Good Old VOR/LOC/GS Antenna question!

[Bill_H]

We have a steam gauge RV9a. Updating to an EFIS soon. But we also have an unused VAL INS422 we could put in it for enough IFR capability to get out on a low day. The INS has a built in indicator. So it needs a VOR/LOC/GS antenna and a diplexer to split that signal to the separate VOR/LOC and GS inputs on the VAL. The marker beacon antenna and input would be optional but used MB antennas are pretty cheap and it mounts on the bottom of the plane straightforwardly.

My question - this RV9A is already built. What is the EASIEST (and cheapest) way to add the VOR/LOC/GS antenna, and what type? Do NOT want to do a Cessna-type bent wire at the top of the rudder and run all that coax. Do not really want to do the same in a wingtip, I imagine that might be difficult (?) Suggestions please!

Is it possible to mount that Cessna bent-wire-type on the underside of the plane with just some kind of insulating spacer? Do not really care about enroute long-range VOR reception. THANKS!

 

[BobTurner]

Yes, some have mounted a V dipole at the bottom of the tail. You need to pull coax thru the plane, and be careful of children, dogs, and yourself any time you are under the tail. Those things are eye-poker accidents waiting to happen. Personally I’d recommend the Archer type wingtip antenna, as long as your wing tips are not riveted on. All the work is external except for pulling the coax. You will have to use a stick to hep with that, gain access thru access plates to secure the coax, etc. But the antenna itself works fine.
I don’t know about TX but here in CA MB’s have gone the way of the Dodo, hard to find anymore. Same with timed VOR approaches. No problem for ifr departures, but for many ILS or LOC or VOR approaches you may need something else to locate the FAF (e.g., DME, VOR cross fix, radar fix, etc). Best to see what extra (not MB) capability you need to actually fly your local approach(es). Is the VAL like the SL30? My SL30 can display 2 nav signals at the same time, e.g., LOC and VOR cross fix, so I can fly the ILS at a nearby airport with just the SL30. But I cannot shoot the ILS at my home airport if my gps fails, because the ILS at LVK requires gps or dme, I don’t have a dme.

 

[blaplante]

There's some conflicting requirements here.
Cheap, easy, unobtrusize...
But going to be used in actual IFR, ergo it had BETTER WORK.

I've seen copper tape in the lower cowling. Cheap & easy, but dubious signal quality if the station is behind you.

The mentioned wingtip Archer type. No experience w/it. Seems like it would be blind on one side.

The v-type eye poker. At the VSTAB top, or on the fuselage bottom at rear.

Another possibility is a loop or blade style Comant CI120
It seems like the tubing type one could be home-made by someone handy enough to build an RV. See the Comant CI 205. Both of these go on as a pair, one on each side of the VSTAB. Look for used one.

Also there's a "flying V" antenna that was used on some Pipers I think.

 

[Bill_H]

Bob Turner: Thanks. For "real IFR" we will eventually get a Garmin 175. This is just to get us out from under a 700 ft overcast that might hang around a few days and on to a VFR destination. And we have a "free" INS422.

I'm looking at the Archer (Bob, not Piper, now I get it) antenna. Great reading on the Archer antenna in other threads.

Question - for the bottom mounted "V" dipole type everyone says as far aft as possible. And then the antenna wires stick out past the fuselage side. Why is aft better? Why not more forward so they do not stick out past the fuselage? Is there that much of a performance hit? Or is it interference with the com antenna normally in that vicinity?

In the used market there are antennas that originally stick out horizontally from the vertical stabilizer. Sometime one on one side, sometimes a pair. I imagine that one of those can't be mounted on the bottom of the plane in a vertical orientation???

THANKS!

 

[BobTurner]

As you may have guessed, I have an Archer wing tip antenna, and like it a lot. It is not quite as good as the rabbit ears up high on the tail (or the "towel bar" type on either side of the vertical stabilizer). By not quite as good, I mean that if a towel bar can pull in a VOR that's 100 miles away, my Archer might only be able to do 70 miles. And at that range there is some directional sensitivity. As another poster suggested, at 70 miles I might only get 50 miles if I turn and place the station is on the opposite side of the airplane. But it always has worked for me on approaches, where you typically are much closer to the station.
VOR signals are horizontally polarized (unlike com signals, which are vertically polarized). In a theoretically ideal world a vertical antenna will not pick up a VOR signal at all. (same for localizer).
The rabbit ears are always, as far as I know, designed to have two identical "radiators" on each side of the vertical stabilizer, which acts as a "ground plane". When you place them under the tail, with only a vertical ground plane between and above them, not below, they still work okay. But under the "fat" part of the fuselage the ground plane is mostly above them, not between them. They're not designed to work that way; the directional pattern, the impedance, etc., are all likely to be "off", resulting in poorer performance. And as you suggested, even though the com antennas are vertically polarized, you're asking a lot of your nav receiver if you put your VOR antenna too close to a transmitter antenna.
The Archer antenna relies on the relatively small end rib to act as a ground plane. It has to be small because it also relies on signals coming from behind the rib ('east', if the antenna is in the 'west' wing) to diffract around it and get to the radiating element. It seems to work better than you might think, which is a tribute to the sensitivity of modern receivers.

