Different installations of the Imax antenna at different heights.

[The DB]

For a while I (well me and some others on this forum) knew that there was a capacitor on the radiating element of the Imax antenna, we even knew where it was, but we didn't know its size. 357 came through with its size a while back, and I made an initial model based on that, and its results were surprising in several ways. The capacitor actually caused a beneficial change in the currents on the raiding element of the antenna. It also caused several other interesting effects to look in to. I intended to do more with the antenna model, but haven't really had the chance to do anything then, and when I did my mind was on another topic, and this was put aside.

More recently some questions have been brought up in another thread, and still another thread the actual model was brought back to the forefront, along with some what if questions, as well as some questions about this antenna and common mode currents.

This is part one of my looking at this antenna model, it is not complete, it is just where I am right now. A part 1 if you will.

I am going to look at the Imax antenna with multiple mounting options, and at multiple heights, to see their effects on the model.

I had a version of this post up earlier today, but I deleted it, so if you saw it and wondered where it went, that is what happened. Upon showing the currents and phase, I noticed I made a mistake, an easy mistake to make, and one that before now has never made a difference in the radiation pattern. Note the words "before now"... Some of the models had a wildly different radiation pattern after correcting for this. To keep bad information from getting out to far from a mistake, I removed the entire post.

In the antenna patterns below, there are three antennas.

• In green we have an antenna with an attached mast and no radials.
• In red we have an antenna with an isolated mast and no radials.
• In blue we have an antenna with an attached mast and four full length radials.

Lets start low and work our way up. The antenna mounted six feet over ground.

Next is eleven feet.

Sixteen feet.

Twenty one feet.

Twenty six feet.

Thirty one feet.

And finally 36 feet.

Results.

1. Looking at the blue and red plots, they stay pretty consistent throughout the comparison. The largest separation between them is 0.52 dB, and are typically much closer together. In some cases one has slightly more gain, while in others the other has slightly more gain. They remain close enough together that you won't notice the difference between the two of them.
2. The model with an attached mast and no radials has by far the most chaotic results. Ranging from slightly better than the other two at lower heights, to being about 3 dB down at 16 feet in height. It bounced back somewhat, but never quite caught back up to the other models presented here. In most of these models you won't notice the difference, but it shows that is it possible for this method of mounting the antenna to occasionally produce sub par results.

When it comes to recommendations based on these results, the order I would put these mounting methods are:

1. Full length horizontal ground plane
2. Isolated mast and no ground plane
3. Attaching the antenna to a mast

The reason I chose the full length horizontal ground plane first is it is able to maintain its stability even if there is no choke on the feed line at the feed point. If you make a mistake, or just leave the choke off, this setup will compensate for it. The other two models is assuming a good RF choke is installed at the feed point, as stated, with this setup it doesn't matter so much. The biggest downside to this setup is it is the hardest to accomplish, so is the least likely to be done.

When it comes to isolating the antenna from the mast, this is what I have recommended multiple times in the past, and this far easier than the first option to implement on one of these antennas. My usual recommendation is to get a thick one inch diameter piece of fiberglass and use that to mount the antenna.

The third option is by far the most common method of installing this antenna, and with the addition of the antennas capacitor, actually did surprisingly well in most, but notably not all, cases. The models above suggest that you can get away with it for the most part, and in some cases actually get slightly more gain than the other methods used, but there are also situations that this setup might not live up to the antenna's potential. I won't fault anyone for setting an antenna up this way, but I can't recommend it myself.

---
Marconi, next I will talk about the "non-apparent collinear" effect you are asking about...


The DB

 

[The DB]

Marconi, your questions in another thread about the phase of the common mode currents on my first of these models having a possible "non-apparent collinear" effect. The fact of the matter is, the antenna models with a mast attached and no radials that I modeled above at the various heights (the green patterns above) have an in phase lobe on the mast near the antenna itself. Here we will look at two of those cases.

The model at sixteen feet above ground is the first of these two examples.

