IMAX 2000 continuity query

[copperhead556]

I picked up an IMAX 2000 the other day "scratch and dent" basically shipping damage to fiberglass which was easily rectified. Admittedly I am by no means an electronic or antenna whiz but while checking out unit after repairs I checked the masts end to end with my meter and continuity tester and discovered NO continuity from center pin of SO-239 to end of primary mast. Nor was there any continuity from base to end of secondary mast. I didn't even mess with the final mast. This was done with unit disassembled, no coax attached and no power fed thru it....is this normal or a flaw?
Thanks in advance.
Doc

 

[camaro1]

normal,, there are capacitors in the base and the middle mast section and does not show dc continuity end to end.. you should see dc continuity from the center pin to the outer threads of the so-239

 

[357]

I never heard of caps in the middle section.
My new Imax show no dc connection on the middle section either, and it shows no capacitance....I'm just about to start trippin'

ok I did more testing and I show 42pf on the middle section....why are the putting a cap there now?
That "imax exposed" guy did a half assed job, he didnt even show this "capacitor"


Near the inside middle of the center section, there is a high power capacitor-coupler (Shown below), and yes it radiates and couples the top half of the antenna. This allows the center section to radiate a larger pattern than most verticals do

 

[The DB]

Near the inside middle of the center section, there is a high power capacitor-coupler (Shown below), and yes it radiates and couples the top half of the antenna. This allows the center section to radiate a larger pattern than most verticals do

Is this part a quote from elsewhere? If so can you give a link?

The cap is 42 pF then? Plugging that into a model I get a surprising result...

Starting with a reference model currents + phase. This is what we would expect from such an antenna with no capacitor.

Playing with this model before, nothing I tried for a capacitance would benefit the model, but 42 pF is different... Here are the currents + phase for the antenna with a 42 pF capacitor in the middle of the antenna... NOTE, where the capacitor is on the antenna does make a difference here, quite a big one actually as only one location actually benefited the pattern any, most others had minimal effect, or a negative effect.

While it isn't making the current on the upper part of the antenna stronger, more of the antenna is in phase, which will have other benefits, for instance, low angle gain should increase as there is not as many out of phase currents present...

So I compared the radiation patterns...

The model with the capacitor made the red plot, while the reference made the blue plot. As expected from the change in radiation pattern, more low angle gain and less high angle gain. The angle of radiation has not, however changed, for those who are interested.

Also, with the capacitor in line, the R value the antenna presents at the feed point skyrockets from 86.7 to 1084. This would potentially put it in the range, if not very close, to an end fed half wave matching impedance, so if they also just happened to also make an end fed half wave antenna they could potentially drop its matching section into this antenna design with minimal if any needed changes...

Additionally, for those who care, the reference antenna has a radiation efficiency of 60.83, while the capacitor version has a radiation efficiency of 59.1, so the capacitor is causing a drop in efficiency. However, when looking at directivity, the reference has an RDF of 5.34, while the capacitor model has an RDF of 6. For this metric the higher the better. Essentially, the directivity the capacitor is adding is overcoming the additional losses the capacitor is also adding to the antenna.

Thanks for the capacitor data, it really helped.


The DB

 

[Marconi]

DB, for me this is the best, easy to understand modeling work, I can recall seeing from you. Your words are also very descriptive of your ideas...and seem right on point of this issue.

Thanks, to 357 too.

 

[357]

/panic attack

 

[rumblefish]

Is this part a quote from elsewhere? If so can you give a link?

The cap is 42 pF then? Plugging that into a model I get a surprising result...

Starting with a reference model currents + phase. This is what we would expect from such an antenna with no capacitor.

Playing with this model before, nothing I tried for a capacitance would benefit the model, but 42 pF is different... Here are the currents + phase for the antenna with a 42 pF capacitor in the middle of the antenna... NOTE, where the capacitor is on the antenna does make a difference here, quite a big one actually as only one location actually benefited the pattern any, most others had minimal effect, or a negative effect.

While it isn't making the current on the upper part of the antenna stronger, more of the antenna is in phase, which will have other benefits, for instance, low angle gain should increase as there is not as many out of phase currents present...

So I compared the radiation patterns...

The model with the capacitor made the red plot, while the reference made the blue plot. As expected from the change in radiation pattern, more low angle gain and less high angle gain. The angle of radiation has not, however changed, for those who are interested.

Also, with the capacitor in line, the R value the antenna presents at the feed point skyrockets from 86.7 to 1084. This would potentially put it in the range, if not very close, to an end fed half wave matching impedance, so if they also just happened to also make an end fed half wave antenna they could potentially drop its matching section into this antenna design with minimal if any needed changes...

Additionally, for those who care, the reference antenna has a radiation efficiency of 60.83, while the capacitor version has a radiation efficiency of 59.1, so the capacitor is causing a drop in efficiency. However, when looking at directivity, the reference has an RDF of 5.34, while the capacitor model has an RDF of 6. For this metric the higher the better. Essentially, the directivity the capacitor is adding is overcoming the additional losses the capacitor is also adding to the antenna.

Thanks for the capacitor data, it really helped.


