-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Question for the guru's
- Thread starter JCH
- Start date
Hey DB, remember I'm from Missouri, and your words are way too brief on the solution you have found, in fact you don't say a word about what you've come to realize. All I really know for sure is that we disagree on something, and what I've claimed so far...nobody seems to understand my point.
I don't know
about all the other's that might be looking in with interest, but I would like to know what specifically answered what you were wondering about, please.I think I have an answer to for this dilemma, but since you went first with a possible soultion...you're in the barrel now DB, so out with it young man.
op:Well I haven't been just sitting around on my hands all day. Like I said I would earlier, I did my Marconi 5x antenna testing at 9', 14', 19', and 24' feet high, with the 102" radiator on a conductive mast, and 3 horizontal 102" radials with maybe a 10* degree slant due to gravity. I mention it, because it might make a little difference.
I don't have room to physically do testing at or near the ground. I have fences all around, so I can't test that aspect of this dilemma, but my models will let me get down to a foot high with horizontal radials.
There are a few problems with my testing that I will explain later. I hope that bit of news doesn't thrill my non-protagonist followers on this forum into getting all excited, and over-indulging themselves in adult beverages, and waking up with a terrible headache at some point.
I plan to post my results...whether it confirms my idea or not. I haven't digested the numbers yet, but I have some new models to hopefully compare, even though nobody believes them either. They may also prove to be of little to no value toward member understanding either. But I don't consider that my fault.
Today I realized just how fruitless this argument of mine may be to argue...even though there are a few reports on the Internet that I posted earlier and suggest agreement with the issue I raised: regarding the impedance of a resonant 1/4 wave radiator, with a suitable ground plane attached. And in particular to find out if there is any validity to the new claim that this impedance is generally 35 ohms only when over a suitable and infinite ground plane. Just to be clear here...I have not seen or read anything that supports this idea, and my model I posted the other day showed there was no difference to the model just because it was set over Eznec's idea of a Perfect Earth.
CK, could you please post one of your most reputable references that you claim supports your point of view here?
I will also try and prove that I'm not just a schmuck at the keyboard using Eznec Antenna modeling software...as some would like all to believe.
I'm going to try and duplicate the ideas presented by Captain Kilowatt in his last post noted above, and see just how close my model comes to his description of one of his experiences at work.
Last edited:
I messed up who said what. It was not you asking if there was a formula around as I thought. It was theDB that asked that and I replied as if it was you that had asked it. My bad. Too much going on in my personal life right now and no sleep and just started nights. No sleep etc.....yeah excuses but they are honest excuses. LOL
Not to worry CK, I understand you're busy. I figured you just had misread something along the way, just like I thought you failed to read my original post in this thread. I really thank you for talking this out...even though we might disagree on some points. Discussion is the best way to flesh out what is on our minds sometimes...and I consider that good. Most guys seem to just get mad.
Good question. Length to diameter ratio can affect it some. Not only is the resonant length shorter for a fat conductor the impedance will be effected a bit too due to the difference in ohmic losses. I suspect this may be part of the reason why we see a certain value for a 10m antenna and yet a different value for a 2m antenna with the same size material used. There is a difference in terms of wavelength and unless even wire size is scaled the outcome would be different. The material used will affect the outcome as well. Copper, aluminum, and stainless steel all have different electrical properties so that will make a bit of a difference as well. How far the feedpoint is from the groundplane will make a difference too as the ground lead has to be factored in. Something I don't think is being taken into account either is that that figure of 35 ohms, and that is a "nominal" 35 ohms not a hard and fast definite value, (sort of like 50 ohms coax being somewhat near 50 ohms but not exactly) not only assumes a perfect groundplane but if memory serves me correctly it also assumes that perfect groundplane is in free space which eliminates any coupling effects to anything other than the groundplane. Antenna installations are so varied that it is impossible to say something will be X value all the time everywhere in all cases. IMHO worrying about whether it is truly 35 ohms or 27 ohms or 41 ohms is simply a waste of time. The fact remains that it must be matched to 50 ohms for most applications and using the nominal value of 35 ohms is as good a starting point as any.
