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Antenna Question




 
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K1JJ
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« on: March 26, 2005, 07:45:31 PM »

Here's the scenario:

You have three RESONANT 1/2 wave dipoles and want to feed them all in-phase.  They are spaced 1/2 wavelength apart from each other, all broadside.

Using openwire, you connect to the first dipole and then flip the open wire over 180 degrees 1/2 wavelength away to feed the second, then flip it again and feed the third.  Everything is FB and all are now in-phase - each sees zero degrees relative to each other.  [Dipole #1 is where the power is fed and the openwire feeds the other two as described] And, for the sake of the discussion, lets assume that the three dipoles are folded dipoles all properly matched to the open wire line, thus no SWR.

Now, assume you lengthed or shortened the first or second dipole so that it was no longer resonant... and it now has some inductive or capacitive reactance.  

Question:  Would the third dipole still see zero degrees of phase angle or would it change to a different angle like +10, -10 degrees, etc.  ie, Does making one of the earlier dipoles reactive cause a phase shift down the feedline chain?

I realize that the power distribution would change due to a different impedance balance between the three, but I am mainly interested in what would happen to the phase angle, if anything, down the line...

73,
Tom, K1JJ
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« Reply #1 on: March 28, 2005, 09:16:43 AM »

Tom,
I would think the pattern would shift left or right with the phase shift change. My phased set up had a better SWR pointing Northeast because the front dipole was a foot shorter than the back one. I had a broad side
pair of dipoles once and could pull the pattern side to side by adding a delay in one of the dipoles.
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K1JJ
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« Reply #2 on: March 28, 2005, 12:00:47 PM »

Frank,

I'm not sure if we are talking about the same thing....  Your dipoles are fed individually and not in a progressive chain.

But anyway... let's say you added a third dipole and they were in a chain, one after the other.. Put a scope probe on the first dipole to get a reference. Now put a second scope channel on the third dipole. Look at the phase relationship between the first and third on the scope.

Now, if you were to lengthen or shorten the second dipole, would the phase advance, delay or stay the same on the third dipole?

I'm trying to determine if individual loads along the feedline affect the actual phase angle as it progresses down the line.

Re-read my first example as show with 1/2 wave spacing and resonant antennas to keep it simple using a progressive chain - that's what I'll be doing here, but in four mini-bays.

I've axed a number of antenna gurus so far and none could come up with the answer...  The reason I need to know is for tuning  sixteen 2el Yagis for 6M that are stacked from 180'> 30' on the tower...  Since a tuning error on each Yagi MAY be CUMULATIVE within each mini-bay, I would like to know before I start.

T
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« Reply #3 on: March 28, 2005, 12:13:33 PM »

The phase relationship is a function of physical length of feedline. Any reactance from mis-match would not change that but would still change the ant. pattern due to the magnitude of physically seperated current nodes in the individual antenna (which is where "gain" comes from in the first place). In other words you are not delivering power to the load so the gain effect sought from stacking will diminish. Each reactive load will also have an effect on the impedance presented to the transmitter. Sounds good anyway.    :roll:
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« Reply #4 on: March 28, 2005, 12:28:14 PM »

I suspect it would shift the take off angle slightly in your case. It doesn't matter how you get phase shift the effect is the same.
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K1JJ
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« Reply #5 on: March 28, 2005, 12:35:07 PM »

Quote from: Dave Calhoun W2APE
The phase relationship is a function of physical length of feedline.    :roll:


OK Dave -

Yep, I was hoping that was the answer. It makes sense that with random mismatched antennas, some would hog the current while others would get less, affecting the overall pattern.

So what you're saying is that capacitaive or inductive loads ACROSS a feedline does not affect the phase to the other end. [in contrast to a SERIES reactance inserted in the line certainly affecting it]


While we're at it....

I read that when feeding a progressive chain on dipoles [or Yagis] that some of the energy gets radiated BEFORE it reaches the next Yagi. So, this creates a problem with current/power division and adds minor pattern lobes that would normally not be there.

Any feel for what percentage of power gets radiated by each dipole before it reaches the end in a four dipole chain? I can model this if I knew the percentage radiated.

The solution is individual "power divider" feeding, but that gets so complicated if done correctly and matched. A simple 180 degree flip on feedline is so easy in a longer chain....

T
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K1JJ
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« Reply #6 on: March 28, 2005, 12:41:33 PM »

Quote from: WA1GFZ
I suspect it would shift the take off angle slightly in your case. It doesn't matter how you get phase shift the effect is the same.



The LOWEST lobe take-off angle stays at 2 degrees no matter what the phase. Its amplitude will change if the phase gets way out of wack. Lowest take-off angle is a function of height above ground with antennas above 1/2-3/4 wavelength. Below this height, then boom length plays a big part too.

But with a long stacked array like this normally there is only one large lobe on the horizon with everything else supressed at least 20db down. When the phasing starts deviating, then minor lobes start creeping in above 2 degrees and mucking up the clean pattern.  But, I've found the overall phase can be off as much as 30 degrees! before it becomes a problem, in modeling.

