Sirio 827 with longer radials

[bob85]

Connor you probably expected different magnitudes of cmc with different length masts,
that's what usually happens with your typical uk install,

If im reading what Eddie posted correctly he put a choke in the masts to reduce cmc so mast length will have very little effect in this comparison.

 

[Marconi]

Connor you probably expected different magnitudes of cmc with different length masts,
that's what usually happens with your typical uk install,

If im reading what Eddie posted correctly he put a choke in the masts to reduce cmc so mast length will have very little effect in this comparison.

You might be right Bob. I don't know what Conor expected with his shorter mast length ideas, but if I add a Feed Line to these models, it would simply provide another conductor path, of unknown length, for CMC to flow on the model.

The models I made only tried to demonstrated what an A99. with these short masts lengths might look like, if there were no CMC flowing on such a system, and instead had a coaxial choke or other type...that worked as intended.

I could also isolate (ISO) the mast from the antenna at the top an inch or two and see similar results. So, ISO by itself has limitations also.

I'm sorry I mislead you guys with these models. IMO and not to be confused, I don't think they are what they seem...surely when compared to a Real Earth antenna installation.

Personally, I don't think this idea of shortened mast is likely to produce any advantages over a grounded type installation and it maybe just be a convenience.

I don't like the idea of installing CB size antennas on or attached to my home, but I learned that the hard way. And down the road I paid big money to fix those problems.

I also see CMC increasing a little on the masts in the both models I presented with shortened masts, when compared to the full length mast model. In that case the simulated choke did not work the same with all 3 models and maybe that should not be surprising.

 

[bob85]

Eddie,
the way i look at it is we can never know the electrical length of our feedline common mode path so theres no point including it in a model,

BUT we should remember that the coax is exactly the same as the mast with regards cmc & just because we don't include coax in the model does not mean it won't have an effect,
you will be lucky if it does not if you take no measures to isolate the outer braid,

a mast will show cmc & pattern skew the same as coax outer braid, the mast is something we know the length of & if its grounded or not so its ideal for your models,

can you remove the choke from the masts so we can see the cmc & pattern with different length masts ?
i don't expect much with a 1/2wave but a99 can cause lip burning rfi,

not as bad as no radial 5/8waves.

 

[Marconi]

Good idea Bob.

 

[Marconi]

Here are the 3 A99 models with no Chokes.

Bob, I am matching these 3 - EFHW antennas using the Eznec 40:1 transformer feature at the feed point.

This is not quite the same as the 1/4 wave variable mutual transductance matching that the A99 uses. So, I guess we could say these models are not real A99's, if that make a difference.

Burning the lips coming up. Here are the 3 tuned models with the Choke removed. I was surprised that the patterns and the currents were not that much different, excepting for the model with the 16.4' foot mast.

Can't explain why yet, except to suggest maybe it is the transformer effects.

This is getting deeper into "Questions" than I imagined at the start, but that makes it more interesting to me.

 

[bob85]

Eddie
we know that in the real world an a99 works & shows a low vswr on an analyzer with no mast so there must be enough conductor for return currents using the short coax & mounting tube to keep kirchhoff happy even when the a99 has no mast or coax braid,

a99 & imax have a foot or so of coax with a foot or so of tube folded back over it, the top of the mounting stub near the rings is a couple of feet or more away from the feed-point electrically,

so its not exactly the same as a model with only a matching transformer that can only use the mast or your coax braid for return currents because they are the only conductors available,

that does not mean 1/2wave end-feds are immune to cmc,

give it a low impedance path for return currents & any end-fed can have cmc on the mast or feed-line that spoils the pattern & more important nowadays picks up more local noise than you would like,

The low gain model looks like a 1wavelength dipole,

"END-FED antenna"

Disambiguation,
The known electrical length of ONE part of an antenna system with an unknown electrical length.

 

[Marconi]

I checked the model with 16.4' foot mast and the radiator is 17.3'. I didn't find any problems, so you're right Bob.

The PDF files below:

1. shows the A99 that is 21' feet to the feed point, with a 40" inch mast, no ISO, no Choke, with 40:1 transformer.

2. shows the same as #1 with the 40" mast removed from the model.

3. shows the same as #2 with the transformer removed from the model.

I set each model to Free Space, so I could see the Average Gain error report at the bottom of the Eznec Control Screen. The gain for all 3 models were under-stated a little. So I made a note, adding the error value to the gain for each model in the Far Field pattern report...that shows us the gain and angle reported by the model.

This tells me that my model with the 40" inch mast and with the transformer needs some work, because the error of -0.50 db is out of range for good accuracy.

This is like working on a real world antenna, it can be full of surprises, and is not always easy.

 

[bob85]

Eddie,
what is the model using for return current when you remove the mast ?
or when you remove the mast & transformer,

looks like a source with nothing to push against, look at the crazy voltage

 

[Marconi]

Bob, it's not a good model.

Your wife makes you the best Apple Pie, because she puts her skills, love, and caring into the process...while others might just take short cuts.