Loop antennas may be constructed in many forms including horizontal full wave loops in square, rectangle or triangle (delta) shapes. They can also be in the vertical plane and are most often in the same shapes with the delta being very popular as it has both vertical and horizontal polarization. The impedance at the loop resonant frequency is approximately 100 ohms but will very slightly on harmonics. Loops are “quiet” antennas compared to verticals and dipoles and are omni-directional. The also exhibit gain on harmonic bands. Once you try a loop antenna you will know why they are so popular with old timers but still a secret to newcomers. Shown below are some typical designs:
Full Wave Horizontal Loop Antenna (a.k.a Skyloop)
This antenna is horizontally polarized and should be mounted as high as possible but works well at low heights of 10-30 feet. They are quieter than a dipole or a vertical, have a broader bandwidth and will usually out perform a dipole antenna.To determine the approximate circumference in feet of a full wave loop antenna use the formula:
1005/Freq in Mhz = length in feet.
The feed point impedance of a full wave loop antenna is theoretically in the vicinity of 120 ohms and requires a 2:1 impedance transformer (for single band loops or a 4:1 balun to match on multiple bands) with 50 ohm line. You will also need a feed line choke or better yet, just get the Hybrid 4:1+1:1 in a single box.
Vertical Delta Loops
Vertical delta loops can be oriented several way but the most popular is to have the “pointy” end at the top (usually a single support) and the lower horizontal ends just out of reach of humans and animals. Best feed point is 1/4 wavelength (246/f(mhz)) from the top point down one side. Vertical delta loops use the same 2:1 baluns as the horizontal loops or 4:1 for multi-band operation.
Icom 746/756/765/775/781 RFI kit is designed to be installed on your radio transceiver to reduce Radio Frequency Interference (RFI) caused by common mode current on the coax output of your radio, the AC/DC power, USB, ALC/Send, mouse,mic and additional interconnect cables. Standard 756PRO Kit has 4 ferrite ring filters and 10 ferrite snap on filters and instruction manual or you can choose the optional CMNF-500-50 coax noise filter option for one or two antenna ports.
This kit will help reduce or eliminate:
1. “Hot mic” RFI caused by poor grounding of antennas or common mode currents from linear amps feeding unbalanced (coax fed) antennas without baluns/line isolators.
2. RFI to neighbor’s electronic devices including audio/video systems, computers, telephones, garage door openers, etc.
3. RFI to your other radios, audio/video systems, computers, telephones, etc.
A side benefit is a reduction in noise floor in your receiver depending on the amount of noise being introduced by common mode currents (which are blocked/reduced by the chokes in this kit).
These chokes use a special mix of ferrite core material, Mix 31, that is effective in suppressing radio frequency interference from transmitters and high noise floor on receivers.
The split beads beads are easy to use, don’t require modification of the protected equipment and work in almost all cases, even when plug-in filters fail.
Use of the chokes often helps cure SWR problems between transceiver and a linear amp and between transceiver and antenna tuner or direct to antenna. The DC power line choke helps keep common mode current out of the power line which could cause interference to other devices connected to the same power line if not suppressed.
Purpose. This kit is designed to choke common mode currents going into or exiting from your transceiver. Use of the chokes often helps cure SWR problems between transceiver and a linear amp (if used) and your antenna.The AC/DC power line choke helps keep common mode current out of the power line (or into your radio) which could cause interference to other devices connected to the same power line if not suppressed.
The Deluxe RFI Filter Kit includes the CMNF-500-50 coax common mode noise filter option which is highly effective in suppressing common mode noise carried in (or out) on the coax shield braid external surface.The ring filters used on the coax connector, the I/O cable filters and the DC power supply filter are effective from 1 to 300 MHz.
|
RFI Kit Part # |
CMNF-500-50 Coax Noise Filter |
F240-31 Ring |
FSB31-1/2 Snap On |
|
RFI-ICOM-756PRO |
None |
4 |
10 |
|
RFI-ICOM-756DLX |
1(*) + DMBC |
3 |
10 |
|
RFI-ICOM-756DLX2 |
2 (*) + Coax jumper |
2 |
10 |
*CMNF Connector Note:
First CMNF-500-50 comes with Double Male Barrel Connector (DMBC) for connection to chassis of radio.
Second CMNF-500-50 comes with 6 in RG-400 coax jumper for connection to radio since the spacing of the antenna jacks is too close for both noise filters to be connected.
Typical Installation Guidelines
RF I/O:Standard Kit: Ring filter – 8-10 turns of ¼”coax cable thru center
Deluxe Kit: Use CMNF-500-50 Coax noise filter with double male barrel connector or short coax jumper cable
AC/DC Input: Ring filter – 3-7 turns of AC/DC power cable
Send/ALC Amp Cable:Snap On – multiple turns through choke – ½” diameter hole
Band Control: Snap on – multiple turns through choke – ½” diameter hole
Audio I/O: Snap on – multiple turns through choke – ½” diameter hole
Digital Interfaces: Snap on – multiple turns through choke – ½” diameter hole
Note: if you have more I/O cables then ferrite filters, you can put more than one signal type wire through the same ferrite snap on.The ferrites filter the RFI/noise current common to all wires, but do not affect the differential signals internal to the wires.
Also make sure you connect a good RF ground to the transceiver ground post.Use multiple turns wherever possible to increase RFI suppression effectiveness.
If you use a linear amplifier use an amplifier RFI kit for additional RFI interference suppression.
Note: Icom 756PRO picture shown which is typical of filters and connection cables for all Icom and many other brand radios with similar I/O ports.
