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Author Topic: Should I add a balun to force equal and opposite current and voltage anyway  (Read 8958 times)
W5WSS
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Posts: 1436




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« on: August 15, 2012, 01:10:50 PM »

while I am at a summer vacation cabin I use a homebrew non resonant vertical made of #14 thhn wire that is low base end height apx 4ft high from earth surface. The antenna base height being 4 ft I feed at the base where the radials are connected around it and slope downward to run along atop of earth surface radially and symmetrically.
I suppose the radials are considered ground mounted ,detuned by surface contact and proximity affects etc, but leaves me wanting to experiment with a balun.
Even though I am using an L auto tuner at the feedpoint (yes it is wx proofed) I am curious about the 1:1 current balun vs a 4:1 current balun since I press this system into multi band service from 10-40m.
The tuner handles the impedance excursions so I thought perhaps for common mode attenuation and decent system balance I would opt for the 1:1 version as a common mode choke and see if the device baluncing". forces a detectable noticeable increase in Field strength. I am not worried about insertion loss as the feedline is very efficient.
I wonder what if any advantage a 4!1 current balun could offer other than maybe a 4!1 step down or up impedance that the tuner doesn't seem to be having difficulty with any suggestions? I know that it depends but am open to your thoughts.

Thanks.
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KD2CJJ
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« Reply #1 on: August 16, 2012, 12:35:52 PM »

I could be wrong, but unless your tuner is not able to bring the antenna into resonance then a 1:1 would be better... If your tuner is unable to bring the antenna into resonance then a 4:1 or even a 9:1 would be used (depends on the amount of impedance mismatch)...

Now, you can help this also either by placing more radials down or elevating the verical and sloping the radials at 45 degrees.  (Anything more it does not help)...the downfall is that you will need to have 2 or more tuned per band you want to transmit on for it to be efficient - even this may require a 4:1 OR 9:1 - again this depends on length of the radiator...

Of course you could make the feed-line resonant - but that becomes an issue as you would need multiple feed-lines per band you wan to transmit on..  Generally if you can get your feedline (ladder line) between 80 - 100 feet typically most bands will tune with an internal tuner...







while I am at a summer vacation cabin I use a homebrew non resonant vertical made of #14 thhn wire that is low base end height apx 4ft high from earth surface. The antenna base height being 4 ft I feed at the base where the radials are connected around it and slope downward to run along atop of earth surface radially and symmetrically.
I suppose the radials are considered ground mounted ,detuned by surface contact and proximity affects etc, but leaves me wanting to experiment with a balun.
Even though I am using an L auto tuner at the feedpoint (yes it is wx proofed) I am curious about the 1:1 current balun vs a 4:1 current balun since I press this system into multi band service from 10-40m.
The tuner handles the impedance excursions so I thought perhaps for common mode attenuation and decent system balance I would opt for the 1:1 version as a common mode choke and see if the device baluncing". forces a detectable noticeable increase in Field strength. I am not worried about insertion loss as the feedline is very efficient.
I wonder what if any advantage a 4!1 current balun could offer other than maybe a 4!1 step down or up impedance that the tuner doesn't seem to be having difficulty with any suggestions? I know that it depends but am open to your thoughts.

Thanks.
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73

Mike
KD2CJJ
KB4QAA
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Posts: 1991




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« Reply #2 on: August 16, 2012, 01:54:17 PM »

You are thinking correctly.

There is no advantage of a 1:1 vs. 4:1 BalUn.  Choose whichever gives you the best impedances your tuner can match across your desired bands.   One problem that can crop up is when the antenna impedance is moderately low and a 4:1 BalUn transforms it much below 50 Ohms, some tuners gag because they can't handle very low impedances.

Of course coaxial cable can't be cut for resonance, though one can moderate a high SWR  by increasing the coax length and thereby increase losses.

Just checking the obvious your arrangement is:   Txcver=>Balanced Tuner=>BalUn=>UnBalanced Vertical Antenna

Now this assumes that your Tuner is designed for balance input, NOT a typical autotuner like an LDG used with coax input from a dipole with output to coax.  Most hams do not have a wild configuration like this.

If you are using an ordinary unbalance autotuner at the antenna feedpoint, then there is no need for a BalUn anywhere.   The vertical antenna is unbalanced, the feed method should be very short and direct or very short unbalance coax, the tuner is unbalanced the output line is unbalanced coax to the unbalanced radio input (unless you are using a boatanchor with balance input).

If you were not using a tuner at the feed point, but rather in the shack you could use either type Balun, as below:

Txcver=>Tuner=>coax=>BalUn-Isolator=>short coax=>Antenna

This is a more typical setup.  The isolator keeps RF off the coax so it doesn't radiate and can transform the impedance if needed as we already discussed.

bill

73, Bill
« Last Edit: August 16, 2012, 02:07:33 PM by KB4QAA » Logged
WB6BYU
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Posts: 12773




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« Reply #3 on: August 16, 2012, 02:52:05 PM »

Many tuners are more efficient matching higher impedance loads, so I'd use a 1 : 1 current balun
instead of a 4 : 1.

