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mfj 259 right at the mobile antenna

B

bighammer

Active Member
Nov 5, 2006
151
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DC
Well you guys asked for it a while back and i just now redid my coax. Took down the antenna and cleaned all the grounds just for safty sake. any how i have stated befor that I trim my coax to read 50 ohms on the channel i center to. And we have been through the whole thing of coax doesnt matter, coax is a specific length per frequency, bla bla bla. well thats not the overall topic here.

I was told that im fooling my system by trimming the coax to get better swr. And I was asked to take a mfj reading right at the antenna. Well this is what I did. mfj, 3 in of coax to the mount. Low and behold. 1.0 swr, 49 to 53 ohms ( 1 to 40 ) x was 0 across the band. antenna is a 17 in shaft 10k.

who said a 1/4 wave antenna cant be 50 ohms? i do know that when you work with the counterpoise around the antenna you can get this reading. if i had it in the middle of the van i know for sure it would never read 50.

well here we are are, test is done. im getting the popcorn out.

after my mobile was done i found an artical 55 wrote stating that location of antenna does make a diff. and other topics that could have saved me time. its great when the time, effort, and testing come down to perfection.
 

bighammer,
Where the antenna is will make a difference, of course. That also includes what's near the antenna, such as yourself when you were taking that reading. Also why an electrical 1/2 wave coax is used to get 'you' further from the antenna so that 'you' don't affect the readings, you know?
Usually, an antenna's location lowers the input impedance, not raises it in the case of 1/4 wave antennas. And that certainly doesn't take everything into account, so can only be thought of as a very 'general' rule.
Like it or not, and no matter what that analyzer says, a 1/4 wave antenna's input impedance is never close to 50 ohms without something else 'going on'.
- 'Doc

Having said that, got any more of that popcorn?
 
W5 i dont need popcorn with your post. your right whats going on is the groundplane the antenna is seeing. which is why location is so important. or height.

when i took the reading i was inside the van with the doors closed.

main reason of the post was not another 5 page debate on what a 1/4 wave antenna is by itself and what it becomes in a mounted location.

i was asked to report my findings and as such i did.
 
Mounting location has something to do with it as well as ground losses. The antenna radiation resistance and the ground return losses,which are both measured in ohms, add together to make up the total impedance of the antenna.It is not unusual to see up to 15-20 ohms of ground losses in mobile situations.The standard 35 ohms of a 1/4 wave plus a dozen or so ohms of ground losses puts you pretty close to that 50 ohms.It is interesting to note that just last week I was sweeping my 8 foot whip on my vehicle to see what the actual impedance was across the entire HF band where I use it with a Yaesu FC-40 auto tuner and lo and behold right around the bottom of 10m I saw 37 ohms of R and 1 ohm of X. ;) This was with the tuner bypassed BTW.
 
Bighammer, I don't understand. Are you telling us that at one point you trimmed your coax to get your antenna to 50 ohms at your frequency of choice? If that is so, then doesn't that suggest that your antenna is set at something other than 50 ohms? Then after doing this you take a reading at the antenna and now your antenna is matched perfectly. That defies logic.
 
Marconi. it just proves my point that any length coax isn't a fine tuned install.

antenna only to the mfj. 50 ohms.

shortest length needed to the antenna didn't get 50 ohms on the mfj. 14.5 ft did. even though it might only be 9-10 ohms off I wanted it to be as perfect as can be. and i wanted my boxes to be as happy as can be. and in previous test i have noted that if i had 60 ohms compared to 50 the reflect was slightly different also. ( more ) i have also stated this is nitty gritty tunning.

it doesn't matter if i typed a 5 page post about my findings in extreme detail, until you did the same test and had the same findings you still wouldn't believe it.
 
