want to do it myself

Pointless to discuss further you do not understand the concept nor the math of Complex conjugate matching. The corporations worth hundreds of millions of dollars like say Firestick, Wilson, and Francis to name a few (in their big money days) had many engineers who obviously knew less than you. Why over decades have they specified a coax length to go with their antennas. Think of all the millions in 100K+ salaries they could have saved in the last 40 years if they had just called you instead of hiring all those college degreed engineers.

 

The problem is with all the degreeded engineers no one gets the idea there is no reason to have 50 ohm coax if you can't build your antennas to work with it .....

BTW right out of the box 1.1:1 Swr

 

I've tuned many antenna on many vehicles. As well as installed base stations. And I have found out all one needs is enough to go from the antenna to the radio. And you being an Amateur Extra should have learned that also. As for the 18 ft myth it came about in the 70's because someone felt it would be enough to reach the radio. Just like the one you always need to have coax in 3 ft lengths. Is this one always correct? How about my base antenna many 3's make 50ft. And if you really do your homework you will find out it's just called marketing. And some of your statements I don't think that you understand a lot of things by some of your comments. Maybe you need to re-read something.
Coax length does not matter if an antenna is tuned correctly. The best way to tune an antenna is with no coax at all. If that isn't practical, just install the antenna and whatever length of coax cable you will be using permanently, and tune the antenna with the random length of cable. If it tunes to 1:1, you're done. Who cares how long the coax is? If it doesn't tune well, then retune the antenna until it does.

 

None of you grasp this is only so if there is no reactance in the system. Also no matter how well you tune a mobile CB whip it is not 50 ohms, closer to 36. To tune the antenna to resonance you are canceling out reactance, meaning the antenna appears resistive at resonance. However at this non reactive, purely resistive point of resonance, the feedpoint impedance is not 50 ohms. If you do not know this it is you who needs to read more and this conversation will forever be pointless. When the ground is not sufficient to create the other half of a dipole (needed for radiation to occur) part of the coax length becomes part of the overall resonant antenna system due to unbalanced line currents. So while the length is not important meaning the SWR in the line is everywhere the same you are faking out the radio so that it 'sees' an impedance closer to 50 ohms. In effect this is the same statement Bruce made about moving the meter. The point is more of the power from the radio is radiated even though part of this radiation is from the coax itself. Bottom line is the final in the radio runs cooler than it would using just any length of line to an antenna without enough grounding. The reason is this power radiated from a portion of the coax would have otherwise appeared as heat in the final. Not an ideal situation but often a driver is stuck with less than ideal.

 

True, but sometimes that is as good as it gets. But it can make the radio where you can at least use it if a different length coax changes the SWR. (as I stated about the Cascadia situation earlier.)

I too have installed many CB antennas on all kinds of vehicles, for myself and others. And length of coax can and does make a difference many times. Should it make a difference? No, not in a perfect world, but that is hardly ever the scenario you are dealing with. I can't argue the technical stuff technically, but I have enough practical knowledge/experience to know what will work and what won't. You guys saying it don't matter really makes me question how many CB mobile units you have ever set up. Especially on trucks, more especially on the newer model trucks. Sorry to say that, but I can't come up with any other conclusion knowing what I have done and seen over the years in regards to this subject while setting up many CB mobile units. And so far, neither you or Bruce have given AB7IF a counter argument that even comes close to convincing me you two know more about it than him........but I'll keep watching!

 

I'm sure not convinced that he's right. If you look around you'll can enough info about antennas & coax that will say he's wrong. Why do most mobile antenna makers recommend 18 feet of coax? It's to reach the radio. They also claim you should use 1/2 wavelength multiples of coax. 18 feet isn't even close to being a 1/2 wavelength in any 50 Ohm coax you will find. Check some commonly used coax. RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet. And I'm not trying to know more than him. What I'm saying is a fact from someone that's not a Brain who claims to know it all. If coax make a difference there would be something wrong with your antenna. And you are fooling the radio. And not solving the problem.

 

" Also no matter how well you tune a mobile CB whip it is not 50 ohms, closer to 36 "

Still a 1.5 : ! match and that would be over some form of ground plane. Coax is a infinite number of inductors and caps depending on just where you are on a wave length. This also changes with the input/output load and its R/C values .... so the bottom line is you match at the output load ( AKA antenna feed point ) not by playing with using the coax as a matching device.

The other problem is not all coax has the same values per foot and with the cheap Chinese junk being sold like the LMR400 clones you have no idea what it real values are ..... I got burned by a clone which was not even close to 50 ohms on 144 mhz.

I have KU4AB squailos from 28 to 440 mhz and none is worse that 1.4:1 right out of thre box and can be tuned to under 1.2:1
I had 2 and 4 stacked 144 mhz ones under 1.5:1 that even when wet changed very little again MATCHING was at feed point.

The new one on 50 mhz was 1.1:1 and it's not done yet ...... There is no reason why a antenna should not work right out of the box with any length of coax you need .....