 

[scrollF4]

I second the Archer antenna

 

[wjb]

The rabbit ears are always, as far as I know, designed to have two identical "radiators" on each side of the vertical stabilizer, which acts as a "ground plane". When you place them under the tail, with only a vertical ground plane between and above them, not below, they still work okay. But under the "fat" part of the fuselage the ground plane is mostly above them, not between them. They're not designed to work that way; the directional pattern, the impedance, etc., are all likely to be "off", resulting in poorer performance. And as you suggested, even though the com antennas are vertically polarized, you're asking a lot of your nav receiver if you put your VOR antenna too close to a transmitter antenna.

The rabbit ears actually are a true dipole antenna and as such does not need a ground plane. Ideally, for best radiation pattern, you would like it to be in free space as much as possible ... such as the top of the tail (traditional), or far down the tailcone.

I just put a Rami antenna under the tail of my -7, right in front of the bulkhead where forward VS attaches, and it works great. There is just enough space via the holes in the VS deck to get tools in to install a backing plate. Running the cable forward was real pain though ... best done at condition inspection time.

 

[blaplante]

Question - for the bottom mounted "V" dipole type everyone says as far aft as possible. And then the antenna wires stick out past the fuselage side. Why is aft better? Why not more forward so they do not stick out past the fuselage? Is there that much of a performance hit? Or is it interference with the com antenna normally in that vicinity?

In the used market there are antennas that originally stick out horizontally from the vertical stabilizer. Sometime one on one side, sometimes a pair. I imagine that one of those can't be mounted on the bottom of the plane in a vertical orientation???

THANKS!

As mentioned above the V elements must be in free air as much as possible. If you put it under the central belly it would be about an inch away from metal everywhere. Not good.

The one that stick out horizontally from the VSTAB must be mounted horizontally. This is because VOR signals are *horizontally polarized*. COM signals are vertically polarized, hence why we use vertical whip antennas for that.

 

[BobTurner]

The rabbit ears actually are a true dipole antenna and as such does not need a ground plane. Ideally, for best radiation pattern, you would like it to be in free space as much as possible ...

Your first sentence is correct. But, due to the symmetry of the dipole, the plane which bisects the antenna perpendicular to it is an ‘equipotential surface’ - the voltage is the same everywhere. So you can place a conducting plane there, no current will flow, it changes nothing. You get the same radiation pattern.

Clearly your photo shows horizontal or nearly horizontal aluminum close to, and above, the high current part of the dipole - not ideal. It’s just a statement that modern nav receivers are very good, VORs and localizers/GS are moderately high powered.

 

[Dugaru]

Under the tail

I just put a Rami antenna under the tail of my -7, right in front of the bulkhead where forward VS attaches, and it works great.

I’ve got the same setup, and can confirm that it performs really well. On the few occasions when someone has bumped it while walking past, the flexibility of the antennas has saved the day. But it’s certainly not an ideal location.

 

[Carl Froehlich]

The old saying holds, better is the enemy of good.

For VORs, how good to you need the system to be? Yesterday I flew home and consistent picked up VOR stations at 100+nm. More than good enough for the third navigation backup in my plane.

Side note - many VOR stations were dead.

LOC and GS signals are picked up far before the approach phase.

I run a homebrew wingtip NAV antenna. It cost a few pennies in hardware and aluminum from the scrap box.

The antenna experts are right, get a dipole antenna 10’ above the fuselage and you will gain a few miles on the VOR station.

I cringe when I see VOR whiskers poking out from the bottom of a fuselage (nose gear) or from the top of a VS (tail dragger). If you have either of these please put eyeball protectors on them when you park someplace. I do that for my bottom mounted comm antennas. You never know when a kid or dog will be running around the plane. For that matter never assume you will remember where these eye ball pokers are when crawling around under the plane or tying down the tail.

Carl

 

[Smilin' Jack]

I too installed the Rami antenna where bruce installed his. It works perfectly with the Garmin650 I have coupled to the Dynon Skyview EFIS
J

 

[bjdecker]

Not trying to be a pirate...

Like Carl and others, I installed a modified Archer SA01 / Aeroelectric antenna in my left wing tip.

Works great out to 100+NM (depending upon the service volume of the VOR being received), LOC/BC and GS received well outside of the IAF.