Sirio 827 with longer radials

[Marconi]

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

.
Thanks anyway Alexis, I doubt you have the dimensions I need for a complete model of the 827 with the physical matching coil included.

The 827, models I've posted here do not have the matching coil at the base, so I have to add the inductance the coil produces...at the top of the antenna. I don't think it would make any difference if my model has the inductance it needs at the top or the bottom, but I can't be sure. This is why I try and model antennas of interest to me. Getting the right dimensions is always the hard part.

 

[The DB]

My models that were different with and without a mast above, I modeled these at different heights. It seems I just happened on the lucky height at 20 ft originally as at 10, 15, 25, and 30 feet the radiation patterns were much closer to consistent. I'll post their results here for

10 ft
[photo=medium]6396[/photo]

15 ft
[photo=medium]6397[/photo]

20 ft, this is what was shown above
[photo=medium]6398[/photo]

25 ft
[photo=medium]6399[/photo]

30 ft
[photo=medium]6400[/photo]

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.

Question, are you suggesting using only shorter masts than what the height of the antenna would allow? Or perhaps do this in Free Space? We can't put the mast under the earth and I would be hesitant to put it as anything other than straight below the vertical antenna as the mast would skew the radiation pattern. (That is not to say that all real life connections go straight down to the earth from said antenna, for example a chimney mount in the middle of a roof.)

I guess I am asking if you could explain your idea in a little more detail so I can better represent it it in models?


The DB

 

[bob85]

some mast lengths are bad for cmc & some are good depending on if they are isolated or grounded,
I think Connor is interested in how the typical scaffold pole mast isolated on the side of a house effects the antenna pattern,

I avoid isolated masts that are close to odd 1/4wave multiples & go for something close to 1/2wave,

on the other hand if the mast is to be grounded i avoid mast lengths that are close to 1/2wave or multiples thereof including the ground wire & go with odd 1/4wave multiples.

 

[Marconi]

Long ago I read W8JI's article below. He did not tell us what the unlucky mast length was, He didn't even show us the Antenna View of the lucky mast length he suggested further down in his words either.

He just showed us the Eznec Antenna View showing the antenna and the currents with phase turned off. I was curious, so I wondered if I could model his idea and find a mast length that showed the same or similar Antenna View pattern as he posted. Scroll down to W8JI's "Imax 2000 Solarcon A99 Antenna" article below.

https://www.w8ji.com/end-fed_vertical.htm#IMAX

Below the PDf file shows what I came up with after many iterations of various mast length tests.

Models:
1. is my starting version of a 22.5' Imax radiator directly attached to the top of a 67' mast with no isolation anywhere. I choose this mast length because it showed the most similarity to the Antenna View W8Ji showed us. But when I scanned the model for FF Plot I was totally surprised. The pattern looked like the best case scenario he shows us later in the article. So, I had to find out what made the difference between my model and his model.

I also made some scratch marks in black ink at some important spots that demonstrate differences to note in the details for the PDF file images below.

2. is equal in dimensions to #1, but is my version of what W8JI's model really looks like. What got my attention was, I noticed the currents at the bottom of W8Ji's mast did not look like the currents at the base of the mast on my model.

NOTE: I had seen this anomaly in modeling before and to me it indicated the mast was not actually touching the ground...even though the image looked like it touch the Earth. So, my model has no isolation (ISO) and and it shows a good pattern, and his model is isolated at the base and shows a worst case scenario.

Conclusion: This small difference can't be seen in the his image. In my version of his model I used 0.50' feet or 6" inches to create the isolation at the base of the mast and now the Antenna View looks identical to the image he produced as the unlucky mast length. This little change in these 2 models makes the difference we see in the FF Plot patterns that W8JI suggested to the reader...as the difference in a lucky vs an unlucky mast length.

W8JI is a smart man, so maybe he was trying to make a point and used this to demonstrate the difference that CMC can make for antenna installations. But, I see this telling us we have to do some else to fix the problem for CMC...and it is not looking for some illusive mast length.

