Question for the guru's

[The DB]

Your extreme example goes way beyond anything I intended, but your other comment...

You are likely correct that we are talking only a few single percentage points of difference at best. By itself not noticeable.

I had through that several effects by themselves that were not noticeable could add up, however, in this case I think other said not noticeable effects would work counter to this effect to the point of canceling it out...


The DB

 

[bob85]

DB,
i don't know if eznec will agree but in my experience with small cars and cb antennas if you have a small car like a ford festiva you won't see much if any difference between short antennas such as a valor 60" halfbreed and a longer sirio 4000,
a 1/4wave whip has a small advantage but not what i expected,

on a larger car the sirio beats the shorter antennas and gets closer to the 1/4wave,

we used to drive 48 miles from home up a 1000ft hill and did lots of tests with swapping antennas from one car to another then comparing signals & having a keydown to our buddies back home,

a sirio 3000 worked better on a festiva sized car than the sirio 4000,

the 1/4wave whip always won against any loaded antenna on the same car including my 22 & 27" predators but the difference gets smaller the smaller the car or that's how it seems from our tests.

 

[hotrod]

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

ive often wondered the same. i try to stick with 5-6 feet. [maybe slightly more]
and try to never go under 4feet.[unless height resisricted. ]i do notice a bit of
loss when i compared my wifes lil wil to my wilson 5k and sirio z180.so my
guess would be 4feet or less.but im guessing

 

[Marconi]

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).

That may be the theory 24, but my Eznec model of the project that Captain Kilowatt showed us, a 1/4 wave tower over good conductive ground as he describes it…showed us the results he indicated happened at the work site, and it was not over Perfect Earth. My model shows that, but I don’t think anybody has noticed the distinction yet.

So, I’m just not convinced that the theory tells us…that we’ll see ~35 ohms only when the 1/4 wave antenna is ground mounted and over an infinite and perfect ground plane.

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.

I received that Bronco model from Henry and changed the frequency and scaled the model to 27.205 mhz. I’ll try to find that model, and post it 24.

I believe you may be the only one around here that will appreciate what I was trying to show, right or wrong, when I posted those two mobile models in this thread, because nobody has made a comment about the models until you posted above.

Just looking at it doesn't a car body more closely resemble sloped radials than a flat plane?

Yes, and that is exactly the point I was trying to make to Robb with his arrow head mobile he posted. I see this model this way, because all of the vertical elements close to the feed point are flush with currents, and IMO the vertical elements are in phase and the horizontal elements are out of phase. I think the slanting window posts are what raises the impedance we tend to see with a mobile 1/4 wave radiator. We also see very low gains and higher angles as a result too, just like the very low frequency broadcast radio antennas do.

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.

I see the same, there is no change in impedance weather in free space or over real Earth, but I don't believe that is the way it works either. I can't explain this, but I don't see it in my models. This is why I don't believe the 35 ohm theory tells up the antenna has to be over an infinite and perfect ground plane.

I haven’t really studied these mobile models yet, but again that is because nobody seemed interested, and I don't use mobiles any more.

Maybe the model doesn't handle the capacitance with the ground right.

That may be 24, but it does not compute for me. Capacitance is just too important in antennas for modeling to ignore the effects. In fact I originally thought that Eznec did show us some signs of capacitance when I checked out the currents log. However, I did not do an extensive study.

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.

That will be great 24.

I suspect that you will see the resistive part of the match increase at resonance even more, and the pattern will become far less directional.

I try and tune all of my 1/4 wave models to resonance, unless I’m comparing antennas.

When you make the change you suggest above, you’ll also have to shorten the radiator significantly, and that tells me that capacitance is affecting this response in a big way. So, again I think capacitance is working in Eznec.

Didn't I see 65 ohms on one of Marconis pick up models? Mine gives ~70 ohms.

Yes, my model mounted on the bed rail shows ~63 ohms impedances. With
your increase in segments…your feed point actually gets closer to the base, and up to some point in the segment count…the model improves in accuracy, but the performance results may get a bit worse.

Yes you did see a higher impedance with the antenna mounted on the bed rail at about 54” above the Earth. I think that is because the antenna is higher up, compared to my model with the antenna sitting on the bumper at about 12” on my P/U model.

Are the models showing something, did we both make the same errors?

