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mfj 259 right at the mobile antenna

Wow

learned alot tonight. Thanks guys.

Anybody mind if I sum up?

Basically, big hammer trimmed his coax to recieve 50 Ohms at his desired frequency. The antennae and the van and the coax all together gave him 50 ohms and that is what the radio wanted to see. So Master Chief is right (unfortunatly they are very rarely wrong!) and the coax finished a half wave getting us back where we started at the radio.

The issue seems to be that no one thinks that 10K can read 0 reactance all across the frequency spectrum. from 1 to channel 40. What I haven't heard and what I would like to know is what would you expect to equal on channel 1 and 40, assuming you trimmed the coax for channel 20?

I have seen charts that say "start with x length coax for this antennae with this power and adjust swr by trimming." So what is the big deal, why is it so suprising that big hammer trimmed his coax, changing the resistance of an electrical circuit, and got to a point where he got the reading he wanted?

Just wondering

Ron
 
well what ever mfj reading i have, the resutls are fact. If it was a load i wouldnt be able to transmit as well as I do. roll into dc on ch 6 and lock it down is easy. and there were some big dogs out there today also.

also tried a mr coily tonight. same basic results with the mfj so its a consistant reading with different antennas.


i have one question. how many of you have actualy taken the time, tried some situations just to see the results. or on their own set up. and how many of you are going on theory?

I would like to thank all the posters of this post for not letting it run rampid with a bunch of loose lip crap, but to stay on topic.
 
btw for clarity, i did not adjust swr by tripping the coax. the stinger length was set by the mfj and fine tuned with the bird meter. then coax is adjusted in length to read 50 ohms where i wanted it to. stinger length rechecked with the bird meter. maybe a slight adjustment ( 3/16" max )

in the end it was right where i wanted it.
 
just curious, and im not being sarcastic;
how many times did you have to cut the coax, solder on a new PL259, check the SWR, unsolder or cut off the old PL259, and solder on a new one?

this seems incredibly monotonous to do!

and there is no real world results to speak of on the receive end.

man, bighammer, you are definitely a perfectionist.

i just run the coax to my amp from the antenna, solder on the connector, and smile when the SWR is 1.5 to 1 or less.

sure, this is with a 102" whip and the biggest amp i've run on this system is about 800 watts PEP, but its well built, well installed, and well, it kicks ass and makes truckers cry.LOL
(oh, stop. you know you've all done it!)

i like to tackle a project as much as the next guy, but at some point i just wanna talk on the thing.

just opinions here,
and i agree, debate without derision IS possible!
later,
loosecannon
 
bighammer said:
well what ever mfj reading i have, the resutls are fact. If it was a load i wouldnt be able to transmit as well as I do. roll into dc on ch 6 and lock it down is easy. and there were some big dogs out there today also.

also tried a mr coily tonight. same basic results with the mfj so its a consistant reading with different antennas.


i have one question. how many of you have actualy taken the time, tried some situations just to see the results. or on their own set up. and how many of you are going on theory?

I would like to thank all the posters of this post for not letting it run rampid with a bunch of loose lip crap, but to stay on topic.
Click to expand...

:) every chance I get ,Dude..
as for you locking up channel 6 ,lol well maybe out by you
because if you do get in the main man will post you on page 1-3
as what you rated...(and thats for real)channel 11 and 20 well thats not a problem..
 
bighammer,
"i have one question. how many of you have actualy taken the time, tried some situations just to see the results. or on their own set up. and how many of you are going on theory?"

Probably more times than you'd ever believe - lol. Before and after learning the theory. (Why would you think that's a 'naughty' word? Only reason I can think of it that you don't understand that it means.)
I was licensed before getting a formal education in that 'theory', which was both a 'help' and a 'hindrance'. A 'help' in that there were occasions when I knew exactly what they were talking about. A 'hindrance' in that 'they' thought I ought'a know more than I did - lol.
I found out that it's very easy to draw the wrong conclusions, and everybody does it at one time or another. I also found out that 'they' tend to be right a lot more often than 'they' are wrong.
There's nothing wrong with experimenting, in fact, it's fun. With a few of those 'experiments' I've tried, I sometimes wonder just what the @#$% I was thinking to try such a thing, even when they worked. The 'biggy' isn't that they actually worked, but why.
- 'Doc
 
cannon to be honest if it said 50 ohms or 58 ohms i doubt that anyone would hear a difference on the other side. its more so the amps are as happy as can be. there is a change in reflection when your on your centered channel, and your at 50 or 58 ohms.
a semi local here george forman does his stuff basicaly the same way, but belives things work off of 48 ohms better. im not going to argue that. buthe does have real world proof. he is the guy that had the van with 2 8 ft fiberglass whips, which got to the main at the va beach keydown.

i posted about my ch 6 adventure only to show that the system works, the antenna isnt acting like a load or that the readings cant be all that far off.
 