The likely reason he did not show us the antenna image of the lucky mast length, is because both of these models masts shows to be very heavy in currents. And as he states later on in the article, "...the currents on the unlucky length mast are about equal to the currents on the radiator."

IMO, these modeling image show us the currents on the lucky length mast are al;so heavy, and finding a lucky mast length is not the best solution even if a little change in length does show some effects in the real world.

 

[The DB]

I think his mast is touching the ground, but he is using something other than the MiniNec ground, which with all grounds except for Perfect and MIniNec will treat the end of the wire like it is hanging out in space and isn't connected to anything.

At the same time he used EZNec. While it isn't my favorite modeling software, I have used it in the past, and as I recall if you use anything with a direct wire to the ground below and don't use either Perfect or MiniNec ground I recall it popping up a message telling you that your results will not be accurate... I guess it is just one of several things that he has said or written on his web site that I question...


The DB

 

[bob85]

Eddie,
when W8JI talks about "lucky/unlucky" mast lengths i read that as luck with regards to folk who don't think about cmc, use any old length mast they happen to have with any old length wire to a ground rod & any old length of coax,

you would be lucky to pick good lengths that have a high common mode impedance / low cmc,

that's not to say its just luck when you try to minimise cmc on a mast by avoiding certain lengths in favour of others depending on it been grounded or isolated,

running willyynilly ground wires can increase cmc just as much as it can decrease cmc,

coax & everything connected to it has the same cmc issue as the mast & its impossible to know its electrical length, often multiple lengths to different ground points through tuners radios equipment wiring & the beer cooler,

you cannot nor do you need to know, you just need to make it a better length if you have cmc on the coax,

i fixed a few cmc issues ( when it was not convenient to take the antenna down to install a choke ) by trimming the coax or adding a long jumper & trimming for a lower cmc length.

proper isolation fixes the whole shebang, lower noise better signals,
unless you get very unlucky with the choke resonating your coax outer braid,

still fixable by trimming coax length.

 

[Marconi]

[UOTE="The DB, post: 734044, member: 21346"]I think his mast is touching the ground, but he is using something other than the MiniNec ground, which with all grounds except for Perfect and MIniNec will treat the end of the wire like it is hanging out in space and isn't connected to anything.

At the same time he used EZNec. While it isn't my favorite modeling software, I have used it in the past, and as I recall if you use anything with a direct wire to the ground below and don't use either Perfect or MiniNec ground I recall it popping up a message telling you that your results will not be accurate... I guess it is just one of several things that he has said or written on his web site that I question...

The DB[/QUOTE]

You are right...he used High Accuracy. The current pattern at the bottom of the mast now looks like the model he posted. BTW, the Wires screen still shows the mast touching the ground.

So, I will remove the ISO at the base and change the model to High Accuracy and see what happens.

I will be looking for this change to fix the problem of CMC and make the pattern look better, so we might find a lucky mast length to check out on our Old Imax. I may have to do some more tweaking to find another mast length that makes this all work better.

DB, thanks for your heads-up.

Below are the messages I saw pop-up on checking these features. Bad images but maybe the PDF Zoom feature will help.

 

[M0GVZ]

Question, are you suggesting using only shorter masts than what the height of the antenna would allow? Or perhaps do this in Free Space? We can't put the mast under the earth and I would be hesitant to put it as anything other than straight below the vertical antenna as the mast would skew the radiation pattern. (That is not to say that all real life connections go straight down to the earth from said antenna, for example a chimney mount in the middle of a roof.)

I guess I am asking if you could explain your idea in a little more detail so I can better represent it it in models?


The DB

OK so to model typical installs here in Blighty make the antenna with the antenna feedpoint at 21ft. It will be fixed at this for all lengths.

So we have the first mast which is the 21ft scaffolding pole, this will be in contact with the earth. 2" diameter.

Next we have a 16.4ft pole which is another common length so the top of that will be at 21ft, the base 4.6ft above the ground. 1.5" diameter pole.

Another common length is 10ft pole, again 1.5" diameter, the top of it at 21ft, the base 11ft above the ground.

Hope that makes things a little clearer.

 

[Marconi]

still fixable by trimming coax length.