Here we see a significant amount of currents on the mast, and they are in phase with the antenna above. This is an example of a current distribution that would have the effect of a "non-apparent collinear" if ever I saw one, however the results are not what we would expect. I'll post them again here. Again look at the green line and how it compares to the other models.

Looking at this we have a clear collinear radiation pattern, however, not only no more, but significantly less gain. The problem is the lower lobes relation to the earth below. Being close to the earth has the effect of significantly increasing losses. It is part of the reason the saying "height is might" exists. That and the fact that in this model the current lobe near the earth is by far the dominant lobe, this is the results we get. To look at efficiency, the blue model has a radiation-efficiency of 59.22%. The red model has a radiation-efficiency of 51.22%. And the model in question has a radiation-efficiency of 16.61%. The collinear effect is in play, but so are a significant amount of additional losses which are more than compensating for the additional gain we would normally expect to see.

Now to look at another model, this time the model at thirty one feet above earth.

This currents plus phase is closer to the one you linked, and factoring in the information from the example I used above, this one is more likely to have the potential gain you were asking about. So lets look at the results of this model again. I'll post them again here for easy reference.

Looking at the patterns, the pattern (the green one) has two lobes compared to the others antennas 3 lobes. This leads me to think the collinear effect you are asking about is in play on this model. However, other things seem to be getting in the way as well.

Lets dive in to efficiency again. The blue pattern has a radiation-efficiency of 63.39%. The red pattern has a radiation-efficiency of 65.96%. And the green pattern has a radiation-efficiency of 57.38%. Much better than above, but still playing its part in hindering this antenna layout.

Another thing to look at is the lobes in the different patterns themselves, in other words, the antennas' directivity. The red pattern has an RDF of 6.83. The blue pattern has an RDF of 6.77. The green pattern has an RDF of 5.51. (RDF is a measure of directivity, the higher the number, the more of the antennas radiated energy is in a smaller space). Here again, a weakness of this antenna's layout.

In this case, the antenna's lower efficiency isn't helping, and the fact that the antenna is less directive compared to the others. The combination of these effects is again countering the expected gain one might expect to see with a collinear current distribution.

Now comes two questions.

1. Is it possible that there is some height that an antenna in these conditions would benefit enough from the collinear effect to actually get an increased gain?
2. Is it possible, in some situations, to artificially manipulate the antenna system, either the mast or the feed line, to get a noticeable benefit from this collinear effect?

To answer question one, I would say that should such a situation like this exist, it will likely be a narrow range, and you will still not get enough out of it that you will notice the difference. As we have shown above, it takes more than a desire-able phase/current distribution to get beneficial results.

And for question number two. I think, if you can get one of these antennas far enough above the ground, and can control certain aspects of the common mode currents present strategically using mast isolation or a coaxial choke, it might be possible to create a collinear situation that could benefit this antenna to some degree. What that degree would be, however, is also a very good question. I am cautious here as we have seen two situations where such a current pattern has produced less than desirable results. That being said, if you can account for the above variables that ended up getting in the way of the desired results, it may be possible. I guess that study will be for another time...


The DB

 

[Robb]

You are using software to emulate antenna isolation; but in the real world what material are we discussing here?

 

[The DB]

You are using software to emulate antenna isolation; but in the real world what material are we discussing here?

There are three ways I've seen it done. The old way that I don't recommend anymore involves insulating the mast from the antenna hardware. We started with electrical tape, you want to use enough that there is minimal capacitance between the mast and the mounting brackets, so more is better, as long as you can fit the hardware around the electrical tape. Unlike a magnet mount antenna, you want as little capacitive coupling as possible. One person found a rubber sleeve that just fit over the mast, and inside the mounting hardware. It was a tight fit but it worked. I suppose if you have a dense piece of rubber you can wrap that around for the same effect as well. Might be worth looking at some of the flat floor mats used in cars if they still make them, although they might be to thick...

I mentioned above a 1 inch fiberglass rod. You bolt the rod to the mast, and the antenna to the other side of the rod. A two foot rod is generally enough. Depending on where you get them from they can be somewhat expensive. I will generally paint them to help prevent UV from the sun wearing out the rod to quick.

