Another Antenna question

[Nashpdman]

I put my comm antennas in the outer most pocket where the flap rods pass through. I would like to put the xponder/ADS-B antennas in the pockets where the steps are located because I have access panels there. Is this enough distance? Also, when measuring antennas, do you measure from the hole wherethe antenna mounts or from the tip of the antenna? You can see the doublers taped where I'd like to mount the antennas. Ideas and advice please.

Thanks!
Mike

 

[rockyfatcat]

Typically one top, one bottom, I have seen them hidden inside the gear leg fairings.

2 on the bottom install isn’t bad.

 

[BobTurner]

You understand this is part black art, part vodoo. But IMHO: the two coms will work. I’d like them more inboard (remember there’s supposed to be a conducting two dimensional flat plane under these antennas) but more inboard makes interference between the two coms worse. I think the transponder/UAT are too close to the com antennas.Can you move them forward, just behind the firewall a bit? Maybe move the coms back? Also remember at a GHz (transponder, UAT) even good coax is lossy, best keep the runs as short as possible.

 

[Southern Pete]

No, not really. Your placement will *probably* work, but, you will likely be giving up range. That won't be a problem until you are in trouble and all of a sudden it is. Measure from the hole, the vertical part of the antenna does all the work. The Txpdr antenna needs less of a ground plane but also needs to be spaced away from the com antenna - check the install manual for the details.

 

[Nashpdman]

If I'm understanding the "ground plane" correctly, it's basically the aircrafts surface? The metal directly under the antenna base to create a mirror type reflection of the radio wave, is this correct? Once my wings are attached there will be a large area under the com antennas.

Thanks!
Mike

 

[Carl Froehlich]

Some thoughts:
- The comm antennas will work just fine where you have them. I however would not put them there as I’d guess they are in spots that could created a problem with passengers getting in and out.
- I would not put the ADS-b antenna and XPDR antenna in the locations you point to. For me the remote XPDR gets mounted on the bulkhead just forward of the panel, the XPDR antenna gets mounted under the passenger seat. Less coax run. I mount the tiny SkyView ADS-b receiver in the tail cone, and the ADS-b antenna a foot or so from it in the tail cone (aft of the baggage area in the RV-8 and RV-10). The ADS-b receiver connects to the panel via a single, four #22 conductor shielded wire (serial in, serial out, power, ground. No coax running to the panel.
- I suggest you don’t get bent around the axle on “ground plane”. The function of the ground plane is to provide a “counter poise” to the quarter wave length radiation element. It is not a reflector or mirror. The fact that you have them mounted near the edge of the fuselage will have not practical impact.

Carl

 

[sansoneservices]

Please increase the distance between com and xponder ants. Typically we install com on top of the airframe and xponder on the belly. For good reason

 

[BobTurner]

If I'm understanding the "ground plane" correctly, it's basically the aircrafts surface? The metal directly under the antenna base to create a mirror type reflection of the radio wave, is this correct? Once my wings are attached there will be a large area under the com antennas.

Thanks!
Mike

Well, ‘mirror’ is a bit hand-waving. A better thought picture is that the radiated RF will induce currents to flow inside the Al skin that make it appear as if the missing half of a half wave dipole is buried under the skin! I forgot about the wing. Remember about black magic? If the ground plane is perfect you won’t be able to hear or transmit to anyone above you, including the tower if you’re on the ground, so don’t make it perfect!