Sirio 827 with longer radials

[The DB]

When it comes to the antennas with no mast, I expected it to be as similar as it is. Back when I was playing with the a99 configurations that are available, and what people tell you you should use (read full length 1/4 wavelength horizontal radials), with no mast all of the configurations were very close to each other as far as gain and radiation pattern were concerned to the point you would never be able to tell the difference between them. The GPK kit layout actually had the most gain, but it wasn't so much that you would ever notice the difference... Its one of a several things that lead me to believe the designers of that antenna and its bigger brother know more than what many people give them credit for.

Anyway, what I see after adding the masts is a bit unexpected, specifically the blue pattern, with is the 8 radials 55 inches long each. The pattern has two lobes, compared to the other antenna, and both antennas without masts, which all have three. The higher the antenna is the more lobes it tends to have, adding a lobe every half wavelength of height or so. Also, 5/8 wavelength antennas tend to add lobes at lower heights than 1/2 wavelength antennas because part of the antenna is out of phase with the rest (its just one cost in going past 1/2 wavelength). So my the question is, what is causing this one antenna to have fewer lobes than the other antenna, and itself without a mast mounted at the same height? This is normally not something that I would expect to happen, and I can't say I've seen this happen before.

I'll look in to it tomorrow if I have the time.


The DB

 

[M0GVZ]

I'm loving reading this thread. It would be interesting to see the results with different mast lengths too but keeping the antenna feedpoint at a fixed height. 6ft, 16ft and 21ft are commonplace lengths that can be bought here in Blighty.

 

[The DB]

Looking at the model I was curious about above, here is why I think the difference might be happening.

[photo=medium]6395[/photo]

Here we see, immediately under the 8 shortened radials, we have currents that are nearly as strong and in phase with the top 1/2 wavelength of the antenna. These currents should act much like a collinear antenna, having a beneficial effect on the lower angle lobe.

There are still significant currents on the mast that are out of phase, but their strongest part is right next to the earth, which is having an attenuating effect on them. Another way of describing that is, ever work with a ground mounted antenna and test it as you add radials? As you add radials the antenna performs better and better. So for these out of phase currents near the earth think a ground mounted antenna with no radials...

A question that comes to mind is how are these currents as they are? Both sides of the feed point will normally have the same current phase and magnitude, so what is causing the difference here? I think the radials are not acting like a ground plane, but instead acting like a capacity hat. That is the only thing on the antenna I can think of that would cause this change in the current phase /magnitude instead of what I would normally expect to see in an antenna model.

I want to point out that what I am posting is my opinion, nothing more. If someone has a better explanation I would love to hear it.

I would like to look at this antenna at multiple heights as M0GVZ mentioned, but I don't have time right now so I will do that later.


The DB

 

[Marconi]

Connor, is your idea to keep the feed point at a fixed height...suggesting the antenna is supported below and to the ground with a non-conductive mast...like maybe like set on several lengths of wooden post?????

 

[Marconi]

DB I don't understand much of your collinear idea here, but your post is interesting considering the 827 with a mast and 8 shorter radials...acting more like a collinear antenna with a lower angle lobe.

If true, then I guess this is some evidence for the 827 working better with 8 shorter radials.

This also seems to be about the same results as I came up with in my earlier models, the short radials on the 827 model are showing much more gain at a low angel to the horizon. I just didn't realize it could all be due to the antenna showing collinear effects.

This probably helps explains the old CB ads claim about the 5/8 wave antennas being a collinear and it was not just CB BS.

 

[M0GVZ]

Connor, is your idea to keep the feed point at a fixed height...suggesting the antenna is supported below and to the ground with a non-conductive mast...like maybe like set on several lengths of wooden post?????

More like a typical UK installation using a metal mast attached to the side of a house with TK brackets, keeping the base of the antenna at roof height, the bottom of the mast not being in contact with the ground.

