IMAX 2000 GROUND PLAIN?

[Marconi]

isolating the imax and adding a feedline choke without radials expecting that to minimise or eliminate cm current on the coax braid won't work,
w8ji explains why that is & kirchhoff's current law tells us he's right,

the solarcon radial kit can work ok with isolation and a choke to minimise cm current issues, the radials need not be 1/4wave.

Bob, since we talked several years ago on the subject of chokes I also believe it works as you described back then. The idea seems logical and to me makes good sense.

I think I understand that the choke will not work right without radials because the currents must have somewhere to go in order for the choke to complete its work stopping CMC's...at the point in the line where radiation begins.

If this is true, then how do we explain the choke stopping the CMC dead in their tracks that are at work in the bottom of the Gain Master...without there being radials included in that situation?

 

[wavrider]

Morning Marconi,

I am just enjoying the first cup of coffee here and reading the thread.

I am not BOB but I will give it a shot to answer your question.

The gain master is a vertical dipole,( at least I think it is by looking at the manual.)

So it is a balanced antenna per say, no radials needed.

 

[Lil'Yeshua]

How many if any of the turns of a RF choke still has CMC's radiating from it until it stops. I know it sounds like a mute point but I'm wondering about where is the other half if any on a GM? Does the RF choke take up the slack for not having ground plane radials?




And I haven't had my morning coffee yet.

 

[W5LZ]

The key to it all is that 'other half' of an antenna. That 'other half' doesn't have to be a 'groundplane', it can be almost anything, sort of. It has to have someplace to go, that's right. In the case of an 'A99' or Imaxx, that 'other half is the feed line. The common problem with that is that the feed line is close to things that will affect it as the 'other half' of an antenna. So, you minimize the amount of interaction if/when you can.
The radiated signal is on the outside of that feed line, the same as that CMC, right? So how do you get rid of one without affecting the other? That's where the length of the feed line enters the picture. Place a choke at the 'right spot' and it leaves the signal alone but attenuates the CMC. So where's that 'right spot'? Good question, and the answer will be different for every installation, to some extent. Lots of variables in that.
Have fun.
- 'Doc

 

[The DB]

How many if any of the turns of a RF choke still has CMC's radiating from it until it stops. I know it sounds like a mute point but I'm wondering about where is the other half if any on a GM? Does the RF choke take up the slack for not having ground plane radials?




And I haven't had my morning coffee yet.

The other half on a Gainmaster is the bottom half of the vertical part of the antenna. Its not like an A99 or Imax which have a single vertical conductor. It actually runs coax half way up the antenna, then connects the center lead, through a capacitor I believe, to the top vertical element. The "other half" is common mode currents on the outside of the coax that runs half way up the antenna. These common mode currents are why it has a coax choke built in to the design. Think of the layout kin of like a vector, except instead of upward facing radials it has coax shielding. Or if you have ever seen designs for a half wavelength center fed coax dipole antenna using the center conductor for the top vertical element and the coax shield for the bottom half, these antennas also have some form of choke built into their design for the same reason as the Gainmaster.


The DB

 

[Marconi]

Morning Marconi,

I am just enjoying the first cup of coffee here and reading the thread.

I am not BOB but I will give it a shot to answer your question.

The gain master is a vertical dipole,( at least I think it is by looking at the manual.)

So it is a balanced antenna per say, no radials needed.

I agree wavrider, but the bottom of this balance dipole is not just hanging out in space like a typical CF dipole. The RF in the top of the GM is radiating from the wire in the top just like we would expect with any other antenna of this sort, but the bottom half is made up of the coaxial feed line.

IMO the bottom of the GM antenna works using the principal of CMC's to produce the RF that must come from the bottom wire. The choke is then placed in the antenna at a point of the design where the CMC's are to be terminated from radiating in phase with the top of the antenna in order to maintain the balance noted by the design.

I see this choke working precisely like is intended, but it does it entirely and effectively without radials in this case, so why can't a choke in another design work without radials. I don't recall what W8JI wrote on this subject, so that is why I asked Bob the question.

My question is how can the choke work without radials on the GM, but the choke absolutely needs radials to work on the Imax antenna like Bob says W8JI claims and Kirchhoff's law affirms.

isolating the imax and adding a feedline choke without radials expecting that to minimise or eliminate cm current on the coax braid won't work,
w8ji explains why that is & kirchhoff's current law tells us he's right,

the solarcon radial kit can work ok with isolation and a choke to minimise cm current issues, the radials need not be 1/4wave.

 

[Shockwave]

The difference is the GM looks like a center feed antenna and all the others look like an end fed. End fed needs a radial or counterpoise system where center fed does not.

 

[Marconi]

I agree DB, the GM and the CF coaxial dipole antennas are similar in their use of CMC's in the bottom of the antennas, but it looks to me like the big difference is the bottom of the center fed coaxial dipole is physically terminated, and thus needs no choke.

When I use a clamp on choke on a line to keep RF current from passing a certain point there is no other element required to accept the currents to another area or element. If the choke works at all, then placing the choke on the line is all that seems necessary in my experience. Is that different than placing a choke that works on a feed line of an antenna?

What am I missing?

 

[Brain the Dog]

I have the A99. Want to add the GPK to it. But this is the first time I've heard about isolating the antenna from the mast pole... I thought having it bolted to the mast pole directly was actually part of the grounding system?

How does one isolate the antenna from the mast pole? What is a balun?

And if you isolate the antenna from the mast pole, how do you ground the whole thing?

