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Archer antenna performance

M

Mtherr

Well Known Member
Patron
I installed an Archer model 1 in left wing tip IAW instructions.
Connected to a GNC 255 radio.

Tested today. I could hear (weak) Morse code from Mirabel VOR on 116.3 only from 20 NM away. Seems not good to me.

Checked cable. No short between pin and shell. Shell also shows no resistance to airframe.

What could be the issue?
Any test I should do with a NanoVNA?
 
Mtherr said:
I installed an Archer model 1 in left wing tip IAW instructions.
Connected to a GNC 255 radio.

Tested today. I could hear (weak) Morse code from Mirabel VOR on 116.3 only from 20 NM away. Seems not good to me.

Checked cable. No short between pin and shell. Shell also shows no resistance to airframe.

What could be the issue?
Any test I should do with a NanoVNA?
Click to expand...

Not sure what the capabilities of the NanoVNA are (I have one, but I haven't "RTFM" yet.)

At 112.975MHz (Mid point of VOR/LOC Band 108MHz - 117.95MHz) Measure:

1. The VSWR of the entire antenna system (measure the cable at the back of the radio while the wing tip is installed), you want as low a ratio as possible (I got mine down to 1.1:1).
2. Impedance ~50ohms (Mine measures 42 which is too low.)
3. Gamma Match (Capacitance) as close to 100% as possible.

Adjustments to the above can be made by trimming the long leg of the antenna (SWR, Impedance), and the length of the small "stub" or the thickness of the insulating material (capacitance/gamma match)
 
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You should do better than 20 miles range. Is the ground leg of the antenna well connected and close to the end rib?
 
Attaching pictures. I will check how to take measurements with my NanoVNA and will test the antenna tomorrow.
 

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Mtherr said:
Attaching pictures. I will check how to take measurements with my NanoVNA and will test the antenna tomorrow.
Click to expand...

I am not an antenna expert but I have installed several of these antennas. The instructions are very emphatic that the coax cable and the wiring for the lights travel together through the plastic clips along that length of antenna where the wires are running in your picture. Your coax is separated at the beginning of the run. According to the instructions this could be a possible problem.
 
Actually, the instructions show the coax immediately turning inboard, back into the wing. I would not run it out to the leading edge of the radiating element.

Are the screws which connect the wing to the wingtip and the Archer ground leg free from any paint under them? They need good electrical contact to the metal wing.

Is the screw that the center conductor connects to free and clear of any metal on the bottom side? As I recall the gap is pretty small.

Is the antenna ‘backwards’? e.g., Most people orient the angled radiator (where all your wires are) toward the front, looks like yours is toward the rear? Not sure that it should make any difference.
 
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On my RV7 I was never satisfied until I riveted the base element to some Al angle riveted to the outboard rib.
I just assume that there are to many unknowns with the screws etc in the tip.
That way you can also have a straight shot of coax without any connectors.
I assume you have a connector somewhere inline if the antenna is permanently riveted to the tip? That can’t help.
RV10 is the same. 80NM at 8000’ is repeatable
 
The screws connecting the antenna to the wing are in contact with the skin as I removed paint from underneath the screw heads. I also tested continuity.

For the conductor screws, there is no metal underneath them. But I did put some JB Weld to keep the screws from turning while installing the nuts.

I think the wires are routed properly on the antenna…. However, they are running back near (on) the rib to a conduit located behind the spar (midway to rear end of wing).

The wing tip lights are quite next to the antenna…. Instructions says to install the antenna as forward as possible and clear of wing tip lights… is my installation OK from that perspective? (See picture in previous reply).
 
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While we're on the subject...

I've never really understood how (or why) the Archer antenna worked. I attached a simple block diagram + schematic of what I recall the thing looked like from a block-ish diagram and an electrical schematic.

I "think" the magic happens due to the 60° angled legs (creates the aperture to the wave front) and the size (capacitance) of the capacitor formed by the stub and the fiberglass spacer.

I've seen (and built) other gamma match antennas, etc. but I do not recall seeing one where the matching network is open with respect to the reflector/radiator like it is on the Archer.

Bob, Carl, you guys understand the physics a whole bunch better than I, can you shed some light on what's happening here?
 

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I'll try.
In your picture, imagine that the grounding rib is much larger - like the top of the metal airplane, a "ground plane". Imagine that you just had a vertical quarter wave antenna on top of the ground plane, like a typical com antenna. It can be fed at its base with 35 ohm coax, and will work pretty well. But it's tricky to make the base strong and fed the coax there, too. So we can make the antenna solid metal, right down to and bolted to the ground plane for strength. Now, in operation, there will be "standing waves" of voltage and current along the antenna, due to the reflected voltages and currents running back and forth. We go up the antenna a little ways, and find a place where the ratio of the voltage to the current is 52 ohms. (volts divided by amps = ohms). Except the current and voltage are not in phase in the antenna, but are in phase in the coax. This phase difference is 'fixed' by adding a capacitor in series with the coax feed. Finally, to make it fit, we bend the last part of the antenna until it's parallel to the ground plane. This is very common on com antennas mounted underneath the plane, for ground clearance. Since most of the current flow is in the first part of the antenna, this bending doesn't affect the power radiated significantly. There is some extra capacitance against the ground plane, so you have to go back and adjust the series capacitor at bit, maybe the length a bit, etc. Now rotate the whole thing 90 deg, and you have the Archer.
 
