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  #65  
Old 04-19-2013, 03:31 AM
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Originally Posted by Shockwave View Post
Can you elaborate on what "conditions" you are talking about? The vague answer alone that contradicts what I've said is not helpful. What would be helpful is to explain the "why" behind your statement so we can determine if I've overlooked something or if you've misunderstood something.
I doubt it very much Donald,that would involve Doc rewriting even more new laws of physics.

Might be easier for everyone to understand if its explained what that 1/4 wave or odd multiple of actually does, firstly it takes one antenna/load of 50 ohms and inverts it up too 100 ohms, which when done on both sides gives two 100 ohm loads,when you parallel two hundred ohm loads your left with a 50 ohm load at the t connector output to the radio,in an ideal world,if you only use one frequency or two resistive dummy loads or even two non reactive resonant antennas which exhibit 50 ohm resistive load,in this case you could use any random length of 50 ohm coax and see a good match.

However few of us live in that perfect world,chances are the end parallel impedance won't be exactly 50 ohms resistive, if you use a wide bandwidth,antennas that are resonant but have impedance around 36 ohms each, I.E. Two quarter wave whips,therefore any coax with an electrical halfwave (incuding velocity factor) will present the radio with the parallel total load at the t connector output minus losses in the coax (the exact reason decent coax often shows a higher vswr than the cheap pish its replacing).

As Donald says when both loads are parallel 50 ohms inverted up to 100 ohms then any random length add on 50 ohm cable can be added to either the 1/4 wave stubs or the actual run from radio to t connector as vswr will be 1.0:1 therefore there is no standing waves on cable so all points will measure same vswr,minus cable losses.

For mobiles I wouldn't bother with cophasing as it achieves little due to insufficient spacing of antennas, for base I'd use either 2 x 3/4 or 5/4 wave runs of rg11 or better, and 1/2 wave multiples of rg213/u or better,alternatively you could just run two odd 1/4 wave multiples of rg11 or better back to the shack and plug them straight into a t connector that has a pl259 straight into back of radio as it might work out cheaper just buying a reel of 75 ohm coax than individual lengths of 50 + 75 ohms coax.
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Last edited by jazzsinger; 04-19-2013 at 03:44 AM.

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  #66  
Old 04-20-2013, 06:31 AM
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JAZZSINGER,
I've read that the long runs of mismatched coax (75 ohm) have more loss than 50 ohm extensions. Anyone experimented with that?
Thanks for the input, Dennis

[QUOTE=jazzsinger;423194]
for base I'd use either 2 x 3/4 or 5/4 wave runs of rg11 or better, and 1/2 wave multiples of rg213/u or better,alternatively you could just run two odd 1/4 wave multiples of rg11 or better back to the shack and plug them straight into a t connector that has a pl259 straight into back of radio as it might work out cheaper just buying a reel of 75 ohm coax than individual lengths of 50 + 75 ohms coax.[/QUOTE]

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  #67  
Old 04-20-2013, 10:44 PM
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I would suggest you do the loss calculations your self, see what the differences are. That depends to a great extent on exactly what cable is used.
As for re-writing the laws of physics, 'Momma Nature' and I have an agreement. If 'she' likes the re-write things are fine and I get an 'atta-boy'. If not, then we discuss the matter.
- 'Doc

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  #68  
Old 04-21-2013, 09:09 AM
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Quote:
Originally Posted by dennis4040 View Post
JAZZSINGER,
I've read that the long runs of mismatched coax (75 ohm) have more loss than 50 ohm extensions. Anyone experimented with that?
Thanks for the input, Dennis

[QUOTE=jazzsinger;423194]
for base I'd use either 2 x 3/4 or 5/4 wave runs of rg11 or better, and 1/2 wave multiples of rg213/u or better,alternatively you could just run two odd 1/4 wave multiples of rg11 or better back to the shack and plug them straight into a t connector that has a pl259 straight into back of radio as it might work out cheaper just buying a reel of 75 ohm coax than individual lengths of 50 + 75 ohms coax.
Dennis long runs of odd 1/4 wave multiples of 75 ohm coax are still matching transformers,or impedance inverters the same as a 1/4 wave would be,only losses are the actual losses in the cables themselves,it don't matter if a cable is 50 ohm or 75ohm, if both have 3db loss at 100m then no matter which type you use its the distance and loss per metre that dictates losses, not the impedance.

