How to tune?

[153]

I am in limbo. I am no antenna genius by no means so I rely on other experts. I have a mfj analizer to use. The question is when I ck a length of coax with only 1 end on it should I try to get it to 50 ohms by trimming? What should the reading be? Is says 49 ohms and 3.0 on swrs. NOW, when I hook the open end to my beam I tuned the gamma and got a reading of 51 ohms and 1.1:0 swr. Is this the correct way or not? I prefer the mfj because a regular dosy or other meter only shows swrs but no impediance. I know you "fool" a inline swr meter. Any help will be useful.

[Circuit Breaker]

There is no way to change the impedance of a piece of coax. Whether it is 50 ft or only 5 ft, the impedance is the same. The reason you get an SWR of 3 is because there's nothing connected to the other end. All you should be concerned with is what the SWR is with the coax connected to the antenna. If the meter reads 49 or 51 ohms, the radio won't care. It isn't enough to make a difference on your SWR when it's connected. The antenna should present a load at or close to 50 ohms. If it does, then your SWR should be good as well. If it isn't, then tuning will be required. The MFJ should also tell you what frequency the antenna is resonant at...if you have the device I'm thinking of. This will help a lot when tuning the antenna. Based on what you said though, your antenna is fine...the impedance is fine and you have a good match so you're good to go.

I'm not sure what you meant by fooling an inline SWR meter...the only thing I can think of that would do that is an inline antenna tuner which matches a non-resonant antenna to your radio by finding the correct values of L (impedance) and C (capacitance) to present a 50 ohm load to your radio. It "fools" your radio because there's 50 ohms between the radio and tuner but the mismatch is still there between the tuner and the antenna. Because coax is considered an unbalanced line, some of your power is going to disappear between the tuner and the antenna because of the mismatch. But there really isn't any reason anyone on CB would need a tuner...unless they're going WAY above or below the normal 40 channels. If you need a tuner to get the antenna to work on just the normal 40 channels, then there's something else wrong that needs to be addressed.

[153]

OK. Here is my next question. I am tuning on 26.975 I started out with 100ft of 213u. A guy who i "thought" was an expert told me to trim it till it hit 50 ohms. Sure enough when i began the mfj said 39 ohms, so i trimmed a foot off and it jumped to 43 ohms, then i trimmed inch by inch till it read 50 ohms on 26.975. Now this is with one end open. If length don't matter why does it change? I don't unerstand. I know the radio is susposed to be 50 ohm load and the antenna is susposed to be 50 ohm, but if i understand you right coax length don't matter.
The next thing is on like a dosey test center hooked up in line I have gotten a different swr reading with different length jumpers. Why is this if coax length doesnt matter? I just am confused

[Circuit Breaker]

The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors, and on the characteristics of the dielectric material between the inner and outer conductors.

The following formula can be used for calculating the characteristic impedance of the coaxial cable: (formula taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page 24-21)

impedance = (138 / e^(1/2)) * log (D/d)

Where:

* log = logarithm of 10
* d = diameter of center conductor
* D = inner diameter of cable shield
* e = dielectric constant (= 1 for air)

In a nut shell the characteristic impedance of a coax cable is the square root of (the per unit length inductance divide by the per unit length capacitance). For coaxial cables the characteristic impedance will be typically between 20 and 150 ohms. The length of the cable makes no difference whatsoever in regard to the characteristic impedance.

If the frequency is much too high for the coaxial cable, then the wave can propagate in undesired modes (i.e., have undesired patterns of electric and magnetic fields), and then the cable does not function properly for various reasons.

You can also find a lot of info here:
[Please login or register to view this link]

[Circuit Breaker]

Here's some real good info -

How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistant SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistant SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistant SWR readings when the SWR meters position on the coax or length of the coax is varied. In practice, generally you will find varying the coax length seemingly effects the SWR reading. Most SWR meters (built into radio and external type meters) and impedance "humps" in coax lines and connectors will cause minor variations in SWR as jumpers and coax length are varied. In reality, the mismatch at the antenna's feedpoint / coax junction is unchanged. Therefore - the actual SWR is unchanged.

Another reason SWR could vary is from the situation where the coax is acting as part of the antenna. Not a favorable or normal situation. The signal is traveling back down the outside of the of the coax braid (note power should only be traveling on the inside on the coax braid). Therefore, the coax is part of antenna system and changing the coax length will change the SWR. This situation is more likely to occur in mobile installations. You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidily between two values, you might have a intermitant (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change.