 

[The DB]

If you are gong to use a ground plane kit you don't normally need to worry about isolating the mast or choking the feed line. Most ground planes do a very good job of those two things. If you have a ground plane, I wouldn't bother with either of those unless you had a problem.

When it comes to isolating the antenna from the mast, you can get a solid 2 foot fiberglass rod between the mast and the antenna. What most people call a balun for this purpose isn't actually a balun. What a balun does is take an unbalanced source and force it into balance. *Some* baluns provide isolation as well, or resist/prevent unwanted currents from flowing on the outside of the coax. Not all baluns, however, do this.

Something else that will resist the flow of unwanted currents on the outside of the coaxial feedline is called a choke. Some people call them "ugly baluns". A specific number of turns on a form of a certain size diameter does an excellent job at preventing said currents from flowing. The diameter and number of turns are very specific, more "just is case" is not better, and can actually lead to problems of their own. Generally though, when someone refers to a "balun" when referring to a vertical antenna, what they actually mean is "choke", and generally they don't even know it.

When it comes to grounding an isolated antenna, you can use a polyphaser, or other lightening arrestor that attaches inline with the coax and has a terminal to attach directly to a ground rod. The purpose of such devices it to prevent a static charge from building up on the antenna, and thus help minimize that chance of lightening strikes. It is not full proof, but then neither is grounding the antenna directly to the tower. In either case, if you take a direct hit, or even near hit, your antenna will be lost.

It should be said, when it comes to radio systems, there is more than one ground type in play. Each type of ground has its own requirements and idiosyncrasies. An RF ground for an elevated antenna, for example, doesn't act anything like how most people think a ground system acts. Not even close. In a bad situation, the mast/tower section could actually radiate, affecting various aspects, not the least of which include how the antenna system radiates, how much noise is picked up, how it appears to be tuned, ect. Further, a lightening ground would do better with a single very deep ground rod, while and RF ground would actually work better with more shorter ground rods spread out over an area in a fan configuration...


The DB

 

[Sonar]

I knew a guy that used the ground plain kit upside down on his Antron 99 ....he swore up and down that it helped reduce a lot of his TVI .....He always sounding great !! ......I wasn't going to argue with him or doubt his theroy , but boy did that antenna look silly up ther in the sky like that !! ......but I always though those sigma 4s looked kind of strange as well but boy did those antenna's kick some a$$ out there on the airwaves !!

I haven't owned many antenna's but when I had my sigma lV up on a ten foot mast mounted on top of my two storie home (1983) I got out so much better than I did with the antenna I replaced it with.
Granted that antenna was a big stick but the difference was tremendous! I actually liked the way the sigma lV looked..
Is there an antenna like the sigma lV still being produced?
If so I'd like to do a little reading.. 73

 

[Blackcat630]

I haven't owned many antenna's but when I had my sigma lV up on a ten foot mast mounted on top of my two storie home (1983) I got out so much better than I did with the antenna I replaced it with.
Granted that antenna was a big stick but the difference was tremendous! I actually liked the way the sigma lV looked..
Is there an antenna like the sigma lV still being produced?
If so I'd like to do a little reading.. 73

http://www.sirioantenna.com/index.php?main_page=product_info&products_id=190

 

[Brain the Dog]

If you are gong to use a ground plane kit you don't normally need to worry about isolating the mast or choking the feed line. Most ground planes do a very good job of those two things. If you have a ground plane, I wouldn't bother with either of those unless you had a problem.

When it comes to isolating the antenna from the mast, you can get a solid 2 foot fiberglass rod between the mast and the antenna. What most people call a balun for this purpose isn't actually a balun. What a balun does is take an unbalanced source and force it into balance. *Some* baluns provide isolation as well, or resist/prevent unwanted currents from flowing on the outside of the coax. Not all baluns, however, do this.

Something else that will resist the flow of unwanted currents on the outside of the coaxial feedline is called a choke. Some people call them "ugly baluns". A specific number of turns on a form of a certain size diameter does an excellent job at preventing said currents from flowing. The diameter and number of turns are very specific, more "just is case" is not better, and can actually lead to problems of their own. Generally though, when someone refers to a "balun" when referring to a vertical antenna, what they actually mean is "choke", and generally they don't even know it.

When it comes to grounding an isolated antenna, you can use a polyphaser, or other lightening arrestor that attaches inline with the coax and has a terminal to attach directly to a ground rod. The purpose of such devices it to prevent a static charge from building up on the antenna, and thus help minimize that chance of lightening strikes. It is not full proof, but then neither is grounding the antenna directly to the tower. In either case, if you take a direct hit, or even near hit, your antenna will be lost.

It should be said, when it comes to radio systems, there is more than one ground type in play. Each type of ground has its own requirements and idiosyncrasies. An RF ground for an elevated antenna, for example, doesn't act anything like how most people think a ground system acts. Not even close. In a bad situation, the mast/tower section could actually radiate, affecting various aspects, not the least of which include how the antenna system radiates, how much noise is picked up, how it appears to be tuned, ect. Further, a lightening ground would do better with a single very deep ground rod, while and RF ground would actually work better with more shorter ground rods spread out over an area in a fan configuration...


The DB

Thanks. Could you tell me exactly the best way to mount my A99 with GPK? My plan is to have a metal pole about 36 feet tall, cemented about 5 feet into the ground. I'll attach the A99 to the top of that. I already have a grounding rod 8-feet into the ground, and I will use that in this setup. So based on this premise, can you advise me exactly what I should do?

Isolate antenna from pole?

Balun, ugly balun setup? (I use RG8X coax, btw...)

Thanks!!