What I'm trying to say is if you add the 50 ohm extension too either the t connector to radio or even on both sides of the T or if you run it in a v shape straight to radio with 2 x 75 ohm odd 1/4 wave multiples or even as Donald suggested running 75 ohm quarter waves then 50 ohm cable from there, then as long as the cable losses per metre are the same for all cables it will be the total cable length that gives your loss figures,and that's only if you've got 2 parallel 50 ohm resistive loads to start with, make those loads reactive allsorts of weird and wonderfull things will appear

One thing I would warn is be very careful what coax your buying,rg8,rg11,rg213,etc are no longer mil spec or are older cables with a mil-c-17f style coding and rg number,now they are stamped M17/28 for rg5! Etc.

Be especially carefull of cables that say rg213/u type,that small word at the end type has a huge meaning,fuck all like the real thing made with low quality materials by a 13 year old in outerfucking gibrovia.copper is now very expensive so becoming more and more worth faking in far east sweat shops.

No point having the best antenna/rig in the world if the bit connecting them is inferior, too many people are hung up on swr and cable loss,both of which can be compensated for,

most important property of coax is shielding/preveting leakage and longevity/weatherability of the jacket.99% of radio problems came across were down to poor installation or poor quality coax,more often than not made worse by poor quality connectors. You only get what you pay for and sometimes your not even getting that.
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  #69  
Old 04-24-2013, 03:15 AM
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Quote:
Originally Posted by W5LZ View Post
I would suggest you do the loss calculations your self, see what the differences are. That depends to a great extent on exactly what cable is used.
As for re-writing the laws of physics, 'Momma Nature' and I have an agreement. If 'she' likes the re-write things are fine and I get an 'atta-boy'. If not, then we discuss the matter.
- 'Doc

(I think jazzsinger is jealous.)
I'm jealous as fuck mate,mother nature wisnae very kind to me,I can believe you have a deal going with her, but I'd love to be a fly on the wall during those disscussions,I'm mentally picturing the Grim Reaper asking Satan for instruction.pmsl.
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  #70  
Old 06-25-2013, 03:34 PM
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Default Using 50 ohm coax between antennas and co-phase harness

Shockwave,

I thought everyone might appreciate some feedback from this project.

I finally completed the harness for 2 Maco 103c vertically stacked 27.5' apart.
It seems to me that SW was correct about adding odd but same length 50 Ohm coax between the 75 Ohm harness and the antennas.
I have posted a video of me checking the tune of this array with a 259b, hopefully the link will work as I have not used photobucket much.

Unless I am missing something, (this is my first time to use the MFJ), it appears to be resonant at very close to the center CB channel. And it did drop down from the resonant frequency of the individual antennas just as Shockwave predicted. Can't remember how much but I may have that in my notes if anyone is interested.
Each individual antenna tuned at 1.3 - 1.6 SWR, as you can see in the video together they dropped to 1.0 - 1.1.
Is this an anomaly or good tuning?
The sad part is due to the limitations of my mast and tower arrangement I was unable to get the thing into the air.

Did learn a lot and talked some DX with it on the ground. :-)
Thanks to everyone that contributed.

CB Radio Photos by dennis4040 | Photobucket
dennis

Quote:
Originally Posted by Shockwave View Post
The Co-Phase harness you need to stack the antennas at 34 feet apart is no different than what you would use at closer spacing. All you have to do is add equal 50 ohm lines off the 75 ohm harness to reach the pair of antennas. Once you pass through the 1/4 wave of 75 ohm coax lines, you're back to 50 ohms. It doesn't matter if the antennas are plugged directly into the harness here or pass through any random equal lengths of 50 ohm coax first.

Another problem people usually face when phasing antennas together is called "Mutual Coupling". The effects of mutual coupling cause the resonant frequency of antenna to lower when combined together. For example, on the FM broadcast band, a dipole tuned to 98.1 Mhz. as a single antenna will usually drop its resonant frequency down to 97.9 Mhz when combined with second dipole that was tuned at 98.1 Mhz. by itself.

To compensate for this we simply tune the individual antennas to be resonant 200 Khz. higher than the desired frequency. The offset in frequency will probably be a little different on 11 meters with two yagi's but the effect and direction of frequency shift will be the same. For maximum performance it's important that we correct this problem by tuning the antennas and not mistake this as needing to compensate by adjusting the length of the phasing harness.

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  #71  
Old 06-26-2013, 06:00 AM
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Quote:
Originally Posted by dennis4040 View Post
Shockwave,

I thought everyone might appreciate some feedback from this project.

I finally completed the harness for 2 Maco 103c vertically stacked 27.5' apart.
It seems to me that SW was correct about adding odd but same length 50 Ohm coax between the 75 Ohm harness and the antennas.
I have posted a video of me checking the tune of this array with a 259b, hopefully the link will work as I have not used photobucket much.