Just as a real example, back in 1986 I moved an AM transmitter site to a new location. The old site being encroached upon by developments and had the original 1948 tower which was a short top loaded affair that was unstable electrically with changes in the weather. I had a 160 foot steel tower installed with a 24 inch face at the new location on a piece of prime dyke land with very good conductivity. The frequency was 1450 KHz making the 160 foot tower a ful 1/4 wave. The ground system consisted of 120 radials spaced equally around the base and each radial was a 1/4 wavelength of 10 gauge bare solid wire. The radials were flat with the exception that about 1/4 of them sloped downwards slightly for about half their length due to terrain. Directly at the base of the tower was a 20 foot by 20 foot ground mat made from 6 gauge stranded wire with the wires running in both directions and spaced one foot apart and silver soldered with Silfoss wherever they crossed. This was done to maintain a more stable ground in the immediate area of the tower. When all was said and done and the tower self impedance measured we ended up with 38 ohms R and just a few ohms of reactance.This was before the electrical cables and clearance lights were added as well as the Austin toroidal lighting transformer at the base. I merely present this as a real world example of an installation and what the results were. Nothing more.
BTW when I said "And yet there are countless examples all over the internet on countless sites that agree with what I said. Gee ....go figure." it was not meant to be against anything you said but merely an observation of facts. I believe we are on the same wavelength to use a very bad pun.
Below I have tried to duplicate a model at the specifications you noted above. I did not try and duplicate the 20' x 20' screen, but here is what I did to see if my model would prove the results you saw at the time. This is not proof for you, but it might help some take a different track on what I try to do with modeling that nobody else is doing here at the moment. I would like to encourage others to an interest in modeling, but we've heard so many tales that it is hard to sell any idea about modeling. This is not the first time that one of my models came very close to duplicating exactly what another claimed, including LB Cebik who posted in one of his litany of works, as an adviser to Roy Lewallen and his Eznec product, a piece on the 5/8 Wave Mystique.
For those that don't know Eznec, we are limited as to the description we can use. I have tried to describe the models with some understanding, but that can fail for other's that are looking in. This is all to say there is a method to my madness.
Models:
#1. is my attempt to do as you describe in your post. Eznec will not allow us to model on the ground, so this one is at 1' foot. I include for each model the antenna image, the total far field gain and angle report, and the Source Data that shows the feed point impedance values for resistance, reactance, and SWR.
This is just basic stuff and you don't have to know everything there ever was to know about method of moments math. You don't even have to know all the RF theory that you CK, have formerly studied for...as a career choice in real RF work. I think this first model comes pretty close to what you described for the field results you had on that project. There is no mystery here, and this is all the model shows me at least. It probably shows 'Doc a lot more, but that is another story not to be told...for my own self respect.
I will say this, so as to speculate without knowing for sure however, That here we see a 1/4 wave at 1.45 mhz showing a natural match that may not be good enough to operate in you line of work without better matching, but this does provide us a distinction, if we care to understand one that I can draw. It suggest why it is not always effective to demonstrate theory for use at very low frequencies...if our interest is higher up in the CB band. To me this is exactly why and maybe how so much CB BS gets started in the CB business.
My reasoning here is, I can model a CB antenna using 5 x 102" elements using similar math and the model will not produce a natural match even close to what we see with the model I made with your limited dimensions. I can give you the math, but then everyone's eyes might go crossed, and they might just give up and leave without a comment or contribution to the discussion.
Instead I will just give you the values that this CB model's source data provides at 1' foot in height, with 1 x 102" radiator, 3 x 102" horizontal radials, and without a mast or a feed line: R=34....X=-30.1....SWR=2.30.
This is more or less the extent of my claim, where I don't see the generally accept idea that a 1/4 radiator will always show us 35 ohms of resistance at resonance, while the idea comes from theory that was likely a source from the area of antenna use...just like your example...broadcast radio.
Beyond this I cannot explain much or why. Maybe your mention of the wavelength to element diameter ratio has something to do with why. I was talking about CBer's and what they have said for years that I consider mostly CB BS concerning there being near 35 ohms showing up on their mobile setup. Maybe when they get their setup close to right...maybe then they will see about 35 ohms...being that close to the ground. That said, I was not referring to mobiles.
#2. is your model #1 that I set over the idea that Eznec uses for a Perfect ground plane, and I removed all losses to make it perfect. We see here the same as the CB model I did at DB's request of a model over an infinite ground plane. There is no change in matching values worthy of note, but we won't ever be seeing this model in our neighborhoods either. But, I would like to know where this idea of testing over Perfect Earth relates to this theory or idea...about the match at resonance for a 1/4 wave radiator being 35 ohms on a consistent bases if not all the time? CK, do you have such a link that is from a reliable source?
