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Author Topic: Inverted V - Legs not perfectly 180* or opposed  (Read 2487 times)
KE6TDT
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Posts: 73




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« on: November 30, 2012, 08:40:20 PM »

My plan is to build a 102' inverted V, mounted in tree with apex at about 48', fed with coax and no balun.  Target is 10-80 meter operation with 100 watts out. No amplifier.     

The problem is the usual, >available space<.

I can get the legs to come down at about 45* angle from the apex no problem, but they would have to be moved out slightly away from 180* from the apex.   

My little diagram below is viewing the antenna from the side at ground level. The "I" is the mast or tree which is basically 90* and the "\" would be the approximate angle of the legs coming down from the apex, or in this case, slightly away from, or out away from the apex, depicted below. 

      I\

At my venue, there is no way around this.

I understand the leg ends should be almost perfectly opposed or 180* out from the apex.  My question is, how would this affect the antenna in receive or transmit and to what degree?

Any comments or tips? 
 
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KF7DS
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Posts: 191




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« Reply #1 on: November 30, 2012, 09:47:16 PM »

How high will the ends be?

Don KF7DS
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KE6TDT
Member

Posts: 73




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« Reply #2 on: November 30, 2012, 09:55:13 PM »

How high will the ends be?

Don KF7DS

About 10' Don.
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WB6BYU
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Posts: 13288




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« Reply #3 on: November 30, 2012, 10:44:13 PM »

Most antenna books show nice, straight antennas.  They certainly are easier
to analyze mathematically than more typical dipoles that run from the chimney
to a tree and down to a corner of the fence in one direction, and down along
the side of the garage roof and around the corner in the other.  That doesn't
mean that a less-than-perfect dipole can't make a lot of contacts - it
actually takes a lot of deviation from a straight dipole to cause much loss
in signal strength.

My first 75m dipole had a 90 degree bend in the middle where it ran along
the peak of the roof and then turned to go out to a tree in the back yard.
Others over the years have made use of whatever supports happen to be
available, which are rarely placed to maintain a straight wire.  You just do
the best you can and don't worrying that it isn't perfect.


However, in the case you describe where you have an inverted vee
that you are using on multiple bands, the VERTICAL angle of the wires may
be more important to effective operation on the higher frequencies than the
azimuth angle between the wires.  The flatter the antenna - the higher
the ends can be off the ground - the better it will work on 20m through 10m.
On 80m and 40m you may not notice much difference.
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WX7G
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Posts: 6079




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« Reply #4 on: December 01, 2012, 02:30:32 AM »

Your antenna will work but not very well fed with coax on 80-10 meters. Since you are using a tuner ladder line or even 300 ohm TV twin lead would be much better than coax.

   
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NQ3X
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Posts: 64




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« Reply #5 on: December 01, 2012, 06:09:20 AM »

I second that.  Balanced line will show much less loss on non-resonant frequencies (which in this installation is virtually all of them).  If you can't bring the balanced line into your shack, put a short length of low-loss coax (like 9913) just as far as you need to get it outside to a balun.  From there use balanced feeders.

Cheers,

Bob WP2XX
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M6GOM
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Posts: 915




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« Reply #6 on: December 01, 2012, 09:59:51 AM »

I understand the leg ends should be almost perfectly opposed or 180* out from the apex.  My question is, how would this affect the antenna in receive or transmit and to what degree?

Any comments or tips?  
 

I assume that you're talking about having a dipole that instead of being say exactly north/south would have one leg pointing north and the other off at an angle from south.

If that is the case, all it alters is the directivity of the antenna. The main gain will be on an angle intersecting the horizontal angle so of the legs were bent to say 90 degrees as follows with the vertical line being north:

|
|
|
_______

Then the directivity would be NE/SW. As yours is an inverted V, the change in directivity will not be as pronounced.

I actually used this characteristic of a dipole to my advantage at a friends whose long thin yard faced E/W and therefore the wrong way for working the USA. I did it so one leg pointed East/West and the other to the South therefore putting the most gain NW/SE as the USA shortpath is NW from the UK.
« Last Edit: December 01, 2012, 10:04:00 AM by M6GOM » Logged
KE6TDT
Member

Posts: 73




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« Reply #7 on: December 01, 2012, 10:33:01 AM »

Most antenna books show nice, straight antennas.  They certainly are easier
to analyze mathematically than more typical dipoles that run from the chimney
to a tree and down to a corner of the fence in one direction, and down along
the side of the garage roof and around the corner in the other.  That doesn't
mean that a less-than-perfect dipole can't make a lot of contacts - it
actually takes a lot of deviation from a straight dipole to cause much loss
in signal strength.

My first 75m dipole had a 90 degree bend in the middle where it ran along
the peak of the roof and then turned to go out to a tree in the back yard.
Others over the years have made use of whatever supports happen to be
available, which are rarely placed to maintain a straight wire.  You just do
the best you can and don't worrying that it isn't perfect.


