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Antenna placement question.

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  • #16
    Now that's something to chew on. Thanks.
    Now I have another twist. My fuel injection CPU sits on the floor, under the seat. Will the antenna and CPU so close cause problems? Should I now consider the mast as the mounting point?
    Thank you everyone for helping out.
    Mike

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    • #17

      I went back and found the seller on Ebay. It was " antenna2-waydirect "
      he has a NMO mount with BNC connector listed for $16.99 and an antenna's for about $12.00. If you send him your requirements, he will cut the coax to the length you want and trim the antenna to a length you want. He was real easy to work with. So, it looks like about $30 and it will show up in a few days in your mailbox.
      David McCutchen
      615-390-2228
      Bensen B7m, 90 hp Mac
      Dominator Tandem, 100 hp Hirth
      Kolb Mark III Classic, 80 hp Verner
      Certified - Advanced Master Beef Producer
      EAA Member #0511805
      PRA Member #28866
      PRA Chapter 16 Member
      Secretary & Treasure - PRA Chapter 16
      President / Sylvia - Yellow Creek Volunteer Fire Dept.
      Chairmen - Dickson County Veteran's Day Committee
      Volunteer - Dickson County Airport Aviation Day Committee
      2 busy 2 No!

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      • #18
        That seems very low priced for sure.

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        • #19
          Which one are you thinking Dave? I don't know squat about antennas.

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          • #20
            Mike,

            It is not impossible, but complicated, to get a vertical antenna of any type to work when mounted to a vertical mast. (And it needs to be vertical for aircraft COM work.) You'd need to model the antenna in software which could also model the structure to which it's mounted. The software's not terribly expensive, but an accurate virtual build of the gyro's frame would be very time-consuming. (Guess how I know!)

            If you use a ground plane antenna extending down from the pod, you can make it work near your ECU, but it will have to be tuned precisely, or there will be signal radiated from the coax. That may rule out the Red Tail, because there's no way to tune it. I'd suggest placing the center of the antenna mount about midway between the ECU and the nose gear fork.

            Cheap antennas can work great, but if you're putting it near the ECU, quality coax and optimized ground radials become critical. The radials not only allow correct tuning, but they choke RF current which could otherwise flow on the outside of the coax shield, increasing the odds of interference with other onboard electronics. I'd be a little worried about that $17 NMO mount. Cheap coax often has a braided shield with big gaps between the copper strands, which allows more leakage from the cable. If you're ordering one online, find out what spec, if any, the coax meets. You want 50-ohm coax with at least 95% shielding. If the seller lists the cable as RG-58, it may be cheap CB coax which will have poor shielding, unsuitable loss and leakage at aircraft frequencies. Preferred types are RG8M or RG8X. Some types of RG58 used in the early days of ethernet may have good shielding, but it's getting hard to find. (I recently saw some advertised with 48% shield coverage.)

            A half-wave dipole loses some of its effectiveness if the two halves are in environments which are not symmetrical. I made a dipole and mounted it vertically along the door frame of the Sport Copter's pod. It was a big improvement, but it was detuned significantly as I moved my leg closer to the lower half of it while seated.

            The right tool for tuning work is an antenna analyzer. Many ham radio clubs have an MFJ Model 259 for loan to members, or members who have one, and it is a great tool for this job. Many of these guys would love to bring their analyzer out to help, just to see your aircraft.
            Last edited by PW_Plack; 03-13-2018, 08:12 PM.
            Paul W. Plack
            Private ASEL, SP Gyroplane
            Secretary, URA & PRA2
            Editor, Western Rotorcraft

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            • #21
              Thanks Paul.

              I have seen the antenna tuner that you mentioned. They usually go for $300. And since it does sound like I will need to trim the antenna I think that I will get one. Pricey, but if that eliminates the possible noise issues then I have no problem with that. That makes total sense the if the signal cannot radiate properly from the antenna that the coax start leaking to near by components. Now I just have to understand the ground plane concept and add it in a way that doesn't look bad and doesn't cause me to repaint my pod.