#3. is model #1 raised 1/4 wavelength and here we see what I was seeing working with a CB 1/4 wave base antenna to try and understand why guys often fail to get their whips to work right and others say they get their 102"-108"whips to work perfect with a 1.01:1 SWR, with no problem. Here we see the problem I was claiming...a really low resistance, but this model is not resonant, so therefore my idea is not as revealing as I once thought.
I speculate that maybe if we tuned this one somehow that low impedance might shoot right up to 35 ohms, and maybe we could still retain the horizontal radials...that might even work better than slanting the radials down to improve the match. Again IMO, this is where the slanted down radials otherwise radiate some and tend to raise the higher secondary lobe in the pattern. This, IMO, takes away some from the primary lobe no doubt due to the lack of cancellation in the radial field of the slanted down radials...forget the matching benefits...we seldom get something for nothing in antennas.
I posted examples of two mobiles earlier, and it shows why and how this 1/4 wavelength can and will show a near perfect match when mounted correctly on some mobile body designs.
Typically mobiles have a terrible or less than desirable ground plane. Besides this possibility, very few folks can visualize the ground plane effects from a mobile. We most often hear complaints that the GP is not enough. Seldom do we ever hear that the mobile produces way too much of a ground plane relative to what the radiator needs to work effectively and efficiently. IMO, some time the GP can actually produce all the effective radiation, and sometimes that risks being the opposite polarity from the antenna design itself. I can imagine some mobiles potentially doing this.
Those two basic mobile setups I posted earlier are a work of art IMO. How else can a mobile get that match and performance results?
Well, I'll never tell how and what I did to Henry's model. Because when he sent it to me, it was a mess and I almost was never able to get it to work. I think I was just lucky on that model.
The main reason is it is probably not as big of secret as most believe, but more importantly I'm not sure we can duplicate the models results with a real world setup. Some good contesters my agree however that some things work along these lines however.
This is the way I see things, but I'll change my mind in a minute if I ever hear and see something more convincing and if that happened I would feel blessed, because I learned something that I did not know.
I hope I made this clear enough for easy understanding. I choose not wait on DB like I suggested, he has to get up and work for a living, and I have things I just feel like doing as well.
<gotproof>View attachment Captain's 1.45 mhz project.pdf
Last edited:
I stand corrected on the one issue I requested of CK to provide a reference that connected the idea in this issue between the impedance and being over a perfect ground plane.
Here Cebik explains exactly this issue and goes on to explain the dilemma. I agree CK, I really don't think we were that far apart on this issue, and our talk here cleared up much for me at least. Maybe this is the same thing that the DB realized too.
This may explains why Captain Kilowatt's 1.45 mhz project showed him the nice match he recounted for us. If my models are right...then we will never see this happen in the CB range, even if we choose to mount the 1/4 wave right on the ground with 120 radials. However I will test this idea with a 102" radiator, and 4 x 102" horizontal radials, compare it to one with 120 radials, and post the results for you guys to see.
Here Cebik explains exactly this issue and goes on to explain the dilemma. I agree CK, I really don't think we were that far apart on this issue, and our talk here cleared up much for me at least. Maybe this is the same thing that the DB realized too.
Just like I my notes show, although I did not realize it at the time...it is the height of these 1/4 antennas and our trying to compare them to ground mounted 1/4 waves is what I think is at issue. I think I talked about this above, but Cebik said it much more clearly.Cebik said:
This may explains why Captain Kilowatt's 1.45 mhz project showed him the nice match he recounted for us. If my models are right...then we will never see this happen in the CB range, even if we choose to mount the 1/4 wave right on the ground with 120 radials. However I will test this idea with a 102" radiator, and 4 x 102" horizontal radials, compare it to one with 120 radials, and post the results for you guys to see.
Well guys, I see I was wrong in my thinking again.
You guys are right. If we put our 1/4 wave right on or close to the Earth, we will see about a 36 ohm match at resonance whether we use 4 radials or 120.
This issue that produced my original idea that the 1/4 wave ground plane would show varied matches at resonance was misread by me, because all my testing was at heights at or above 9' feet, where I saw varied impedance and some below 25 ohms.
I just never considered testing a 1/4 wave CB antenna on or near the ground because they are so easy to raise up high. This is also why Cebik reported similar low impedance with his 2 meter testing in the link I posted above.
