All this talk about SWR, and not one word about where X = 0 until post #35.
Jus Sayin
Jus Sayin
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Pringles Can CMC Choke
- Thread starter Chicago Savage
- Start date
You apparently have a ham license and thus you likely have understanding well above that for a typical CB operator. I also missed any discussion here about guru's as well. When I first noticed this thread...I had the thought it might be sort of a joke of sorts, with somebody considering it cute to make a choke former out of a potato chip container.
Does your screwdriver antenna provide a separate control display that allows you to select the tune for maximum signal and/or the SWR, or do you use some meter functions on your radio to change bands/frequencies, and control the antenna auto adjustment?
Well sure, a lot of things could be going-on that could effect Chicago Savage's 9' foot antenna antenna, but one thing is likely for sure...he doesn't have an electric motorized tuneable screwdriver antenna. Apparently he doesn't have a ham rig either. Other than making the 9' foot whip shorter or longer, there is little he could do...in order to effectively tune this radiator. So, he does what he can to see a better match for his radio and amp and probably figures any loses, out of sight out of mind.
What would you advise him to do in order to see the inefficiencies you claim?
You rightfully acknowledge there likely isn't much loss in such a short run of coax. But, I have no idea what he has done with his coax, coiled up or not. However, I would think if it caused a problem then he might realize it. But even if he didn't realize a problem and had his coax coiled-up...what do you think could happen in such a situation? And if you know...what could he do about his problem?
I doubt that.
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Hey Savage, I don't think I can doubt anything that you've alluded to here. You told us what you did to make your match better. If your setup is working for you...more power to you.
I know some get all flustered hearing the experiences of others...but IMO working with your antenna setup can be informative and I think at some point you will find it is a learning curve process as well. With antenna work we're often dealing with very small differences in results as we get closer to a perfect setup.
Below are two Eznec models of a 104" stainless steel whip, and 4 x 102" in radials, each having a bit of slant at a downward angle to help closer simulate an automobile roof.
These models are set with the bottom of the 104.5" whip at 9' above Earth, again to help simulate closer to a mobile.
Models:
1. is a model with a 1.40 SWR match at the feed point, which is similar to the 1.50 SWR that W9CII talked about in his mobile. This model shows a maximum gain of 0.83 dbi at 18* degrees of elevation to the horizon.
2. is the same antenna as #1 with the match forced to show a perfect SWR = 1.00. This one shows us a maximum gain of -0.12 dbi at 18* degrees of elevation to the horizon. I did not change the physical antenna. CS this model is suggestive of what may be happening in your situation by supposedly tuning the antenna using the coax.
If you have any questions regarding these simple models...just ask. These models only simulate a little of your mobile setup.
Results show the 1.40 SWR match with more gain possibly similar to what W9CII was talking about with his auto tune screwdriver mobile antenna and what Road Squawker mentions in his post #46 above regarding the resonance X factor in modeling antennas.
IMO, the phenomena going on here is in your using the coax to improve the match of your antenna you have basically added losses to the system and in this modeling example we see the results with a slight reduction in gain.
The point I leave with you is, in spite of your good matching results your antenna is still showing some mismatch, and your efforts to fix it just hides the loses.
Does this really matter? There is an argument to be made both ways IMO. It is not very much difference is obvious, but working an antenna down low to the Earth naturally shows lower gain that we see noted for CB base type antennas mounted up higher. So with a mobile and with ground mounted antennas we need every bit of gain we can muster even though the difference is small.
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Next you need to figure out a way to hang that can horizontally so you can also use it as a waveguide. Home brew Pringle can antennas were popular several years ago as a means of extending the range of your WiFi, but personally I had better luck experimenting with an old metal colander.
Awesome. You guys rock! I would pay anything to be able to understand these types of things, and it seems like I never will. I've read, watched and listened for a long time, and these things are out of reach for me. I can put a rig together, put up a tower, and antenna, but electronics, and antenna theory, especially SWR, CMC, and impedance is something I may never understand. What I do is experiment, record and report my findings, and ask the experts what they think. The only thing I truly believe is, is that it's called Antenna Theory. That means everything is a theory. So many strange things can occur, between the transmitting station and your antenna, and again between your antenna and your radio that all theories can be crushed. Peace, brothers!
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Remember,
Just cause you got a good match, your antenna hasn't changed. Your true "SWR" is what the antenna reads at the feedpoint which typically is around 1.4 in the center of the CB band.
Marconi's model numbers are about the same as a 102 whip I measured with an analyzer with no matching.
A 102 or 108 whip isn't a 50 ohm antenna, it's about 37 ohms so any matching/coil loading configuration doesn't change this fact. It only changes the way the radio sees it.
Nothing wrong with trying to get a better match and if it reduces CMC, all the better!
Just cause you got a good match, your antenna hasn't changed. Your true "SWR" is what the antenna reads at the feedpoint which typically is around 1.4 in the center of the CB band.
Marconi's model numbers are about the same as a 102 whip I measured with an analyzer with no matching.
