[TowerTalk] Balun Recommendation

Sat Apr 21 00:38:40 PDT 2012

Gerald,

You analysis is correct!

Of course, the CM impedance "looking into" the coax braid at the 
feedpoint varies depending on the coax length: if the coax is well 
grounded at the shack end and is an odd multiple of a quarter-wave long, 
the CM impedance will be very high and you probably don't need a choke; 
but if it's a multiple of a half-wave long the impedance can be very low 
and far more current will flow along the braid than into the dipole leg.

I've been doing some work over the past day or so to try to understand 
better what the common-mode impedances are; a typical worst case with 
the coax 3/4 wave long was 16+j0 - that made the CM current about 10 
times the current into the dipole leg!!

Understanding the range of CM impedances we're likely to encounter has 
implications for the choke design. I'll start a new thread to outline 
what I've been doing on that topic.

73,
Steve G3TXQ

On 21/04/2012 02:56, TexasRF at aol.com wrote:
> Hi Ian and others:
>
> Is common mode current the same as rf current flowing on the coax shield
> because it is connected to one half of the antenna?
>
> If so, no wonder the feed point is unbalanced; there are two return current
>   paths for any voltage present on the shield side of the antenna. The "hot"
> side  of the antenna reflection sees the only the coax center conductor and
> associated  impedance. The other side of the antenna reflections sees the
> inside of the coax  and associated impedance in parallel with the outside of
> the coax and it's  associated impedance. The total impedance then has to be
> less than the "hot"  side.
>
> So, there is a balanced antenna feedpoint with say 50 ohms referenced to
> ground on one side and maybe 35 ohms on the other side reference to ground.
> The  voltage would have to divide 35/50 in that case.
>
> Providing a means to reduce the current on the shield has to improve the
> voltage ratio on the two halves of the antenna as that makes the impedance
> referenced to ground more nearly equal. Series choke losses also have a
> parallel  equivalent that would tend to degrade the balance. So, it seems the
> choke can  help balance to a large degree but never completely restore balance.
>
> Any corrections to this understanding are cheerfully accepted.
>
> 73, Gerald K5GW
>
>
>
>
>
>
> In a message dated 4/20/2012 11:57:10 A.M. Central Daylight Time,
> gm3sek at ifwtech.co.uk writes:
>
> Gerald  wrote:
>> Hi All, at the risk of really showing  some ignorance, a question:
>>
>> If the choke is doing a good job  and located at the feedpoint, isn't the
>> common mode circuit quite  short?
>>
>> Seems that the common mode current is the cause of the  feed point
> imbalance
>> rather than the imbalance causing the common mode  current. Chicken first
>> or egg  first?
>>
> Neither; or  both.
>
> The only parameters that can truly be labeled "first causes" of  antenna
> behavior are the physical dimensions of the antenna elements and
> feedline, their location relative to ground and other nearby objects,
> and the frequency and power level of the applied signal.
>
> These are  the parameters that constrain the antenna's behavior according
> to the laws  of EM physics. It is no coincidence that these are also the
> inputs  required by an antenna modeling program, which then computes the
> one-and-only solution that will fit all the facts supplied.
>
> Antenna  currents, feedline currents (both CM and differential) and all
> the related  voltages and impedances are all related parts of the
> antenna's behavior -  but they are all *consequences*, not causes.
>
> Because these are all  *related* consequences, a change in one will be
> accompanied by a change in  all the others. But "chicken or egg"
> arguments are futile because none of  them has been the root cause of the
> changes we're talking  about.
>
>
> So here we are, with an antenna system in which some  unwanted CM current
> is flowing on the feedline. If we now install a CM  choke, we are adding
> completely new constraint upon the system's behavior,  in the form of a
> high impedance at that particular location (which  modeling software
> would call a "load").
>
> The key concept is that we  are deliberately forcing the ENTIRE
> antenna-feedline system to behave  differently from the way it did
> without the choke. Everything changes to  accommodate this new
> constraint, so NONE of the RF currents and voltages  will be the same as
> before.
>
> Many of the "chicken or egg" arguments  that we've seen in the past few
> days contain a hidden assumption that CM  voltages or currents will be
> the same both before and after the choke is  inserted. They won't! The
> only valid solution is to recompute the behavior  of the entire
> antenna-feedline system. Unfortunately that will only  produce an answer
> which is valid for a particular situation; but it also  explains why
> anecdotal reports can be so variable.
>
>
>
>
> --
>
> 73 from Ian  GM3SEK
> http://www.ifwtech.co.uk/g3sek
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