Much more recently, and still being experimented with, we have used one of the half inch diameter high density cookie sheets. We cut it to size, and use clamps to bolt it to both the mast and the antenna with separate but close hardware. So far the two I have seen done this way are holding up well, although I want to give it a little more time before giving it my seal of approval. It is cheaper in general than using a fiberglass rod, and I paint these for the same reason as I paint the fiberglass rods...


The DB

 

[The DB]

OK, on to part 2. Here I am going to add in several configurations of angled radials. The patterns will be as follows:

• Pink will show 1/2 wavelength angled radials. Asked about by Needle Bender specifically.
• Green will be 1/4 wavelength angled radials. This was on my list of things to test, however, Needle Bender also asked about this.
• Red will be the shortened angled radials that you can purchase for this antenna.
• Blue will be the same blue patterns from the models above for comparison purposes.

There will be no model in this group at the height of six feet. Also, the half wavelength angled radials will start at twenty one feet in height.

So lets get started. Eleven feet in height.

16 feet.

Twenty one feet.

Twenty six feet.

Thirty one feet.

And finally 36 feet.

One of the models, there is an anomaly with one of the patterns. At twenty one feet, the 1/4 wavelength angled radials model is at a higher angle of radiation than the other models. I don't know why this is. I have looked at the model closely and cannot find any problems or errors. Perhaps it is just something about this configuration at this height?

What I am surprised at here is the 1/2 wavelength angled radials. It has as much, and sometimes more gain than the other models, however, it has a significantly weaker high angle lobe.

When it comes to angled radials, you can get a kit, or you can make them, use them for guy wires and such if you have to, insulate to the electrical length you are interested in. I would put the shortened angled radials such as you find in the radial kit, and the 1/4 wavelength angled radials that you can make and use as part of a guy system after horizontal radials. They don't affect gain much, or the angle of radiation (except for one anomaly) but they both control common mode currents rather well. I would put the half wavelength radials after the shorter angled radials mentioned above as they don't control common mode currents nearly as well, although they do still have some effect. I would put the isolated mast after all of these angled radials.


The DB

 

[Robb]

1/2 w/l radials shows a marked improvement of radiating most of the energy horizontally.
Cool; but practical?!?

 

[The DB]

1/2 w/l radials shows a marked improvement of radiating most of the energy horizontally.
Cool; but practical?!?

I wouldn't call it a marked improvement, in both cases that the 1/2 wavelength radials has the most we are still taking about 0.12 dB or so over the second place model. That isn't enough to notice.

If the radial wires are doubling as guy wires it isn't to hard to set up.

I really only included the 1/2 wavelength angled radial models because they were requested.


The DB

 

[Marconi]

DB here is my wires description for your green model "6ft-m-nr" with a mast attached and no radials, mounted @ 6' feet above Earth.



Here is the pattern I get.



Here is what I get for the pattern if I isolate the mast using Average Earth.



And, here is the pattern I get if I isolate the mast over Extremely Poor Earth instead of Average Earth.



My model now is very similar to what you get for your model "6ft-m-nr" with a little bit less gain at 2.96 dbi vs. your gain of 3.22 dbi for the green model.

Here is my wires description for my model with the mast isolated not connected.



You did a lot of work here and a lot of little details. I had trouble just trying to duplicate what you've done for us. I hope there is an easy explanation. I've only done the first model in green, and I sure would hate to go back and recheck all these models again, which I've already done on my end...just to get my one model at the various heights to 36' feet. I hope I'm wrong and I messed up somewhere here. That is why I posted my wires descriptions for you to check maybe.

Note the only difference between the two wires descriptions is in (Wire 2 End 2) for the 6' vs 5.5' mast where I isolated the antenna.

I think this is a very important topic for us to try and understand a bit more. Thanks!

 

[Marconi]

DB here is my results in the PDF file below. Sorry, but the green is a bit hard to see.
The asterisk is next to the model with the green cursor at the maximum gain and angle noted in the details below on the right side of the image.