A 102 or 108 whip isn't a 50 ohm antenna, it's about 37 ohms so any matching/coil loading configuration doesn't change this fact. It only changes the way the radio sees it.
Nothing wrong with trying to get a better match and if it reduces CMC, all the better!
I have a question which the answer provided in layman terms might help me finally understand what w9cll and others say. "Don't be a slave to the SWR meter", and I have "tuned an inefficient system", and "why so much coax?" I know there is some truth here. I have always heard, and do believe that even with a perfect SWR, your antenna might not be performing as efficiently as you might think. I kind of get it. So on to my dumb question...
I'm only referring to mobile antennas. Mobile antennas of all types, most of them have the manufacturer's recommended cable length. It's usually between 16 and 19 ft. So aren't they using the length of the coax to tune the antenna? Is it being said that in fact all mobile antennas are inefficient? Why is a person trying to tune a whip antenna of any length by varying length of the coax a bad thing? I'm sure we would all love to have 1 ft of coax going from the radio to the linear, and just enough coax to get from the linear to the antenna, but in my experience it has never worked that way. In my case it's only 4 feet from the linear to the antenna feed point. I tried a one foot lmr-240 jumper, and a four foot lmr-240 run to the antenna, and my SWR was 3.6. Totally unacceptable. Then, I tried a 3 foot jumper, and 9 foot run to the antenna, still using lmr-240, resulting in a 2.5 SWR. Still unacceptable in my mind. Finally, I switched to cheap-ass RG8X, nine and 18 feet runs (which by the way are quarter and half waves), and my SWR dropped down to perfection. So coax length absolutely matters.
Oh, well. I'm out of breath here.
Gracias amigos, and good night!
A teacher once told me my learning curve is a complete circle. He was certainly right in some instances.
Dean
I'm only referring to mobile antennas. Mobile antennas of all types, most of them have the manufacturer's recommended cable length. It's usually between 16 and 19 ft. So aren't they using the length of the coax to tune the antenna? Is it being said that in fact all mobile antennas are inefficient? Why is a person trying to tune a whip antenna of any length by varying length of the coax a bad thing? I'm sure we would all love to have 1 ft of coax going from the radio to the linear, and just enough coax to get from the linear to the antenna, but in my experience it has never worked that way. In my case it's only 4 feet from the linear to the antenna feed point. I tried a one foot lmr-240 jumper, and a four foot lmr-240 run to the antenna, and my SWR was 3.6. Totally unacceptable. Then, I tried a 3 foot jumper, and 9 foot run to the antenna, still using lmr-240, resulting in a 2.5 SWR. Still unacceptable in my mind. Finally, I switched to cheap-ass RG8X, nine and 18 feet runs (which by the way are quarter and half waves), and my SWR dropped down to perfection. So coax length absolutely matters.
Oh, well. I'm out of breath here.
Gracias amigos, and good night!
A teacher once told me my learning curve is a complete circle. He was certainly right in some instances.
Dean
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Chris ( w9 ) , you know I respect you , You are probably one of the first people on here that helped me ! Thank You for " Bowing Out " , I've said it before & I'll say it again Marconi is my go to guy for antenna's , he does the " Antenna Math " No disrespect Chis , What we decide works for us best whether 11m like myself or a Ticketed Ham Op , It's our choice .
Some one on this forum emailed Sirio about that in the past, and their response was that was the length they felt was needed to get to the radio no matter where on the vehicle the antenna was installed. They did not intend for their mobile antennas to be tuned that way, at least according to the quoted email. I can't speak for other manufacturers but I can say this, in a mobile environment, if you are having such problems that changing the length of the coax appears to change SWR, then simply routing the coax differently will do the same.
This is true. All mobile antennas are inefficient. This has to do with several factors. One, most vehicles do not make adequate ground planes (hence why many people use RF bonding, this improves the vehicle's ability to act as the antenna's other half), and two, the biggest enemy of mobile antennas is how close they are to the earth below, and mobile antennas are always to close.
First off, you are not actually tuning the antenna, you are only getting the appearance of tuning the antenna. What is happening is common mode currents are on the outside shield of the coax. These are currents that are supposed to be on either the antenna of the ground plane. If they are on the coax between the antenna and radio they are not where they are supposed to be. They are also not benefiting you as much, and in many cases are actually working against the antenna. The presence of common mode currents is only an indicator that something isn't working as it should, nothing more. So, in short, changing the length of the feed line so that SWR appears to drop does not change the SWR of the antenna, and only hides a problem that can come and bite you later, when it comes to RF burns, take this literally.
Any install that I have done I use the minimum amount to get to the amplifier, and the minimum amount again to get to the antenna. My current setup has 14' between the radio and amplifier (it is in the back with a remote power switch), and then 6' to get from the amplifier through the headliner to the antenna on the roof of the SUV. I have a ferrite based choke on the 6' length right by the antennas feed point because when I added a short length of coax SWR changed.
The DB
The resonant point will always be where the reactance is lowest even if the swr is not.
That is a good point oldtymer.
CS, do you have any idea what your <2.00:1 SWR bandwidth is?