View attachment A CB atenna mounted on Earth.pdf
RES = resonant
I learned something new today and I thank you guys for provoking me to get back to work and maybe test more.
You guys are right. If we put our 1/4 wave right on or close to the Earth, we will see about a 36 ohm match at resonance whether we use 4 radials or 120.
This issue that produced my original idea that the 1/4 wave ground plane would show varied matches at resonance was misread by me, because all my testing was at heights at or above 9' feet, where I saw varied impedance and some below 25 ohms.
I just never considered testing a 1/4 wave CB antenna on or near the ground because they are so easy to raise up high. This is also why Cebik reported similar low impedance with his 2 meter testing in the link I posted above.
View attachment A CB atenna mounted on Earth.pdf
RES = resonant
I learned something new today and I thank you guys for provoking me to get back to work and maybe test more.
Sorry for the delay Marconi, I was camping over the weekend and wasn't in range of any form of internet connection, not that I would have said much on the small screen of my phone if I were... My thanking CK was literally on my way out the door, which is why it was so short.
I was thinking of a ground mounted quarter wavelength antenna for the 36 ohm impedance figure and apparently you weren't modeling exactly that?
I'll take some time to think on what has happened/been said since I left on Friday, perhaps I'll have something to add or ask about...
The DB
I was thinking of a ground mounted quarter wavelength antenna for the 36 ohm impedance figure and apparently you weren't modeling exactly that?
I'll take some time to think on what has happened/been said since I left on Friday, perhaps I'll have something to add or ask about...
The DB
DB, I can only guess, but I think very few CB'rs ever ground mount their CB vertical antennas. And, due to the height of commercial broadcast radio antennas...they don't have much choice with their very tall antennas, so they mount them on the ground...just like CK's example.
So the theory or idea here, that a 1/4 wave vertical over a suitable ground plane shows us an impedance near 35 ohms at resonance, was probably developed some years ago for Commercial Broadcast Radio...at very low frequencies.
I do see a CB 1/4 wave antenna with horizontal radials mounted on or near the Earth producing a nice match and near 35 ohms of resistance at resonance too, but as soon as you start to raise that antenna up you start to see the match start to get lower than 35 ohms. I saw this in my testing because I hardly ever tested below about 9' feet, and with my mobile I was always well above 3' to 4' where the resistance began to get lower.
I believe you guys are right in your thinking about the theory or idea being discussed being true in the technical sense, but there is a distinction to be made in understanding where and when this happens. It likely won't happen on a mobile install or mounting a CB vertical 1/4 wave antenna. IMO this happens and it also has little to do with the idea that the antenna has to be over Perfect Earth, or another idea suggested earlier regarding the ratio of element diameter and wavelength.
Can we all agree with this much about our making some miscues in this discussion so far?
I admit I presented my message badly, because I did not understand or consider the distinction about the theory only applying if and when the 1/4 wave was mounted on the ground or really close either. My models show being on are very near the Earth is the only situation where the antenna shows to perform as you guys have noted in this discussion and nobody described that specifically...thus there was some confusion and heistation in replies.
Again, my mistake was...I was only testing 1/4 waves at resonance, with slanted down radials, that were raised well above ground level, at 5'-6' feet or more.
Friday when I built my Marconi 5x, with 3 x 102" horizontal radials, I was thinking such results might help prove something on the issue, but what I found was I could not make the antenna resonant without some matching device. That did not fit the 35 ohm idea for 1/4 wave ground plane at resonance.
As I understand the issue at this point, a 1/4 wave for the CB band with a suitable horizontal ground plan will not show a workable match at resonance when raised starting at about 4' feet. The resistive part of the match begins to drop below 30 ohms without some form of matching device to maintain resonance.
I'll post these models that IMO support this 1/4 wave idea close to the Earth, because I think they will be informing of what else might be going on around a mobile install. These are not mobile models however. I've already posted my mobile models earlier in this thread, but they did not create any conversation in spite of the fact they show near perfect matching. I always wonder if a mobile is capable of a perfect match, but I guess that is of no interest to anyone here. Based on the fact these are mobile models...this amazed me, but no one else apparently. I think you guys are just afraid to even try and discuss anything about modeling.
Here are 7 models with a hand written recap of my 1/4 wave with no mast, at resonance, with 4 radials, set from 1" up to 72" inches, in 1 foot increments. They show the match values for R, X, SWR, and the effects on the element lengths to maintain resonance for each model. They also show the gain and angle.
