Antenna tuning

I have the Bob archer Sa-001 COM antenna in my right wingtip. While I know that the performance will never be as good as some of the other, external, antennas, I am still getting sub-par performance. I have the radio really for ATIS and other non-critical com functions. However, the radio has constant static when receiving and will receive ATIS at a distance of about 1 mile. I computed the SWVR to be 2.01, which is not great, but not all that bad. Can anyone share how they successfully installed (pics would be great) or tuned their antenna for max performance? Right now the radio is essentially useless and it is next on my to do list of things to correct. Thanks.

Eric

Eric,

I have the same antenna in the wingtip of my -10, installed per the Archer instructions. Connected to a SL-30, I routinely pick up ATIS 30 miles out. I know that doesn't answer your question, but I don't find anything second rate about the Archer antenna.

Jim Berry
RV-10

Actually VSWR is not all that critical on receive.

More info

Thanks for the replies so far. I do not question the capabilities or quality of the antenna and certainly hope I did not make it look that way. Otherwise I would not have purchased it. I am sure my issues are installer error and I am trying to remedy that.

Mel, The reason I was checking VSWR was because when I would transmit on the ground my autopilot would kick off and in flight when I tried to engage it it would then give me an error saying that I was over the maximum airspeed and it would not reset until I removed power via the circuit breaker. Took me awhile to make the correlation after talking to my avionics that work for me. In flight sometimes when I try to transmit it would do the same thing but have not been able to nail down specific conditions or frequencies. When I first measured VSWR I was at 3.4. Just tweaked the antenna in the wingtip and got it down to 2.0. Right now I basically took the trailing edge of the antenna and basically curled it up and tacked it to the top of the wingtip to try and get a little more of the antenna in the vertical plane. That got my range from about 100 feet to the mile I get now. I am just trying to match antenna position and layout with others that are seeing positive results so I can continue troubleshooting my issue. If it helps I am transmitting on the SL-30 at 4.5 watts with about .5 watts reflected as measured at the wing root bulkhead connector.

Eric

5 watts RMS seems awfully low- min spec for the SL30 is 8 watts.

How did you measure (not compute) VSWR? VSWR does actually matter as it is a measure of antenna impedance at the frequencies you are using it for and hopefully testing it at. However, a VSWR of 2 should be acceptable. Assuming that you followed Bob's installation instructions it should work.

Has this installation ever worked well with any antenna?
Cable problems? What cable did you use?
Connector end at the radio made up properly? Small mistakes here can make a big difference.
How did you terminate at the antenna end?

Just some thoughts.

eric.kallio said:

Mel, The reason I was checking VSWR was because when I would transmit on the ground my autopilot would kick off and in flight when I tried to engage it it would then give me an error saying that I was over the maximum airspeed and it would not reset until I removed power via the circuit breaker.

Click to expand...

Sorry. For some reason I was reading this as a receive only antenna. My bad!
Must have had my head up and locked.

A high vswr is just as bad for a receive antenna as a transmit antenna. The only real difference is the receiver will not be destroyed if the swr is high where as the transmitter could be ruined. A bad vswr is just plain bad all around.

I am the US service rep for XCOM avionics. Often I have had to troubleshoot radio installations and have generally found the Archer COM antenna in the wingtip works very poorly. Several customers of mine have changed those antennas out. I recommend a belly antenna from Don Pansier http://www.deltapopaviation.com/Home.html. Just this afternoon I tested one of his bent whip antennas on a friend's Onex with my antenna analyzer and saw a VSWR of 1.1 on 123.00. Can't get much better than that.

Test equipment

RVDan said:

How did you measure (not compute) VSWR? VSWR does actually matter as it is a measure of antenna impedance at the frequencies you are using it for and hopefully testing it at. However, a VSWR of 2 should be acceptable. Assuming that you followed Bob's installation instructions it should work.

Has this installation ever worked well with any antenna?
Cable problems? What cable did you use?
Connector end at the radio made up properly? Small mistakes here can make a big difference.
How did you terminate at the antenna end?

Click to expand...