Unless I am missing something, (this is my first time to use the MFJ), it appears to be resonant at very close to the center CB channel. And it did drop down from the resonant frequency of the individual antennas just as Shockwave predicted. Can't remember how much but I may have that in my notes if anyone is interested.
Each individual antenna tuned at 1.3 - 1.6 SWR, as you can see in the video together they dropped to 1.0 - 1.1.
Is this an anomaly or good tuning?
The sad part is due to the limitations of my mast and tower arrangement I was unable to get the thing into the air.

Did learn a lot and talked some DX with it on the ground. :-)
Thanks to everyone that contributed.

CB Radio Photos by dennis4040 | Photobucket
dennis
Nice work Dennis. Sometimes the education gained by experimenting is more valuable than the project itself. When you say "Each individual antenna tuned at 1.3 - 1.6 SWR", where you tuning them above the desired frequency to compensate for mutual coupling? Do you also mean that one antenna showed a 1.3 and the other showed a 1.6 SWR? If that's the case the two antennas were not evenly tuned and they should be to work balanced.

Both antennas should be identical. To keep this consistent through the process of tuning the gamma match, you should mark the gamma rod and its slider bracket at equal intervals using a tape measure. That way it's easy to match what you've done on one antenna to the other. If you try tuning them together without marking them evenly, you are more likely to have an imbalance because one can compensate for the reactance of the other.

I like to tune the antennas individually for a perfect match but this requires knowing how much offset in frequency mutual coupling is going to cause. That is easy to figure out. Tune both antennas individually then connect them to a known good phasing harness. The resonant frequency is going to drop. Measure the difference between the frequency you tuned them at individually and the new resonant frequency when they are combined. Add that difference to the desired frequency and retune both antennas by themselves for this calculated frequency and they should be right where you want when combined.

As I watched your video I noticed you may have tuned these antennas very close to the ground and in the horizontal position. If you were to raise this antenna more than 10 feet off the ground now, I suspect you would see a noticeable change in resonant frequency and SWR due to the effects of ground reflections. If you must tune a beam antenna this close to the ground prior to a higher installation, you should point them straight up in the sky with the back reflector right on the ground.

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  #72  
Old 06-26-2013, 10:17 AM
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Shockwave,

Thanks for the additional tuning tips.
I originally set both beams horizontal on separate masts 9' off ground spaced 27.5' apart in a clear area 40' from anything as Maco suggests. It sounds like the measurements I took there would be more accurate than on the mast 2.5' off ground.
Next time I will try to point them to the sky. Although my next stab at this is to put all six elements on a home built mast setup to Maco 106c specs. It might be hard to point that straight up. I don't think this 6 element will give me the signal to noise ratio I was hoping for with the vertical stack but should still be a 3.5 DB improvement over a single 103c according to the information on DX-antennas.com where they have impartial comparisons of common antennas.
Below is some additional information from my tuning notes:
Vertical Stack 2 Maco 103c vertical polarization 27.5' apart.
Both beams elements were measured and setup to be resonant @ 27.205 Mhz per Maco instructions.

Antenna A has been up by it's-self for about 9 months and really working exceptionally well.
Lowest SWR I was able to get was 1.6 @ 27.260Mhz straight off the Gamma Match connector to 259b.

Antenna B is new and I was able to get SWR 1.3 @ 27.331 Mhz straight off Gamma Match to 259b
Note: When using a 2' jumper from harness to 259b SWR changed to 1.5, it does make a difference when using other than 1/2 WL coax.

Both setup horizontal 9' off ground and 27.5' spacing
SWR 1.6 @ 27.230 Mhz

Both on mast laid horizontal about 2.5' off ground.
SWR 1.0 50 Ohm X=3 26.790 Mhz
SWR 2.0< 28.015 - 23.268
SWR 1.5< 27.823 - 23.507
SWR 1.0 26.820 - 26.670
52 Ohm SWR 1.1 26.905
99% efficiency 26.545 - 27.180

I did not keep good notes on resonant change when both antennas were together vs: single but it did drop lower.
Thanks, dennis

So pr
Quote:
Originally Posted by Shockwave View Post
Nice work Dennis. Sometimes the education gained by experimenting is more valuable than the project itself. When you say "Each individual antenna tuned at 1.3 - 1.6 SWR", where you tuning them above the desired frequency to compensate for mutual coupling? Do you also mean that one antenna showed a 1.3 and the other showed a 1.6 SWR? If that's the case the two antennas were not evenly tuned and they should be to work balanced.

As I watched your video I noticed you may have tuned these antennas very close to the ground and in the horizontal position. If you were to raise this antenna more than 10 feet off the ground now, I suspect you would see a noticeable change in resonant frequency and SWR due to the effects of ground reflections. If you must tune a beam antenna this close to the ground prior to a higher installation, you should point them straight up in the sky with the back reflector right on the ground.

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