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mfj 259 right at the mobile antenna

Beetle said:
"Two points you need to keep uppermost in your mind when doing this: First, as 'Doc said, a "resonant" antenna is one with X equal to zero; doesn't matter what the R value is. Second, any real-world antenna will be "resonant" (X=0) at only ONE frequency.
Click to expand...

Yes I am being picky but my longwire antenna has several points where it is resonant.X=0 in a few places but the R is anything from 3 ohms to 200 ohms at those same points.I get a kick out of telling folks my antenna is perfectly resonant on 1.840 with an SWR of 16:1 (R=3 X=0) but it's better on 14.150 with just a 4:1 (R=192 X=0) and having them tell me my antenna is not resonant if my SWR is that high. :roll:
 
Did the mfj get grounded at the antenna base when tested? If not, the readings will be off... The source always needs to be at the same potential as the load or the readings are meaningless.

As stated by many others, the ideal input impedance of a 1/4 wave properly ground planed antenna is about 36 ohms. The 50 ohms we are all so accustomed to comes from losses and being slightly off-center of the band the antenna is cut for, and is not an ideal value.

Plus, anytime temporay objects are in the near field of the antenna the antennas reading will change when the object moves.

That is why testing is typically done at multiples of 1/2 wavelength of coax away so some of the variability can be removed.

Ken
 
QRN said:
Beetle said:
"Two points you need to keep uppermost in your mind when doing this: First, as 'Doc said, a "resonant" antenna is one with X equal to zero; doesn't matter what the R value is. Second, any real-world antenna will be "resonant" (X=0) at only ONE frequency.
Click to expand...

Yes I am being picky but my longwire antenna has several points where it is resonant.X=0 in a few places but the R is anything from 3 ohms to 200 ohms at those same points.I get a kick out of telling folks my antenna is perfectly resonant on 1.840 with an SWR of 16:1 (R=3 X=0) but it's better on 14.150 with just a 4:1 (R=192 X=0) and having them tell me my antenna is not resonant if my SWR is that high. :roll:
Click to expand...

Yeah - I meant to say "any real-world HF mobile antenna". Blame it on age, or lack of coffee, or lack of aged coffee ... ;)
 
sorry for the time away from the post.

Cannon no i wasnt lieing.
X was 0 from 1 to 40
ch 1 was 49 ohms ch 40 was 53 20 was 52
the other factor to get to the 50 ohms is ground plane.
this is easy to test yourself. and most know this already. take a wilson mag mount. for instance a 5000. move it around on a roof and check the readings on the mfj.
actual set up. wilson 5000 mag, full size van.
mount it in the middle and 60+ ohms
in the back 58 ohms
6 in behind the axel 50 ohms
the change was ground plane.
i have learned that there are alot of theory for cb or RF related set ups. but there are alot of variables.

every frequency has its own specific length. might only be a 1/16th of an inch but they do. so why wouldnt a piece of coax be the same per frequency??

try this link for a reference. do feet, 27.205 .85 and 1 wave length. then do 27.225 and there is a change. do 1/4 wave and you will not end up with the magical 18 ft.

coax link

when you tune the antenna to best reflection on a certain channel. even if x is 0 and you dont have 50 ohms on the channel you want to tune to. relfection will be higher then if you had 50 ohms on the channel you tune to.

i know this post will stir up even more post of no way no how.
these are my findings in my many installs I have done. take it how ever you like.
 
with further playing on the coax page ( link ) the basic std "18ft" is only correct with a .66 velocity factor. those of us that run coax with a .85 velocity factor require a different length.

so coax length can matter
 
well if you tune thantenna first. then the coax wont matter period.

if you do ajust coax to some other freq's then your changing the antnna's middle freq.

so tune anttena first then the coax length does not and will not make a differnce. or at least not on the rx end.

or something like that.


just hook it up and run what you brung and i will key on you everytime since you believe this bullshit. LOL
 
line up then

and ill chop ya up.

local amp builder thoguht his 24 pill was bad till i sent him to the swamp( with a 16 pill ) both one antennas both on vans
 
bighammer,
Basically, what you are saying it the same thing all them ol'farts have been saying all along (almost). That 'theory' everybody 'poo-poos' is just what them ol'farts wrote down from when they did about the same thing. Since it more or less was the same results for everybody, they got to wondering why, and that 'why' turns into theory. If you want to go through the same process, experimenting, re-inventing the wheel, then that's fine. Or, you can do it the easy way and pay attention to that theory. [That experimenting thingy is a part of any 'formal' electronics education. 'They' make you do that so that you will understand that there really is some reason for learning all that theory, don't have to do all that experimenting again.]
To really understand that theory you gotta have the basics. Just exactly like building a house. If you don't start with a good foundation, the @#$ thing will fall down at some point. (Nobody goes from poking out 'Chopsticks' to Mozart in two easy lessons! Or assembling tinker-toys to rocket engines in a week! (except for us 'rocket scientists', we always knew all that stuff.)
Reactances determine resonance. '0' reactance, 'x' = '0', is the definition of resonance. Just like 'Bell' said on his pages, 'shorter' antennas get away from resonance a lot faster than longer antennas do. That means that if, in your case, 'x = 0' for the whole 40 channels, something just ain't totally right, and also like 'Bell' said, that antenna is not the most efficient because of 'losses' in the antenna system. Forget the 50 ohms thingy for now, just talking about the 'x'. I don't know of any antenna who'z 'resonance' range is close to 1 Mhz wide without there being losses involved. Sorry, just don't happen. So, that 'x = 0' finding from ch-1 to ch-40 is sort of suspect. It's just not something that you are likely to see. Doesn't mean a 'miraculous' antenna, it just means there's something 'not right' somewhere.
- 'Doc

PS - Reactances don't radiate. That's why you want '0' of them.
 
Because everything between the meter and the antenna contributes to what the meter will read.
If the feed line between the antenna and the meter is electrically 'neutral', has characteristics that allow it to not even appear to be there, then it's just like connecting the meter to the antenna with nothing between the two. That just happens to be one of the characteristics of an electrical 1/2 wave of feed line, and why it's typically used to do measuring.
When 'x = 0' it means no reactances present, or it's resonant. The further you move away from the resonant frequency the less resonant an antenna becomes. That means that 'x' starts to be something 'other' than '0'. For the 'x = 0' over the whole 40 CB channels it means that the antenna isn't very efficient, it has 'losses' of some kind that introduce reactances that 'counteract' the reactances that are a result of the frequency move. That may sound like a good thing, but it isn't. A reactance doesn't produce power or radiation, only resistance does that. So the more reactance present means less radiation. The power that would have been radiated is changed to a -loss- because of the reactances being present.
An increase in reactance is completely normal for any resonant antenna when you move away from the resonant frequency. So because of that measured 'x = 0' thingy, the whole measurement thingy starts to be questionable. Just too good to be true.
- 'Doc
 
My dummy load will read 50 ohms with "0" reactance over a very wide freq. range,
BUT! it is NOT going to radiate your signal very well.

just because you get a reading of 50 ohms and X=0 does not mean the 10K is an efficient radiator.

this is where the theory comes in handy.
you do not need an MFJ instrument to tell you that a 102" whip is a more efficient antenna design than the 10K.

bighammer, is sounds to me like your 10K is more of a "load" than a "radiator". the antenna analyzer will not tell you which you have.
antenna theory will.
loosecannon
 

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