I don't think anyone is disputing what you are saying you got as readings. If anything, the dispute is over the interpretation of those readings.
If the antenna is a 1/4 wave, and if there is no impedance matching system incorporated in the antenna it's self or between the meter and the antenna, and if you are taking the readings at the input of that antenna, then there's a 'problem' somewhere. Where? I have no idea. A 1/4 wave antenna's input impedance is never 50 ohms all by it's lonesome, just doesn't happen, sorry.
- 'Doc
 
what does not stack up is if the antenna was 50ohms the coax length would not transform that 50ohms to 60ohms unless the coax is not 50ohms coax or the method of connection was causing a problem,
something aint right?
 
maybe i am missing something here, (been known to happen)
but why is it surprising that bighammer got a 50 ohm reading when using 3" of coax and sitting inside the vehicle?

if the antenna is installed properly, and has a suitable groundplane under it, why would it not equal close to 50 ohms?

sure, remove it from the vehicle and it would be very different.

"shortest length needed to the antenna didn't get 50 ohms on the mfj. 14.5 ft did."

this reminds me of the stuff i used to read in Lou Franklin's books about measuring SWR at the antenna end and at the transmitter end, and how the readings could be different.

to me, it seems like what bighammer experienced was this phenomenon.
he tried a length of 14.5 feet, and it happened to be one of the lengths that give an accurate interpretation of what the antenna's impedance is.

50 ohm antenna, 50 ohm coax. does the SWR meter at the radio end show 50 ohms?
maybe, maybe not.
is the antenna's impedance 50 ohms regardless of what the SWR meter says?
yes.


this is all my opinions based on the few things that i have learned.
if i am wrong, i will be happy to learn the truth.
loosecannon
 
loosecannon,
"if the antenna is installed properly, and has a suitable groundplane under it, why would it not equal close to 50 ohms?" - Because a 1/4 wave antenna with no impedance matching will have an input impedance of less than 50 ohms, more like 20 - 40 ohms, and that's being optimistic. Sure, you can do that matching by 'drooping' the radials of a groundplane, but that's just not all that 'possible' for a mobile antenna. At least, I don't think I've ever seen that done.
A 1/4 wave antenna not over a ground plane, just the 'stick' by it's self also won't have an input impedance of anything close to 50 ohms. That's not what I meant by "all by it's lonesome" (if that's what you were refering to, sorry). I meant that the thing had to have 'help' from something or somewhere. That 3" section of coax just wouldn't make enough difference to make any difference. It would affect what the meter read a bit, but it certainly wouldn't make it 50 ohms impedance (much less with '0' reactance).
- 'Doc


bighammer,
You wouldn't remember what the 'X', reactance was, would you?
 
The same mobile antenna on different vehicles will have different readings because the system is not balanced. The antenna may be 1/4 wave, but the vehicle portion of the antenna system may be much larger.

If you get a reading of 50-ohms with a 1/4 wave antenna, there is "something" going on. Its not a good thing or a bad thing......its just a thing. Either way, its not perfect but will work just fine.

Coax is almost never a perfect 50-ohms. The same coax on a 500' spool may have different readings every 100'. In a perfect world, an electrical half wave length will repeat the readings of the feed point, at the opposite end. If the coax is not perfect, the physical length may be different than the "math", but still be the correct electrical half wave length.

IF the feedpoint and the coax are NOT a perfect match, then the coax acts as a transformer. If the antenna was perfect then as we know, the length of the perfect coax would not matter. This is not what we are seeing here.

So what are we seeing here? A non-perfect antenna system being fed with a coax transformer......and a happy radio.
 
M.C.,
And if that coax just happens to be a characteristic 50 ohms, the transformation you end up with will NOT be close to 50 ohms if the antenna is a typical 1/4 wave. The coax transformer has to be of an impedance greater than the desired impedance if the load is less than the desired impedance. Right?
- 'Doc
 
the impedance required of a 1/4 wave matching section is the square root of the product of the two impedances to be matched.

1/4Z = sqrt(Z1 X Z2)
1/4Z = sqrt(50 X 25)
1/4Z = sqrt(1250)
1/4Z = 35.355

Z1 represents the source and Z2 the load

1/4Z = sqrt(50 X 100)
1/4Z = sqrt(5000)
1/4Z = 70.710

i'm amazed that no one has figured out what's happening here.
 

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