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Author Topic: Is a 1:1 balun really needed on RX-only antenna if preamp is connected directly?  (Read 4541 times)
NOCALLSIGN9000
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« on: June 21, 2012, 08:38:56 PM »

Is a 1:1 balun really needed on an RX-only balanced 75-ohm UHF antenna if the unbal. coax preamp is connected directly to the antenna: one lead of the antenna would go straight to pin of the pre, the other to the shield of the pre with the output sent down the coax downlead?

The pre's shield is common on both in and out. The downlead would pass through a grounding block.

This opens another question: Is a balanced antenna literally balanced around true ground? If so, would that cause one lead to drain into the coax shield's grounding block rather than into the preamp?

Thanks for reading
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WB6BYU
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« Reply #1 on: June 21, 2012, 10:04:46 PM »

Many people connect coax to a balanced antenna without a balun.  It might or might not
work as expected, depending on a number of variables, particularly the length of the
coax and what all is connected to it, because it then becomes part of the antenna
(even when the preamp is mounted right at the terminals - one of them is still common
to the coax shield.)

If you absolutely need it to work first time, and performance is critical, then using a
balun makes it more reliable.  If performance isn't as critical and you can afford to fuss
with it if needed to get it to work right, go ahead and try it without a balun.
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KB4QAA
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« Reply #2 on: June 22, 2012, 05:36:25 PM »

An antenna or feedline is balanced or unbalanced relative to itself.  System or earth ground has nothing to do with it.

If an engineer designed a preamp so the signal was routed to ground (thus attenuating the signal) that would not be a very good amplifier!  Wink

If your installation is such that signal level is weak enough to require a preamp (expense and power supply complications), then it only makes sense to pay attention to basic details like not losing signal strength from impedance mismatch.  Use a 75ohm to 50ohm BalUn!!!  And yes the BalUn should be in line between the antenna and preamp.

bill
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NOCALLSIGN9000
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« Reply #3 on: October 02, 2012, 09:25:07 PM »

Thanks for the replies. It turns out my understanding of balanced/unbalanced transmission lines was not quite right.

My understanding now is that coax, although having 2 conductors, has 3 signal paths: Outside surface of shield, inside surface of shield, and surface of pin. The purpose of a BalUn is to ensure that all of the currents meant for the shield are channeled to the inside surface of the shield and not the outside surface. I'm assuming that most end-equipment throws away any signal on the outer surface of the shield as it is polluted with environmental noise. Furthermore, a lack of balun could cause the noisy outside surface currents and phase-smeared original signal reflections to flow back to the dipole (or whatever), mix with the phase correct signal, and make its way into the inner shield, adding heavy interference.
« Last Edit: October 02, 2012, 10:18:30 PM by NOCALLSIGN9000 » Logged
N4CR
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« Reply #4 on: October 02, 2012, 09:36:14 PM »

The RF on the coax shield still radiates. It just doesn't go where you expect your antenna to send a signal and so it's sometimes called pattern pollution.

Take a J-Pole with RF coupled onto the shield. Even if it radiates all the power you send it, the effective height of the antenna is reduced since some of the radiated power is much lower in height than the antenna itself.

On a beam, it might radiate sideways or rearward which would negate some of the forward gain and corrupt the pattern of the beam.

Since it's probably not a tuned radiating element, sometimes a lot of it will travel all the way back to the shack and make your radio and shack environment high in RF. This introduces it's own set of problems that aren't antenna pattern related.

Many people refer to a dipole fed by coax without a balun as a tripole.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
NOCALLSIGN9000
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« Reply #5 on: October 02, 2012, 10:17:44 PM »

"Many people refer to a dipole fed by coax without a balun as a tripole."

haha  Grin
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KI4SDY
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« Reply #6 on: October 03, 2012, 05:40:11 AM »

N4CR knows what he is talking about. Around here, when he speaks, we listen!  Smiley
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W8JI
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« Reply #7 on: October 03, 2012, 08:22:34 AM »

Is a 1:1 balun really needed on an RX-only balanced 75-ohm UHF antenna if the unbal. coax preamp is connected directly to the antenna: one lead of the antenna would go straight to pin of the pre, the other to the shield of the pre with the output sent down the coax downlead?

Yes. It a balun should be used.

Quote
This opens another question: Is a balanced antenna literally balanced around true ground?


No. It is balanced to what is around the point you are looking at.

