common mode currents/decoupling

[freecell]

you can't make any common mode current measurements if the antenna is still in the box or not up in its intended elevated mounting location.

you can't make any common mode current measurements if you don't have access to an rf antenna current probe, although there are some symptoms that have been long established as the result of the presence of common mode currents in the system, which by the way are seldom if ever a problem in properly installed ground mounted arrays.

 

[Bushmaster]

Quote:

"which by the way are seldom if ever a problem in properly installed ground mounted arrays."

This particular installation is not ground mounted. The base of the antenna is going to be 20 feet off the ground.


Quote:

"If you're talking about a groundplane antenna, where the radials take the place of earth ground, there's really no point in doing that isolation thingy. The mast/support would only be adding to the ground, which ought'a be a good thing."

That's just it! I keep being fed contradictory information. I keep seeing this same thing being stated, over and over: That it is "NOT a good thing" to have your mast ADDING to the ground. I've seen it stated over and over, that if you do not "isolate the antenna from the mast", then you distort the pattern of the antenna, and introduce more "ground loss".

I've seen it time, and time again. People saying "keep an elevated vertical isolated from the ground". "Do not ground the radials".

So which is correct?

Is it a "good thing" to have your mast act as another "radial", or is it better to PREVENT continuity between the antenna base (on an ELEVATED antenna) and the ground? And if it is better to ISOLATE the antenna radials from ground, HOW does one do this? Electrical tape wrapped around the mast top? And if so, then how does one ground this antenna against lightning?

This question (and the answer I received) is why I am STILL confused about this. I'd really like to put this question to rest in my mind, because I still don't know which is correct, and HOW to isolate the antenna from ground (if that is the way to go).


Anyone??

 

[bob85]

doc, my buddy was pulling his hair out trying to stop the rf,
the a99 was mounted on a 21ft aluminum scaffold pole,
i isolated the a99 from the conductive pole then added 2 wire 1/4wave radials and an ugly balun at the feedpoint,
it gave increased signal strength comparable to a doubling of transmitter power on my jrc signal meter and a marked reduction in rf in my buddies shack, its hard to believe even for me and it was me that did it,
i had a brainwave and it seemd to work much better than anybody local expected, rf problem gone and locals saying your signal has gone up what have you done, nothing scientific just an observation from several local stations including my own, i dont get it so i went looking for answers,

i dont know how much this guy knows but he seems to agree, what i did met his criteria,

[FONT=Times New Roman, Times]Summary [/FONT]

[FONT=Times New Roman, Times]The cure for common mode problems caused by less-than-perfect grounds is inserting a 1:1 choke balun in the system at the antenna feedpoint. The coax should also be kept away from the radials as it exits the area of the radials and the antenna. An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly). There is no exception to this![/FONT]

http://www.w8ji.com/verticals_and_baluns.htm __________________

if hes got it right then the vast majority have got it wrong.

 

[Beetle]

W8JI knows a bit about radio. He designed a lot of the amplifiers formerly and currently in production at Ameritron, including the AL-811 and -811H. He knows what he's talking about, and you can take it to the bank.

 

[Marconi]

Bob and Beetle, read the following which preceeds the Summary notation and text that follows in Bob's post.

"Longwire and Windom antennas really aren't much different than verticals. They are a form of Marconi antennas, and require a counterpoise or ground of some sort. As with verticals, common mode current flowing into the antenna must be balanced by current flowing into a ground system.

Instead of bringing the longwire directly to an antenna tuner, a better solution is using an RF ground system independent of the station safety ground, and keeping that ground isolated from the station safety ground."

Then read the summary below again and tell me if the article is suggesting that a grounded mast should be insulated from the radials or that a grounded mast should be independant (insulated) from common point ground for the station? Could it be that the word radial in the text below should not be there?

Summary

The cure for common mode problems caused by less-than-perfect grounds is inserting a 1:1 choke balun in the system at the antenna feedpoint. The coax should also be kept away from the radials as it exits the area of the radials and the antenna. An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly). There is no exception to this!

 

[freecell]

the use of the term radials is proper when referring to antennas designed to operate wholly independent of earth ground.

on the other hand both longwire and windom antennas are rarely seen in combination with any counterpoise or radials operating in the same fashion and are designed to operate against an earth ground reference.