I tested with a wattmeter. Since it only measured one direction, I had to then calculate VSWR. I have not checked this antenna on another radio, but the radio works on the other antenna. I used RG-58 cable and connectors from ACS. Cable checks out as best as can be with a multi-meter and I have made a few dozen of these connections in my days and while I do make errors, the connections seem to be good. At the antenna it is connected per the instructions with the braid to one terminal and the center wire to the other. I am just concerned that I do not have the antenna installed in an adequate plane to match the frequency band. The VSWR did not change over the different frequencies tested.

I am trying to avoid any other external antennas primarily because to place another comm antenna on the belly and I will not have adequate spacing. I want to try and make this work and just want to see how others have mounted theirs that are getting decent results.

Eric

I have an Archer clone in my -10 wingtip, fed by a G 420W com.
1. It is not as good as my SL-30/belly whip, but not bad. At 20 miles tower calls my belly whip 5x5, the Archer 5x4. I too use it mostly for ATIS. At my home field the belly whip gets readable atis at 30 miles; Archer at 25 miles.
2. The high current part of the antenna - where most of the power is radiated - is the short leg that is attached to ground, not the far end. To get more energy into vertical polarization, that's the part that you want to bend down, not the far end. Just like the bent belly whip, you want the end near ground to be vertical.
3. I riveted the ground side under the wingtip nutplates along the top edge, placed so the short arm of the radiator (referred to in#2) is located at the thickest part of the wingtip. This puts it back quite a bit from the front.
4. As close to the nutplates as possible, I bent the entire antenna down until the long arm of the radiator hit the bottom of the wingtip (a few pieces of fiberglass and epoxy holds it there). This gives me some vertical component where it counts the most.

Continued.....

Continued...

Okay, now your system.
Something is very wrong. I think you could attach a coathanger to an SL30 and pick up ATIS more than a mile out.
Have you checked your coax and connectors for continuity and no shorts?
Is your wingtip painted with metalic paint?
Does the antenna have a good ground? I rely on the wingtip attach screws making good contact with the wing skin. I was careful to not have paint on the wing skin under the screw heads.
Make sure you haven't accidently shorted out the little parallel plate capacitor where the coax connects. The nylon screw cannot be replaced with a metal one.
The SWR should change with frequency (ideally minimum around 126 MHz).
Next time, use RG400. RG58 is okay here, but why give up any performance to save $20?.
More...

More...

You asked about 'tuning'. Here's how an amateur radio guy would do it.
Make the far end adjustable, so you can slide it longer or shorter.
Make the point where the piece attached to the coax attaches to the antenna adjustable, so you can slide it toward the ground end or toward the far end. Remove the parallel plate capacitor (the aluminum-bakelite-aluminum sandwich) and replace it with a variable capacitor.
Now put an SWR meter in the line, turn on the transmitter. Adjust the far end in and out to minimize SWR. Adjust the capacitor to minimize SWR. Move the feedline attach point one way and repeat. Keep iterating until you find the configuration that minimizes the SWR. Then bolt down the adjustable pieces, and replace the variable capacitor with a fixed one of the same value.

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Here's a photo of mine.Notice how much vertical component there is in the antenna arm closest to ground.
I also made an effort to keep it away from the landing light and the light heat foil. Not sure how important that is, but that's what I did.
(At the time of the photo there were cleco's where the coax attach screws go (one metal, one nylon).

BobTurner said:

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Here's a photo of mine.Notice how much vertical component there is in the antenna arm closest to ground.
I also made an effort to keep it away from the landing light and the light heat foil. Not sure how important that is, but that's what I did.
(At the time of the photo there were cleco's where the coax attach screws go (one metal, one nylon).

Click to expand...

I'm having trouble seeing where the ground plane is grounded to the airframe. It MUST be grounded, preferably by several of the wingtip mounting screws.

Notice how the ground plane of my antenna is UNDER the nut plates:

Exactly the same, just hard to see. The ground element is bent lengthwise, the long way. The part that is under the nutplates is hard to see, but notice the nylon clamps, where they are attached, is in line with the nutplates and is part of the ground.

As I mentioned earlier, make sure the wing tip attach screws can conduct to the wing - no paint under them.
Maybe this photo is clearer:

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I mounted my Archer to the outboard wing rib, rather than to the wing tip. I think it makes it a little easier to pull the wing tip, without having to futz with disconnecting the coax. Admittedly an infrequent occurrence.

Jim Berry
RV-10