These models might also suggest how important it might be to get your mobile as efficient and effective with as few ohmic losses as possible in the system. Mobiles suffer the same as Broadcast Radio Stations do...because with antenna close to the Earth, and the low gain that looks to be all that is achievable at these low heights...we need every little bit of advantage we can get for our mobiles to work best. I see this as a lot of worry by CB r's regarding the antenna they choose, and very little regarding getting the ground plane as good as we can in order to help reduce all the ohmic losses we can. I use to complain that guys trying to improve their mobile body parts by bonding was just a lot of hooey, but I'm not so sure anymore after seeing this unfold with these mobile models I posted. Is this maybe something worth discussing? Is this something worth considering?
View attachment .25w's at various heights.pdf
Last edited:
Agreed, although I've seen it happen, and even then it is not over a theoretical "perfect" ground...
That and every book I can find on the subject. There is a caveat here, the books and the 35 to 37 ohm figure is in relationship to a near "perfect" horizontal ground plane that is nearly a half wavelength radius from the base of the antenna in a commercial environment.
This appears to be a big part of where our differences were, when I asked about the impedance measurement over a theoretical "perfect" ground I assumed the vertical element would be mounted using said ground as its ground plane instead of elevated with a separate ground plane of its own. That ended up being a misunderstanding between us, nothing more, and because of it I was looking for possible explanations as to why you were getting different results.
I agree that we are referring to very specific set of conditions with regards to the antenna and antenna theory. A "lab" environment if you will, or an environment that has been specifically built to meet said requirements. I would also propose that very few people in the CB world would attempt to meet said requirements, if they even know what those requirements are.
There was some confusion between what you were doing and we were saying. I wouldn't worry about it to much, we all make mistakes...
That information is good to know.
One thing we have to remember is the numbers given in antenna theory are only a starting or reference point as far as mobile environments are concerned. These environments have features that you simply don't normally study in said theory. Environments where a quarter wavelength antenna mounted on the middle of an SUV roof will have different impedance, reactance, and radiation resistance (among other things) from the same antenna mounted on the left rear bumper of a Ford Fiesta. We are playing with a compromised set of variables even with the best mobile setup. "Return Loss" as shown on my VNA has been a most helpful tool in such circumstances, unfortunately that information is not available on most antenna analyzers...
The DB
DB, my computer is acting weard today. So I did try and dovetail my responses into your comments above. It will not be interesting reading for others, but I'm tired of trying to get the text like I wanted it. I hope my comments are listed in the same order as you have listed.
During my search for commentary on the 35 ohm idea, I saw some suggestions about Perfect Earth, so I know the idea is out there. I just don’t think it applies to this 35 ohm issue, because I don’t see it present in my Eznec models, and CK’s example for the 1.45 meg ¼ tower certainly wasn’t over Perfect Earth. So don't think this applies as theory, it is just what happens in this situation?
Well DB, I discussed a little with CK about his ideas regarding the Perfect Ground idea, and I mentioned we see it used in theory, but I contended when it is used in theory and the CB’r gets wind of the idea…all references to the Perfect Earth or infinite ground reflection goes out the window, in recounting the theory as it applies to CB. And I gave CK an example.
CK claimed this, I reported I saw the reference too, but none of this is proof. So try and think about this issue in light of the fact that CK produced for us an idea that demonstrated this issue about how and where we'd see 35 ohms for a ¼ wave at resonance to a “T”, yet nothing was noted that indicated the 160’ foot tower CK used was over Perfect Earth. He did say it was on very conductive ground however.
I asked CK this same question. Can any of you show me a reputable link that says this theory or idea only manifests when over a perfect ground plane of any size?
I agree, but if you had mentioned ground mounted, that might have given me a clue where this idea happens, or at the very least caused me to pause and think. All I had in my mind was my testing of resonant ¼ waves that were elevated some, and again my interest in this issue was raised by hearing this 35 ohm rule banded about by mobile operators and their antennas are typically raised up too. So, I wasn’t thinking about ground mounted antennas. If you knew this all along, or questioned the idea was ground mounted…why didn’t you raise the issue in that light.
DB, I don’t think any of us were clearly thinking this issue out clearly in the beginning.
Well again, in light of CK’s tower project proving the only real way this 35 ohm idea applies to a ¼ wave…I don’t think this idea was ever considered in antenna theory…simply because there was no perfect ground plane in the realization of what was going on when it was realize the feed point impedance was 35 ohms for a ground mounted ¼ wave radiator at resonance. IMO, the resonance part here is the key issue in this idea, and this is experience anybody can experience…not just in a lab setting. I disagree.
