[TowerTalk] current balun question

Tue Mar 14 17:02:32 EDT 2017

Hi Chris,

If your coax is well shielded, there's relatively little need for chokes 
like this along the feedline, but you DO need a very good one right at 
the feedpoint, and what you describe sounds quite good. If your antennas 
are stacked arrays, you need a choke on each antenna, as close as 
possible to its feedpoint.

As it happens, I just answered private email on the same topic for OZ7UV 
about a stacked pair of 2M Yagis. The general answer is this. Go to the 
Fair-Rite website and study these pages. You want a core that has its 
resonant peak near the operating frequency. That's #61 for 440 MHz, #31 
for 2M. Determine that by studying the impedance curves.

http://www.fair-rite.com/product-category/suppression-components/round-cable-snap-its/higher-frequencies-200-1000-mhz-61-material-round-cable-snap-its/

http://www.fair-rite.com/product-category/suppression-components/round-cable-snap-its/lower-broadband-frequencies-1-300-mhz-31-material-round-cable-snap-its/

2631102002 is resonant at about 200 MHz, 2643102002 at 300 MHz, so this #31 core would be the better choice on 2M. You can see these curves by clicking on the part number in each table.

73, Jim K9YC

On Tue,3/14/2017 1:10 PM, Christopher Brown wrote:
> 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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