Additional Photos: 
A very useful application for a feedline isolator is installing them in series with a yagi's normal feed system. The proper location is between the antenna’s matching device/system and the feedline. Doing this will not affect the antenna and prevents the feedline from acting as part of the antenna. In beam installations, using a feedline isolator in series with the antenna’s feed system can substantially improve the antenna’s front-to-back and front-to-side ratios. It does this by providing the antenna with balanced current at the feedpoint and by effectively preventing the coax shield from acting as part of the antenna.
If you have a vertical you should be using one to prevent the vertical element from using your coax as a radial and to decouple the feedline for reduced noise. Installing an additional isolation/choke balun outside the near field of the antenna will help eliminate common mode currents induced by RF and EMI on the coax shield.
It is a seldom appreciated fact that feedlines which are not adequately decoupled can act as efficient vertical antennas that degrade an otherwise excellent radiation pattern. The addition of a quality feedline isolation ("choke") balun can significantly reduce feedline radiation and dramatically decrease RFI and TVI.
Beam antennas especially, benefit from improved balanced drive and superior feedline isolation, but even simple dipoles benefit from properly selected and installed Feedline Isolation baluns. In addition, receiver noise may also be reduced by eliminating stray EMI picked up by the coax shield.
Here are the radiation plots of the same dipole with and without a balun. Notice how the pattern is distored without the feedline being isolated by the balun:
Key Design features:
Utilizes specifications of Jerry Sevick, W2FMI, the authority on baluns. His spec utilizes a bifilar toroidal Guanella current balun which he modified to provide higher efficiency. By utilizing a new toroid mix, the bandwidth of his design has been increased.
Special mix toroid with low permability allows broad frequency coverage.
Isolation balun has significantly higher common-mode impedance and larger effective core area than other similar designs. It is much more effective than types with ferrite beads, ferrite bars or wound air-core coax baluns.
Windings are heavy gauge Thermaleze wire with a minimum of 2000v breakdown voltage each. This is NOT the less expensive enameled or Formvar wire like many of my competitors use. It may appear similar but both of my windings are coated with an expensive Polymide covering. I also wrap one winding in an additional layer of Polyimide tape to ensure a uniform input impedence of 50 ohms. When combined with Polyimide tape, total breakdown voltage is increased to over 7500 volts.
Typical insertion loss is less than 0.2 dB
Power handling of 3kw continuous, 6kw intermittent upto 35mhz. May be used to 50 mhz with lower efficiency (1-2%).
SO-239 connectors are silver plated with teflon insulation.
All hardware is stainless steel
N connectors and Alternate mounting options are available in Accessories.
Very high efficiency. Will not heat up or saturate like many of the typical cheap current baluns.
All Baluns are sealed in weatherproof 4" X 4" X 2" Nema Box which makes an excellent outdoor enclosure.
Regardless of the antenna you're using, an isolation balun can provide numerous advantages
- Preventing unwanted RFI by eliminating feedline common mode currents and radiation
- All power goes to the antenna, improving efficiency
- Reduces noise or EMI picked-up by your coax shield
- Power is balanced between driven elements of antenna
- Overcomes a less than optimal ground system
One Exception!!!
It must be pointed out that a 1:1 balun should never be used on the second harmonic of a half-wave center-fed dipole fed with coax (like an 80 meter dipole being used on 40 meters). The impedance can be as great as 10,000 ohms creating very high voltages which can bring about voltage breakdown and/or excessive heating. This does not apply to Off Center Fed dipoles or dipoles fed with ladder line.
Also, please note that although this balun will tolerate very high SWR, efficiency drops markedly if your SWR is 4:1 or above.
While the most common advice is to improve the station's RF ground, the root of the problem is in the poor isolation of the feedline from antenna currents. If you would like to reduce feedline radiation and improve reception by reducing noise, feedline isolation baluns are an excellent choice. Adding an additional isolation balun at the point where the feedline exits the near field area of the antenna will substantially reduce unwanted feedline radiation and reception of EMI without the need for improved station grounding.
If you are using a vertical antenna you need to read this
If your antenna SWR is already low and you wish to reduce feedline radiation and improve reception, a feedline isolation balun is recommended. Adding one at the base of a vertical antenna will substantially reduce unwanted feedline radiation (RFI) by preventing your antenna from using the your coax feedline's shield as a radial. This can also reduce the need for improved station grounding.
With a ground-mounted quarter-wave vertical, regardless of the radial situation, but especially with poor radial systems, there is the need for a feedline isolation balun to keep current off the feedline.
When quarter-wave antennas are constructed over a good radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over a less than optimal radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced onto the feedline.
With my SteppIR vertical I use two of these baluns. One at the base of the antenna and another midway along the feedline after the near field of the antenna. You can also install the balun where it enters your shack if you would rather not break the feedline run. It balances the current at the antenna, is terrific for keeping RF out of my equipment and greatly reduces the noise inherent in a vertical by decoupling the coax shield.
This isolation balun will handle continuous power of 3kw and tolerate high SWR for extended periods.
It will not heat up or saturate at rated power, under extended duty cycles or stressing load conditions.