We agree here completely, and now I think there is some valuable considerations to be made for this issue, but I also realize it is not an Earth scattering revelation.
I say again DB, that before I was thinking this issue was possibly based in theory, but I hedged a bit and called it an idea instead, because I wasn’t sure. But after Captain Kilowatt posted his detailed information on his 160’ foot 1.45 mhz project, and I modeled the idea, I realized the truth and where it probably came from…Broadcast Radio studies and testing at the turn of the last century.
Thanks for you comments.
Marconi
Last edited:
We are in disagreement here. As an example, in a book "Antenna Theory Analysis and Design" by Constantine A. Balanis, which was written as a broadcast engineering textbook, has a section "4.7 Linear Elements near or on Infinite Plane Conditions" which specifically talks about a "perfect" infinite size and conductive groundplane.
Antenna Theory Analysis and Design said:
Then later in the paragraph...
Antenna Theory Analysis and Design said:
Note: Both the Geometrical theory of Diffraction and the Method of Moments are covered in chapters later in the book.
A bit later in the chapter...
Antenna Theory Analysis and Design said:
It talks about "perfect" ground conditions, then talks about factoring in the imperfect conditions of the real world. The theoretical "perfect" ground plane is at the heart of the theory. It is my opinion that what CK gave you was nothing more than a reasonable approximation of such an environment in the near field of the antenna.
Many in the CB world take whatever the best possible theoretical possibility and try and apply it to everything, including things that have no relevance to what they are referring to. They have no concept that what they are stating and what is actually happening is like apples and oranges. They don't care, they believe what they want to believe, and, most importantly, are convinced that they are absolutely right based on what someone who is often just as uninformed told them because "they know". When talking to such people I find it easier to smile and nod.
I have a quote above that shows that this manifests over a "perfect" ground. To say that such an occurrence only happens over a "perfect" ground would, I believe, be a fallacy. Some things raise the antennas impedance and others lower it. It is not beyond possibility that you have the right amount of both giving the appearance of said impedance for the wrong reasons.
I was under the impression that my request for your figures over a "perfect" ground assumed just that. Honest mistake caused by different assumptions from both of us. I did, ironically, bring up the possibility of a varying impedance from raising the vertical element of the antenna higher above said "perfect" ground as a possibility, and later asked CK about it directly.
In addition to a "lab" environment I also mentioned "or an environment that has been specifically built to meet said requirements." I also mentioned above that it is not impossible for various different effects based on the actual environment around the antenna also causing such an impedance, effectively for other reasons.
For me it is a root part of antenna theory when a ground plane of any kind is in play, then modified for the real world conditions for use in broadcast radio, ham radio, cb radio, ect. It is a bit different point of view from yours, but in the end I think they should come very close to the same results.
No problem, any time.
These posts are getting long... If we continue in this way perhaps we should break them up a bit...
The DB
Evening all, just to add my two penneth (2 cents) the whole discussion about a 1/4 wl antenna having 35 ohm impedance came about asa result of one respondent suggesting it was 70 ohm, so I chirped in with the 35 ohm response, little did I realise what would follow, however I'm currently overseers in Turkey and only using my phone to access the Web, I'll take my time to read the replies when I get home, but it looks like a consensus has been reached re ground mounting the antenna, something that many amateurs do, but as Marconi suggested, not many mud ducks
Evening all, just to add my two penneth (2 cents) the whole discussion about a 1/4 wl antenna having 35 ohm impedance came about asa result of one respondent suggesting it was 70 ohm, so I chirped in with the 35 ohm response, little did I realise what would follow, however I'm currently overseers in Turkey and only using my phone to access the Web, I'll take my time to read the replies when I get home, but it looks like a consensus has been reached re ground mounting the antenna, something that many amateurs do, but as Marconi suggested, not many mud ducks
I first posted in this thread was post number 28, 8 post after your first post with a question for Marconi and a general response based on what I've read and a statement that I would look up that 35 ohm figure. Somehow I went from that to being one of the biggest posters in this thread...
It is a discussion that I think we would have had eventually anyway so don't fret. I am interested in your take on all this...
The DB
I was the respondent who asked where 35 ohms came from. OK so ~35 ohms is from a 1/4 wave with horizontal radials (and perfect ground).
