Question for the guru's

[Marconi]

Marconi, could you model a quarter wave vertical antenna over a "perfect" horizontal infinitely large and infinitely conductive groundplane and see what feedpoint impedance you get?

I seem to remember reading that this was the basis for the 36 ohm baseline figure that I personally use for quarter wavelength antennas. On my end I'll see about cracking open a few books and try to verify that.

The DB

Yes I can set the model over what Eznec refers to as a Perfect ground type, and remove all other losses due to wire type and plot type, and that changes the feed point match so little as to be insigficiant, like noted below:

23.55 +0.04104 with losses and over real Earth
23.44 -0.06411 with no losses and over a Perfect ground type.

As I understand the theory...the impedance does not changed unless there are physical changes to the antenna system and bending the wires is quite common.

A resonant dipole doesn't change much if setup properly at resonance unless you bend the wires, but a ground plan can be made resonant in many different radial configurations.

That is what this project I presented shows. This was what I was thinking, and why I asked if any body sees the ground planes at resonance getting even close to showing 35 ohms of resistance. I also posted a CFD model to show that Eznec can produce a match value that is very close to what theory reports almost all the time.

I did not add a mast or feed lines to these models, maybe if those were included we would see the 35 ohms being discussed. Most of the time this subject of a 1/4 wave showing 35 ohms of impedance comes up in conversations about mobiles, and there no body can predict how the ground plane will be involved, and for sure the variables and the weak condition presented by moblies...may all but excluded us from ever seeing a true 35 ohm impedance at the feed point.

 

[Marconi]

Maybe I should have used the word, "suggested" instead of "...are you telling us..."

You also got the owner of Eznec all wrong too, it was not Cebik. Roy Lewallen is the developer, but I would like to ask Cebik some questions. 'Doc are you speaking for Cebik now that he is departed this Earth? Be careful though, after Road Kill gets out of the corner in a bit...he might call you a "Name Dropper." He is renowned in that respect for picking up those little tidbits...as he tries to read other guys words and misses the meaning...because he has a reprobate mind working full bore on his imagination.

I also wasn't referring to me changing my mind 'Doc. If you read that post again you'll note I was referring to CBr's, like our good buddy the Road Cleaner. I some times change my mind at the drop of a hat...and for sure when I figure I'm wrong, but thank you, thank you very much for checking out my opinions. <gotproof>

 

[Captain Kilowatt]

Marconi, could you model a quarter wave vertical antenna over a "perfect" horizontal infinitely large and infinitely conductive groundplane and see what feedpoint impedance you get?

I seem to remember reading that this was the basis for the 36 ohm baseline figure that I personally use for quarter wavelength antennas. On my end I'll see about cracking open a few books and try to verify that.


The DB

35 ohms is what you get over a perfect groundplane. It is usually more than that due to ground losses which add to the nominal 35 ohm impedance for a 1/4 wave GP.

 

[The DB]

35 ohms is what you get over a perfect groundplane. It is usually more than that due to ground losses which add to the nominal 35 ohm impedance for a 1/4 wave GP.

That is what I was thinking, however Marconi's modeling software came up with a different figure.

23.44 -0.06411 with no losses and over a Perfect ground type.

I've seen several graphs while looking through my available resources, and for 0.25 wavelength they all show a little above a 35 ohm radiation resistance. In most cases this is meant to be a baseline before factoring in things like height, width, inadequate ground plane, ect. I have yet to see any math to get to the 35 to 37 ohm figures that are used to make the graphs, and one source specifically states that their information was found via using quarter wavelength vertical antennas with 120 half wavelength or longer radials and real world measurements.

I was hoping for something a little more specific on the theory side, such as the formula used for the graphs, to reference but if it exists I haven't found it yet...


The DB

 

[Marconi]

That is what I was thinking, however Marconi's modeling software came up with a different figure.



I've seen several graphs while looking through my available resources, and for 0.25 wavelength they all show a little above a 35 ohm radiation resistance. In most cases this is meant to be a baseline before factoring in things like height, width, inadequate ground plane, ect. I have yet to see any math to get to the 35 to 37 ohm figures that are used to make the graphs, and one source specifically states that their information was found via using quarter wavelength vertical antennas with 120 half wavelength or longer radials and real world measurements.

I was hoping for something a little more specific on the theory side, such as the formula used for the graphs, to reference but if it exists I haven't found it yet...


The DB

DB, if you could post the images of the 1/4 wave model you saw that showed the impedance at 35 ohms then maybe I can detect something that would fix my model, or help me explain what we are seeing.

I'll go look for some old Cebik 1/4 wave models and post them if I can find such showing 35 ohms. I too always believed the 1/4 would show 35 ohms at the feed point, but in 2009, I did some testing of the 1/4 wave antenna and that is when I saw the real world antenna never show 35 ohms with 3 or 4 horizontal radials. In the process of doing this testing with 102" whips I developed the idea for my Marconi 4x and the 5x with 4 radials using 102" ss whips.