Quote
If so, would that cause one lead to drain into the coax shield's grounding block rather than into the preamp?

You are looking at  the feedline as a transmission line in all modes, including common mode, when it is really a transmission line and an antenna rolled into one.

As for balance, read this:

http://www.w8ji.com/common_mode_current.htm

73 Tom

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G3TXQ
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« Reply #8 on: October 03, 2012, 08:55:07 AM »

It's worth remembering that the third "leg" of that "tripole" is still there during reception! The house/shack end of that leg likely comes near some RF noise sources, and the other end is directly connected to one side of the feedpoint where any noise picked up can be injected into the signal path.

In other words, a 1:1 current balun (choke) can also be of benefit on Rx.

73,
Steve G3TXQ
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AA4HA
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Posts: 1482




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« Reply #9 on: October 03, 2012, 02:49:10 PM »

Thanks for the replies. It turns out my understanding of balanced/unbalanced transmission lines was not quite right.

My understanding now is that coax, although having 2 conductors, has 3 signal paths: Outside surface of shield, inside surface of shield, and surface of pin. The purpose of a BalUn is to ensure that all of the currents meant for the shield are channeled to the inside surface of the shield and not the outside surface. I'm assuming that most end-equipment throws away any signal on the outer surface of the shield as it is polluted with environmental noise. Furthermore, a lack of balun could cause the noisy outside surface currents and phase-smeared original signal reflections to flow back to the dipole (or whatever), mix with the phase correct signal, and make its way into the inner shield, adding heavy interference.
Wink  It's ok, the theory of antennas and feedlines is poorly understood, even by the experts. Really your entire antenna system begins at the radio, the feedline, baluns, ground system and antenna are one fairly complex arrangement of different circuits. Essentially everything is acting like part of the receiver, even that coax cable.

For a receiver it is "nice" if you can have perfect impedance matches and as you mentioned there is some 75 ohm stuff in there as well. You did not specify what brand of radio (receiver?) but it can be all over the place. I have stuff that wants a 125 ohm doublet antenna (twinax cable), others that want 300, 450 or 600 ohm balanced line and then the 50 and 75 ohm feedline (commonly called coaxial).

As has been mentioned, for a pre-amp there is already going to be some impedance matching going on if it was designed by any engineer or experimenter with a lick of sense and pride in what they are doing. Many amplifier devices have a very high impedance input and a pretty low impedance output and as a circuit may have a balanced or unbalanced input with most outputs being unbalanced.

Unfortunately Dallas Lankford has cut the world off from access to his excellent writings on antennas and receiver amplifier designs. I consider him one of the greatest and actively followed everything he was doing through his Yahoo group.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W5WSS
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« Reply #10 on: October 13, 2012, 12:49:42 PM »

The receiver is the load. The network tuner at the receiver and the pre amplifier and balun up at the antenna should do nicely in this case one could use two baluns at each end one at the antenna and the second at the receiver will terminate the signal through the balun, network tuner and into the load/ receiver.

This system treats common mode noise ingress and offers the preamp with better differential mode signal purity to be pre amplified.
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KB4QAA
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« Reply #11 on: October 13, 2012, 04:16:39 PM »


Unfortunately Dallas Lankford has cut the world off from access to his excellent writings on antennas and receiver amplifier designs. I consider him one of the greatest and actively followed everything he was doing through his Yahoo group.
Tisha,
What has happened to cause him to do this?  I can't find any indication why from searching.

bill
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G8HQP
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Posts: 124




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« Reply #12 on: October 14, 2012, 08:59:44 AM »

For best results use a balun at the antenna. Exceptions would be if the preamp has a balanced input and is directly connected to the antenna, or if you decide to use a balanced feeder (unlikely at UHF).

Don't be confused by people introducing different issues like impedance matching (at UHF you want a good noise-match not impedance match), or a second balun at the bottom end (won't help and might make things worse).
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W5WSS
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« Reply #13 on: October 14, 2012, 10:46:51 AM »

Why would the second identical 1:1 current balun at the bottom make things worse?
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W8JI
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« Reply #14 on: October 14, 2012, 01:53:39 PM »

The only reason to add a second balun or isolator in the shack is if the feedline has some problem outside someplace, or unless there is a noise source in the shack causing noise to follow the feedline out to the antenna's area where it gets back into the antenna. If that is the case, the isolator belongs at the building entrance, not inside the building.
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