I got the numbers ( the 70 ohms) from a model with 379 wires and 690+ segments very much like Marconis blazer model only car shaped and the whip is mounted like a gutter mount on the rear passenger side.
Just looking at it doesn't a car body more closely resemble sloped radials than a flat plane?
The impedance doesn't change much whether in free space or over ground.So I don't think the ground is responsible for the high (~70 ohm) impedance reported .
Maybe the model doesn't handle the capacitance with the ground right.
I'm curious now what dead center of the roof reports. When I get a chance I'll move the whip from the rear corner to the center of the roof.
Didn't I see 65 ohms on one of Marconis pick up models?
Mine gives ~70 ohms.
Are the models showing something, did we both make the same errors ?
Marconi how many wires or segments can your EZNEC version handle ?
I got the numbers ( the 70 ohms) from a model with 379 wires and 690+ segments very much like Marconis blazer model only car shaped and the whip is mounted like a gutter mount on the rear passenger side.
Just looking at it doesn't a car body more closely resemble sloped radials than a flat plane?
The impedance doesn't change much whether in free space or over ground.So I don't think the ground is responsible for the high (~70 ohm) impedance reported .
Maybe the model doesn't handle the capacitance with the ground right.
I'm curious now what dead center of the roof reports. When I get a chance I'll move the whip from the rear corner to the center of the roof.
Didn't I see 65 ohms on one of Marconis pick up models?
Mine gives ~70 ohms.
Are the models showing something, did we both make the same errors ?
Marconi how many wires or segments can your EZNEC version handle ?
As you noted a car is a much more difficult environment to simulate than a "perfect" ground system. Depending on how big a car is, its shape, and where the antenna is virtually anything is possible. If the antenna is mounted on the roof the sloping down and lower planes (hood and trunk of the vehicle) will raise impedance in a similar way to angling radials down. If the antenna is mounted on the bumper, or the trunk lid for that matter, or any place other than the roof, the planes that are higher than the antennas mount (roof, possibly the top of the trunk, ect) will lower the feedpoint impedance in a similar way to the radials on a Sigma-4/Vector 4000.
This isn't even the half of it. We also have other things to worry about, such as the vehicle body as a whole acting as a capacitive ground system with the earth below, which is the true ground plane for the vehicle's antenna. Beyond that we have issues with modern cars that don't have good electrical connections at RF frequencies between the body parts and chassis which hurts the capacitive ground system and with most vehicles as well as makes the loss problem that vehicles even worse. I could go on here...
On a side note... Something I would like to see a study on is a shortened antennas over an inadequate ground system (such as a car) compared to a full sized antenna and their efficiency numbers. I know a shortened quarter wavelength antenna will never be as efficient as a full length quarter wavelength antenna over a "perfect" ground plane, however, according to theory, shortened quarter wavelength antennas do not need as large of a ground plane as a full length quarter wavelength antenna to simulate a near "perfect" ground conditions. I am wondering if this difference will keep a smaller antenna closer to the efficiency numbers of a larger antenna in such a mobile environment, and if so how much you can shorten an antenna for a given type of car before additional losses in said situation become significant.
The DB
This isn't even the half of it. We also have other things to worry about, such as the vehicle body as a whole acting as a capacitive ground system with the earth below, which is the true ground plane for the vehicle's antenna. Beyond that we have issues with modern cars that don't have good electrical connections at RF frequencies between the body parts and chassis which hurts the capacitive ground system and with most vehicles as well as makes the loss problem that vehicles even worse. I could go on here...
On a side note... Something I would like to see a study on is a shortened antennas over an inadequate ground system (such as a car) compared to a full sized antenna and their efficiency numbers. I know a shortened quarter wavelength antenna will never be as efficient as a full length quarter wavelength antenna over a "perfect" ground plane, however, according to theory, shortened quarter wavelength antennas do not need as large of a ground plane as a full length quarter wavelength antenna to simulate a near "perfect" ground conditions. I am wondering if this difference will keep a smaller antenna closer to the efficiency numbers of a larger antenna in such a mobile environment, and if so how much you can shorten an antenna for a given type of car before additional losses in said situation become significant.
The DB
DB, here's an example taking it to an extreme, I use a Scorpion SA680 on my car, fully retracted it is around 1/4wl on 10m, however fully extended on 80m it is no where near 1/4 wl, so it is a shortened antenna on an inadequate ground, we are probably talking single percentage points efficiency on this particular band