W8JI simply fails to make the distinction between the two, as he also fails to inform us that less-than-perfect ground is only one cause of common mode currents flowing on the outer shield of a feedline or support structure.

the primary cause is that we're attempting to feed a balanced antenna with an unbalanced feedline, hence the use of the balun, (BALanced antenna > UNBALanced feedline)

that's only one of many reasons why the ZeroFive antennas operate so well when ground mounted because the earth ground reference + radials is "more perfect" than an elevated radial system with fewer radial elements operating independently of earth ground. when you choose to operate it in an elevated mode you've just created more work and expense for yourself because to fully uncouple the radials from earth ground the feedpoint will have to be raised quite high above the ground, especially at the lower operating frequencies.

stray capacitive coupling between earth ground and an above ground radial system is still quite noticable at heights up to just under one wavelength above ground, not to mention the fact that now you have a support structure that will conduct common mode current where in a ground mounted antenna there is no such path. if you still want to use it in an elevated fashion i suggest that you take all of this into consideration and calculate the height above ground at one wavelength at the lowest operating frequency you intend to use and see just what you're getting yourself into.

 

[bob85]

eddy, maybe i am reading it wrong but i take it he was refering to verticals with insufficient radials to constitute a perfect ground and that such an antenna should not be grounded to a conductive mast at the junction of the radials and feedline outer shield,
a no radial endfed or 4 radial vertical groundplane fits that description to me,
some would say i make it read how i want it to read and that may also be true but it is unintentional,
i would hardly compare 4 radials or no radials to the 120 wire ground mounted radial system let alone the perfectly conducting infinate ground no mast and feedline that is often used in antenna models when we see the huge difference between 1/4 1/2 and 5/8waves in the polar plots which dont seem to be born out in real world tests over real ground and typically grounded mast no balun installs,

i dont fully understand it maybe i dont understand it at all but what i did clearly altered the pattern of that a99 in a positive way and fixed the rf problems in the shack,
it is something i will be experimenting with in the future.

 

[Marconi]

Bob don't take my previous words as an agrument questioning your results.

When I read the words "common point" I associated the term with electrical grounds and building code requirements. I read the W8JI article and I found the statement you posted at the bottom of the article. So, I went backwards in the article and noted that it was in fact talking about safety grounds and such, so I figured that was my alert and responded.

I did not understand that the reference to the words "common point" could also refer to radials and antennas too. You learn something new ever day.

Never mind!

 

[bob85]

eddy i did not for a moment think you were making any argument, its not in your nature,
if i am reading what the guy is saying incorrectly i want to know about it

 

[W5LZ]

What do radials do? They take the place of 'ground', dirt, the other 'half' of the antenna. Radials are elevated ground/dirt. Will it hurt to insulate that elevated 'dirt' from real dirt? I very much doubt it. Can a radial be of any length? No, but they are not as length critical as some people think. Why? Because RF currents seek the path of least resistance (actually, impedance, since we're talking about an alternating current). Also why radials of several different lengths are used on multi-band verticals. Does that mean that the radiation pattern isn't as symmetrical as it could be? Yes, but that pattern isn't as skewed as you might think. If those radials are distributed in a sort of symmetrical way, the resulting radiation pattern will be too. What do you do if the skewed pattern is skewed too much? Rotate the @#$ radials a little till it's pointed correctly!
Put the silly thing together. Put it up in the air and -see- if there's anything to worry about. Then worry about it.
- 'Doc

 

[Bushmaster]

Quote:

"that's only one of many reasons why the ZeroFive antennas operate so well when ground mounted because the earth ground reference + radials is "more perfect" than an elevated radial system with fewer radial elements operating independently of earth ground. when you choose to operate it in an elevated mode you've just created more work and expense for yourself because to fully uncouple the radials from earth ground the feedpoint will have to be raised quite high above the ground, especially at the lower operating frequencies.

Stray capacitive coupling between earth ground and an above ground radial system is still quite noticable at heights up to just under one wavelength above ground, not to mention the fact that now you have a support structure that will conduct common mode current where in a ground mounted antenna there is no such path. if you still want to use it in an elevated fashion i suggest that you take all of this into consideration and calculate the height above ground at one wavelength at the lowest operating frequency you intend to use and see just what you're getting yourself into."





The reason I am mounting my Zerofive 43 multiband vertical in an elevated position, is because I have no other choice. My property allows no other possiblity. And I definitely do not have the real estate necessary for a low-band horizontal antenna. No way to even use an 80 meter inverted V here. My only choice is the vertical, and my only choice is to mount it on 20 feer of pipe (mast) bracketed to the eave at the end of my house. I will be able to use a decent number of radials for all bands (except 160), and only a few for 75/80.

Tom at Zerofive told me that I should make a radial for 80M about 87 feet long, to make the antenna act like a vertical off-center-fed dipole. (43 foot antenna +87 foot radial = 130 feet) A half wavelength on 80 meters. I don't know if this would be better, or whether it would be better to add several 65 foot (quarter wave) radials for 80M. What do you think?

But one thing I'd love to FINALLY understand, is HOW does one isolate the support mast from the antenna base? See, Bob even says this again:

"i isolated the a99 from the conductive pole then added 2 wire 1/4wave radials and an ugly balun at the feedpoint"

But he doesn't say HOW he "isolated the A99 from the conductive pole". This is driving me nutz! I've asked this question dozens of times, on various forums, and STILL cannot get anyone to explain this (seemingly) simple concept.

