Here are some Eznec models for an Imax with 72" horizontal radials at 9' up to 72' feet. I raised the model up 4.5' in each iteration so I could track the results for 1/4 and 1/2 wave intervals. The idea was to see what Eznec might show for maximum gain and angle, and in some cases the secondary lobe gain and angle. I also show the number of lobes, and gave my subjective idea as to the state of the currents on the mast and the nature of the patterns produced. The A99 also had 72" horizontal radials and was modeled using the same process in raising 4.5' feet at a time.
In the left column I gave my opinion of the model as good or bad, and I also indicated the wavelength in multiples.
The last lines highlighted in yellow was a special comparison I added to show how the Imax at 72' feet compared with the A99 at 76.5' feet where the two antennas were equal in height to the tips. The gain looks about the same, but surprisingly the Imax angle at 5* degrees bested the A99 at 6* degrees. That could have been my error in setting the cursor for the Eznec angles too, because IMO, under this situation, the angles should be the same.
Personally I doubt there is any real gain difference to be noted if we tested using our radios, but there might be something to consider about the current trends on the mast relative to a particular height, liken to a "worst case scenario." This may also be relative to the feed line length from the ground to the feed point and whether common mode currents are maximum or not. Models do not have feed lines installed, but the mast could be considered for that purpose excepting for its much larger diameter possibly. This project was done to try and determine some trends for the currents, gain, and angle...and not meant to represent perfect accuracy in my modeling.
As a testimony to real world testing, in my personal experience I was always confused at the changes I saw in antenna responses, as I raised my various antennas up and down to different heights. So, my seeing these models display such sensitive responses to changes...suggest to me that maybe this is what I was seeing at the time, and I just didn't understand. This may also help explain a little for why we hear so many different ideas from folks on performance.
Real world testing would be necessary to be sure, so your mileage may vary.
I hope this hand written stuff can be read clearly.
View attachment Imax A99 at various heights. 1.pdf
In the left column I gave my opinion of the model as good or bad, and I also indicated the wavelength in multiples.
The last lines highlighted in yellow was a special comparison I added to show how the Imax at 72' feet compared with the A99 at 76.5' feet where the two antennas were equal in height to the tips. The gain looks about the same, but surprisingly the Imax angle at 5* degrees bested the A99 at 6* degrees. That could have been my error in setting the cursor for the Eznec angles too, because IMO, under this situation, the angles should be the same.
Personally I doubt there is any real gain difference to be noted if we tested using our radios, but there might be something to consider about the current trends on the mast relative to a particular height, liken to a "worst case scenario." This may also be relative to the feed line length from the ground to the feed point and whether common mode currents are maximum or not. Models do not have feed lines installed, but the mast could be considered for that purpose excepting for its much larger diameter possibly. This project was done to try and determine some trends for the currents, gain, and angle...and not meant to represent perfect accuracy in my modeling.
As a testimony to real world testing, in my personal experience I was always confused at the changes I saw in antenna responses, as I raised my various antennas up and down to different heights. So, my seeing these models display such sensitive responses to changes...suggest to me that maybe this is what I was seeing at the time, and I just didn't understand. This may also help explain a little for why we hear so many different ideas from folks on performance.
Real world testing would be necessary to be sure, so your mileage may vary.
I hope this hand written stuff can be read clearly.
View attachment Imax A99 at various heights. 1.pdf