However, in the case you describe where you have an inverted vee
that you are using on multiple bands, the VERTICAL angle of the wires may
be more important to effective operation on the higher frequencies than the
azimuth angle between the wires.  The flatter the antenna - the higher
the ends can be off the ground - the better it will work on 20m through 10m.
On 80m and 40m you may not notice much difference.


Thanks for the great response Dale!
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KE6TDT
Member

Posts: 73




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« Reply #8 on: December 01, 2012, 10:42:52 AM »

Your antenna will work but not very well fed with coax on 80-10 meters. Since you are using a tuner ladder line or even 300 ohm TV twin lead would be much better than coax.

   

I can do that but I've seen a lot of conflicting opinions regarding baluns etc. 

Question:

1. If I go with ladder line, how long should the ladder line be with an apex about 45 feet?

2. I have a MFJ 949D turner with a balanced connection, with a 4:1 built in balun, should I go straight into that with the ladder? (I am concerned with TVI, lots of wiring in the home)   

3. Would it  be better to come off the ladder line with a short coax, about 10' length connected to  a balun "outside" the window and come into the turner with coax, straight to the coax input on the turner.

I've seen so many conflicting opinions regarding this, it's rather confusing to be honest.

Thanks Dave
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KE6TDT
Member

Posts: 73




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« Reply #9 on: December 01, 2012, 10:44:57 AM »

I second that.  Balanced line will show much less loss on non-resonant frequencies (which in this installation is virtually all of them).  If you can't bring the balanced line into your shack, put a short length of low-loss coax (like 9913) just as far as you need to get it outside to a balun.  From there use balanced feeders.

Cheers,

Bob WP2XX

Bob, I have aluminum window frames, and why I am thinking of coming in with coax.  Lots of wiring in the compound and want to avoid TVI etc.

Thanks Bob.
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KE6TDT
Member

Posts: 73




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« Reply #10 on: December 01, 2012, 10:49:00 AM »

I second that.  Balanced line will show much less loss on non-resonant frequencies (which in this installation is virtually all of them).  If you can't bring the balanced line into your shack, put a short length of low-loss coax (like 9913) just as far as you need to get it outside to a balun.  From there use balanced feeders.

Cheers,

Bob WP2XX

Sorry Bob, you lost me with "Just as far as you need to get it outside to a balun."

You mean put the balun outside the window with coax going to the tuner or radio?
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KE6TDT
Member

Posts: 73




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« Reply #11 on: December 01, 2012, 11:59:22 AM »

You just do
the best you can and don't worrying that it isn't perfect.

I think that's where I'm at Dale. What's that old song...Don't let the sound of you're own wheels drive you crazy... 
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N6AJR
Member

Posts: 9908




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« Reply #12 on: December 01, 2012, 12:07:37 PM »

Any antenna is better than no antenna at all.  SO like he said, get it up get on the air and improve it when you can.  look at some of the weird antennas sold in the past, like the whole Isotron series, which were basically a large capacitive hat on the end of the coax.
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K9SRV
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Posts: 121




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« Reply #13 on: December 03, 2012, 12:27:57 PM »

Take it Easy ~ Eagles  Cool
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WB6BYU
Member

Posts: 13288




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« Reply #14 on: December 03, 2012, 01:37:43 PM »

Quote from: KE6TDT

1. If I go with ladder line, how long should the ladder line be with an apex about 45 feet?



If you are trying to recreate G5RV, then use about 32' of ladder line times the
velocity factor.  (Probably around 28'.)   Looking at VK1OD's analysis here:
http://vk1od.net/antenna/G5RV/index.htm
the SWR on the coax and associated losses should be reasonable on 80m,
40m and 20m (though the impedances will be somewhat different with an
extreme slope to the antenna.)

Otherwise, the ladder line should be long enough to reach to your tuner, or as
close as you an get it.  Occasionally you may encounter an impedance that your
tuner won't match due to a particularly combination of antenna and feeldine
length, but you deal with those cases individually when they come up by adding
feedline, switching a coil across the balanced line, etc.


Quote

2. I have a MFJ 949D turner with a balanced connection, with a 4:1 built in balun, should I go straight into that with the ladder? (I am concerned with TVI, lots of wiring in the home) 



Running straight to the balanced output on the tuner is fine.  (Don't forget to add
the shorting wire from the single wire output jack to the balun.)  Radiation from
properly balanced open wire line is quite small - less than what you can have on
the shield of coax cable with many antennas.  If you run one side of the ladder
line against the case of a computer or TV it will pick up more signal, but there
is a simple fix - don't do it.  Otherwise running ladder line into the shack
is no more likely to cause TVI than any other feedline.

Your primary cause of RF interference is fundamental overload due to the
transmitted signal from the antenna being stronger than the equipment can
handle - that isn't the fault of your feedline or your transmitter. 



Quote

3. Would it  be better to come off the ladder line with a short coax, about 10' length connected to  a balun "outside" the window and come into the turner with coax, straight to the coax input on the turner.



The length of the coax between the external balun and the tuner is
generally more important than the length of ladder line from the balun to
the antenna because the losses in the coax will often exceed those it the
rest of the system combined.  The ideal coax length is generally "as short
as possible", preferably measured in inches rather than feet.

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