Does height above Earth effect the common mode current response?

[Marconi]

" ...... I just now tried readings on my stationary field strength meter with a 14' coaxial antenna, and when adding 4 feed of coax to my Starduster shows a 100% increase in FS indicated vs. the regular feed line. ...... "

what is that %100 equal to in DB ? also , what were you measuring ? the signal strength from the antenna or a increase in CMC's ?

and what happens when using a vertical with proper 1/4WL ground elements ?

Booty, my FS meter is a digital RF sensor, and it reads up to about 1005 units, whatever the units are...I'm not sure.

Like most CB FS meters, it is just a reference. It read 97 with the feed line before I added a 4' jumper, and 192 after. I don't know what the readings mean in DB's either, the value just increased about 100% with that simple little change. I was surprised I saw that much change even thought I was expecting something to show up.

It was just reading RF field strength in the area around this meter and antenna which I have hanging near the ceiling in my shack. I have no idea what type of signal it is sensing, antenna or CMC. Maybe it is a combination of both.

If you have a FS meter that is sensitive enough to register somewhere in your shack, you can try the same comparison and see what you get. Let me add that neither value effected my PC speakers which usually tells me I have a problem with RF in my radio room.

I was transmitting on my Startuster up at 46' feet to the hub. What do you mean when you ask,

"...what happens when using a vertical with proper 1/4WL ground elements?"

 

[Marconi]

I think you would find just moving the source to favor the opposite side of the dipole would shift the entire curent imbalance to the other side of the dipole too. This is because a majority of these problems are the result of poor antenna design or poor matching that places standing waves on the coax line.

I know for example you can take a Sigma and tune it for a perfect 50 ohm match and experience very little if any CMC problems. If you detune that match the slightest bit you can clearly see the results of the standing waves on the coax with increased CMC problems.

This is demonstrated by the fact the position or length of the feedline shows no change in the resonant SWR when tuned correctly. When detuned even the slightest change in the position of the coax in relation to the mast can show measurable changes in reflected power. Proving we have now made the feedline a substantial part of the radiating antenna.

If the Yagi element were fed in a balanced manner and simply had its center grounded to the mast the maximum current would be in the center and trough the element with virtually no RF potential on the mast. Of course things like coax and real world matching devices don't typically allow this to work perfectly.

This makes perfect sense Shockwave, but the antenna feed point impedance also changes on changing height, so excepting that Nature doesn't require us to have a perfect setup...we can likely never get thing right.

This is why I was suggesting this idea, so folks could consider what might be go on when changing the antenna location, height, or design. Get the antenna match perfect at 10' feet and raise it to 30' feet and the match changes.

I will add, that if your antenna is structurally causing CMC and TVI, nothing will mitigate that except to fix the problem.

This is more to this puzzle to consider.

 

[BOOTY MONSTER]

" ...... I was transmitting on my Startuster up at 46' feet to the hub. What do you mean when you ask,
Quote:
"...what happens when using a vertical with proper 1/4WL ground elements?" "

thanks for clarifying your FS meter readings .


i was asking because this thread started about a horizontal dipole on top of a metal mast and insulated from it . FWIU CMC's are typically (but not always) caused by the lack of a elevated ground-plane on omnis . i was wondering if the H dipole was seeing the mast as a potential ground element and if a typical vertical with proper ground elements would satisfy the antennas need for a ground-plane . but since you were using the starduster now that negates my idea .

BTW , are your models earth grounded ? FWIU a ground wire from a antenna to a ground rod is electrically seen as a mast .

 

[Marconi]

" ...... I was transmitting on my Startuster up at 46' feet to the hub. What do you mean when you ask,
Quote:
"...what happens when using a vertical with proper 1/4WL ground elements?" "

thanks for clarifying your FS meter readings .


i was asking because this thread started about a horizontal dipole on top of a metal mast and insulated from it . FWIU CMC's are typically (but not always) caused by the lack of a elevated ground-plane on omnis . i was wondering if the H dipole was seeing the mast as a potential ground element and if a typical vertical with proper ground elements would satisfy the antennas need for a ground-plane . but since you were using the starduster now that negates my idea .