I don’t know the answer to this. I think we are both seeing something happening that needs more discussion however, but I was never able to get anybody to discuss these models, right or wrong, until you popped up with some questions.

For now, I’m probably the only one that even has a clue to what you’re talking about, but hopefully this conversation will bring others out of the wood work.

Marconi how many wires or segments can your EZNEC version handle ?

I have the version that allows only 500 segments and I’m just below that limit. I try to get my segment distribution as close to the same as possible, and I strive for about 6” per segment with my mobile models and a large number of wires. On my 1/4 waves I shoot for about 2.5” per segment.

On the mobiles I use about 50 wires. I haven’t yet tried more or less wires to see how that affected the models.

Thanks for your comments on my mobile models. I will send you this Bronco model if you want it.

 

[Marconi]

GHZ, right now I figure I'll miss the chance to discuss modeling with you, but I understand not wanting to get into this mess with all these guys. They have no faith in modeling at all...and that is mostly because there was probably a campaign set-out in the beginning to stop the idea from taking off.

For the most part these guys don't know how to use the products, and they don't want to appear stupid in front of this group...a group that is just waiting in the wings...ready to jump all over your work and ideas.

If you're not a professional or an expert around here, no idea is even worthy of consideration, and they'll try and run you off and given you the grief on the way.

I finger'ed modeling out in spite of being a stupid old man among all these young punks...who can't hack a real problem, and don't even know how to ask questions in order to maybe learn something new.

I don't give a hoot what they think anymore.

BTW, Marconi is just a CB handle guys. That is not really my name. <gotproof>

 

[Captain Kilowatt]

What a gluteus maximus.

 

[Captain Kilowatt]

Whoa! Somebody closed the thread? Well I just reopened it. It could get interesting and may need closing again but lets see first.

Eight days this thread lay dormant and then out of left field comes Marconi with his last post above. ^^ ab v c^^

Ever notice how Marconi is the one always saying CBers are stupid and that they don't know crap? Almost like he is trying to put words in someone else's mouth but we all know he would never do that. You are the one beating up on yourself Marconi. I can onlu take your reference to " professionals" to be a dig at me. I suggest you throttle back the derogatory comments including calling people "young punks". I could call you a name or two but have not done so until my previous post which IMO fits.

 

[Road Squawker]

Whoa! Somebody closed the thread? Well I just reopened it. It could get interesting and may need closing again but lets see first.

Eight days this thread lay dormant and then out of left field comes Marconi with his last post above. ^^ ab v c^^

Ever notice how Marconi ....

yeah, thats why he is on my IGNORE list.

seems that he just loves to stir up s%#$ and try to play one person against another.^^ ab v c^^^^ ab v c^^^^ ab v c^^

 

[Captain Kilowatt]

As an admin I do not have anybody on an ignore list.

 

[Shockwave]

In all fairness I think some of this animosity was directed towards me in response to the other thread on the Sigma. I say this because I had lost some patience at the time in responding to the topic and used many of the exact words Marconi refers to.

GHZ, right now I figure I'll miss the chance to discuss modeling with you, but I understand not wanting to get into this mess with all these guys. They have no faith in modeling at all...and that is mostly because there was probably a campaign set-out in the beginning to stop the idea from taking off.

No one should feel opportunenities to discuss ideas in this forum come at some great risk or that anyone would receive the same aggressive responses you have at this point. It's only when you revisit the same troubled ideas of the past and expect a different response that a few members here with more experience on a particular topic will speak up loudly.

I have faith in software modeling of antennas any time we can build a model in the program that simulates what can be reproduced in the field. Problems have clearly been identified in how EZNEC interprets the radiation currents and phase in the Sigma design. If you built a collinear model in EZNEC and it showed another 2 dbd, how much faith would you have in the model once you got in the field and realized you had to remove over 8 feet of conductor in the phasing network that EZNEC produced?

Don't be paranoid, No one has set up a champagne against you to discredit most computer modeling. So far, this design is the only one I can find such huge discrepancies with.

For the most part these guys don't know how to use the products, and they don't want to appear stupid in front of this group...a group that is just waiting in the wings...ready to jump all over your work and ideas.

I've admitted I'm no expert in using these software programs. Your geometry skills and how that relates to building models is probably far better than mine. However, we can't really connect that with stupidity and should not overlook my advantage with measurements in the field. Jumping all over ones ideas usually only happens when the idea has been thoroughly discussed and then recycled again looking for a different result.

