Slightly Unbalanced dipole?

[John Oppenheimer]

On 1/13/19 11:24 PM, Jim Brown wrote:
>> The instrument used to measure this is quite simple.
>   I'd be very interested in how you're measuring the current at the
> antenna feedpoint. :) That is NOT so simple.

The common mode current can be measured 1/2 wavelength from the
feedpoint. Here is an example:

https://www.kn5l.net/Balun-CM/

MFJ has a clamp-on current meter which can be used.

John KN5L
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[NK7Z]

It is one hell of a nice read Jim, THANK YOU for publishing it.

73s and thanks,
Dave (NK7Z)
https://www.nk7z.net
Award Manager, 30MDG Grid Contest
ARRL Technical Specialist
ARRL Volunteer Examiner
ARRL OOC for Oregon

On 1/13/19 9:24 PM, Jim Brown wrote:

> I'm not going to go beyond this on the reflector, but will refer
> interested readers to that tutorial, and to my latest work, a major
> update of the Choke Cookbook published in the first edition of the
> tutorial. The update is k9yc.com/2018Cookbook.pdf
>
> 73, Jim K9YC
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[Barry LaZar]

Vic,
     The simple answer is that the common mode chokes available to hams
are of the unbalanced type. That means they are designed to be
unbalanced in to unbalanced out. Some companies make baluns that are
both current baluns that act for choking common mode currents and
transform a balanced load to an unbalanced load. If you place a common
mode choke at the antenna feed point, the system will be unbalanced,
usually.

     Next, most tuners that have built in baluns use voltage type baluns
and can be less than good in a high SWR situation. However, using a good
current balun at or neat the tuner usually prevents common mode issues.
However nothing is 100% when dealing with RF.

     Putting reactance in the line to counter what is already there is
good when dealing with the impedance, but you are correct about
complexity. The very simplest configuration that I know in a multiband
situation is to erect a dipole for the lowest frequency to be used and
feeding it with ladder line, or equal. Where you enter the house mount a
4:1 current balun and connect coax from there to the tuner. As long as
your tuner can handle the SWR and the coax run is short, you need not
worry about losses due to SWR; I am speaking to the 1 or so db losses
over just the transmission line loss. If you use the tuner in your
Elecraft gear, I just wouldn't worry, period. Elecraft tuners appear to
be able to handle anything I've put in my backyard so far.

     The Johnson matchbox line of tuners were excellent. I no longer have
mine; selling mine was a less than brilliant move on my part, in
retrospect. However, I don't usually recommend this approach. Getting
through some of the walls/windows in the houses around here with
balanced line can be problematic. So, I usually recommend the approach
above for overall simplicity and assurance of maintaining feed balance.

73,
Barry
K3NDM

------ Original Message ------
From: "Vic Rosenthal" <[hidden email]>
To: "Al Lorona" <[hidden email]>
Cc: [hidden email]
Sent: 1/14/2019 1:12:37 AM
Subject: Re: [Elecraft] Slightly Unbalanced dipole?

>Why would a common mode choke at the antenna feedpoint present more problems than the common practice of feeding a balanced line through a single-ended tuner and balun?
>I’ve done a lot of experimenting with the latter and have learned that a) the best configuration is with a 1:1 balun, and b) the balun will become lossy and heat up if the impedance it sees is highly reactive. The solution to b) is to cancel the reactance by putting the appropriate opposite reactance in both sides of the line between the balun and the antenna. Of course this means added complexity.
>I would love to figure out how to make a choke that would work with balanced line at the antenna feedpoint, on multiple bands without any additional components.
>FWIW I’ve switched to a real balanced tuner (Johnson Matchbox).
>
>Victor 4X6GP
>
>>  On 14 Jan 2019, at 1:42, Al Lorona <[hidden email]> wrote:
>>
>>  By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by.
>>
>>  Al  W6LX
>>
>>
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[Gwen Patton]

I got this unbalanced common-mode choke from Amateur Radio Supplies. It
completely clobbered all of the "bite" I was getting in the shack, and is
keeping stray RF out of my bench computer. I got it when I was having
trouble with the keyboard I had connected to my PX3 -- when I transmitted
CW or PSK31, the RF would cause random characters to get stuffed into the
keyboard buffer, and the transmission wouldn't stop until I stopped it
manually. It helped, but the way I finally got around it was by using a
2.5ghz non-Bluetooth wireless keyboard for the PX3. Any wired keyboard I
used would result in garbage in the transmission as the RF induced current
in the keyboard cable.

https://www.amateurradiosupplies.com/product-p/iso-max.htm

73,
Gwen, NG3P

On Mon, Jan 14, 2019 at 12:03 PM Barry <[hidden email]> wrote:

> Vic,
>      The simple answer is that the common mode chokes available to hams
> are of the unbalanced type. That means they are designed to be
> unbalanced in to unbalanced out. Some companies make baluns that are
> both current baluns that act for choking common mode currents and
> transform a balanced load to an unbalanced load. If you place a common
> mode choke at the antenna feed point, the system will be unbalanced,
> usually.
>
>      Next, most tuners that have built in baluns use voltage type baluns
> and can be less than good in a high SWR situation. However, using a good
> current balun at or neat the tuner usually prevents common mode issues.
> However nothing is 100% when dealing with RF.
>
>      Putting reactance in the line to counter what is already there is
> good when dealing with the impedance, but you are correct about
> complexity. The very simplest configuration that I know in a multiband
> situation is to erect a dipole for the lowest frequency to be used and
> feeding it with ladder line, or equal. Where you enter the house mount a
> 4:1 current balun and connect coax from there to the tuner. As long as
> your tuner can handle the SWR and the coax run is short, you need not
> worry about losses due to SWR; I am speaking to the 1 or so db losses
> over just the transmission line loss. If you use the tuner in your
> Elecraft gear, I just wouldn't worry, period. Elecraft tuners appear to
> be able to handle anything I've put in my backyard so far.
>
>      The Johnson matchbox line of tuners were excellent. I no longer have
> mine; selling mine was a less than brilliant move on my part, in
> retrospect. However, I don't usually recommend this approach. Getting
> through some of the walls/windows in the houses around here with
> balanced line can be problematic. So, I usually recommend the approach
> above for overall simplicity and assurance of maintaining feed balance.
>
> 73,
> Barry
> K3NDM
>
> ------ Original Message ------
> From: "Vic Rosenthal" <[hidden email]>
> To: "Al Lorona" <[hidden email]>
> Cc: [hidden email]
> Sent: 1/14/2019 1:12:37 AM
> Subject: Re: [Elecraft] Slightly Unbalanced dipole?
>
> >Why would a common mode choke at the antenna feedpoint present more
> problems than the common practice of feeding a balanced line through a
> single-ended tuner and balun?
> >I’ve done a lot of experimenting with the latter and have learned that a)
> the best configuration is with a 1:1 balun, and b) the balun will become
> lossy and heat up if the impedance it sees is highly reactive. The solution
> to b) is to cancel the reactance by putting the appropriate opposite
> reactance in both sides of the line between the balun and the antenna. Of
> course this means added complexity.
> >I would love to figure out how to make a choke that would work with
> balanced line at the antenna feedpoint, on multiple bands without any
> additional components.
> >FWIW I’ve switched to a real balanced tuner (Johnson Matchbox).
> >
> >Victor 4X6GP
> >
> >>  On 14 Jan 2019, at 1:42, Al Lorona <[hidden email]> wrote:
> >>
> >>  By the way, whenever Jim says, "...a very good common mode choke at
> the feedpoint of an antenna...," he means an antenna fed with coax. For the
> rest of us, of course, that choke would (should) go at the output of the
> antenna tuner, whether in the shack or close by.
> >>
> >>  Al  W6LX
> >>
> >>
> >>  ______________________________________________________________
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> >>  Post: mailto:[hidden email]
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--

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http://quarktime.net
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[Jim Brown-10]

On 1/14/2019 9:02 AM, Barry wrote:
> The simple answer is that the common mode chokes available to hams are
> of the unbalanced type.

Huh? We're hams -- we're not limited to products we can buy! And we can
build far better than we can buy. In 2010, I published the design for a
common mode choke to cover the HF bands using a pair of #12 THHN wound
on a 2.4-in o.d. #31 toroid.  k9yc.com/2018Cookbook.pdf has dozens of
designs using THHN, teflon pairs, and RG400. These chokes are fairly
easy to wind, and all can be built for less than about $35, including
connectors.

A member of our contest club is building a serious contest station, and
asked me to measure a "balun" from a well-known manufacturer. It came in
a nice weatherproof non-metallic enclosure, and was nicely made. But it
looked NOTHING like the data sheet that WAS on the website at the time
(and which has been removed since I showed my data to a few engineering
colleagues, one of whom obviously asked the mfr about it.)

The problem with choking a two-wire feedline is NOT one of balance -- if
it's a good choke, the choke takes "balance" out of the equation. What
matters is dissipation in the choke due to excessive voltage or current
that is present because the antenna is not matched to the line.