BTW , are your models earth grounded ? FWIU a ground wire from a antenna to a ground rod is electrically seen as a mast .

Booty, I haven't rechecked all the posts, but I think you were the only one that raised an issue about isolation or insulation of the dipoles...maybe in your first or second post when you asked a what if question.

Except for the first two horizontal models, without a mast, that I used to demonstrate how the currents remained balanced and symmetrical all the way across the radiator at both heights used...my models here were all connected to the ground and Earth grounded.

When I did the dipoles, my thinking was to use a simple antenna that was less complicated and naturally had a good enough match to help compare how Eznec handle the currents with and without a mast.

If you check Shockwave's last post to me you will see that he raises a mismatch question already about the match I got using the dipoles, and they are known to be off and unbalanced just a little bit. Imagine what he would be using as an argument if I posted the 5/8 wave models to start...having a match with a 23.3:1 SWR.

My focus in this project was to demonstrate this idea using 5/8 wave models, but demonstrating that one would be like opening a can of worms.

To be clear Booty, the Eznec ground symbol is indicated with a mast (a wire) connected to a small circle with a black dot in the middle. This symbol is typically noted at the bottom in the Antenna View.

I don't think I'm getting my ideas across here, and as simple as it is...I don't think anybody will try and duplicate the idea or check it out.

 

[Shockwave]

When I did the dipoles, my thinking was to use a simple antenna that was less complicated and naturally had a good enough match to help compare how Eznec handle the currents with and without a mast.

If you check Shockwave's last post to me you will see that he raises a mismatch question already about the match I got using the dipoles, and they are known to be off and unbalanced just a little bit. Imagine what he would be using as an argument if I posted the 5/8 wave models to start...having a match with a 23.3:1 SWR.

You may have found that in the EZNEC models, impedance and SWR seem to have little to no effect on any of the performance characteristics. This is because the program simply reports what the impedance is at a given frequency allowing you to design the appropriate matching device. If you fail to correct that match in the real world, it's effects on coax radiation can be much more apparent.

 

[Marconi]

You may have found that in the EZNEC models, impedance and SWR seem to have little to no effect on any of the performance characteristics. This is because the program simply reports what the impedance is at a given frequency allowing you to design the appropriate matching device. If you fail to correct that match in the real world, it's effects on coax radiation can be much more apparent.

I have thought about that before SW and what you say makes sense, but I couldn't figure out why that was or how to fix it. None of the Eznec attached models seemed to discuss or demonstrate fixing the match, and those that I thought did such...did not look good to me.

I also see the currents and phase not making sense sometimes, and I can't figure that out either. So, I figured I just didn't understand well enough, or else it was like a lot of other stuff we hear about concerning radio operations: like when we hear that a modestly high SWR, too much reflection, CMC's, lack of radials, improper radial length, or to much reactance is a big problem. Then after a lot of discussion we sometimes find out this all could just as well be so small a difference as to not really matter just using our radios.

Thanks for your comments.

 

[Marconi]

Shockwave, I decided to add a feed line instead of using the mast, just to see if that changed what I was seeing in the models I posted. Doing it that way I could make the line longer/shorter without changing the height. So, I could also test the height idea separately from the line length.

I didn't see any major changes in what I was seeing earlier with the dipoles at 36' vs. 40.5', but I think I can confirm that the worst/best case scenario ideas that W8JI talks about sometimes...where guys can just get lucky with the height of their installs is surely possible in the real world. Thus I think we can help mitigate CMC by using the right height and feed line length. He didn't elaborate on the subject, and I think I know why...the idea and CMC's can be complicated. There is more to it, so it might not be practical to even try the idea, but it might be worthwhile to try and understand how the line length and height can effect these currents like it does the impedance, angle and gain. No doubt it may be best to use a suitable choke if you have such problems however.