With a tuner at the feedpoint, I'd use a 1 : 1 current balun = feedline choke between the tuner
and the coax to the rig.  That helps to decouple the outside of the coax for RF.
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W5WSS
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Posts: 1436




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« Reply #4 on: August 16, 2012, 05:26:37 PM »

Hello Dale, prior to this antenna design solution I looked at my auto tuner and noticed, that the shield path per both SO-239 connectors do not pass through the auto tuner but only the center conductor. Therefore Then I connect the antenna radials directly to the tuner ground lug which is common to each SO-239 shield and chassis. The vertical antenna connects directly into one of the SO-239 center conductor socket.
This technique places the auto tuner directly At the base feed point of the vertical antenna. The Tuner is located directly inside at the bottom corner of the window sill. The radials fan away symmetrically downward and the antenna wire heads up vertically to a tag line holding it taught in the vertical orientation.
The radials and the vertically oriented antenna wire both originate at the tuner so as a result I have no feedline between the tuner and the antenna.
The auto tuner tunes properly and couples the antenna and equipment nicely.
Since the radials and the antenna originate at window sill level about 4ft above earth surface" there is some chance that the radials and antenna relate as a variant such as an low elevated 1/4 wave ground plane that really is a vertical dipole hence leaves me some room to experiment with a 1:1 current balun.
I have a T-match wide range tuner but opt for the Marconi method of feeding .In either case I suppose a 1:1 current balun is probably prudent.
Incidentally the antenna has performed as is but I wonder If I should force equal and opposite by placing the 1:1 balun between the antenna and radials and look for better radiation strength from the vertical portion after all we are expecting the radials to help the vertical portion Field strength by minimising ground return losses.
Although I can use this system as is and am doubtful that any increased Field strength as a function of the addition of a good 1:1 current balun would be detectable there is still some empirical work and questions remaining what do you think?
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KD2CJJ
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« Reply #5 on: August 16, 2012, 06:01:32 PM »

Thats interesting - so really what your saying... try to match... if you cant get it below 2.0 then go with a 4:1?  Isnt there a more scientific (mathematical approach) to choose the proper balun?



Many tuners are more efficient matching higher impedance loads, so I'd use a 1 : 1 current balun
instead of a 4 : 1.

With a tuner at the feedpoint, I'd use a 1 : 1 current balun = feedline choke between the tuner
and the coax to the rig.  That helps to decouple the outside of the coax for RF.
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73

Mike
KD2CJJ
WB6BYU
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Posts: 12773




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« Reply #6 on: August 16, 2012, 08:00:06 PM »

Quote from: KD2CJJ

Thats interesting - so really what your saying... try to match... if you cant get it below 2.0 then go with a 4:1?  Isnt there a more scientific (mathematical approach) to choose the proper balun?




No, that isn't what I was saying.

First, we have an unbalanced coax feeding an unbalanced antenna, so a BALUN is
somewhat out of place already.  There are actually two functions that we are
considering:  a wide-range impedance matching device between the tuner and
the antenna, and a feedline choke, which is the same thing as a current balun,
to reduce the amount of RF flowing on the outside if the coax.  Let's consider
these two functions separately.

First, the matter of impedance matching.  The impedance of a single wire radiator
will vary over a wide range when used over a range of frequencies, such as
160m to 10m.  It might be 25 ohms at quarter wave resonance, lower with some
capacitive reactance when shorter than a quarter wavelength, and up to several
thousand ohms (depending on the radiator diameter) when it is a multiple of a
half wavelength.  The tuner has to match this impedance on the bands of
operation.

If you use a theoretically perfect 4 : 1 transformer, the 25 ohms will appear to the
tuner as 6.25 ohms.  You can use W9CF's handy tuner simulator

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

to estimate the tuner loss in the two cases:  you'll find that the losses are lower
when matching 25 ohms.  In fact, many tuners tend to be more efficient matching
higher impedances than lower ones, though it does depend on the specific tuner
design, frequency, etc.  So if you have the tuner right at the antenna feedpoint,
and a long as it has sufficient range, efficiency generally is higher if you don't use
a transformer to step down the impedance.  In the real world it gets more complicated
because we don't have perfect transformers that operate over that range of
impedances and frequencies

When the antenna is fed via coax to a tuner in the shack, then using a 4 : 1 UN-UN to
step down the impedance may give lower coax losses in some cases.  The cable length
will affect the impedance seen by the tuner, so one can't generalize whether it will make
the impedance more or less efficient to match.