 

[Marconi]

DB, the main error I felt I made in my earlier 827 model with 55" inch radials, over real Earth, had the radiator physical length from the radials/feed point to the tip of the antenna set at 264.8" inches, and that is about 6700 mm as per the manual for overall length.

I think this overall length in the manual includes the bottom mounting bracket that I figure is about 15" inches long. So, if we subtract the 15" mount length it makes the radiator physical length equal 249.8" inches or 20.8' feet. However, without the added length for the mounting bracket and the added inductive length for the coil the model would not tune to resonance for me at 27.205 MHz.

This net length also does not include the inductor matching coil, which also adds some length back to this short radiator. DB, I didn't include the coil in my model and it looks as though you didn't either. So, in my case I apparently added <>15" inches back to the radiator length during my process of trying to tune the model to resonance and also show a good resistance.

Note: I don't think tuning is necessary for good results in modeling...as long as the Free Space Average Gain results for the model is close or equal to the value of 1.

Some folks may not understand this concept, but in such cases we do not need to try and match a model in order to get reasonable and workable results. The only problem I see is...we can't successfully scan the model for SWR or Bandwidth...two simple checks that folks can do with their antennas on their rigs at home.

Over time I've collected 827 dimensions at 27.205 MHz from several kind folks, and they were each different. Most of the errors were omitting the overlap for some tubing connections and showing the full lengths for the tubing instead of exposed lengths.

In my tuned model at 36' feet I used 264.8" inches for the radiator with an average tubing diameter of 0.80" inch, and the model shows nearly a perfect match.

I hope I didn't muck this up.

Below is a PDF file for my tuned 827 models with and without a mast for comparison.

I will fix this model with 4 x 108" radials and post them later for comparison to the model with 55" radials with and without masts.

i show here everything I did to the models to help with the tune, help eliminate Common Mode Currents, to scan the SWR Bandwidth Curves, noting the antenna resonant frequency, and overlays to compare the antenna patterns, gain, and maximum angle close to the horizon.

DB, if you don't mind, what were your lengths for the radiator and the 1/4 wavelength radials you used?

 

[Marconi]

More like a typical UK installation using a metal mast attached to the side of a house with TK brackets, keeping the base of the antenna at roof height, the bottom of the mast not being in contact with the ground.

Connor just curious, what do these folks suggest happens or doesn't happen with these variable mast lengths below the antenna while the bottom of the mast is not touching the ground?

 

[Marconi]

Here are the 2 models I fixed and the 2 overlays for the Sirio 827 with 8 x 55" radials vs. 4 x 108" radials to compare. These models are at 36' feet elevation.

Remember that the (*) beside the title for the antenna and under the word Primary is the active antenna that is showing the gain and angle in the pattern view overlays.

 

[Alexis Mercado]

Marconi, according with your modelings, it’s not worth using longer radials.

They behave practically the same ?

 

[M0GVZ]

Connor just curious, what do these folks suggest happens or doesn't happen with these variable mast lengths below the antenna while the bottom of the mast is not touching the ground?

Its to try to address something that comes up often where you'll get two completely different opinions of an antenna and often the only real difference between the opinon of person A and B is the length of mast and the coax running from the antenna. W8JI also makes a note of this in one of his articles about end fed antennas, in particular the Imax 2000, and RF grounding where depending on the combination of lengths of mast and coax one person can find an antenna working considerably better than another person does.

 

[Marconi]

Connor, Is this the article you are referring to?

 

[Marconi]

Marconi, according with your modelings, it’s not worth using longer radials.

They behave practically the same ?

Alexis, do you happen to have any 827 dimensions, in inches or mm at some frequency showing your best tune?

My dimensions are only assumptions. I have the manual and it is of little help.

 

[Alexis Mercado]

Alexis, do you happen to have any 827 dimensions, in inches or mm at some frequency showing your best tune?

My dimensions are only assumptions. I have the manual and it is of little help.

Slowest SWR is on post 18 and dimensions on post 21.

If the info you request is not there, please be more specific and I will tell you.

 

[Marconi]

Radiator length: 253”

Thanks Alexis.