A couple of years later I was challenged to learn Eznec and when I made 1/4 wave models with 102" elements...I did not see 35 ohms at the feed point when the antenna was resonant again. For me this confirmed my real world findings.

If I'm wrong, then maybe your links to the observations you saw will set me straight.

 

[Robb]

BTW - not to omit the fact that 1/4w vertical with ground planes set at a ~45 degree angle brings it back to ~50 ohms.


But this post is about a mobile install; there are lots of things that can influence its impedance if the length is resonant but the surroundings of the install affects the capacative or inductive reactance - or both. No doubt that it is the install that needs to be dealt with.

 

[Captain Kilowatt]

BTW - not to omit the fact that 1/4w vertical with ground planes set a ~45 degree angle brings it back to ~50 ohms.


But this post is about a mobile install; there are lots of things that can influence its impedance if the length is resonant but the surroundings of the install affects the capacative or inductive reactance - or both. No doubt that it is the install that needs to be dealt with.

EXACTLY!

 

[Marconi]

You're are right CK, this thread is about mobiles.

I think I tried to make some reference to the fact in my first post here. That was post #26, and I talked a little about this idea for mobiles, but I just got carried away a bit. These questions about mobiles was how I got interested in antennas long ago and discovered that testing mobiles was not fruitful, there are way too many variables.

I don't want to appear to hijack this thread in favor of a 1/4 wave antenna base idea. I can start a new thread to try and unwind the confusion going on here. So, I'll wait on the DB first, and see if he can post the links to his sources showing the impedance for a 1/4 wave is typically 35 ohms though like all the claims are suggesting here.

I have looked back at some of my old notes in 2009 when I did the 1/4 wave work and I see a few 33 ohm reading with a little reactance...but back then I was using a feed line and that is no way to really test any experiment on antennas.

 

[The DB]

DB, if you could post the images of the 1/4 wave model you saw that showed the impedance at 35 ohms then maybe I can detect something that would fix my model, or help me explain what we are seeing.

I didn't see any models, I saw graphs that plotted the radiation resistance in relation to the length up to a wavelength over a theoretical "perfect" ground plane. I've seen these graphs from multiple sources, but not the math or whatever the method that was used to determine the information in the graphs.

I'm not saying your right or wrong, but every source I have thus far looked through and that I remember reading from before agree that a 0.25 wavelength antenna over perfect ground presents 36 ohms of radiation resistance, give or take an ohm, at resonance. As I mentioned earlier this is often presented as a baseline for other charts that vary the length, width, height, ect. of the antenna. Further, the same thing has been stated on this and other forums by many people as fact.


The DB

 

[The DB]

So, I'll wait on the DB first, and see if he can post the links to his sources showing the impedance for a 1/4 wave is typically 35 ohms though like all the claims are suggesting here.

I can see about scanning the graphs I'm talking about from books, although most of those are fairly thick and hardback so not very scanner friendly. I'll see what I can do... likely tomorrow.

If you want to create a new thread go for it, I'll find it.


The DB

 

[Marconi]

I can see about scanning the graphs I'm talking about from books, although most of those are fairly thick and hardback so not very scanner friendly. I'll see what I can do... likely tomorrow.

If you want to create a new thread go for it, I'll find it.


The DB

Just put the book on the platen and cover the open spaces around it from light. I do the Arrl Antenna Book often, and I get good results. Sometimes I can't even lower the cover at all, but the printer still works. Sometimes I scan a copy, if the image is going to be upside down, and then scan the copy to my hard drive.

There has been a topic or two other than about mobiles being discussed by the some who show us nothing but words, so I think I'll hang out right here until DB can show me some reliable info that proves the end of a 1/4 wave element is always near 35 ohms. Graph info is fine too DB.

I read some W8JI stuff on 1/4 waves, but with W8JI you always have to have total faith in what he says...because he never produces any significant modeling results. We're lucky if he shows us a pattern for some antenna out of the blue.

I found a couple of topics by Cebik that I printed out a long time ago that were talking about soil conditions, and another about 40 meter stuff, but now Cebik is a pay for site now, and I won't spend a dime to see his stuff anymore. He sometimes shows enough info for Eznec models he has done, but them good old free days are all over.

I'm going on the Internet an see what I can find there. It is common knowledge that everything on the Internet is true, so we'll see.

Words sometimes are not very convincing without some sign of work, and some results to support those words would always be informative...one way or the other.

To say again, the main reason I test base antennas only, is because unlike with testing mobiles...I can see and measure the only ground plane around the antenna. With a mobile that is not possible. You may be able to see what you think is the ground plane, but you cannot be for sure, and you can't change it without moving the antenna any way.

I know somebody out there with a mobile and a 1/4 whip on top can somehow show us guys from Missouri...that your setup shows us a feed point impedance of about 35 ohms on all your installs.

I showed you guys my proof, and now I'm waiting on some evidence that will support your opinions. Isn't that fair?