You can model the antenna, transformer and feedline, and calculate the resulting
impedances and losses for each band to see which works best for you.  Or you can
install such a system and measure the actual field strength to compare the two
methods if you want to know exactly which is better on each band.  But, for the
end-fed radiator with a tuner right at the base, personally I would choose not to use
any sort of transformer between the tuner and the antenna unless you had problems
getting a match on one or more bands, and I'd consider adjusting the radiator length
and/or the tuner matching range before adding a transformer, because the impedances
where you have matching problems are going to be ones where the non-ideal
characteristics of the transformer tend to become most apparent.


Regardless of where the tuner is located, the coax shield from the antenna feedpoint
to the shack is connected in parallel with the radial wires, and will act like a radial
with current flowing on it in many cases.  That's the reason for using the coax
choke, e.g. current balun,  in the feedline near the feedpoint.  On one hand it
doesn't matter which side of the tuner it is on, but if the antenna goes right into
the tuner then it goes on the coax side by default.  It certainly needs to be between
the radials and the main feeder, and if the radials are bonded to the case of the
tuner, then it can't go in the lead between the tuner and the antenna.


So we come to the conclusion that, with a proper tuner right at the antenna feedpoint,
putting a feedline choke in the coax between it and the rig is likely to be the most
efficient installation.  You can run models or experiments to see if that is really the
case for your specific antenna length and tuner circuit, but in general it is a good
starting point.
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W5WSS
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« Reply #7 on: August 16, 2012, 09:52:33 PM »

OK Dale, fb. I suppose I could as you suggest,or I can add a male to male coaxial adapter and the 1:1 current balun/choke balun between the output of the tuner and the feedpoint to allow the device to treat the radials equally .This will do what we want and keep everything correct .WE want the vertical to radiate and not the radials.
I was not questioning the positioning of the device or the common mode choking but questioning whether or not forcing the system balance would return detectable field strength increase radiated out into the environment feasible for the added expense.
What we know about the relationship between a radial set and a vertical element is that when the return power in RF current is presented to the vertical element of the RF ac circuit system as a radiation resistance to be radiated without RR+IR squared additional loss we are meeting our design goals as an efficient antenna system. I do however place a concern on whether or not a common mode displacement potential difference in voltage above or below neutrality at the station operating position is remedied as a priority above and beyond signal strength increase but see both as a worthy goal though I may already be there with a rather nice RF radial set for which the vertical may push against. I would surely want to avoid the system behaving as an out of whack off center fed dipole since the radials are elevated and slope downward to insulated tent stakes so i find it prudent to prevent them from radiating horizontal or some orientation of radiation......they can because remember in the original post I stated they were probably behaving as ground mounted? I think I will seriously consider re routing them to immediately drop the 4ft to earth surface then run along atop of surface as above ground radials. rather than sloping down and outward to insulated stakes.
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WB6BYU
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« Reply #8 on: August 16, 2012, 10:35:53 PM »

Keeping the radials spaced away from the ground is likely to reduce your ground losses.

In theory, it doesn't matter which side of the tuner you put the choke on, as long as
the radials return to the feedoint, not to the tuner case.  In practice, since a choke is
a length of coax, putting it on the side with a low SWR puts less stress on the choke
and minimizes loss, impedance shifts, etc. due to the length of the coax in the choke.

For the radials to be effective, they MUST have RF current flowing in them, just like
the radiator.  What keeps them from radiating is the fact that they are symmetrical,
and any radiation from a wire on one side of the ground plane is cancelled by the
out-of-phase radiation from a wire on the opposite side.

If the feedpoint was high enough in the air that you could slope the radials down
at 45 degrees, then the horizontally polarized radiation would cancel, but the vertically
polarized radiation would be in phase from all of them, and it would contribute to the
overall radiation from the antenna.  In fact, if you bent them all down to 90 degrees
from horizontal, you'd have a dipole, with both halves radiating equally.  That's a good
thing, at least when the radiator is 1/4 wave long.  (It isn't for a 5/8 wave whip,
however, as in that case the current in the radials is out of phase with that causing
the bulk of the radiation from the vertical.)


The only difference that adding a choke makes is that it prevents the coax shield from
acting like one of the radials.  With a low impedance radial system, that doesn't make a
lot of difference in the radiation pattern.  (There may be other side effects of common
mode currents, however.)  But when the radials have a high impedance (such as elevated
radials that are a half wavelength long) most of your return current may be on the coax,
so it could radiate nearly as much as the antenna itself, depending on how it is routed.
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W5WSS
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« Reply #9 on: August 17, 2012, 07:42:46 AM »

Hello Dale yes we are in agreement. In this installation  The junction that is to say the location where the radials connect and also where the antenna element connects is physically side by side it just also happens to be "at the tuner because the tuner is directly at the antenna base radial junction all this is located at 4ft above the earth surface so there exists no feedline linking the tuner to the antenna system.
I think I will go with a quality 1:1 current balun directly between the tuner output and the antenna feedpoint...and there will still be no feedline in this non tuned zone. Tuner  to balun directly via a male to male UHF adapter then balun connecting to antenna radials to proper terminal of balun and antenna lead to proper terminal of balun.
The question still remains will there be any increase in antenna system Field strength as a result of the action of the balun?

