Tuning jumpers and coax length

[Master Chief]

It can, but only if you NEED them. Its best to realize that they are NOT necessary for 99.9% of the operators out here.

On the other hand, having knowledge can never be a bad thing, especially if it dispells myths!

 

[2at124]

coax length

I agree 100%

 

[Switch Kit]

Sounds very strange there Cheech for you to be blowing finals so quickly like that ....hell !! even if you didn't have any antenna in line ! So you had a 1.3 but then you tuned it to 1.7 and everything was good ? OK ....IM just glad for your pocket book that you finally got that all figured out......I think in this case .......better you then me. It's just a damned good thing for me that I haven't needed a antenna analizer over the years or a history lesson in why ? Interesting to say the least all the same. IM starting to think that outfits like MFJ and others should be sponsoring these Forum sites ....Einstien's theroy's of coax/antenna relativity has got to be good for business. IM very glad you got that stuff all figured out Cheech ....I for one would not be a very happy camper if I was blowing stuff up left and right . IM mostly just grateful to the RF Gods that I have been able to get away with keeping it simple all these years ....the mathamatic's would have screwed the $hit out of me a long time ago. (-: Peace

 

[freecell]

http://www.ibiblio.org/obp/electricCircuits/AC/AC_14.html

good link. i still get the feeling however that the author still doesn't understand fully the reflected power aspect but even given that, he should rememeber the conservation of energy rule in that energy is neither created or destroyed. even if the reflected power situation is never rectified, the transmitter will still and always deliver ALL AVAILABLE POWER to the load, regardless of the mismatch loss at the transmitter. it's just that as long as the generator mismatch is ignored, the transmitter is NOT developing its full power potential into the load. just so no one is confused here, the transmitter has more power to deliver when it's looking into a matching impedance than when it is not.

i refer particularly to this statement. "........since reflected energy is energy not delivered to the end device." if not then where does it go?....read:
http://www.firecommunications.com/mismatch.txt

here's an excellent example because this particular swr makes the numbers easy to work with. if we have a 1.5:1 swr measured at the feedpoint of the antenna then we can assume 1 of 2 things. the approximate impedance at the feedpoint is either 75 ohms or 33.33 ohms, one or the other. if we attach an electrical 1/2 wavelength to the feedpoint and take an swr measurement at the opposite end of the line and come up with the same swr as at the feedpoint then we can rightly assume that the impedance present at the feedpoint is in the neighborhood of 75 ohms. that's because when the load impedance is greater than the characteristic impedance of the feedline a maximum impedance point occurs every half wavelength from the antenna and a minimum impedance point occurs at every odd quarter wavelength point between. when the load impedance is less than the characteristic impedance, the maximum and minimum points swap places. at all points between the odd quarter wavelengths and the even half wavelengths exists an entire range of impedances in between the upper and lower limits. here's something else interesting. in the example presented here you will also find that at every odd 1/8 wavelength down the line away from the load that approximately 50 ohms can be located. this 50 ohm figure can be found approximately every 42.375 inches along the line away from the feedpoint. this figure can and will vary slightly from one sample to the next. reactance will most likely be present in one form or another but that can be cancelled without much work. sometimes a difficult to match situation can be helped by changing the line length. doing so changes the impedance seen by the transceiver. generally speaking, the higher the swr the wider the range of impedances we have to work with. and in addition, cutting the feedline and terminating it with a connector at these wavelength related intervals does not change the swr or the range of impedance present along the remaining length of the line.

 

[Chuck Bravo]

Freecell, you are going to have to explain for me what a conjugate match is.

 

[FL Native]

Definition of conjugate:

noun: a mixture of two partially miscible liquids A and B produces two conjugate solutions: one of A in B and another of B in A
verb: unite chemically so that the product is easily broken down into the original compounds
verb: add inflections showing person, number, gender, tense, aspect, etc.

8)

 

[freecell]

impedance matching for maximum power transfer (also called conjugate matching) is used to maximize the power dissipated by a load impedance driven from a fixed source impedance.

 

[Chuck Bravo]

impedance matching for maximum power transfer (also called conjugate matching) is used to maximize the power dissipated by a load impedance driven from a fixed source impedance.

Are we talking about matching stubs? Pieces of feedline extending beyond the antenna feedpoint that are left open or shorted? :?:

 

[freecell]

actually when i was speaking of stubs earlier i was talking about using them to cancel positive values of reactance (inductive or capacitive) to a value of 0 at the point where the source (transmitter) connects to the line after the length of the line is corrected to present the required 50 ohms.

shorted stubs under 1/4 wavelength (inductive) to cancel capacitive reactance and open stubs (capacitive) to cancel inductive reactance, depending on what the measurements call for.

 

[Cheech]

So as long as the antenna has an impedance of 50 ohms then coa length dont matter? Okay think I got that but when your tunning the antenna to get the match right coax will not afect this as well? I think Im geting it slowly! :roll: Just as long as you get it close to 50 ohms as possible at the radio right?

 

[2at124]

tuning coax

yeah now your gettin it

oh hey chuck bravo who tha tin your avatar it looks like it could be a pic of BNT from the 50's J/K

 

[Chuck Bravo]

If the coax is not a half wavelength (or multiple), changing the length of the antenna can cause the entire circuit to be resonant, but the effect is to cause the coax to be part of the resonant circuit and hence radiation from the coax occurs.
In addition, the antenna itself will not be a resonant circuit; hence, the resultant high impedance of the tuned circuit will prevent maximum current flow in the radiation resistance. This is not true if the antenna radiation resistance is the same as the characteristic impedance of the coax.
Another effect is that the value of radiation resistance measured at the transmit end of the coax is not the same as the value at the antenna if the coax is other than 1/2 wavelength and the radiation resistance is other than 50 ohms.


Hey N.J. 124, the avatar is Ricky from the comedy TV series "Trailer Park Boys".

 

[2at124]

coax

cool chuck bravo

 

[W5LZ]

Chuck Bravo,
If the antenna's input impedance isn't the same as the feed line's impedance, and if neither is the same as the output impedance of the transmitter, then I'd agree that you can't measure the radiation resistance at the transmitter end of the feedline. But then, you can't measure it at the antenna end either. Radiation resistance can be calculated, but not measured, period. Change that radiation resistance to characteristic resistance and I'd agree with you.
- 'Doc
PS - "Picky"? Yes, but also true.

 

[Chuck Bravo]

Sorry Doc,
You can use an RF bridge to measure the resonant frequency and the antenna radiation resistance at the antenna feedpoint.