http://elecraft.85.s1.nabble.com/Slightly-Unbalanced-dipole-tp7648044p7648104.html
I got this unbalanced common-mode choke from Amateur Radio Supplies. It
keeping stray RF out of my bench computer. I got it when I was having
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
> 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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