BTW DB, I used Eznec's automatic "conservative" segment feature to ensure that all models were segmented similarly.

 

[The DB]

Did I not mention the ground stats? Just to make sure we are on the same page, ground conductivity is .001, and its dielectric constant is 5.

Normally our models are pretty similar, but there are some distinct differences this time. Is it the capacitor casing these differences? With the change in the current distribution the capacitor causes in the antenna, I can see that change making the difference. On standard 5/8 wavelength antenna models I would expect to get results more like yours.

For my antenna mounted six feed off the ground, no radials and mast attached I have the following. Remember, for me segments don't mean much so you will have to find that data out yourself. This (and all of the above models) were made using inches.

Wire 1
x1 0, y1 0, z1 72, x2 0, y2 0, z2 349.1564, diameter 0.5

Wire 2
x1 0, y1 0, z1 0, x2 0, y2 0, z2 60, diameter 0.5

Wire 3
x1 0, y1 0, z1 60, x2 0, y2 0, z2 72, diameter 0.5

Wire 1 is the antenna itself, Wire 2 is the mast, and Wire 3 is the mounting tube. when I isolated the mast, I lowered the Z2 side of Wire 2 by six inches, making it 54 instead. In this diagram you can see that the mounting tube on my models is actually below the height of the antenna. This should get to at least real close to my dimensions.

I have a few more of these to do when I get a chance, and then I want to collect and compare data from across all of these models. After that I will run some of these models without the capacitor and see how that affects said models. For this project I have been assuming that the capacitor is necessary to model this antenna, I want to confirm that.

Still a lot to do...


The DB

 

[Marconi]

DB, when I noticed the pattern with no radials and the mast connected...I knew something was different. I checked the ground type right then and started to do my models with 0.001 and 5, but I changed my mind on that and left it at 0.005 and 13 using Eznec Average Earth feature for Ground Description.There is a difference, but that is not the difference I see between our patterns.



If you used the capacitor in the Imax antenna, then it is obviously having and effect of making all your antenna configurations look similar with a couple of exceptions as you've noted.

If the capacitor has that much effect on the current distribution in the real antenna...then my ideas about the currents on the mast is out the window, but I will be checking that out on a more conventional model to be sure.

I also added your wires and I still get the same pattern, I just didn't have the 12" tube for the mount in the model. So, I can only figure the cap makes the difference.

Here is the difference my setting the segment length to 3" inches.


I also had my radiator set at 271.4" inches and you at 277.1564" inches. That change did not change the shape of the pattern...just the gain changed. So the segments, the length, and adding the mount to the radiator did not make the difference. It must be the load you added...what are the values you used?

BTW, somebody mentioned recently that the Super Penetrator 500's mount design may create a capacitor type of function (tube inside a tube) similar to the Maco V58, and a few other antennas. IMO this is to allow some adjustment at the base of the antenna for the real antenna, but it could be more than that if your idea of adding capacitance to the Imax produces performance gains that would not be there without such an object added to the antenna. I think I recall you or somebody else talking about this as a possible modeling problem if the feature has consequences like the Imax or maybe it was Bob.

Did that cap mod' to your Imax model help with performance?

Do you have a model of the SP500 and if so did you model the mounting bracket that raises the radials up about 12" to 14" inches above the bottom of the mount? I see that feature in my model making a notable difference in performance.

Yesterday I was working on my Big Mack and checking everything I could find on the Internet. I found someone that described part of the base coil area, saying there was an insulator inside of a tube that held the base of the radiator. IMO that can add capacitance and maybe a model would have to have this object included in order to work as intended like you may be suggesting on the Imax. If so, then a 5/8 wave might not always be a 5/8 wave...so-to-speak.

What is the value of the capacitor you used on the Imax and how did you add it to the radiator?

Sorry to launch off into the weeds, but I figured I might have your ear a little with this new work you produced going on.

Good luck.

 

[The DB]

It must be the load you added...what are the values you used?