T
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« Reply #7 on: March 28, 2005, 12:42:58 PM »

Tom,
You also have resistive losses between each antenna that makes one think the antenna at the top gets the least power. You could spend a lifetime tuning it perfect. A power splitter and a single feed line to each
antenna has different losses to live with not to mention interaction.
Present method is simple and may be the best way to go.
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K1JJ
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« Reply #8 on: March 28, 2005, 12:45:27 PM »

Quote from: WA1GFZ
Tom,
You also have resistive losses between each antenna that makes one think the antenna at the top gets the least power. You could spend a lifetime tuning it perfect. A power splitter and a single feed line to each
antenna has different losses to live with not to mention interaction.
Present method is simple and may be the best way to go.


Good points, OM!

T
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« Reply #9 on: March 28, 2005, 01:30:46 PM »

Quote from: K1JJ
Quote from: Dave Calhoun W2APE
The phase relationship is a function of physical length of feedline.    :roll:


OK Dave -

Yep, I was hoping that was the answer. It makes sense that with random mismatched antennas, some would hog the current while others would get less, affecting the overall pattern.

So what you're saying is that capacitaive or inductive loads ACROSS a feedline does not affect the phase to the other end. [in contrast to a SERIES reactance inserted in the line certainly affecting it]T



The phase relationship between the different loads is set by the feedline length. The physical length of feedline will shift the wave's amplitude points across time. It takes time for your R.F. to travel through the phasing.
I could be wrong but the reactive mismatched load will just radiate less juice and the phase relationship will be the same. Now if you look at this stuff at microwaves then I could see the mismatched load's reactance actually having an effect on the phase relationship of power delivered by the loads. Time for either a Smith chart or a talk with someone named Smith.

 
As far as feeding a whole bunch of loads, why not check out some of those big broadcast FM deals and see what they have to say. Probably doesn't mean a whole lot in the real world. Where ya gonna find a clean pattern with stacked Yagi's anyway?
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« Reply #10 on: March 28, 2005, 03:10:29 PM »

Dave there is a phase delay but he reverses the feed line to make up for it
Voltage is split off every 1/2 wave with a 180 degree pahse shift resulting in all antennas fed in phase. The only loss would be resistive in the feed line. The load mismatch just complicates the power splitting by introducing
different phase shifts from the loads. Tom needs a motorized trombone line at each load to tune things.
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K1JJ
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« Reply #11 on: March 28, 2005, 03:44:09 PM »

Yes, the trombone slider.... :lol:

Each 2el Yagi driven element is 50 ohms+ J0.   I am using a balanced  "T" match to bring it up to 350 ohms. The openwire is #12 enameled and spaced at 3/4" apart with Teflon spacers giving 350 ohm line.

So each Yagi will see a good balanced match to the feedline.

If I got anal about it, I could feed two Yagis in a power divider split using two 350 ohm feedlines - I would then need to combine these lines into a pair of 525 ohm 1/4 wave legs that join into a 'T', correct?   This will bring it back to 350 ohms to start another bay connection, and so forth.

The above procedure will give a nice power split without worrying about phase [assuming the lines are the same lenghth to each Yagi.]

So, you can see why using a simple 1/2 wave spacing and flipping the line over 180 degrees is so much easier - assuming phase and power division can be controlled reasonably.

Btw, I'll be building this whole sixteen, 2el Yagi array on two 70' masts - on the ground - so it can be tuned and adjusted and then put up like a pre-fab. The two "masts/booms" will be raised and mounted vertically on the tower. They will hang on swing gates [like a door] giving about 75 degrees of wide horizontal coverage, but rotatable around the tower about 330 degrees. Basically most of the USA will be covered with one heading - and a single vertical lobe on the horizon at 2 degrees takeoff.   Ougta be sporting if it works out.

T
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« Reply #12 on: March 29, 2005, 07:40:44 AM »

An interesting scenario, Tom.  I wondered about one thing though regarding the take off angle if you shorten two of the dipoles in the 3-dipole array.  

Wouldn't changing the length of a 1/2 wave dipole change its ground coupling characteristics and therefore affect take off angle?
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K1JJ
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« Reply #13 on: March 29, 2005, 11:35:28 AM »

Quote from: Paul, K2ORC
An interesting scenario, Tom.  I wondered about one thing though regarding the take off angle if you shorten two of the dipoles in the 3-dipole array.  

Wouldn't changing the length of a 1/2 wave dipole change its ground coupling characteristics and therefore affect take off angle?



Good question, OM.

Yes, but the effect would be extremely minimal.  This is because lenghtening the 1/2 wave dipole to add a small amout of inductive reactance would amount to only inches on 6M - plus 1/2 wavelength above ground on 6M is only about 10'.  So, most antennas are far above ground in terms of wavelength where ground effects are getting into the "free space" realm.

In contrast, when someone goes from a simple 1/2 wave long dipole on 75M at 60' high [1/4 wavelength high] compared to 2 half waves in phase, the ground effects can be quite pronounced causing a reduced take-off angle as you suggested.

But, generally, if an antenna is above 3/4 wavelength above ground, the take-off angle is little affected by element length....  ground becomes the main factor in take off angle based upon the reflections forming the eventual far field pattern.  Even boom length stops having much effect above 3/4 wavelength high.  ie, the lowest take off angle is about the same on a 3el yagi as a 20el Yagi when above 3/4 wavelenghth high - though more energy is contained in the lower and lowest angle lobes on the 20el.

That's why 6M is a great band for big gain antennas. Or, if you are limited to heights below 3/4 wavelength high [on ANY band for that matter], a long boom Yagi is a great way to go for lowering take off angle..

T
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