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@audiosystemsgroup.com
<mailto:jim@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@contesting.com <mailto:TowerTalk@contesting.com>
http://lists.contesting.com/mailman/listinfo/towertalk
<http://lists.contesting.com/mailman/listinfo/towertalk>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|