I did some tinkering though, and I think I can also confirmed what Jay in the Mojave posted in his I-10K manual, where he recommended a particular coax and the length for proper tuning of his antenna.

A long time ago I told Bob about my idea on CMC, and that I thought Jay was on to something...when he recommended a 50' foot feed line. IMO Jay probably figured it out testing the tune on his I-10K and saw it jump all around unpredictably. I did some testing back when?, using a 50' feed line and saw some very good results that placed my match right at the resonance I wanted. For some time Bob and I discussed this problem with our I-10K antenna, but when Bob heard the comments above from me...he rejected the idea as just smoke and mirrors, so I let the idea go.

Except I kept in the back of my mind that my old radio mentor use to always use 50' foot feed lines for 11 meters, and he use to caution me to keep my CB feed lines closer to random non resonant lengths...instead of 1/2 wave lengths. I think Bob might remember. He remembered the other day that I have tried to raise this issue before.

I think Jay might have figured out his issues with CMC's by testing, tuning, and eliminating CMC as best he could using the feed line and the length he recommended to us, because when he started to develop his beam antenna I recall he was acutely interested in trying to stop the bad effects of CMC on his tune. So he had CMC on his mind.

Now with the feed line added, I can make the feed line 50' long and still keep the antenna at 36' feet high, and cut the currents down on the mast to almost nothing...compared to the 36' foot mast length earlier. That also confirms that you are right, the feed line length is probably the primary actor here.

If this new approach continues to produce something interesting, I'll post some models with some ideas latter.

I started a new medicine yesterday and it's making me feel bad this morning.

 

[bob85]

yes i remember you talking about jays 50ft of coax, i don't recall it been offerd as a cure for cm currents,

when the topic of isolation and chokes was talked about the desert rats denied the i-10k can have cm currents, jay said the i-10k needs no isolation nor chokes because the radials decouple the mast/feed-line,

why would jay think the 50ft of coax was to avoid cm currents they believe don't exist?

you are going over old ground with nothing new with regards what length coax or mast grounded or ungrounded is likely to give the most common mode current, its been covered more than once in old threads

 

[Marconi]

yes i remember you talking about jays 50ft of coax, i don't recall it been offerd as a cure for cm currents,

when the topic of isolation and chokes was talked about the desert rats denied the i-10k can have cm currents, jay said the i-10k needs no isolation nor chokes because the radials decouple the mast/feed-line,

why would jay think the 50ft of coax was to avoid cm currents they believe don't exist?

you are going over old ground with nothing new with regards what length coax or mast grounded or ungrounded is likely to give the most common mode current, its been covered more than once in old threads

Bob, I don't think Jay recommended this coax idea as a cure for CMC's either. I just remember Jay having issues with CMC's while tuning his new beam, so I assumed he may have been aware of how CMC's work on the feed point during his development of his I-10K too. Maybe he ran into a similar problem or maybe he just found that the 50' line worked best by accident, or some CB guru told him something.

I reported to you I had some good luck using a 50' footer too. I was assuming then as well. I was never able to know his thinking. I was just thinking out loud that it could be why he mentioned the idea and the specs he gave in his manual. We can't deny he said what he said, and now I see that 50' foot feed line length tending to work best in my model too. I wouldn't be talking about Jay if I wasn't surprised at this result. Here is what he published on tuning in his manual:

View attachment Jay's manua; on tuning..pdf

I may try and do a video to show how this develops, if I can get it legible enough so we can see the numbers on the screen. I'll post some models shortly and try again to explain how I see this issue.

I gave up once when you rejected the idea as smoke and mirrors, in spite my seeing some good results and producing a couple of my Antenna Work Sheets that indicated I was able to get my match and resonance at about the same frequency. That was also in spite of the fact my old radio mentor always recommended using 50' feed line for 11 meters.

