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I think you meant to address that to Phillip. I believe you and I are are on the same page with the possible exception that I am lazier than you and wouldn't bother with any balun unless I believed I would notice the difference.
Steve Stearns AB0KB
Boulder / Longmont CO
Steve I would not know how to use ferrite at those frequencies. A coax balun is easy to make especially if you use 1/8” UT141 semi-rigid coax which solders nicely. See attached.
You can make a folded dipole with a 4:1 balun or a simple dipole with a 1:1 balun. Either way it would work well. Either way you are making a 75 ohm antenna which would have a slight mismatch with our 50 ohm systems.
Cozy IV N197DL
Cannon Creek Airpark 15FL
"With your dipole arrangement, what did you use to decouple the feedline? Did you use ferrites etc.? "
I didn't use anything. But then my dipole is receive-only and the feed line is very short and it's going into two paralleled high impedance inputs (two SDR receivers). I've found that, generally, dipoles, though not 50 ohms at resonance, still provide low enough SWR to not be a problem for transmitters when on 50 unbalanced feedline. I expect our aircraft environment is also acceptably tolerant of some shield current but I haven't explicitly tested this configuration. Jim Weir seemed to like-feed through (aka "current") baluns in his stuff, Bob Nuckolls seems to think they usually don't provide much benefit except for lab measurements and, on other projects where I've wanted them, I've designed full up matching baluns rather than used so called "current baluns".
In short, your mileage may vary. :-) If I went this route for transmit on 900 MHz, I'd check the SWR. I've got parallel dipoles (again, no balun) fed with 50 ohm coax for my ELT antenna (406MHz and 121.5MHz) and its SWR is lower on both frequencies than the supplied antennas (both the monopole and the "rubber ducky" portable).
With your dipole arrangement, what did you use to decouple the feedline? Did you use ferrites etc.?
Cozy MKIV RG Powered by Subaru IO-200 (SN 0030)
P.S. Note that the measurements shown in my picture are for a wide-band (to cover both bands) receive only antenna whereas an antenna for transmit/receive on the 900 MHz band should be a little shorter. (i.e. twice the length of your monopole antenna)
On Wed, Jun 24, 2020 at 7:52 AM Steve Stearns <steve@...> wrote:
"You're less likely to go wrong sticking with a more conventional solution"
It looks like you have enough room for a dipole antenna (see my previous picture) if you ever decide to upgrade. It will at least give you better coverage for the volume on the other side of the virtual ground plane and should be more cylindrically symmetric.
On Tue, Jun 23, 2020 at 9:04 PM Marlon Gunderson <sondergun@...> wrote:
Thanks James, you're right, this is totally experimental and may not work reliably for someone else. I have not done any kind of formal testing, just flown with it for 3 years and checked the FAA ADSB performance test a couple of times a year. It hasn't shown anything other than 0% failures (of about 10K pings/hr) when I've checked but there's nothing to say I won't see some failures in the future. You're less likely to go wrong sticking with a more conventional solution.
On Tue, Jun 23, 2020 at 6:36 PM berkut13 <berkut13@...> wrote:
It may indeed “work”, but the radiation pattern is going to be significantly affected by the asymmetrical ground plane radials. Could cause some reception issues by ground stations in certain directions. No way to really know without extensive testing.
Virtual ground plane in the strake cavity has worked well for me for an Echo UAT. Good separation from the Mode C antenna under the front seat.