 

[Marconi]

Here is a link to Walter Fair's Website on 1/4 wave ground plane verticals. It might be helpful to some that suggest a 1/4 wave radiator with a horizontal 1/4 wave radial at 90* degrees shows a feed point impedance of about 35 ohms.

Antenna Notes

In answer to CK suggestion that the 35 ohms at the feed point comes into play when the antenna is over perfect ground. CK idea here is not supported by the article above, or by my model (.25w 4 HR 32') that I posted earlier.

Below is this same model which I have set over perfect ground plane just like CK suggested, and the impedance changed just a little bit as noted, and it is still not near 35 ohms, but instead down at 23.44 ohms...almost identical to what Walter Fair's article reports.

CK, I also added the wires report, in case you would like to input them into your Eznec program. However, this will not work for you if you have the Demo Version.

I'm off looking for more verification for what I see in my modeling and in my real world testing back in 2006 and 2009.

Here is another Website that again shows what I've claimed is true: http://ve2bmc.com/ANTENNA.htm

Here is another link, but it agrees with CK and others who claim the feed point impedance for a 1/4 wave ground plane is 36 ohms: http://www.chem.hawaii.edu/uham/radials.html

Here is another that CK and others will agree with: http://www.tomthompson.com/radio/EHam_Articles/DipolevsVertical/DV.html

 

[Marconi]

Here is Tom Raunch, W8JI proclaiming a 1/4 wave has a feed point impedance near 30 ohms.

http://www.eham.net/ehamforum/smf/index.php?topic=37511.0;wap2

Here is another link in agreement with CK and others.

http://en.wikipedia.org/wiki/Monopole_antenna

Here is one take you pick. He goes either way.

http://www.qsl.net/d/df3lp//projects/vertical/

Here is Cebik reporting what I've reported:

http://w4rnl.net46.net/58-2.html

Another that agrees with me.

http://www.radio-electronics.com/info/antennas/quarter_wave_vert/quarter_wave_vertical.php

Here is another guy that don't believe the feed point is 35 ohms either.

http://doityourselfchristmas.com/fo...1-4-Wave-Verticle-Antenna-for-the-FM-02/page3

Another CK fan: http://www.bencher.com/pdfs/00361ZZV.pdf

There is more, so take your pick.

 

[The DB]

Marconi, your from Missouri? That makes two of us... With the questions you have asked in the past I'm not surprised that you to hail from the show me state...

After some playing with the scanner I managed two scans... As one of them was practically in the binding of the book I only got about 90% of that one, but it should be enough for our uses.

Guess its my turn to show and tell then...

Caption:

Figure 2.28 - Feed point impedance for the same antennas as in Figure 2.25, but calibrated in wavelength rather than frequency over the range from 0.132 to 0.300 λ, above and below the quarter-wave resonance.

A note on the caption: I think that the reference to Figure 2.25 is a misprint and they actually meant to refer to figure 2.27. I don't know that it is relevant for what is being posted here, but wanted to mention it in case I or someone else posts either or both of those figures in the future.

Reference in the text:

Figure 2.28 shows an expanded portion of the frequency range above and below the λ/4 resonance, but now calibrated in terms of wavelength. Note that this particular antenna goes through λ/4 resonance at a length of .244 λ, not exactly .25 λ. The exact length of resonance varies with the diameter of the wire used, just as it does for the λ/2 dipole at its λ/2 resonance. The range shown in Figure 2.28 is from 0.132 λ to 0.300 λ, corresponding to a frequency range of 2.0 to 5.9 MHz.

Moving on to the other figure I scanned.

Caption:

Figure 2.29 - Radiation Resistance (at the current maximum) of monopoles with sinusoidal current distribution. The chart can also be used for dipoles, but all the values must be doubled.

Reference in the text:

The variation of a monopole's radiation resistance with electrical length or height is shown in Figure 2.29 from 0° to 270°. Note that for the λ/4 monopole (a length of 90°) the radiation resistance is 36.6 Ω, one half the radiation resistance of a λ/2 dipole. The radiation resistance is measured at the current maximum, which for monopoles longer than λ/4 will be above the base of the antenna.

Hope this helps...


The DB

 

[hammer0630]

I can't get into the technical aspects of a radio or antenna but I do own a TOYOTA TACOMA and they may have done a much better job making a bed liner fit and have boxes built in and etc. but is the floor and sides only plastic or is there steel behind it? seems like there would be. Mine is a 99 TRD with a traditional bed liner and I know you are talking about a much later year just by the bed liner you are talking about. I have a Gull-Wing type tool box that mounts immediately behind the rear window and has four bolts holding it in place and also making a ground for the antenna, if I had no box I would be using a very small diameter box steel several feet just enough to clear the cab and bolting to the top rail and through the bottom inserting threads toward the rear of truck to keep the gap between cab and bed open and positioning it directly in the middle of the rear window but that would be for a coil wound antenna