[TowerTalk] Fair rite materials for choke baluns

[Jim Brown]

Roger,

Both you and Rick understand things as I do. Rick is simply speaking 
figuratively, using exactly the same words I have. That is, an effective 
choke nearly eliminates the common mode connection between the feedline 
and the antenna. But no choke is perfect, so there will be some small 
current if there is imbalance in the system.

73, Jim K9YC

On Mon,7/4/2016 11:40 PM, Roger (K8RI) on TT wrote:
> It's my understanding that a choke of  (theoretical) infinite Z does 
> not disconnect the antenna. It would reduce the current flowing on the 
> outside of the coax (common mode) to zero at that point. It has no 
> effect on the interior currents
>
> The choke does nothing to the balance of the antenna. It just stops 
> (more correctly reduces) the CM current caused by a reasonable 
> imbalance such as a sloping dipole, or that caused by nearby objects.  
> The greater the imbalance the greater the power dissipated in the core 
> material.  The difference between sloping, center fed dipoles on 80 Vs 
> 40 is very pronounced
> Different core materials will likely have different dissipation values.
>
> OTOH it does not prevent that same exterior shield from picking up 
> signals if long enough, or of the right (or wrong) length. Then a 
> second and possibly third choke of smaller values  might be advisable 
> making the shield appear as if it were broken up with insulators.
>
> Unless referring to a specific antenna, I'd not expect Z to be 
> relatively important. For a specific type of antenna Z & R vary with 
> height, particularly on 160 and 75.   Sometimes by large amounts.
>
> 73
>
> Roger (K8RI)
>
>
> On 7/4/2016 Monday 2:08 AM, Jim Thomson wrote:
>> Date: Sun, 3 Jul 2016 11:23:10 -0700
>> From: "Richard (Rick) Karlquist" <richard at karlquist.com>
>> To: Steve Hunt <steve at karinya.net>, Reflector -tower
>> <towertalk at contesting.com>
>> Subject: Re: [TowerTalk] Fair rite materials for choke baluns
>>
>>
>> On 7/3/2016 10:05 AM, Steve Hunt wrote:
>>> Rudy,
>>> http://www.karinya.net/g3txq/temp/52_mix.png
>>>
>>> It achieves Rs>5000 Ohms from 10Mhz thru 28MHz, and Rs>8000 Ohms from
>>> 12MHz thru 21MHz.
>>>
>>> Steve G3TXQ
>>>
>> It is more useful to think about Rp than Rs.  Rp determines the
>> power dissipation for a given voltage.  Although Z is a figure
>> of merit for how well the antenna works, Rp is a figure of merit
>> for how much power can be handled.  On the G3TXQ curves, note
>> that where Z=Rs, it is also true that Z=Rp.  Away from that
>> frequency, the general trend is that Rp monotonically increases
>> with frequency, at least up to resonance.
>>
>>
>> It's nice to brag about extremely high Z values, but even if
>> Z=infinity, all you have accomplished is that you have
>> disconnected the feedline from the antenna.  It doesn't
>> guarantee there will be no feedline currents.  Also, it merely
>> PERMITS the antenna to be balanced, if there is no external
>> conductor otherwise perturbing it.  It does not guarantee
>> balance.
>>
>> Rick N6RK
>>
>> ##  Ok, Im lost.   I thought  we wanted  Z >  RS > XS .
>> Put another way, RS ideally should be the same as Z... or as close as
>> you can get to Z.   And  RS  should definitely be >  than XS.
>>
>> What is RP  ??
>>
>> Jim   VE7RF
>>
>>
>>
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