Did another flight tonight. Seems things may be better than I had observed. This evening, I was tracking the localizer of St-Hubert airport from about 35 NM away while being at 3000 feet. What I noticed that i did not earlier today is that the navigation tracking works from much farther than the audio signal presenting the Morse code….

I will still take measurements tomorrow.
 
Are you sure the morse code filter is not on? I’ve seen lots of radios where this filter is not very good, takes a strong morse code and makes it weak but still audible.
 
bjdecker said:
I've never really understood how (or why) the Archer antenna worked. I attached a simple block diagram + schematic of what I recall the thing looked like from a block-ish diagram and an electrical schematic.

I "think" the magic happens due to the 60° angled legs (creates the aperture to the wave front) and the size (capacitance) of the capacitor formed by the stub and the fiberglass spacer.

I've seen (and built) other gamma match antennas, etc. but I do not recall seeing one where the matching network is open with respect to the reflector/radiator like it is on the Archer.

Bob, Carl, you guys understand the physics a whole bunch better than I, can you shed some light on what's happening here?
Click to expand...

Bob provided a nice response. I’ll add a couple of thoughts:
- The 60 degree triangle has nothing to do with the antenna itself, it just provides a way to feed the antenna.
- As with all quarter wave antennas, the base is the high current point, the tip is the high voltage point (current and voltage are 90 degrees out). So the base impedance is ~0 ohms and the tip impedance is infinite. Keep in mind this is RF impedance at the operating frequency, not DC resistance. RF and DC are very different animals.
- Now that we know the antenna impeadance is zero at the base and infident at the tip, someplace between the base and tip will the impedance will be 50 ohms. This is the point we want to feed the antenna with our 50 ohm coax. This is the point where the “triangle arm” connects to the antenna. We use a gamma match network to allow the addition of a capacitor to balance out the inductance of the arm that connects to the antenna.

Some thoughts on how to mount these antennas:
- I would never run NAV/Strobe or landing light wires alone the antenna as shown in the Archer instruction. You are asking for trouble. Just don’t do it.
- I suggest that mounting it further aft and to the end rib solves this light wire issue, as well as everyone’s obsession to achieve a perfect DC ground (in my opinion misplaced as the antenna wants an RF ground, not DC - but that is a much longer discussion).
- The attached photo is how I mount my wingtip antenna. I make the antennas to stick out further into the wing tip. The most expensive part of this antenna is the hardware. I have VOR ranges well beyond 100nm on these antennas, and ILS way beyond practical ranges.

Note, do not attempt to make this antenna unless you have access to an antenna analyzer. To make these antennas sing, you need to tune them (as in length and adjusting the gamma match capacitor). I strongly suggest verifying all antennas with a real analyzer - so a great shared tool for you local EAA chapters.

Carl

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Mine works great

Don’t remember how I exactly mounted it, but it is mounted in the right wingtip. Placed the antenna grounding side under a doubler that I put nut plates on to secure the tip to the wing. I did add another grounding wire to it and connected it to a spot on the last rib.

Now, as for performance. As I fly over the Sierra Nevada west bound, I am getting a valid nav signal from over 110 miles to the north and south, so I must have done something right.

FYI, it is connected to a SL-30.

Brian
 
BobTurner said:
Are you sure the morse code filter is not on? I’ve seen lots of radios where this filter is not very good, takes a strong morse code and makes it weak but still audible.
Click to expand...

Wunderbar! This was the issue. In Garmin language I needed to activate NAV Ident. Today I would hear the Morse code farther than the Nav signal worked with the EFIS.

Thanks!
 
I tested the antenna with my NanoVNA. Either I do not understand how to use this tool properly (which could very well be the case as I am totally new to this) or that antenna setup is not impressive. But it seemed to work pretty well today.
 

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Notams?

I may or may not have read ALL the Notams pertaining to the particular VOR I was using to test my VOR with archer Antenna. Playing with all the buttons during a flight I happened to pull up 'nearest' VOR and was able to get a good signal and CDI. Only then I went back and found out that the local VOR I had been using, despite being with in eyesight of the airport, was effectively useless below 5000 AGL (yes I did know it was a H-VORTAC beforehand)

Good luck
 
Mtherr said:
Wunderbar! This was the issue. In Garmin language I needed to activate NAV Ident. Today I would hear the Morse code farther than the Nav signal worked with the EFIS.

Thanks!
Click to expand...

Consider forwarding some fraction of what you would have paid a shop for this advice to VAF.
 
BobTurner said:
Consider forwarding some fraction of what you would have paid a shop for this advice to VAF.
Click to expand...

Unsure when was the last time I did, but I checked and see my name is on the list. Will definitely continue to contribute.
 
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