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W8JI
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« Reply #10 on: August 17, 2012, 08:16:24 AM »

I would never feed a counterpoise system without feedline isolation. A current balun is EXACTLY the same in function as a line isolator or choke.

Also, a 4:1 ratio can reduce SWR excursions and might reduce system losses. A properly built 1:1, on the other hand, will handle mismatches better.

A balanced system, by definition, has equal and opposite terminal voltages to "earth".

An UNbalanced system , by definition, has one terminal at zero potential to "earth".

BOTH systems have equal and opposite currents into and out of the terminals.

Very few systems are perfectly UNbalanced, and very few are perfectly balanced. Most are in between. Since systems are often neither balanced nor perfectly unbalanced, and since a common mode choke and current balun are identical in function and design, there is little reason to get all hung up on names.  :-)

73 Tom
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W5WSS
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« Reply #11 on: August 17, 2012, 08:48:13 AM »

Lol Hello Tom what took so long? glad to hear from you and I agree.
Dale and I were boiling my system to the root. This system is a budget all wire vertical with a half radius(circle) of symmetrically installed slightly elevated radials(I prefer a full semi circle but is not possible) sloping downward to tent stakes from 4ft height downward to 6" above surface of ground. The vertical proper is apx 10 degrees off of plumb unavoidable and the whole system originates at the window sill where the L-network auto tuner is located. I am going to add the balun to facilitate proper system feeding methodology.
Tom as you may have read in my original post that the whole subject revolves around the depends question of 1:1 or 4:1 current balun to better facilitate a multi band 42ish long #14 wire and half circle radial set symmetrically installed in a half semi circle around the vertical element.....I would opt for a trap vertical but not possible from here so I am efforting to make this the best it can be and agree that needs balancing regardless of whether it is a half shape or a full circle shape of symmetrically oriented radials and will probably choose a metal enclosed design with the tuner type nomenclature.
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WB6BYU
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« Reply #12 on: August 17, 2012, 09:41:46 AM »

Quote from: W5WSS

Hello Dale yes we are in agreement...

...I think I will go with a quality 1:1 current balun directly between the tuner output and the antenna feedpoint.



That's not exactly agreement:  I'd put the choke between the tuner and the rig rather
than between the tuner and the antenna.
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W8JI
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« Reply #13 on: August 17, 2012, 09:54:26 AM »

Tom as you may have read in my original post that the whole subject revolves around the depends question of 1:1 or 4:1 current balun to better facilitate a multi band 42ish long #14 wire and half circle radial set symmetrically installed in a half semi circle around the vertical element

Where is the tuner in relationship to the feeder, and what type of feeder?

A 4:1 ratio will reduce average SWR over wide frequency ranges on a low impedance feeder run to a tuner, but the 4:1 has to stand some pretty significant impedance changes.

Quote
That's not exactly agreement:  I'd put the choke between the tuner and the rig rather
than between the tuner and the antenna.

The stress on the balun core, and choking effectiveness, is identical at either end of the matching system. I'd put any balun on the output side, so the tuner does not have to be ground isolated or a source of e-field coupling problems.
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W5WSS
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« Reply #14 on: August 17, 2012, 11:19:25 AM »

Yeah OK Tom exactly. In this special case There exists no extension of feederbecause the antenna leads are brought and directly connect to the tuner.Consequently, I need one or the other balun types inserted directly between the output of the L-network auto tuner and the 43 ft non trap vertical. I suppose I can accomplish the connecting via a male to male adapter or a short coaxial quality jumper. The auto tuner has been tuning the system without the needed balun temporarily and permanently adding the balun will help the system performance.
Note it is true as evidenced by noise ingress that when I ground isolated the tuner and counterpoise radial set the noise ingress was changing so I know I need to rectify the system further, just to be clear; The radials do not connect back to the station via a earthing rod but rather are elevated and isolated from a dc path. That approach will remain unchanged as I add the balun between the output of the tuner and the vertical/counterpoise system to properly complete it and all techniques work together to produce a well working somewhere between balance and unbalance lol. I need line isolation and system current and voltage equilibrium and believe the 1:1 current balun will accomplish the task and if the insertion of the balun causes the auto tuner problematic issues I will need to re evaluate In this case The source of the problem will not be adding the balun but rather out at the antenna especially in this case where we have a questionable amount of radials to properly serve as completely effective counterpoise system.
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