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Not to start an arguement just some thin s Ive learned over time.

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  #41  
Old 02-20-2012, 11:43 PM
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eddie,
if you used odd multiple electrical 1/4waves of 75ohm coax terminated with 50ohm pure resistance input impedance seen looking into each line becomes

Zin = Zo^2
---------
Zl

the characteristic impedance of the line (75ohm) squared, divided by (50ohm X=0) load

if the loads are not 50ohm X=0 the phase relationship between current and voltage along the 75ohm lines shifts and all bets are off.
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  #42  
Old 02-21-2012, 01:33 AM
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Quote:
Originally Posted by Needle Bender View Post
Hi Marconi,

You turkey!

Sometimes you take things personally that are never meant that way, not by me anyway. You're the most enthusiastic user on this forum and the only one who addressed my question on topic.

...but stop calling my momma bad names!

OK, the 100 ohms is what you should get at the other end of a 75ohm electrical 1/4 wave matching transformer coming from a 50ohm load. (Don't even begin to try to make me think you don't already know this)

Two of them, each from a 50ohm antenna into a T connector should show right about 50 ohms at the single input, like your test, except double the impedance back to 50ohms.

5 out of 4 Doctors recommend wine over Chicken McNuggets.
I'll have to dig around in the shop and see if I can find one of my old harnesses, and see how it acts hooked up to my dummy loads.

You may be right, but I have never seen a 50 ohm complex load from a 1/4 wave radiator attached to a mobile. Sometimes it's even difficult for me to get such a load on a real 1/4 wave base radiator, with slanted down radials unless I have real good control over the slant of the radials below 50* degrees.

Check out this 1/4 wave with 50* degree radials. This is the Z I usually see and worse when reading the match at the feed point on a mobile. When I check through the feed line, this match does often get better due to transformation. This is why I think guys typically tell us they have a perfect match in their mobiles with a flat SWR that is low and hardly moves the needle. IMO many mobiles can't come close to this angle and provide the necessary ground plane. We found that installing these co-phase setup closer to the ground, on the bumper of pickup trucks seem to improve performance just using old Mother Earth on the roadway. This was the reason for my work with the 102" whip over the years, long before I could make a model.

Attachment 6665
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  #43  
Old 02-21-2012, 02:05 AM
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Quote:
Originally Posted by bob85 View Post
eddie,
if you used odd multiple electrical 1/4waves of 75ohm coax terminated with 50ohm pure resistance input impedance seen looking into each line becomes

Zin = Zo^2
---------
Zl

the characteristic impedance of the line (75ohm) squared, divided by (50ohm X=0) load

if the loads are not 50ohm X=0 the phase relationship between current and voltage along the 75ohm lines shifts and all bets are off.
Good morning Bob. I use this test formula that I've used for years, and it is base on the feed point impedance for a single installed radiator. Of course both antennas should be the same of very similar for this to prove out and work right the way I understand it.

I don't know if it's good or not, and I don't know the source. Could be my old antenna Mentor buddy gave it to me when we were building co-phase harnesses years ago.

Z = SQR (load impedance ohm x transformer line impedance)

Example of 37 ohm 102" antennas using RG6, 75 ohm, or RG71U, 93 ohm.

37x75=2775 SQR = 52.7 ohms results at the TX end of harness. So, if using a 50 ohm feed line in system, then further transformation can occur, and the following is supposed to calculate the new impedance value using the results from the first calculation.
52.7x50=2635 SQR = 51.3 ohms.

Then we trimmed the pig tail ends of our harness to the lowest SWR and I figured, if everything was good, maybe the Z value for the TX end of the match should be close to 51.3 ohms.

I never questioned the idea, and I really don't understand all the nuances of this business. So, if you find this in error or totally off base...then let me know.

Do you see anything wrong with the images I posted above? Is that pretty much what I should be seeing with such a setup?
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Old 02-21-2012, 02:23 AM
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im not familiar with that formula eddie,

the only thing i see wrong with your pics of the 50ohm lines are the rubber and glass where your lcd display connects needs cleaning with alchohol to get the missing segments working.
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  #45  
Old 02-21-2012, 07:36 AM
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Originally Posted by bob85 View Post
im not familiar with that formula eddie,

the only thing i see wrong with your pics of the 50ohm lines are the rubber and glass where your lcd display connects needs cleaning with alchohol to get the missing segments working.
Well I see my parallel 50 ohm harness shows about half of the two parallel 50 ohm loads in my example above, so I guess 'Doc and NB were correct. I stand corrected, a transformer should show 100 ohms if the load is 50 ohms, and when run in parallel the net load results will be 50 ohms.

Sounds reasonable to me, but I always imagined this thing backward I suppose.

What is a great tip Bob, thanks. No wonder Autek is so willing to fix it for a nominal charge, over $50 dollars plus shipping both ways. I'll try to clean mine.

Are you telling me that this part of the meter is rubber, like latex?

Thanks for the tip and the advice.
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  #46  
Old 02-21-2012, 07:40 AM
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Closer would be 70.7ohm coax (squared) = 4998.5 / 50 = about 100ohms, but it seems like I recall reading that 75ohm coax is actually closer to 72ohms.

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Old 02-21-2012, 08:01 AM
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Originally Posted by Needle Bender View Post
Closer would be 70.7ohm coax (squared) = 4998.5 / 50 = about 100ohms, but it seems like I recall reading that 75ohm coax is actually closer to 72ohms.
I see your point NB, thanks.
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Old 02-21-2012, 10:15 AM
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eddie,
it would be unusual if the display did not sit on conductive rubber strips, most lcd displays work that way,
over time they get contaminated from airborn crud,
all it takes is a slight bit of resistance and the polarising angle of the segment changes, sometime the bad segments can look ok when you tilt the display to a different angle,

if you can get at the screen look to see if its sat on rubber,
if it is, clean both edges of the rubber, the glass where the rubber makes contact and the contacts on the board with alcohol, then reassemble without touching the contact parts with your fingers,

if its not sat on rubber strips you are unlucky,

doc correct? surely not, let me go look at this, i don't believe it lolol
good luck.
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