Bob.. (or anybody)... HOW did you "isolate" the antenna from the pole? Electrical tape?

How?

 

[bob85]

i used the only insulator that was handy, a strong piece of wood wrapped in insulation tape and hammered into the 1.5" id scaffold pole, high grade phenolic or fiberglass rod would be my first choice especially if the antenna was expensive,
bear in mind this was only an a99 on 11mtrs.

 

[W5LZ]

Bushmaster,
You're getting into that 'isolate/insulate' thingy here. Electrical tape will insulate the antenna from the mast, but it won't 'isolate' it. Best 'isolator' would be distance to reduce any capacitive/inductive reactance between antenna and mast. Convert that 'distance' to a longer insulator (non conductive mast?). How long? Whatever you can manage without breaking the bank and is still reasonable/practical electrically and mechanically, sort of. After that, you are 'stuck' with whatever it happens to be. While it ~can~ be, it doesn't necessarily have to be a 'big deal', and you won't know till you try it.
Half the fun of it is in the 'doing', not the results...
- 'Doc

 

[Bushmaster]

Quote:

'Half the fun of it is in the 'doing', not the results..."

While I do appreciate the sentiment, the fun for me starts the day I can stop climbing up on my roof, and start using my radio.

I guess the thing that frustrates me the most, is that I keep getting TOTALLY conflicting info, everywhere I turn. Most places I look, say to "make sure to isolate the radials from the mast, and don't ground them". Other people say it doesn't matter. In fact, Tom (at Zerofive) told me that his experiments do not support the "non-grounded radials", and the "isolate the radials from the support mast" contention. And he is quite aware that I am talking ELEVATED radials here.

Most people say that you should "decouple" the radials from ground, on an elevated vertical. I've gotten to the point that I don't really care one way or the other. In fact, I think I'll ground the hell out of, just for spite.

 

[freecell]

Zero-Five Antennas
43 FOOT 10 THROUGH 160 METER
MULTIBAND VERTICAL
INSTALLATION NOTES

Radials are required

Your ground radial system is the most important part of vertical antenna performance. When installing ground-mounted radials use radial lengths between 20 and 43 feet, with 43 foot being the choice for best performance. If you have to use shorter length radials, put more down. good place to start is a minimum of 32 with 120 being the best. When choosing radial wire, #14 insulated stranded wire should be used. You can buy it at Home Depot or most local hardware stores. When installing radials on a ground-mounted vertical, they do not have to be cut to resonance. Only when using elevated radials isolated from ground do the radials need to be cut to length for the various bands. A radial plate should be used and all connections soldered and coated with liquid electrical tape.

COAX CHOICE

A good low loss coax should be used with this vertical. For runs up to 150 feet, Bury flex or LMR 400 works great. For longer runs or near-legal-limit operation use LMR 600 or ½ inch heliax. Please remember that this antenna system is NOT resonant on any of the amateur bands. As a result, the feed line is operating at a higher than normal SWR most of the time. This can cause additional stress on the coax run especially under high-power conditions. Be sure to use a coaxial cable heavy enough to carry the power you’ll be using plus an additional safety margin due to the high SWR conditions.

ANTENNA TUNER CHOICE

A good quality antenna should be used with this vertical. Examples are Palstar, Dentron, Ameritron ATR 30. Some SGC remote tuners will not match some bands on this vertical and should be avoided. Most rigs with built in antenna tuners will have no problem with matching this vertical if the radio power only will be used. If your tuner doesn’t match the antenna on a particular band, try adding 10 to 25 feet of coax using a barrel connector and try matching again. Sometimes, certain lengths of coax cable will cause an antenna to not match on one or more bands. This is function of the length of the coax cable NOT a design defect in the antenna itself. See almost any ARRL Antenna Book for a discussion of feed line lengths and proper coax use.

A BALUN IS REQUIRED

This antenna REQUIRES a 4 to 1 balun be used at the feed point if you are feeding this antenna with coaxial cable. The balun should be rated for the power level you intend to use. For full legal limit installations we recommend that the balun be capable of around 5 KW for best performance and safety. Baluns with a lower power rating may fail due to heating of the balun under high power levels and extended transmitting times. Again, the balun and feed line are under some stress during normal use so don’t skimp when choosing a balun or coax cable especially under full legal limit conditions. If a “tuner” version of your favorite balun is available, please use that. Tuner versions of baluns tend to tolerate the higher SWR better and have less heating under high power conditions.

PLEASE REMEMBER THAT THIS ANTENNA SYSTEM IS NOT RESONANT ON ANY OF THE AMATEUR BANDS. As a result, the feed line is operating at a higher than normal SWR most of the time.

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