Bob, I know your right, but when we hear that trimming coax is a fixer...it makes arguing that coax length does not matter...a real challenge...and often without a very good explanation that sounds plausible to the operator.

 

[Shockwave]

View attachment 37742

Yes it is. Here is a picture of my working conditions: See pics.

Sirio cover all con band incluidos the si called marines to all the aerials put the beginning of 10 meter band.


What impresa me is the bandwidth of the Gain Master, almost the triple of the 827zsee picture

Just want to point out the Gain-Master does not like another horizontal antenna directly underneath it. Being a balanced design, its TOA is pushed above the horizon more than an end fed groundplane would be in this type of installation. Just adding a large TV antenna 6 feet under mine, shaved off some performance that I could measure. Very little change in VSWR but I suspect if I was checking the bandwidth ends, they would have also shifted.

 

[bob85]

Eddie,
coax length does not matter if & only if you don't have cmc flowing on the outer braid,

when you do have cmc on the braid the current path is seen in parallel with the load which changes the impedance terminating the coax & changes vswr on the inside of the coax,

the currents inside the coax are always equal magnitude & opposite phase,
anything you do outside the coax with masts chokes radials will be balanced as described in kirchhoff's current law,

when trimming coax changes cmc/rfi in the shack it will also change vswr,

that's hardly a situation where coax length does not matter because it clearly does matter,
not just to the vswr but to how much rfi you get into your base mic / tv speakers,

if you trim to a length that carries less cmc you force more current into the radials or mast or something else connected to the coax braid to keep total return current in balance with current entering the antenna/load,

imho cb antennas stuck in the past, the pre nec days when people had no clue about mast & coax currents,
even the couple that bamboozle with pretty graphics omit the mast/coax making the model useless,
as a result none of them will be installed correctly if you follow the makers instructions,

properly isolating the antenna fork handle style, widing a correct size choke & giving the antenna radials for return currnts to flow where they won't disrupt the pattern fixes all the issues people have with incorrect antenna installs, or it has so far for us,

proper installation & same tip height is an amazing leveler of performance.

 

[Alexis Mercado]

Just want to point out the Gain-Master does not like another horizontal antenna directly underneath it. Being a balanced design, its TOA is pushed above the horizon more than an end fed groundplane would be in this type of installation. Just adding a large TV antenna 6 feet under mine, shaved off some performance that I could measure. Very little change in VSWR but I suspect if I was checking the bandwidth ends, they would have also shifted.

After all this time testing this antenna on top of the yagi, it doesn’t perform like I thought.

What I am planning is to replace it with an I-Max 2000 I have. Distance between yagi and top of mast is around 3 feet.

What I still don’t have clear is the use of a line isolator or not. Some people say that not using the line isolator will allow the I-Max 2000 to use the yagi as a ground plane, making it more efficient.

Any comments are welcome from anyone about an I-Max performance on top of a yagi.

 

[Marconi]

Ok Conor, here are my 3 A99 models with no radials set at 21' to the feed point with masts lengths set at 21', 16.4', and 10' feet below the antenna.

Models have no isolation and no feed lines included in the models.

1. these 3 overlays from my A99 matched models, using a 40:1 transformer and a simulated choke 51" inches down on the mast to help mitigate CMC.

2. I also added 3 overlays for the A99 model that was not matched for comparison.

 

[M0GVZ]

Nice, interesting. Thanks for doing it, work is stupid so no longer have the time to do it myself any more. Very little difference. Not sure what I was expecting but that wasn't it. Hmm...that'll give me something to ponder whilst i'm staring into darkness 400 miles a night.

 

[M0GVZ]

What I still don’t have clear is the use of a line isolator or not. Some people say that not using the line isolator will allow the I-Max 2000 to use the yagi as a ground plane, making it more efficient.

Any comments are welcome from anyone about an I-Max performance on top of a yagi.

Always use a RF choke. The antenna system will decide whether to use the yagi as a ground plane or the mast or the coax depending on which it sees as the easier route for any RF current to flow down at the frequency you're on. We use a RF choke to ensure it isn't the coax with all the problems that presents.