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Build Your Own Antenna - Discuss topics for building your own antenna.

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  #21 (permalink)  
Old 11-06-2014, 3:12 AM
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My LPDA project is posted on the website of Italian A.I.R.
http //air-radiorama.blogspot.it/2014/11/antenna-lpda-225-470-mhz-di-iz7bwz.html
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Old 11-12-2014, 9:19 PM
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Quote:
Originally Posted by red-dog View Post
I have done a mistake by 'stupid', has been the rush to mount the antenna or old age? Thanks for the advice, today I will to move items !!!!!
Stupid doesn't make mistakes, stupid doesn't do anything. you on the other hand are an experimenter you made something if it works for it's intended purpose remains to be seen but you tried and that's all you can ask of anyone. just think where we would be if no one tried something. just keep up the good work.

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Old 11-13-2014, 6:02 AM
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K3CFC Thanks for the encouragement, in fact it is. . . who does not gets nothing. . . not wrong !!
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Old 11-14-2014, 3:21 AM
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K3CFC Thanks for the encouragement, in fact it is. . . who does not gets nothing. . . not wrong !!
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Old 05-10-2016, 10:49 AM
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Why the hell didn't I find this thread BEFORE I put my LPDA on top of the pushup mast!

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Old 10-29-2016, 5:35 AM
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so what kinda results have people had with this? I'm getting ready to build one, I'm still in research mode....
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Old 06-18-2017, 3:28 PM
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I made one up and am going to used it vertically oriented.
Does it matter which side of the boom; top, bottom or sides), that the feed line is attached to?

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Old 06-18-2017, 5:09 PM
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Sorry, red-dog, my post about the on-line calculator was not aimed at you - 747mech asked about dimensions.
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  #29 (permalink)  
Old 07-03-2017, 6:40 PM
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Quote:
Originally Posted by w4rtt View Post
I made one up and am going to used it vertically oriented.
Does it matter which side of the boom; top, bottom or sides), that the feed line is attached to?

Randy
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No.

Working the coax through one boom is best to make it invisible to the antennas structure.
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  #30 (permalink)  
Old 07-04-2017, 9:54 AM
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A log periodic is balanced so there is no polarity to the feedline connection. Since it is balanced you want to keep the feedline insulated from either boom and if you run the feedline inside one of the booms to the feed point, its common to put rubber grommets on the coax inside the boom to keep it centered.

Its also good to use a common mode choke where the feedline exits the antenna and for a VHF/UHF log, covering the feedline with about 6 or 8 inches of #43 ferrite beads or snap on's is a good start.
prcguy

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Originally Posted by w4rtt View Post
I made one up and am going to used it vertically oriented.
Does it matter which side of the boom; top, bottom or sides), that the feed line is attached to?

Randy
W4RTT
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  #31 (permalink)  
Old 07-04-2017, 6:51 PM
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I'm sorry to say that you cannot connect an unbalanced coax directly to a balanced LP antenna. You'll ruin the function of the antenna just as much as putting all left elements to one boom and all rights to the other boom and not alternating them. You must use a balun, as all factory made LPs have, to make the antenna work properly. As it is done now the coax shield is "hot" and a part of the antenna.

/Ubbe


The Secrets of the Log Periodic Antenna
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  #32 (permalink)  
Old 07-04-2017, 9:54 PM
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The ferrite beads on coax I described is a 1:1 choke balun. It will have about 20dB of isolation (100:1 ratio) between input and output which is enough to isolate the feedline and force a balance at the antenna end. There are more elaborate ways to construct a better balun but what I described is known to work well.
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Originally Posted by Ubbe View Post
I'm sorry to say that you cannot connect an unbalanced coax directly to a balanced LP antenna. You'll ruin the function of the antenna just as much as putting all left elements to one boom and all rights to the other boom and not alternating them. You must use a balun, as all factory made LPs have, to make the antenna work properly. As it is done now the coax shield is "hot" and a part of the antenna.

/Ubbe


The Secrets of the Log Periodic Antenna
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  #33 (permalink)  
Old 07-05-2017, 12:21 AM
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I am curious the results of this antenna. You might try turning it toward fixed stations, measuring the signal, then turn 180 degrees , find the null , measure signal again and estimate the front to back ratio.

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  #34 (permalink)  
Old 07-05-2017, 2:00 PM
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Quote:
Originally Posted by prcguy View Post
The ferrite beads on coax I described is a 1:1 choke balun.
As the name implies it's only for choking the RF on the coax shield. It will not magicly add the current from the shield to the coax inner lead. You will loose half the signal from the antenna right there.
The elements connected to the shield will work as stubs and ruin the wideband function of the LP and instead work as some sort of unpredictable multifrequency antenna.