I know we disagree on this issue, but I thank you for your comments.

 

[bob85]

im not sure what we agree or disagree on eddie, i measured my 10k at the feed-point and with 1/2wave of coax, neither showed resonance to be at the same frequency as minimum vswr,
when i asked jay about it he had no answer,

looking through the 200ft+ of coax i must use gives similar results with a wider vswr bandwidth.

 

[Marconi]

im not sure what we agree or disagree on eddie, i measured my 10k at the feed-point and with 1/2wave of coax, neither showed resonance to be at the same frequency as minimum vswr,
when i asked jay about it he had no answer,

looking through the 200ft+ of coax i must use gives similar results with a wider vswr bandwidth.

I should have said we might disagree. I also talked to Jay about this and he said to focus on an increase in bandwidth and the issue would settle out if I could get the BW over 2.0:1 using an analyzer. My best match shows 2.0:1 on the money and 2.5:1 using my in-line meter and a 50' cable.

The match is close, but not perfect.

 

[Marconi]

Update on CMC's project

These models all have an Eznec feed line added instead of using the mast as I noted in my original post to this thread.

The first two models did not change much if at all by this modification.

The 1st model has a mast 36' feet tall and the feed line is also 36' feet long. There is a note on this one at the top indicating how Eznec indicates the magnitude of currents and the differences between wire #1 and wire #2. This difference is what causes Common Mode Currents to flow.

Image #2 model has a mast that is 40.5' tall with a feed line 40.5' tall. Note this model shows the currents at the radiator are balanced and symmetrical and thus CMC will be minimized as noted, no currents noted on the mast/Feed line using a random non-resonant length feed line and raising the antenna just 4.5' feet higher.

Image #3 model has a 36' foot mast like the first model, but the feed line is 50' feet long, a non-resonant random length feed line. Note the similar balance and symmetry to image #2. In this 36' foot high model we see the random feed line appears to mitigate the CMC using a non-resonant length 50' feed line.

View attachment Update of CMC's issues..pdf

Here is the feature that adds the Eznec feed line. Sorry the image is not clear

View attachment Eznec front page..pdf

 

[Marconi]

Ok guys, I've been suggesting that some combinations of coax length and feed point height might have bad effects with CMC's, and that some other combinations might help us mitigate such problems.

However, due to some of the questions that I've received here, I decided to check closer into my use of the transmission line feature that I presented recently. I now find that changing the data changes the results...just as one might expect. Now I'm sure that what I was originally seeing in these models was just coincidence.

What lead me to change my mind, was making a simple change to the VF value in the Eznec transmission line data feature, and then the results no longer supported my idea.

I can still say that height and feed line length effects how common mode currents are manifested, but I can't continue to say that feed point height and/or feed line length can help us mitigate such currents.

Thanks for all the comments, I probably would not have ever seen the light without some of my friends questions and doubts.

 

[BOOTY MONSTER]

OGP , irregardless of whether or not i agree or disagree with you or sometimes i'm just lost by the lingo or technical aspect of your post/threads i always find them interesting and/or thought provoking . keep posting your ideas

 

[Marconi]

OGP , irregardless of whether or not i agree or disagree with you or sometimes i'm just lost by the lingo or technical aspect of your post/threads i always find them interesting and/or thought provoking . keep posting your ideas

Yea BM, this one was just too much like you note. I got carried away on the idea without checking out all the variables I could imagine...before I posted. It's not like I've never made a mistake though.

I think this project does suggest that maybe mounting our 11 meter antenna at 36' feet to the feed point, or using a 1/2 wave tuned feed line may not always be our best choice however.

And, we still have to take away something from W8JI's words, when he commented in his article on the Imax and mentioned worst case scenario regarding antenna mounting. I don't think he said that for nothing, even though he didn't give us any more details.

Click here: End-fed Vertical and J-pole