If you're not a professional or an expert around here, no idea is even worthy of consideration, and they'll try and run you off and given you the grief on the way.

I finger'ed modeling out in spite of being a stupid old man among all these young punks...who can't hack a real problem, and don't even know how to ask questions in order to maybe learn something new.

Many ideas are worth considering Marconi. In doing so we should consider the entire picture and all of it's variables. The testing conditions you've outlined in the past open your results to the type of criticism you've seen. When your Vector model produces 3.5 dbi at 32 feet above real ground in EZNEC, you should ask yourself why the gain is less than unity once you take into consideration the ground gain EZNEC has already added. How do you think your 1/4 wave ground plane reached figures in excess of the 2.15 dbi unity gain when it was 48 feet over real ground?


I know how frustrating it is to not be able to find the answer when you can spot the problems. We both have brought Roy's attention to specific problems we've found using EZNEC. We got the same response which offered no help and appeared to show he was in denial. I almost wanted to send him the extra 8 feet of bent up aluminum phasing section I had to rip out of the prototype his program suggested would be perfect.

I don't give a hoot what they think anymore.

All we are asking is that you improve your testing methods if you intend to use them regularly in forum debates. Using modes like SSB and comparing antennas mounted several car lengths away from one another do not lend credibility to ideas that are already subject to skepticism.

Haven't you moved your vehicle a mere few feet over the years enough times to see what that can do to the signal? This is not exclusive to mobile antennas or areas of overhead wires and happens with base antennas too.

BTW, Marconi is just a CB handle guys. That is not really my name. <gotproof>

 

[vkrules]

The design changed in 2005, your truck has a metal bed. The bed on the 2005 and newer (at least to 2007) trucks is of a composite fiberglass-like material. The rear quarter panels are bolted to this composite bed. There are vertical steel supports at the rear of the bed where the tail gate connects.

I'll post up a pic of my antenna setup here after while.

Removing the sprayed-on primer from my antenna seems to have helped with Rx sensitivity issue. fyi.

Edit: With this setup, the SWR was a little high, 2.1-ish prior to bonding the rear quarter panels to the frame, the cab to the frame, the hood to the frame. Now 1.3 after bonding.

Question: Is my poor recieve issue due to the antenna's close proximity to the cab, the plastic bed, all of the above?

thanks guys

To answer the question. Yes to all of the above.Try turning the truck around 180 and check the results.On the roof the ant is in the clear and has a ground plane under it 360 deg as well as having the feed point higher than the bed mounted ant.Also I would not like to be sitting in that cab with my head less that a foot away from the bed mounted ant . Ok guys back to your discussion on feed point impedance .

 

[Marconi]

Donald, I want to responed to your post #85, but I was in the process of answering what I could, and we had a storm. I lost all that I had done up to the storm. There are some points in there that are important to me I think. I'll get back on this.

 

[JCH]

Update

I mounted the whip on a tool box which is attached to the bed rail and grounded to vehicle frame via grounding straps, painted the whip with the same primer and it works fine now. The mount being close to the cab, and mounted in fiberglass were my main issues I think. Primer doesn't affect operation as I thought previously. Antenna is 14" or so from truck cab now, very quite receive with 99v2.

Just trying to clear things up.

 

[C2]

I am inspired to build a 1/4 wavelength radiator over a perfect ground plane and measure the impedance/resonance.

Everybody seems to point to the sky as they reference this thing called theory. I never seen one, but I guess they speak.

 

[Marconi]

I am inspired to build a 1/4 wavelength radiator over a perfect ground plane and measure the impedance/resonance.

Everybody seems to point to the sky as they reference this thing called theory. I never seen one, but I guess they speak.

C2, if you're serious, I've been building a home made 1/4 wave antenna using 102" whips for the radiator and radials for some years now. I use a mobile L-bracket attached to an aluminum tube that will allow an A99 GPK hub to slide on...just below for the L-bracket.

I have a hub that will allow three radials at about 25* degrees, and I have a stock A99 hub with 3 slanted radial ports, and 4 horizontal ports. I call this my Marconi x.

It works very well and the match is very good.

Here is a picture of the base of the antenna on a short mast without the whips.