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              • #22
                Is this the model? https://www.gigaparts.com/mfj-259c.h...SABEgKtmfD_BwE

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                • #23
                  Find the seller on Ebay, which I mentioned. Send him a message. He will build the antenna to your specs. He was easy to work with. RG8 will increase the price a little, but it is worth the better shielding.
                  I had him trim mine to 22.8 inches for a 1/4 wave on the radiating element/antenna and specified the length of coax with a BNC connector.
                  I mounted the antenna vertically down under the rear of my pod. I added a piece of metal sheeting in the lower section of the pod to create a counter poise/ground plane. The combination of this created an SWR of 1.1:1 while on freq. 122.8 MHz .
                  However, if you are using a handheld radio, the rubber ducky antenna should be sufficient for several miles.
                  David McCutchen
                  615-390-2228
                  Bensen B7m, 90 hp Mac
                  Dominator Tandem, 100 hp Hirth
                  Kolb Mark III Classic, 80 hp Verner
                  Certified - Advanced Master Beef Producer
                  EAA Member #0511805
                  PRA Member #28866
                  PRA Chapter 16 Member
                  Secretary & Treasure - PRA Chapter 16
                  President / Sylvia - Yellow Creek Volunteer Fire Dept.
                  Chairmen - Dickson County Veteran's Day Committee
                  Volunteer - Dickson County Airport Aviation Day Committee
                  2 busy 2 No!

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                  • #24
                    What did the ground plane sheet look like? How big?
                    Thanks.
                    Last edited by mvadney; 03-13-2018, 11:50 AM.

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                    • #25
                      Mike, yes, that's the current version of the analyzer I've had for about 25 years. Be sure to get the MFJ-7737 UHF-to-BNC adapter, which will allow you to plug the coax lead from the radio right into the analyzer. If you order from GigaParts, the adapter is here:

                      https://www.gigaparts.com/nsearch/?q=BNC+to+PL259

                      The BNC female on the top of the analyzer is an output for a frequency counter, not the input!
                      Paul W. Plack
                      Private ASEL, SP Gyroplane
                      Secretary, URA & PRA2
                      Editor, Western Rotorcraft

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                      • #26
                        Thanks Paul.
                        I appreciate the help

                        Can you describe a small ground plane that I could use if mounting an antenna under the keel? Does the direction of where the antenna is pointing make much difference?

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                        • #27
                          Mike, the last ground plane I built for the aircraft band was made from CB parts I got at a truck stop, with aluminum foil tape for a ground plane. The vertical element was a plain stainless steel whip with the little ball on the end, starting about 23" long before trimming. It was held at its base by a set-screw adapter to a 3/8"-by-24 threaded antenna base. The base itself was a feed-through type used on those 90 angled mirror brackets, but it was used without the angle bracket, and just run through a fender washer for support behind a fiberglass panel. The mount had a CB-type female coax connector on the side inside the panel, but you might be able to find one with a BNC connector, the aircraft standard.

                          I believe the length when we got done testing and trimming was about 22-1/4 inches for resonance centered at 120 MHz, and we got the SWR to about 1.4. Your length will vary based on your unique frame and mounting location.

                          To be sure it responds as a vertically-polarized antenna, get at least the bottom third of it coming straight down from the bottom of the pod before any bend. Keep the tip at least 10" from metal frame members if you can. If it gets closer, the resonant frequency will be lower.

                          It doesn't make much difference which way the bent portion points from a performance standpoint, but it may be a little quieter in the wind if it's bent back rather than forward.

                          These really simple antennas may be prone to collecting high static voltages as they fly. Check with your radio manufacturer to see if that will be a problem. Some radios can continuously discharge this static to ground. Others may be damaged by it.

                          I recently moved, and all my radio stuff is in stacked plastic tubs. I'll post some photos of an antenna made this way when I can find it.
                          Last edited by PW_Plack; 03-15-2018, 08:03 PM.
                          Paul W. Plack
                          Private ASEL, SP Gyroplane
                          Secretary, URA & PRA2
                          Editor, Western Rotorcraft

                          Comment


                          • #28
                            Thanks Paul. I also was communicating with the MFJ company that makes the antenna analyzer too. They were recommending a MFJ-1728B 5/8 wave model. I was also surprised to see that the ball at the end of an antenna actually served a purpose. I always thought that it was just making the end safer in the real world.
                            Wonders never cease.

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                            • #29
                              Hey Paul,
                              How about... http://www.aircraftspruce.com/catalo...p?recfer=24280 ?

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                              • #30
                                On Aircraft Spruce's site I found a fairly short antenna. LYNX MICRO SYSTEM SHORT FLEXIBLE VHF ANTENNA
                                What do you think?

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