 

I give up on the two of you. At 14.1 feet with mini foam you see a mirror image of about 36 ohms on channel 20. The extra 3.9 feet to reach 18 adds around 14 ohms through complex conjugate matching to make the radio see around 50 ohms to keep the final happy. This is why for decades the manufacturers have specified 18 feet of coax. It does not take 18 feet to go from the dash or the central overhead in any 18 wheeler to reach the mirror. This 18 feet spec did not originate from personal vehicles, it is more like 19 in a 70's pickup to reach the back bumper for the 102" SS whip. In the 70's the big companies started specifying 18 feet for their target market 18 wheelers through tests of an antenna mounted on west coast mirrors so high above the chassis that the counterpoise was not sufficient. They actually came up with 18 through experimentation. If you ask most slip seater's a 9 foot coax reaches the mirror. Even routed 12 feet would still be enough to reach, no need to waste the money on extra coax to reach the mirror with an 18 foot length.

You both need to get the writings by Walt WD2U and study them.

 

WD2U
Robert G Witham
1106 Fifth Ave
Spring Lake, NJ 07762-1316
USA

Who is " WALT " ?

 

I swapped the 2 and D. It is Walt W2DU. Here are some of his words when we talked once:

"As a professional antenna engineer with the Astro-Electronics Div of RCA, I published a seven-part article in QST in the 1970s highlighting many misunderstandings concerning transmission-line theory and techniques that were prevalent in both the engineering and amateur communities. The most prevalent misunderstandings centered around the use of SWR and impedance matching between the transmitter and feedline, and how the mismatch at the junction of the feedline and antenna can be compensated with an antenna tuner, thus providing a conjugate match at the junction.

The series of articles I mentioned above was titled, "Another Look at Reflections." When the series was finished the ARRL had many requests for reprints, but rather than make reprints the League asked me for permission to put the article material into a book. I agreed, but added much more material than that from only the seven articles. The result was the first edition (two printings of 5,000 copies each), hard cover. The second edition was printed by Worldradio.

Here's the reason for the 14-year debate on the CM. The original articles and the first edition of Reflections discussed impedance matching and used the wave mechanics of reflections to explain how the conjugate match theorem applies. It must be understood that a conjugate match exists if the power delivered to its load decreases if the load impedance is either increased or decreased. In other words, when you tune and load the tx into the feedline or dummy load to obtain the maximum delivery of power, there is a conjugate match. (Some people believe incorrectly that a CM cannot exist with real elements because they have loss. This is not true. I have added a new chapter in Reflections 3 that prove a CM can exist when real elements having loss are used in a networkl)

However, before the book was published, Warren Bruene,W5OLY, a well-reknowned expert on tx design (actually, he was one of Collins Radio Corp. top engineers), missed a vital point concerning the CM with RF power amps, that although the input to the RF amp is non-linear because the plate current comes in pulses, the output is linear, resulting in a nearly-perfect sine wave, and thus permits a CM to exist at the output. Bruene, however, missed the fact that the tank circuit provides energy storage, resulting in the linear sine-wave signal at the output. He therefore believes incorrectly that a CM cannot exist with an RF power amp. So, before Reflections was published, Bruene contacted the ARRL editors and told them Maxwell's material in the QST articles was all wrong, and that the book should not be published. However, the RF guys then at the League knew better, ignored Bruene's demands and published the book anyway.

In the meantime the knowledgeable RF editors at the League retired, and were replaced by some not so knowledgeable. Bruene then published an article in the Nov 1991 QST that claimed that the CM cannot be established when using an RF amp, thus igniting the 14-year war between those who believe Bruene and those who do not. Bruene's position with Collins so impressed the League editors who didn't know squat about impedance matching or about CM that they told me not to submit any more rebuttals concerning CM, because they believed Bruene's position is correct, even after I submitted references to widely-known technical text books proving Bruene wrong. In addition, several of Bruene's colleagues at Collins knew he was wrong, but couldn't convince him otherwise. I know one of these colleagues personally, so I've been privy to Bruene's activities at Collins.

Bruene and the League hit me hard during those 14 years, but all has quieted down now, and no one now objects to my presentation in Reflections. In fact, CQ Communications is now preparing the 3rd edition of Reflections for publication, and is to be released sometime around the end the this coming Summer. In addition, while at Dayton this past weekend Richard Ross, K2MGA, owner of CQ Pubs, told me that they have just published the list of the new CQ Hall of Fame award recipients, and that I am one of the recipients. I consider that quite a honor.

For those of you who look at my web page at www.w2du.com, I suggest reading Chapter 19 to see what occurs in the matching of an RF amplifier to its load. Since that chapter was written I have performed many more test-bench experiments proving the CM. The results of these experiments are going into Reflections 3 as an addition to Chapter 19.

Walt Maxwell, W2DU"

Look at the material in the link to His site and get back to me. http://www.w2du.com/