I have an MFJ 259b meter that I use to check the swr's on my cb antenna's, my question is just how do you get the r= and the x= down to where you want them?
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MFJ 259b R & X
- Thread starter ctvanover
- Start date
That depends on the antenna.
If you can only adjust the length, as is the case with pretty much all mobile antennas that is all you have to work with. x=0 is the best length in such a case. Although a mobile antenna that is attached with a magnet mount can fairly easily be moved around the vehicle.
Base antennas can get a bit more complicated. A radial system can usually be adjusted for length, its angle can also sometimes be adjusted. Different matching systems also work differently. Anything that can be adjusted can come into play. Just because instructions say something should be the listed length does not mean you can't adjust it when tuning. Remember, instructions for antennas are designed so pretty much anyone can set it up. That being said, if you followed the directions it should be fairly close, but anything that can be adjusted, and usually more than what the instructions list, is fair game.
Don't be afraid to take your time and don't be afraid to keep at it until its right.
The DB
And the thing is... that '259 doesn't change anything, it only tells you what it is. You then get to manipulate/change the antenna so that the 'R' and 'X' are values (impedance) desired.
The part of impedance that makes the most difference is that 'X' quantity, the reactance part of impedance. Why? Because reactance doesn't contribute anything to radiation, there's no radiation produced with reactance, it's only produced with resistance. So, the idea is to get rid of, or neutralize, any reactance -IN- the antenna. That way, everything getting TO the antenna can be radiated.
{Something to try with that '259. Connect it to your antenna system. Dial around until you find where that 'X' is '0'. That's where your antenna system is resonant. It won't matter what that 'R' is and it probably won't be '50 ohms'. But whatever frequency where the 'X' is zero is where the system is resonant. That resonant frequency isn't going to be where you thought it was (unless you are very lucky)! While you're doing that watch the 'SWR' meter. It will not react as you think it might. That resonant point isn't likely to show a '1:1' SWR, no telling what it'll read. It doesn't mean that the SWR meter in that '259 is wrong, or that your other SWR meter is wrong, it only means that an SWR meter can't tell a resonant point from peach cobbler. If that doesn't change your way of thinking about SWR it'd surprise me!}
- 'Doc
The part of impedance that makes the most difference is that 'X' quantity, the reactance part of impedance. Why? Because reactance doesn't contribute anything to radiation, there's no radiation produced with reactance, it's only produced with resistance. So, the idea is to get rid of, or neutralize, any reactance -IN- the antenna. That way, everything getting TO the antenna can be radiated.
{Something to try with that '259. Connect it to your antenna system. Dial around until you find where that 'X' is '0'. That's where your antenna system is resonant. It won't matter what that 'R' is and it probably won't be '50 ohms'. But whatever frequency where the 'X' is zero is where the system is resonant. That resonant frequency isn't going to be where you thought it was (unless you are very lucky)! While you're doing that watch the 'SWR' meter. It will not react as you think it might. That resonant point isn't likely to show a '1:1' SWR, no telling what it'll read. It doesn't mean that the SWR meter in that '259 is wrong, or that your other SWR meter is wrong, it only means that an SWR meter can't tell a resonant point from peach cobbler. If that doesn't change your way of thinking about SWR it'd surprise me!}
- 'Doc
Although it isn't extremely accurate, that 'little gadget' has many functions and abilities. Accurate enough for Hams and CBers. Exploring the manual might amaze you. Not a bad piece of gear at all - if one considers what it sells for.
I've only needed to use about a half dozen of its features so far.
I've only needed to use about a half dozen of its features so far.
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I like the 259b because I screw it onto the bottom of the mounted antenna to check the SWR's so I know when I've got my antenna adjusted right. Then I go to the coax and do the same. So what would be a good SWR meter for use on mostly for mobile antenna's?
Yep, the MFJ works fine with mobile antennas, you just typically don't have as much to tweak. With a base antenna, there are typically more variable parts in play.
I suppose if you really wanted to you could take the antenna apart and adjust the tuning mechanism. I highly doubt any warranty would cover such an endeavor.
I suppose if you really wanted to get both x and r perfect you could learn to use a smith chart and use coax lengths and stubs to tune out the difference. Sounds like a lot of work for what will in a vast majority of cases be a rather minor change in performance.
The DB
Its ok for a casual Ham and CBer. if you want better then you'd better be ready to pull out the wallet!!! :tongue:
I would say its OK for the serious ham as well. That is unless the serious ham has large amounts of money to spend on a Rigexpert. I feel that the difference in possible accuracy will not be seen when the antenna is in use.
It is a low end device, however, it is far more capable than the SWR meters that most people who get an MFJ are using. I am not saying that there isn't better, I could make a list of what could be improved on my current one.
To get much better you need to have much deeper pockets. I guess the question is, is the additional cost worth it to upgrade? That is something that only the individual can decide for themselves.
The DB
Zman,
I agree with you in general. But there comes a point where it just really doesn't pay to throw more money at it. That 'point' is different for everybody for different reasons. I haven't had a problem throwing enough for an antenna analyzer because it makes things easier for me. I did more antenna stuff than the average person for quite a while (not so much now though), and an analyzer was the 'answer' for me. It certainly won't be for everyone, and I wouldn't expect it to be. There's all kinds of very nice specialized equipment I'd like to have! I'm not going to hold my breath till I get it though. And then there's the other part to think about, where the @#$$ would I put it?? Now, if I could borrow it when I needed it... unfortunately, the people who have it are smart enough not to lend it to me. Oh well...
- 'Doc
I agree with you in general. But there comes a point where it just really doesn't pay to throw more money at it. That 'point' is different for everybody for different reasons. I haven't had a problem throwing enough for an antenna analyzer because it makes things easier for me. I did more antenna stuff than the average person for quite a while (not so much now though), and an analyzer was the 'answer' for me. It certainly won't be for everyone, and I wouldn't expect it to be. There's all kinds of very nice specialized equipment I'd like to have! I'm not going to hold my breath till I get it though. And then there's the other part to think about, where the @#$$ would I put it?? Now, if I could borrow it when I needed it... unfortunately, the people who have it are smart enough not to lend it to me. Oh well...
- 'Doc