Jim, the problem is not the radiation of a VHF EM field from the antenna (if I remember correctly it is 'supposed' to radiate when you push the little black button or it becomes difficult to talk to anyone
With the antennas on the inside of the canopy and a 170 pound conductive salt water sack sitting well inside the near field boundary, there are going to be SWR reflections back down the line. Some on the inside of the coax and some on the outside of the shield. THAT radiation isn't going to help bouncing its way around the cabin.
It is that the grounding system of the other devices on the plane is defective.
Grounding? Ehhhh, maybe, but a second order effect if anything.
Certainly the connection at the end of the coax to the antenna connector could be dressed a bit tighter than an extra inch of center lead showing beyond the braid at the antenna screw/solder point. But all that does is raise the VSWR a small amount. (meaningless in the real world) But the grounding eyelet connector soldered to the braid wants to be dressed as short as possible.
I don't know what a "grounding eyelet connector" is --- do you mean the solder lug that goes between the ceramic insulator and the airframe?
If low voltage LEDS are blinking during transmit, then there is poor/no RF bypass at the power connection to those devices. There should be a 0.01 across every power lead on these devices.
Oopsie. An 0.01uf at VHF isn't a very good bypass. If you figure that there may be an inch or so of lead length (20 nH per inch) then an 0.01 is self-resonant around 12 MHz. Anything above that and the capacitor actually looks like an inductor --- not so very good for bypassing.
If you can get that lead length down to half an inch or so (REAL short leads) then a 150 pf capacitor is self resonant at about 125 MHz. and a real good bypass.
And a separate DC ground wire back to the single ground point should be present. Multiple points about the airframe are not a proper ground - contrary to general belief and practice.
And most likely, the builder did not read Aeroelectric and make a single point ground bus for all devices.
Brother Nuckolls and I studied from the same textbook.
A ferrite choke over the coax (ala W2DU) at the antenna base will help by reducing circulating RF on the outer skin of the braid.
In stubborn cases consider adding a 0.1 bypass cap from the braid back to ground immediately before the choke (on the transmitter side forming an L match)
That is the basis of our ferrite-foil antennas. We think of the ferrite beads we use at the base as a balun, but all a balun does is suppress reflections from coax to dipole on the outside of the braid. Unless it is a MATCHING balun, and then you get into squirrel impedances (like from a folded dipole.
Of course, you know all these things I suspect.
I am continually amazed at all that I do NOT know. And students have a better way of probing your knowledge than a dentist with a sharp pick.
A skeleton slot antenna on a GA aircraft will be interesting. Hasn't been done in GA that I am aware of. Military and aerospace use them of course. The main problem for GA will be shadowing.
Yeah, that's my thought also, but to gain a half a dozen knots without wires in the breeze might be a good tradeoff.
cheers
denny-o
And to you, sir.
WX6RST (Weather 6, Readability, Signal, Tone)
k8do
