Geoffrey Preston's Posts (5)

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Controlling Radio Frequency Interference


RFI is often a very frustrating problem but the following information may be of help. You may have RFI and be unaware of it and following these precautions might increase your range.

If possible keep your wire/cable runs away from transmitters, coax and antennas. Never group coax cable together with wiring runs. Use shielding for your wiring runs and if possible grounded metal enclosures for sensitive electronics. Be sure your coax cables have soldered connectors, not crimped. Keep your antennas as far apart as possible. Use dedicated batteries for AV equipment rather than a BEC. Carefully choose which bands you want to employ on your UAV keeping in mind harmonic relationships. Use Low Pass filters for your transmitters to reduce if not totally eliminate interference.

Here is a useful discourse on interference: link to a White Paper from

Adding a grounding system to your electronics is a good thing to do. You can use the copper shielding from large coax cable (knitted wire) to place over your cable runs and then ground that. Good thing about the copper shield from coax cable is you can scrunch it together by the ends to make it larger in diameter to stuff your wires through and then stretch it out to make it smaller in diameter, like a Chinese finger lock toy.


Also you can, if you have enough spare length on your wire or cable runs you can wrap them through or around a ferrite core to cut interference. Although some people say that ferrite cores aren’t really very useful above HF frequencies. Here is a link to an interested discourse about RFI suppression and Ferrite material.

3689577696?profile=originalNot all ferrite cores are created equal and most aren’t effective at the frequencies we use….

All are usable from very low frequencies 500 KHz and up to 150 MHz and beyond. Wurth product id RND-6 is good to 1+ GHz.

These links might get you some useful cores.

      We use a lot of vhf/uhf frequency radios in this hobby and those wavelengths are relatively short and unfortunately so are our wiring runs. Often and inadvertently those cable/wire runs are at a length that makes them resonant to the actual frequency we’re using or at a harmonic to that frequency. This makes the wire/cable an unintended antenna element.  Shielding, filtering, antenna separation and grounding are your tools to eliminate RFI.

Geoffrey KB6WSQ

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More Antenna Alternatives


Came across this great product for antenna building for UAV's. Adhesive backed copper foil 5 mm wide. Great for creating your own dipoles, vee's or helix, easily soldered. Attach it to fiberglass or plexiglass or directly to the foam (prepped with acrylic spray to help adhesion or just taped over with package tape).  Create dipoles or vee's directly on you wings (not over your carbon fiber spars though).

Keep experimenting!

This is available at Sparkfun.    $2.95 for 50 feet

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Antenna Alternatives

   Antennas now has its own section in the Forums



  There are alternative antenna choices for your UAV when space is a consideration. RHCP antennas are without doubt one of the best choices you can make for your video and telemetry signals. I had wanted to use a RHCP antenna for telemetry for my X-8 but the choices I had come across were pretty space demanding. Surfing the web doing some antenna research I ran across these wheel antennas from Kent Britain WA5VJB, a fellow amateur radio operator.  He's got some really useful designs other than the wheel.
   I purchased 2 of the 900 mhz variety for the telemetry, one for in the X8 and one for the ground station . Since the plan is to install all antennas within the X8 these are a great space saving choice over the current offering of cloverleaf antennas. The 900 mhz wheels measure about 4.5 inches in diameter, somewhat smaller than a CD ROM and only slightly thicker. The best part being they are only $ 12.00 each. They can be fed with coax directly or as seen in the photograph you can solder a circuit board SMA connector of your choice to the center in the holes provided.
   These wheel antennas are considered to be horizontally polarized at the plane of the horizon however as your UAV gains altitude the wheel antenna emits a cross polarized signal. Here is a quote from “Around 1990 the first launch of a balloon borne payload in the Denver, Colorado Area took place sporting a Little Wheel as the radiator for the on-board ATV transmitter. It was found to work quite well and had some interesting attributes that were previously undisclosed. Those being that the Wheel antenna although being a horizontal radiator on the horizon displayed properties of cross polarization above and below the array - similar to two in phase horizontal dipoles. This worked to the advantage of the ground station by employing a circularly polarized antenna (Helix) - also a fixed horizontal antenna directly below at the launch site would not see the up to -30 dB nulls as the balloon rotated. The horizontally polarized tracking antennas (Yagis) were receiving a signal of constantly changing polarity as the payload rose to an altitude at which it would start to drift away from the launch site and the horizontally polarized tracking antennas would become more effective due to the horizontal polarization emitted from the sides of the Wheel. The pattern of the Wheel is within +/-0.5dB of being a perfect circle in the E-plane. In the H-plane the pattern is +/-35 degrees of the horizontal axis of the antenna. These figures equate to ~ 4.28 dBic gain or 6.38 dBdc. The dBic gain above and below the array is somewhat indeterminable due to the fact that the circular polarization exhibited is quasi-circular or cross polarized; however, it responds to a Helix in a manner consistent with the gain of a pair of like sense Helices in a system looking at one another. ”

  This antenna does radiate well (60% efficient) and also radiates well in the second and third harmonics so take precautions to provide filtering for your transmitter.
  You’ll want to use a “Wheel” antenna on each side of your link as losses will result if mated to just a dipole when your UAV is overhead. Just mount it in a horizontal position at each end of the link. If it fits inside your UAV that’s great but it could also be mounted flat on the wing as well.


Holes are provided for SMA circuit board jack mount
or direct connection to coax.










This link shows the antenna being tested in a 3D pattern.


 i'll post my actual installation once I get there.

Keep experimenting with radio, get your tech license, it's easy and you'll be contributing not only to this great hobby but ham radio as well. 



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