[TowerTalk] current balun question
Christopher Brown
cbrown at woods.net
Tue Mar 14 18:01:29 EDT 2017
That is pretty much what I expected but was not sure of how much things
might shift with the addition of .4" coax through the cores.
All coax is double shielded, mostly LMR400 and LMR400-UF.
If I was running FM, had a higher environmental noise level, or higher
NF receiving systems I doubt the near end and along the path chokes
would make a noticeable difference. By far the largest improvment was
with the addition of 2 chokes at the antenna end, one as close to
feedpoint as possible, second about 1/4 wave away.
But there is still the matter of conducted noise from the shack and
noise picked up by the coax acting as an antenna being re-radiated even
if the choke deals with the conducted to antenna terminals issue.
Add to this a small urban install with multiple antennas for each all
within a 15x25 foot area.
When I added shack end and along the coax choking (hopefully rending the
whole run fairly "dead") I noticed an additional drop in noise on the
antenna connected to the choked run, but a larger drop on nearby
antennas. For example, I have seperate 2M and 70cm verticals about 20
feet apart at the same height, additional choking on the 70cm vertical
resulted in a small improvement on that antenna, but a larger one on the
2M vertical that had good line of sight to the 70cm vertical and its
feedline.
Max suppression was when I treated _all_ of the feedlines (6M, 2M and
70cm) to the rooftop antennas this way.
The only choke on the 6M antenna that makes a diff to 6M is the one at
the antenna terminals, but the additional ones reduce noise on the 2M
antennas.
And to be clear, none of this additional choking would matter if I was
not running squelch open, headphones on on a low NF system trying to
copy a 25watt station via a 180 mile path that bounces off the side of a
mountain 80 miles away because there is another mountain in the way.
I think I have about $300 in ferrite on the 6M/2M/220/23cm feedlines to
the roof, even though the improvements from the last $200 or so worth of
extra is small compared to the massive improvements from choking at the
antenna it is worth it to me.
On 3/14/17 13:02, Jim Brown wrote:
> 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
>>>>
>>>>
>>>> _______________________________________________
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> TowerTalk mailing list
>>>> TowerTalk at contesting.com <mailto:TowerTalk at contesting.com>
>>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>> <http://lists.contesting.com/mailman/listinfo/towertalk>
>>>>
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
More information about the TowerTalk
mailing list