73, Jim K9YC

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[Vic Rosenthal]

I agree. And I maintain that the problem is exactly the same with the
common solution of a single-ended tuner feeding a balanced line through
a balun. If there is a big mismatch the choke will get hot.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 14 Jan 2019 21:21, Jim Brown wrote:

> The problem with choking a two-wire feedline is NOT one of balance -- if
> it's a good choke, the choke takes "balance" out of the equation. What
> matters is dissipation in the choke due to excessive voltage or current
> that is present because the antenna is not matched to the line.
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[Barry LaZar]

Jim,
     Your comment about hams not being limited, " Huh? We're hams --
we're not limited to products we can buy! And we can build far better
than we can buy.". you may be correct, but from what I see, the majority
of hams, at least around here, buy what they need. Even of our more
serious hams buy baluns and chokes from sources like DX Engineering and
Balun Designs; few are really building these easy to make devices. There
are exceptions to what I just said, but in the main, more hams are
buying rather than building. Sorry, but that's what it looks like to me.

     I do like your paper and have it. I also wish that you continue to
get more hams to actually build more things.

73,
Barry
K3NDM

------ Original Message ------
From: "Jim Brown" <[hidden email]>
To: [hidden email]
Sent: 1/14/2019 2:21:30 PM
Subject: Re: [Elecraft] Slightly Unbalanced dipole?

>On 1/14/2019 9:02 AM, Barry wrote:
>>The simple answer is that the common mode chokes available to hams are of the unbalanced type.
>
>Huh? We're hams -- we're not limited to products we can buy! And we can build far better than we can buy. In 2010, I published the design for a common mode choke to cover the HF bands using a pair of #12 THHN wound on a 2.4-in o.d. #31 toroid.  k9yc.com/2018Cookbook.pdf has dozens of designs using THHN, teflon pairs, and RG400. These chokes are fairly easy to wind, and all can be built for less than about $35, including connectors.
>
>A member of our contest club is building a serious contest station, and asked me to measure a "balun" from a well-known manufacturer. It came in a nice weatherproof non-metallic enclosure, and was nicely made. But it looked NOTHING like the data sheet that WAS on the website at the time (and which has been removed since I showed my data to a few engineering colleagues, one of whom obviously asked the mfr about it.)
>
>The problem with choking a two-wire feedline is NOT one of balance -- if it's a good choke, the choke takes "balance" out of the equation. What matters is dissipation in the choke due to excessive voltage or current that is present because the antenna is not matched to the line.
>
>73, Jim K9YC
>
>______________________________________________________________
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[Wes Stewart-2]

You do realize that if the common-mode current on the line is due to asymmetry
between the antenna and the line, then choking at the feed-point reduces the CM
current at that point, but just like your example said, a quarter wave *down*
the line you have a CM peak.  If asymmetry isn't an issue then open wire is just
fine with a balun at the tuner and I say this while not being a fan of open wire
fed antennas.

I'm also somewhat bemused by the cottage industry that's sprung up in the last
few years to calm the hand wringing about common mode current.  I think there's
been a lot of development of cures that are looking for ailments.  In my
curmudgeonly opinion of course :-)

Wes N7WS

,  On 1/13/2019 10:24 PM, Jim Brown wrote:
> WHERE are you measuring it? You do realize that, like any antenna, current
> varies along the wires that make up the antenna. A choke attempts to force the
> current to zero AT THE POINT WHERE THE CHOKE IS PLACED. You're putting it at
> the tuner, so it forces current to zero there. But a quarter wave up the
> feedline, the current reaches a maximum value.

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[Jim Brown-10]

On 1/14/2019 6:32 PM, Wes Stewart wrote:
> You do realize that if the common-mode current on the line is due to
> asymmetry between the antenna and the line, then choking at the
> feed-point reduces the CM current at that point, but just like your
> example said, a quarter wave *down* the line you have a CM peak.

Yes, but the choke at the feedpoint prevents that common mode current
coupling to/from the antenna, which is how it gets inside the feedline
as a differential mode signal. And the common mode current on the line
below the choke is that induced by the field from the antenna, not from
any imbalance in the antenna.

> If asymmetry isn't an issue then open wire is just fine with a balun
> at the tuner and I say this while not being a fan of open wire fed
> antennas.
And it's also not an issue if you have no local noise sources, a luxury
that I think you said you have.
>
> I'm also somewhat bemused by the cottage industry that's sprung up in
> the last few years to calm the hand wringing about common mode
> current.  I think there's been a lot of development of cures that are
> looking for ailments.  In my curmudgeonly opinion of course :-)

If you had local noise you might view things differently. :) FWIW, I
wasn't the first to realize the relationship between common mode current
and RX noise -- I picked up on it from a long treatise that W1HIS wrote
around the time I was doing my work. And I contributed pieces of the
puzzle that he didn't know about, most importantly, the equivalent
circuit of a choke and the importance of a high resistive impedance.

73, Jim K9YC

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