Tuning a Bob Archer antenna

I have a Val Nav2000 attached to a Bob Archer antenna and have never been able to receive VOR signals more than about 20 miles out and then only intermittently. Val says I need to tune my antenna. Not sure how to do that and my avionics guy doesn't know either. Is there a quick and dirty way, such as, shortening the long arm of the antenna a little at a time and test flying to see if reception improves? I'm pretty sure I could just replace the arm with one having the original dimensions if the experiment doesn't improve reception. John

It should have been tuned when manufactured. Did you install exactly as written in the instructions? I have one in the left wingtip on my -7 and I can receive VOR's 50+ miles away when at 5,000ft and 80+ miles away when at 10,000ft.

If only it were that easy. There are actually 3 variables: the length, the place where the feed arm attaches to the antenna, and the capacitor (the 2" long piece of bakelite). If you have a ham friend, you could borrow an SWR meter and tweak. BUT, Bob Archer did a pretty good job on these things already. 90% of the problems are installation errors. I've even seen one that had the radiating element attached to ground. Make sure the ground arm has a good short electrical connection to the end rib. Can you post a photo?

Installation

I can tell you from experience if it is not installed per the instructions and grounded to the wing properly it will not work correctly.

Installation problem is always possible, but i think i have it right. I started with an antenna in the left wing and when I couldn't make it work I added a second Bob Archer in the right wing thinking interference might be the issue. With the left wing the antenna rests on the bottom of the tip and is grounded to the plate nuts. In the right tip I used aluminum angle and attached the antenna to the rib. My nav light wires are attached to the antenna per Archer's instructions.

archer Nav antenna

I have the archer nav antenna in my wing tip. I don't have an exact distance for the vor to pull the flag, but it works at the normal distances that I have been accustomed to for certified aircraft. The ILS also works great. This is on a GNS430W.
I think I would have an avionics shop hook up a coax direct from a portable test set to your nav receiver and see where that is at. This will help define weather you have a nav receiver sensitivity problem or an antenna problem.

I drilled holes in the base of the Archer antenna to match the wing tip mounting holes. I made sure that the metal on the antenna and the wing metal were bare and alodined. I also made sure it was mounted well aft of the wing tip lights and associated wiring.

These antennas do work best if you take the time to tune them. If you are building the antenna yourself (easy enough and you can optimize the dimensions for your wingtip) then you must tune it.

SWR bridges work only if you provide RF energy (transmit). A much easier and more accurate way is to use an Antenna Analyzer like this: http://www.mfjenterprises.com/Product.php?productid=MFJ-259C

If a local ham does not have one to come over and help you, have your EAA chapter buy one. It also comes in real handy for troubleshooting antenna and feed line problems.

I built 5 or 6 wingtip VOR antennas using scrap aluminum and a piece of fiberglass for the gamma match dielectric. I always attach the antenna to a 0.063" piece of aluminum angle that gets bolted onto the outboard rib. This way the wingtip comes off without detaching the antenna.

Rough dimensions for building your own antenna can be found in the AeroElectric book. I make mine so they extend as far as possible into the wingtip. If you make the length an inch or so longer than called for you have something to trim for tuning.

Tuning:
1. Attach the analyzer to the feed point.
2. Determine the resonant frequency (the lowest SWR point).
3. Trim the end of the antenna to raise the resonant frequency to about 110mhz.
4. Adjust the gamma match (the piece with the fiberglass) to further reduce the SWR. You can see which way to go (more capacitance or less capacitance) by loosening the nylon screws that hole the aluminum/fiberglass/aluminum sandwiched together (this reduces capacitance) and see if SWR goes up or down. If it goes down, then make the piece of aluminum that has the feed line going to it slightly smaller and measure again. This is an iterative process but it only takes a few minutes.
5. After the gamma match is set, trim the overall length of the antenna to have minimum SWR at about 113mhz.

At altitude I get VOR signals at 90+miles. I pick up the nearest VOR (5 miles away) with the airplane inside a metal roof hangar.

While you have the analyzer out, connect it to your comm antennas at the radio end of the coax and verify they are working correctly.

Carl

Thanks!

Thanks for all the good information friends. I forgot to mention one occurrence which was interesting to me. There is a VOR transmitter about 7 miles away from the Baton Rouge Airport. The only time I've seen my nav receive that VOR on the ramp is when a plane passed directly overhead on final approach. It has happened a couple of times.

I'll get my avionics repairman to direct connect his test rig to my radio to see if the radio is the problem before trying to tune my antenna. John

jpowell13 said:

There is a VOR transmitter about 7 miles away from the Baton Rouge Airport. The only time I've seen my nav receive that VOR on the ramp is when a plane passed directly overhead on final approach.

Click to expand...

Not unusual - the passing airplane reflected the VOR signals.

Even though we say VHF is line of sight, RF energy can take many different routes between transmitter and receiver.

Carl