And there is the problem, the definition of words/terms! A "load" is just what's connected to the end of the feed line. Just like a wheel and tire is the 'load' at the end of the drive train.
- 'Doc
 
i understand about wanting to make the amps happy.

im still learning about how the whole reflected power as it applies to SWR thing works.
of course, the books i am using to learn about this are full of, wait for it;
THEORY! LOL

so do you trim the coax like I stated?
by soldering on a new PL-259 onto the cable after each trim?

having never done this, i am curious as to how different people do this.
loosecannon
 
linearone said:
This argument hold special feelings for me since a few months ago I went through all this with my suburban... trying to use the MFJ to tune etc. etc. In the end it was very frustrating and led me on a stray path, got two new whips tuned for lowest reflect, did the math and wound up with a 1.29 swr. Not bad for a 30 min tunbe session. I'm sure I can get it lower if I wanted to. The bird is my friend.

When I was telling this story to an Elmer of mine I was saying how I wanted to use specific connectors that were "50 ohm" and I was hoping I had antennas that would have the right impedance... and he looked at me and said "pfft... heh heh.. impedance..."

Tune for lowest reflect.
Try to use a multiple of a 1/2 wave of electrical length before the antenna... works for me and lots of others.
Click to expand...

Tuning for lowest reflected is actually tuning the antenna for the proper impedance the final stage of your system wants to see. If running a stock radio it will want to see 50 ohms.That is the way it was made and is a standard.This can be done with an MFJ-269 with an electrical 1/2 wavelength of cable and be spot on every time. HOWEVER when using an amp the output impedance can be anything but 50 ohms.Maybe it is 62.5 ohms,who knows.In that case tuning to 50 ohms will still show a higher SWR due to the mismatch between the antenna impedance and what impedance the amp wants to see.This is the reason why you sometimes hear a person say that the SWR actually goes DOWN when using the amp instead of the radio barefoot.The antenna was tuned to the output impedance of the amp not the radio.Amps are supposedly 50 ohms but in reality almost never are,not on 11m anyway.
 
tuning for minimum reflect is what i have done for years but it says nothing about the resonance of the antenna,
just how important do you guys think having a resonant antenna and for that antenna to be 50ohms at the feedpoint is??
as opposed to just tuning for the lowest reflect??
 
bob85,
Tuning for the lowest reflected power is the same thing as matching impedances. Doesn't say anything about making it 50 ohms, but if your radio/amplifier likes whatever the impedance is, who cares.
As for resonance, it can be a 'plus', and then again, it doesn't matter too much (a 5/8 wave antenna is not resonant). If a particular length happens to work for what you want it to do, then it's fine. Doesn't matter if it's resonant or not. In general, resonant antennas do work 'better'. In some cases not as well as some other length, but that depends entirely on what it's supposed to do.
The 50 ohms impedance thingy is as important as what the output impedance of your transmitter is. If it's 50 ohms, the a 50 ohm antenna is a very nice thing to have. If the transmitter's output isn't 50 ohms, or at least close to it, then that 50 ohm antenna isn't too important. Substitute amplifier for transmitter in some cases, same thing, just a different spot.
Hows that for a very straight forward, absolute, but ambiguous answer?
- 'Doc
 
bob85 said:
tuning for minimum reflect is what i have done for years but it says nothing about the resonance of the antenna,
just how important do you guys think having a resonant antenna and for that antenna to be 50ohms at the feedpoint is??
as opposed to just tuning for the lowest reflect??
Click to expand...

Bob85, I always tune for X=0, and not for lowest VSWR, at the geometric center of the transmit band. You are correct, VSWR does not give a useful indication of effective radiated power. However, maximum power transfer does occur when impedances are matched so if the antenna can be made to have a feedpoint impedance of near 50 ohms resistive, then minimizing reflected power is a good thing. For a base station, having an input impedance of near 50 ohms is pretty easy, but for an electrically short 1/4 wave whip, it is almost impossible to achieve and will always be much lower unless there are lots of losses.

ken
 
thats what i am going to try next ken,
i have a couple of 40mtr antennas cut for 45mtrs from my pirate years,
i want to tune for x=0 then match at the feedpoint with a dollar match,
do you think that will work better than just throwing the antenna on the car and adjusting the antenna for lowest reflected power?
 

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