[TowerTalk] Question on choke

[Jim Brown]

On Mon,4/11/2016 7:06 PM, Roger (K8RI) on TT wrote:

> In this case, "I believe" two separate chokes would do much better 
> than adding more cores, although "I think", shifting the external 
> portion of the coils into a bundle should lower the frequency of 
> maximum R.

Yes and no. The problem is that we need to get the choke resonant near 
75M. BTW -- maximum resistance is actually parallel resonance of the 
inductance of the coil with its stray capacitance. :)  The photo shows a 
choke tuned for around 30M. Squeezing the turns together would probably 
lower it to 40M. To get the choke down to 75/80M, you'll need both more 
L and more C.

> Adding 1 or 2 cores?

L increases approximately in proportion to the length of the ferrite(s), 
and as the square of the turns. To lower Fr by a factor of 2, we must 
increase sqrt of LC by 2, LC must increase by a factor of 4.  So going 
from 5 cores to 7 won't be enough without also adding turns. The good 
news is that doubling the number of turns also increases Rp, which is 
the quantity we're looking for. And C increases approximately in 
proportion to the number of turns.

So the solution to the problem is to get as many turns as possible 
through those cores, adding cores as needed. That means winding the 
choke before installing the connector, or using smaller diameter coax.

OR -- look at the bifilar chokes configured as parallel wire 
transmission lines.  Two 12-turnor 14-turn chokes in series would 
probably handle legal limit power assuming CW or SSB and not AM.

>
> The cores are epoxied together with a very thin film so there should 
> be plenty of dissipation. I'd like to have both chokes "up there", but 
> I want to keep the load at the feed point as light as possible.  That 
> antenna is currently under somewhere around 200# tension so even with 
> the choke and roughly 100' of LMR400 there is no noticeable sag. 

In the modeling I've done, and in my experience, a choke at the 
feedpoint and another much farther down the line will be as good for 
power handling as two up in the air.

73, Jim K9YC