The DB

Thank you 357! and thank you DB! This was what i was trying to figure out in another thread. The 4pf value I suggested for the model was way too low. I threw that in because i saw it used in the matching system to couple the signal into the antenna.For all I know the 4pf. cap value may be wrong. I do know now the schematic floating around on the web is wrong. k3dav states there is a coupling cap in the center section and goes on to use the same incorrect schematic????

 

[Marconi]

I just tested the middle section of my Imax and I too find there is no continuity.

 

[Road Squawker]

.... k3dav states there is a coupling cap in the center section and goes on to use the same incorrect schematic????

K3DAV,

there is a thread on here where we smacked him around a LOT...... He doesn't have a clue about a lot of basic RF concepts,


"An RF choke coil is not rocket science. I have seen the many websites that tell you how to build a choke coil. They show charts of capacitance and inductance measurements and tell you the coil should be a specific length and size for different frequencies. They tell you where to mount the choke within the feedline.... blah blah blah blah blah...and so on....."

or, this one,...

actually, based on the number of and diameter of the turns, it may work just as POORLY.

 

[rumblefish]

I just tested the middle section of my Imax and I too find there is no continuity.

Hi marconi, It looks like you've done a great deal of work modeling the imax. I very much appreciate your plots posted for this antenna. Now I'm curious how the ground plane affects the angle of radiation. Do you think there will be a difference with the capacitor in place?

 

[Marconi]

Well DB shows us there is a difference, but it is not much of a difference as noted in the overlay patterns he posted.

My Eznec models are currently lock into an old computer with a bad power supply switch. It will not boot up...until I replace the part hopefully.

 

[The DB]

I did some quick modeling, I made a comparison model with 1/4 wavelength horizontal radials, and a model based on the short angled radials used in Solarcon's radial kit.

Here are all three of these models in one plot for a comparison.

The green color (labled as model gpk below) is the model that simulated Solarcon's ground plane kit.
The red color (labled as model hgp below) is the model that simulates a set of 1/4 length horizontal radials.
The blue line is the original Imax model with added capacitor from abobe.

A lot of people think that adding radials makes a difference in an antenna's radiation angle, as we can see here, this is not normally the case.

To see what the models look like with a mast added, I added a mast to each of these models, and below is the result.

The antennas happened to be 10 meters, or about 32 feet off the ground. As expected the horizontal ground plane model was affected the least, only loosing 0.01 dB of gain. The Solarcon ground plane kit model lost 0.08 dB of gain, and the model with no ground plane lost 0.37 dB of gain.

The cmc test model of the imax with no radials looked different than I expected so I researched that a little further and found this current+phase magnitude of the antenna...

In this model the antenna is mounted 10 meters (or about 32 feet) above the ground. Below the feedpoint there is a small bit of out of phase current, but then a full half a wavelength of in phase common mode currents, essentially acting like a collinear antenna. I cannot, at this point, guarantee that this will happen at every height (or every random length of coax from antenna to radio) but it is worthy of exploring. That, however, will have to wait until I get some more time...


The DB

 

[Marconi]

DB here are my Eznec models on the Imax topic. I also made a copy of your reference model and it shows to be identical to your model.

I cannot duplicate the load you did to your reference model, but I did the Imax with the factory 72" inch GPK slanted down at 45* degrees including the mast. I also did the model with 108" inch horizontal radials.

I added an overlay for comparison.

I support your reporting here.

 

[The DB]

What you can't see in your models that I am seeing in mine with the load is the series capacitive load is adjusting the phase of the antenna in such a way that more of the vertical element is in phase. In much the same way as a series inductive load makes a shorter antenna act like a longer antenna, the series capacitive load in essence makes a longer antenna act like a shorter antenna.

With a minor adjustment to the capacitance of the capacitor, I am seeing slightly more gain, and a current distribution similar to what Sirio advertises for their Gainmaster antenna...

It makes sense, and these are the first models I've seen where something like this was accomplished. If the capacitor is in the wrong place, or you use the wrong capacitance, the current distribution looks very different, and other aspects like efficiency and gain are negatively affected.

Years ago when I studied various aspects of an a99 model I discovered that the GPK that Solarcon sells for said antenna actually outperforms the same model with 1/4 wavelength radials, if only slightly. This was a surprise to me. I also noticed that adding the mounting tube to the model helped the antennas gain as well. With the addition of the capacitor to my Imax models, I'm seeing again a design that shows more intelligence was involved in its creation than many give this company credit. If only they did something to deal with the common mode current problems their antennas have as well, although their GPK actually does a good job with that to...


The DB

 

[Marconi]

DB, your explanation makes a lot of sense and is simply put. I agree it is obvious there is a change in current distribution noted in your model...with the use of the correct capacitor in the right location.

It is remarkable that modeling with NEC engine, with all the bad press and misguided comments from naysayers, demonstrates nicely the modest difference your model shows. This might make more difference than is apparent, and for sure without your thoughts on the subject above.

For a long time I've had a sense that my best working A99 performed exceptionally well for me at times. I can't explain, but I have signal reports that show this, only to have some notable members on this forum...lambast me for such a claim, my using a virtual dummy load for an antenna.

At about the same time the only small difference I could claim was I saw the addition of the A99 GPK produce a more well balance bowl shaped bandwidth curve...than just the A99 alone.

Thank you copperhead556, 357, and of course K3DAV.com for your contributions on this subject. Thank you DB for making sense of it all.