On a 64 segment antenna element of the afore mentioned length, it is on segments 31 and 32, and it is a 25 pF capacitor.

Did that cap mod' to your Imax model help with performance?

On the original model, I don't recall. I haven't done a direct comparison with these models and one without a capacitor yet. As I mentioned above, it is on my list of things to do.

Do you have a model of the SP500

No. I looked at yours some, but I never made a model of the SP500.

and if so did you model the mounting bracket that raises the radials up about 12" to 14" inches above the bottom of the mount? I see that feature in my model making a notable difference in performance.

I can see getting the radials closer to the radiating element making a difference.

Yesterday I was working on my Big Mack and checking everything I could find on the Internet. I found someone that described part of the base coil area, saying there was an insulator inside of a tube that held the base of the radiator. IMO that can add capacitance and maybe a model would have to have this object included in order to work as intended like you may be suggesting on the Imax. If so, then a 5/8 wave might not always be a 5/8 wave...so-to-speak.

Just because an antenna is a physical 5/8 wavelength antenna does not necessarily mean it is electrically a 5/8 wavelength antenna. In this case we have a capacitor that makes the feed point look like it is close to an electrical half wavelength, i.e. it is making the antenna electrically shorter than its physical length. This is just the opposite of how a loading coil works on a mobile antenna. I actually mentioned when I made that initial model in another thread remarking that as far as matching goes, this antenna with the capacitor added has very close to the same feed point impedance as a half wavelength antenna. I postulated that it wouldn't take much at all to adapt the a99's matching system to this antenna.

What is the value of the capacitor you used on the Imax and how did you add it to the radiator?

I said it above, but it is 25 pF, and it is just below the half way point on the antenna, as far as the modeling software is concerned it is on segments 31 and 32 of the 64 segments on the element.


The DB

 

[Needle Bender]

On a 64 segment antenna element of the afore mentioned length, it is on segments 31 and 32, and it is a 25 pF capacitor.



On the original model, I don't recall. I haven't done a direct comparison with these models and one without a capacitor yet. As I mentioned above, it is on my list of things to do.



No. I looked at yours some, but I never made a model of the SP500.



I can see getting the radials closer to the radiating element making a difference.



Just because an antenna is a physical 5/8 wavelength antenna does not necessarily mean it is electrically a 5/8 wavelength antenna. In this case we have a capacitor that makes the feed point look like it is close to an electrical half wavelength, i.e. it is making the antenna electrically shorter than its physical length. This is just the opposite of how a loading coil works on a mobile antenna. I actually mentioned when I made that initial model in another thread remarking that as far as matching goes, this antenna with the capacitor added has very close to the same feed point impedance as a half wavelength antenna. I postulated that it wouldn't take much at all to adapt the a99's matching system to this antenna.



I said it above, but it is 25 pF, and it is just below the half way point on the antenna, as far as the modeling software is concerned it is on segments 31 and 32 of the 64 segments on the element.


The DB

Wait, what happened to the 42pF he measured? Now it's 25pF?

 

[Marconi]

Wait, what happened to the 42pF he measured? Now it's 25pF?

NB, that was probably just a mishap. I learned something new just trying different values and all the while I could see the device was having an effect. Our models were not exactly equal and are still not equals. I was just trying different things hoping to get on track with DB's idea.

Just today I got my reference model to equal what DB did in his original post. That is where I felt I had to start if I was going to have something new to consider about the Imax.

The Imax is not a panacea; capacitor added or not.

The big deal is why Solarcon decided to do this. My new model results using the new reference is showing more gain, but the match went to hell in a handbasket.
So transforming the very high feed point impedance will likely produce added loss. It looks like a wash to me.

 

[BBB]

Could you model two co-phased Imax (with or without radials) spaced at least 21' apart both at a 36' elevation?

I plan on doing such an arrangement with a HyGain Co-Phaser box I have

Would you need more info on the HyGain Co-Phaser box? If so, i don't really have much info on it besides the ad stuff.

http://www.cbtricks.com/ant_manuals/hygain/model_ph1/ad/index.htm