Source: [Please login or register to view this link]

['Doc]

Think of it this way. Impedance is a combination of resistance (R) and reactance (X). An SWR meter doesn't know the difference between that R and X value, can't 'see' that 'X' value at all, has no idea it's even there. An SWR meter 'reads' the combined effect of both of those values. The 'catch' to that is that there are more than one combination of those R and X values that can make the SWR meter think it's seeing a very low SWR when it really isn't. The 'biggy' with that is that reactances, that 'X' number, doesn't produce any radiation, only the 'R' (resistance) part of that does that. So if the R value is not the ideal 50 ohms, the antenna just isn't doing the best it can. All feed lines have reactances. Changing the length of that feed line can also change the values of those reactances. And there are some combination of reactances and resistance that make an SWR meter think it's seeing exactly what should be there, but really isn't. The SWR meter can be 'fooled' cuz it just don't know what it's seeing.
That '259 antenna analyzer CAN tell the difference between R and X, and displays that information for you. But, you have to know what that information means for it to do any good, right? It can tell you if the antenna is resonant (no reactances present, or at least very small amounts of reactance). So when the X value is zero, that's where/when the antenna is resonant, no matter what the R value is. Very few antennas have a resonant input impedance/resistance close to 50 ohms, and never 50 ohms exactly. So, the resulting SWR is seldom really close to 1:1. The 'trick' is to 'adjust' whatever that R value is to something at least close to 50 ohms. And that's when this can really get confusing. Since there are combinations of reactances that mimic 50 ohms resistance, you use that mimicking ability to sort of 'skootch' that R closer to 50 ohms. First, reactances are no good, now they are? Yep, just depends on how/where they are applied! There are two kinds of reactances, inductive (L) and capacitive (C). Coils and capacitors. Oh crap! That's a tuner! And they certainly can be. Also called a gamma, beta, hair-pin, or whatever else kind of impedance matching circuit you want to call it. The best place to put a tuner is where that impedance matching would do the most good, between the feed line and the antenna, in most cases. Sometimes not too practical, but still the best way of doing it. You can also do that with coax length, which when used that way is called a 'Q' section. They do work, but they also have limitations that a 'tuner' or matching circuit doesn't have, and that means that they are NOT the best way of doing things if you can possibly help it. One of those limitations is power handling ability. It really doesn't take much power at all to ruin a piece of coax. That doesn't mean in the hundreds or thousands of watts, but just in the tens of watts range. Oh! For joy! Isn't all this 'stuff' fun? And that's why a lot of them antenna 'experts' don't have much hair left, or have sort of a 'blank' stare at times. (No, I'm not an expert of any kind, by the way. I still got hair. )
Another one of those 'limitations' is that if you adjust coax length and go too far, there just isn't any way to un-do that without a LOT of trouble/expense.
All of the above just 'touches' on how this all works. It isn't a definitive explanation at all. It can give you a sort of different way of looking at the process, and hopefully make it make more sense. Hope it helped.
- 'Doc

[153]

All I know is my head hurts for brain overload and my mfj 259 changes (from 39-50) ohms by cutting coax and I know its not the meter beause my neighbor has a 269 and it reads the same. I know the swrs will be high with open ended coax. My question now is what do I need to do? Got my beams on a test stand, coax hardwired (new) and a connector on the other end. What should my mfj read? Thanks

['Doc]

One 'trick' with tuning a beam antenna on the ground is to stand the thing on it's rear end, the reflector. You should then see something close to what it will be when put up in it's 'assigned space/place'. There's no need to 'tune' the feed line at all if the beam has some impedance matching device at it's feed point, such as a gamma match. You use that gamma match to make the input impedance as close to 50 ohms as you can get it.
I may be 'missing' something, so if that's not what you have, then the above isn't going to make much sense.
- 'Doc

[153]

Thanks, that what i have. So if i understand you i do not need to have them verticle I need them pointed straight up? Do I need to get the ohms close to 50 or the swr low? It currently reads 1.1:0 at 49 ohms on my mfj

[Circuit Breaker]

Based on what your last post, your antenna is good to go. Your match is low and you show an impedance of 49 ohms. Getting it to 50 will make absolutely no difference on the antenna's performance. Put it up and be happy with it. Keep in mind though you may have to adjust the gamma match once the antenna is up because height above ground can and does have an impact on SWR...but it shouldn't change much.

['Doc]

The standing the beam on it's rear end is just a 'trick' to make things 'close' to what they will be when the beam is in the place its going to be used. It definitely is not the 'end all' of how to do the tuning. And as Circuit Breaker said, you might have to do a bit of tweaking once the thing is up, that's normal. Also as Circuit Breaker said, if you're seeing something like 49 ohms input impedance, that's really close enough not to matter at all.
- 'Doc