responding as you would expect with match, reflection, and whatever you can tell with forward gain...I wouldn't worry about phase as CK notes unless maybe the beam acts more like an omni, and then the lack of proper phasing might be the problem.
Zman, just read CK remarks, that will answer your question. Just ignore what I said about phasing. I did talk about your issue of sitting the beam on its end to tune, and I agreed with CK supporting the idea, but just ignore that part of my post too.
My idea about phasing was noted to be theoretical only, and since you're not interested at all in theory, just ignore such ideas. Aspirin works better than than getting all bent out of shape.
Leave your frustrations to when you get to tuning you beam, if you don't get lucky.
Marconi, I may have misunderstood what you meant by "phasing". I was thinking more along the lines of feeding more than one element in an array and not whether the redirected energy was in or out of phase with the main wave. That's why I said about it being easy to just use a field strength meter and not worry about "the degree of phasing" as the meter will show when you hit the proper setting. As for feedpoint impedance, a high gain multi-element array may have a feedpoint impedance of only 10 or 12 ohms at resonance which must be matched to the 50 ohm feedline.
Not to worry CK, I got over my excitement about phasing antennas. I should have just left well enough alone when I proved to myself that a beam I had at 36' didn't hardly change at all when I sat it down a foot for Earth like you suggested.
Again, when I did that I checked the currents and noticed the antenna was not in phase as a beam should be, and that got me started on another subject which no body gives a hoot about.
So, I'm through with that topic as it wasn't fair to the author of this thread, and Zman was having fits with the idea anyway.
Sometimes it is just better to leave a nice smiley face or two and give loud praises and glory. <can not stand it>
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