SNAP On Feed Line Choke
Each end fed antenna needs a feed line choke at the radio end of the coax to stop the antenna common mode current (on the coax braid) from getting into the radio when transmitting and also to stop RFI common mode noise when receiving. A
simple easy to adjust in different places along the coax used as a counterpoise is the snap on ferrite core with a 1" ID (hole) installed as shown in the picture. (coax not included, but you can use RG-8X size for 5 turns and RG-8X size for 3 turns).
Add the Feed line choke (part # SOFLC) to your end fed antenna today today and stop the pesky RFI from getting into your radio and causing RFI, audio distortion or unexpected operation!
NOTE: Coax shown in picture is NOT INCLUDED and is shown as an installation example only. If you want a pre-wound choke check part# JC-1-1500-3 or JC-1-1500-6.
For non-resonant end fed antennas placement of the choke from 10-50 feet away from the antenna feed point to use the coax braid as the counterpoise (since these antennas are really off center fed dipoles with the short side using the coax braid as the count5erpoise. Wind 8-10 turns thru center for an effective 3-30 Mhz choke.
For End fed Half Wave antennas place 3-10 feet from antenna feed point to act as short counterpoise for these voltage fed antennas. Use 8-10 turns.
TECH NOTE: SNAP On Chokes for 80 and 160 meter end fed antennas
If you use RG-8X size cable, use 2 SOFLC if you have 5-6 turns thru the center of each.See picture on left as an example of how to wind the cable.
Measure 40 feet from the matching unun and then wind 6 turns thru the center of the SOFLC.Close it up and then wind an additional 6 turns thru another SOFLC placed adjacent to the first one and on the radio station side of the coax.See picture to left.
If you use LMR-400 or 1/2" coax cable you will only get 3 turns thru the snap on choke and you will need an additional choke (with an additional 3 turns) in series to provide enough choking on 40 and 80 meters. Part# SOFLC.
In lieu of the 2 chokes shown above, you may also use a F400-31 ring (3” ID) with 6 or more turns which provides a much higher common mode suppression and better antenna tuning on 80 meters. See picture for winding example.
A simple ferrite ring filter on the DC power line can help suppress the RFI noise affecting the victim device or keep the DC power cord from acting as an antenna and radiating RFI into other victim devices. This kit is also useful on laptop computer supplies, routers, cable modems, wireless phone charges, etc.
Installation is quick and easy: Just wrap the DC power cord through the center of the ring filter as many times as you can and reinstall. For best results with long DC power lines, use one at the ring filter at the wall wart end (to prevent the wall wart power supply from using the DC line as an antenna) and another at the DC plug end (to prevent the electronics in the powered device from using the DC power cord as an antenna). Typical installation is shown below:
The filter suppresses RFI in from .1-100 MHz. Inside diameter is 3/4 inch/19 mm.
Since most households and office environments have multiple wall warts we offer the Wall Wart RFI Filter kit in bulk quantity packages.
Known RFI Generators include Roku/Sling-TV power adapter - Interference heard in the AM broadcast band and harmonics. Also many ethernet Cable/DSL modems, cell phone chargers, computer power adapters have the same issue: using the AC or DC side of the power supply as a radiating antenna. RFI solution is to use a linear power supply, change adapters or wall wart ferrite filters