[TowerTalk] current balun question

[Christopher Brown]

My VHF/UHF weak signal station is limited by the antennas being mounted
on the roof.

Besides not being nearly as high as I would like, nearly half of the
short feedlines run in proximity to household electronics/power making
it very easy for them to pickup and conduct any house noise to the antennas.

Since the antenna terminals back rcv system NF for 2M and 70cm is about
.7db and 1.1db and local noise floor is almost low enough to make that
worthwhile it was very bothersome.


My solution was to take 10 1.25 long .5 id cores and slip them into 1.5"
id (before shrink) 3:1 heavy duty thick wall adhesive line shrink tube
(used for buried cable splicing in telco and power industry).

When I make a cable...jumper from ant to LNA, feedline from LNA to entry
bulkhead, etc...  I slip a 2 inch long piece of .75-1" id 3:1 HD self
seal, then a choke, then another piece of .75-1"  I do this at each end.

After I put on connectors and test I figure out the best placement and
shrink the .75 on each side of the choke, locking it in place.  Then I
cut 2 3" link pieces of the larger dia, and slip it over choke and
shrink at each end (one half seals to choke body, other half to smaller
diameter on the feedline, sealing things).

I do it this way because of the Alaskan weather...snap-ons or even
solids would get water in there and freeze, breaking them.

Once I route the cable, the weather protected sections usually get a
bunch of snap-ons of equiv size spaced along the length.

Also, I do not limit to one at each end, a few feedlines have 2 or even
3 at each end at ~ 1/4 wave intervals.


As it happens I used type #31 P/N 2631102002 for the solid and
0431164181 for the snaps.

I had alot of these around, and the specs looked very close the the type 43.


On that front, we could really use an expert opinion.

Jim, what is your take on

Type 31 2631102002 v.s. Type 43 2643102002 when around .4" coax?



On 3/14/17 10:55, Jim Brown wrote:
> Hi Svend,
> 
> Yes. Use a lot of #43 clamp-on cores. Buy the longest ones that fit the 
> cable. Study the data sheet and aim for at least 1K ohms. More is better. :)
> 
> On Tue,3/14/2017 11:08 AM, Svend Spanget wrote:
>> Hi Jim.
>> Do you have a recommendation for an effektive choke for 144 MHz?
>>
>> VY 73 de Svend, OZ7UV
>>
>> Den 14. mar. 2017 18.19 skrev "Jim Brown" <jim at audiosystemsgroup.com 
>> <mailto:jim at audiosystemsgroup.com>>:
>>
>>     On Tue,3/14/2017 12:41 AM, Máximo EA1DDO_HK1H wrote:
>>
>>         Same question in a different way is; Is the current choke
>>         forcing to balance differential currents?
>>
>>
>>     Hello Maximo,
>>
>>     An effective common mode choke forces current to near zero at the
>>     point where it is placed. IF it placed at the feedpoint, it will
>>     force current in the sides of the antenna to be equal AT THE
>>     FEEDPOINT. In effect, it disconnects the feedline from the antenna
>>     as an element of the antenna.
>>
>>     If the same choke is placed near the shack, the feedline IS part
>>     of the antenna, with it's electrical length equal to its physical
>>     length, with the choke forcing the common mode current to near
>>     zero at the point where it is placed in the feedline.
>>
>>     By "effective," I mean a choke that has a very high common mode
>>     resistive impedance at the operating frequency.
>>
>>     73, Jim K9YC
>>
>>
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