/Ubbe
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  #35 (permalink)  
Old 07-05-2017, 2:14 PM
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Default LP Antenna

The dual boom antenna (with alternating elements) was designed to use a coax feed. The coax should be run through one boom, with braid tied to that boom and center conductor attached to the second (bot at the high frequency end)

UBBE

the diagram you reference is for a different design, thus different feed.....

I have a construction article somewhere from the 70's describing the dual boom LPA (now if I could only find it....)

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  #36 (permalink)  
Old 07-05-2017, 2:23 PM
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An effective (20 - 30dB or more isolation) 1:1 choke balun will magically force a balance by isolating whats on the input and output ports. It will take an unbalanced feedline that would otherwise skew the pattern of an unbalanced antenna and nearly remove it from the circuit so the antenna will operated as if it were fed with balanced feedline.

Not all 1:1 baluns are designed the same but a good ferrite loaded current balun or actual transformer with isolated primary and secondary will force a balance from unbalanced feedline.
prcguy



Quote:
Originally Posted by Ubbe View Post
As the name implies it's only for choking the RF on the coax shield. It will not magicly add the current from the shield to the coax inner lead. You will loose half the signal from the antenna right there.
The elements connected to the shield will work as stubs and ruin the wideband function of the LP and instead work as some sort of unpredictable multifrequency antenna.

/Ubbe
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  #37 (permalink)  
Old 07-05-2017, 5:01 PM
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Quote:
Originally Posted by prcguy View Post
An effective (20 - 30dB or more isolation) 1:1 choke balun will magically force a balance by isolating whats on the input and output ports. It will take an unbalanced feedline that would otherwise skew the pattern of an unbalanced antenna and nearly remove it from the circuit so the antenna will operated as if it were fed with balanced feedline.

Not all 1:1 baluns are designed the same but a good ferrite loaded current balun or actual transformer with isolated primary and secondary will force a balance from unbalanced feedline.
prcguy

The advantage of an isolated transformer balun is the cancellation of common mode current that not only distorts the pattern, conducts RFI up the feedline to the antenna. So if you have a noisy shack (damn computers) having a balanced feed antenna takes that noise out of the equation.
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  #38 (permalink)  
Old 07-05-2017, 6:25 PM
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It doesn't have to be an isolated primary/secondary transformer type balun to get rid of RFI up the feedline to the antenna. Wrapping the proper amount of turns of coax around the appropriate mix ferrite can give you a huge reduction of common mode currents. Even ferrite beads slipped over the coax can give you a very noticeable reduction of noise conducted from computer equipment in the shack up the coax to the antenna, if you have the right mix and amount of beads.
prcguy

Quote:
Originally Posted by RFI-EMI-GUY View Post
The advantage of an isolated transformer balun is the cancellation of common mode current that not only distorts the pattern, conducts RFI up the feedline to the antenna. So if you have a noisy shack (damn computers) having a balanced feed antenna takes that noise out of the equation.
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  #39 (permalink)  
Old 07-06-2017, 5:23 AM
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Quote:
Originally Posted by prcguy View Post
An effective (20 - 30dB or more isolation) 1:1 choke balun will magically force a balance by isolating whats on the input and output ports.
It will only isolate the shield, stop any current running outside of the shield. You can do that by using toriods or coil up the coax that will inductivly shortcircuit the shields on each turn cancelling out any RF. You have to do that when something is wrong in the antenna system and you get RF on the coax shield.

You must use a transfomer to connect a coax to any type of balanced antenna. All log periodic antennas are balanced.

If you connect a coax directly to the booms of a log periodic antenna it will no longer function as one.

You can stick any metal up in the air and it will receive signals. More metal will receive more. Scanner antennas are often advertised as having a 25MHz-1000MHz range and it will actually receive all frequencies, if the signals are strong enough. When you connect a coax to a balanced antenna without using a balun transformer you will receive signals but the antenna will not perform as it was designed to do.

/Ubbe
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  #40 (permalink)  
Old 07-06-2017, 6:10 AM
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Sorry ubbe, but you are wrong. Blocking common-mode currents in the coax doesn't happen in just the coax shield, but in the center conductor as well. You can't even define "common-mode current" without considering the current in both the shield and the center conductor of the coax. A common-mode current occurs when the sum of the shield current and the center conductor current is not zero.

A properly constructed choke balun forces the currents in the coax shield and center conductor to be equal, but opposite. Any currents trying to flow in the same direction, whether coming up from the shack, or down from the antenna, are blocked. As a result, the coax shield on the antenna side of the choke balun is NOT at ground potential, and a choke balun works exactly the same as as a transformer balun with isolated windings.
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