Logic behind shape of Archer antenna?

Hi all-
I am ready to install my VOR antenna and am planning to build the DIY Archer antenna shown here:
http://www.vansairforce.net/articles/vorwingtipantenna/wingtip_vor_antenna.htm
(Note that a much clearer drawing can be found in the Aeroelectric Connection page 13-16)
My understanding is that VOR antennas are half wave devices. At a mid-range of 113MHz, wavelength is 2.65m, so each quarter wave leg of the antenna would be .66m or 26". The upper skin of an RV4 fiberglass wingtip is slightly longer than 52", so is there any reason why one couldn't simply lay two 26"strips of aluminum in a straight line in the upper wingtip? My understanding is probably overly simplistic, but I can't find an answer to this in either The Connection or perusing VAF threads. Knuckolls mentions that folding back the antenna Archer-style has a "marked capacity effect", but I'm not sure I understand what he means. Any thoughts would be greatly appreciated...
Eric

There is magic in Mr. Archer's antennas - they are much more than just simples trips of metal.....

I originally built the copy of his, and had quite mediocre results. I had the opportunity to change it out for one that HE had made, and it performed much better. The positioning of other wires within the wingtip is critical to performance - stuff that you would never think has an infulence...does!

(I am not an RF engineer - I just listen to guys that are, and Mr. Archer is!)

1. Your proposal, which in radio speak is to put in a half wave dipole running front to back in the wing tip: (1) the metal wing and the metal end rib would have a significant but hard to predict effect, and (2) even if the wing wasn't there, this antenna would have maximum sensitivity at the 3 and 9 o'clock directioms, and none in the forward and aft directions - exactly the opposite of what you usually want, especially for a localizer!

2. So if the metal end rib was infinitely large up, down, front, back, you can make a quarter wave antenna sticking out perpendicular to the infinite ground formed by the infinite end rib. The ground plane magically mirrors the antenna, so it takes the place in a way of the other missing quarter wave to form a half wave dipole, except that nothing is really picked up from the side away from the antenna. Most of the current flows in the part closest to the rib, so it isn't a big deal if you bend the end, as is commonly done on bent whips under the plane. This bent piece does have some capacitance with respect to the ground plane, so it makes the antenna look a bit longer than it is. Finally, you need to find a feedpoint that matches 50 ohm coax and where the current and voltage are in phase. That little arm taps into the 50 ohm point, and the parallel plate capacitor that's formed by the two pieces of aluminum sandwiching a piece of bakelight (the material and dimensions here are important) provide the phase match. Given how far from infinite the end rib is it's amazing it works as well as it does, also that there is considerable sensitivity looking in the direction of the wing.

Thanks

Paul and Bob-
Thanks for your replies. Wow Bob, that was very enlightening! I appreciate you taking the time to explain. I will stick with the tried-and-true Archer.
Eric

Homemade " Bob Archer" style antenna

Here's my experience, FWIW. I built a homemade Archer style comm antenna about three years ago and mounted it inside the right wingtip of my RV-4. It replaced a homemade 1/4 wave ground plane whip antenna mounted horizontally in the same wing tip, which came with the airplane and didn't work well at all. This Archer style antenna works much better than the previous antenna, but probably not as well as an externally mounted antenna. But, it suits me fine for talking with the tower and ground control, listening to ATIS, and talking with the occasional FSS. I've had problems with flight following in the Seattle metro area because the transmitter site orientation to the airplane changes as I fly along, and I can fly into an antenna null and lose a strong signal. Usually, I solve this by disabling the auto squelch so I can receive just fine, but Approach will sometimes complain that I am "weak and unreadable", although they will usually continue to talk with me. (They can lose interest in me if they are busy.) So for IFR it wouldn't be good enough. But for fair weather sport flying it is just fine. I like the idea that I am going faster, maybe two whole knots more, over an external antenna.

This antenna works best, and quite well, in the three to six o'clock orientation to the aircraft, pretty good 12 to three and six to ten o'clock, and poorly at eleven to twelve o'clock, where it appears to have a null. I've thought about remounting the antenna backwards in the wingtip to move the null behind me (don't know if it would actually do that) or making a second antenna for the left wing tip and using an antenna switch to choose the better signal. But I haven't carried the experiment any further. I'm pretty happy with it as it is for the type of flying I do.

Mine isn't a genuine Bob Archer antenna. I built this antenna myself after seeing a picture of the Archer. It wasn't hard to do but I am a Ham and have some test equipment and a rudimentary understanding of what is going on.

One way to get an idea of the antenna's signal strength pattern is to tune in an ATIS maybe twenty or thirty miles away, just far enough so that the signal is pretty weak. Then fly slow circles with a shallow bank angle and note where the signal disappears. This can identify the nulls. Creative use of the auto-squelch can help, too.

Half wave dipole wingtip antenna

Stewie: Re-read your post and realized I hadn't addressed your question about mounting a half-wave dipole antenna in the wingtip. First, I have zero practical experience with that problem, but my Ham gut tells me it wouldn't work well for a variety of reasons. If you want to home brew your own wingtip antenna I'd stick with the Archer design. You could look for a local Ham to help you.

BTW, strictly speaking, I'm not entirely sure if it is legal for us amateurs to home brew our own transmitting antennas. Most gear outside of the Ham bands has to be type accepted by the FCC. I could be wrong.

Archer type antennas won't work as well for com as they do for VOR because the VOR signals are horizontally polarized, same as the antenna, while com ground stations are vertically polarized.
I also built an Archer clone for com. I have an RV10 with fairly thick wing tips. I mounted the ground side to the top of the rib, and then ran the radiator vertically down as much as the wing tip allows, fiberglassing the rest of the antenna to the bottom of the wing tip. Unscientific tests (different radios, my ears) show my external bent whip under the plane works better, but not hugely, maybe 6 dB. e.g., with squelch disabled, if I can make out the ATIS at 20 miles on the external antenna, I get the same level of clarity at 15 miles on the wingtip antenna. I haven't done a lot of directional tests except to note there is a null at 9 o'clock (antenna is in the 3 o'clock wingtip). No surprise.
Once in a while ground has trouble with the antenna under the plane. In those cases the wingtip works better.

PS. I don't think the FCC cares. Certainly not for VOR which is reception only. Their concern is spurious emissions, which have to do with the transmitter (or corroded connections). A poor antenna design just means a weak signal.
Note to others thinking about the com. If your antenna is bad enough it is possible to damage your transmitter, so proceed with caution.

Transmitters today have a "fold-back" circuit to protect the final drive transistors from disconnected or grounded antennas, so experimenting doesn't pose much risk to the gear.

detailed construction diagram of wingtip VOR antenna

I recently built a wingtip VOR antenna using updated design documentation from that noted in this thread. The results have thus far been more than acceptable. Initial testing was just a handheld and no real ground plane. The antenna was elevated in the backseat of the RV-8. Reception at 3,500' was usable at 40-50nm.

I posted some additional detail and links on my blog.

I haven't really researched the Archer design itself, but it reminds me of a modified J-pole antenna, except horizontally polarized.

I know its a great design from the fact that a lot of folks that have them swear by them, and how well they perform. I have one I picked up from a seller on here, that I plan to install in my wingtips. I think that anyone could mimic this design, but you'd have to use an existing Archer to get it all correct. It is not a complex antenna, but it is very precisely made.

CubedRoot said:

it reminds me of a modified J-pole antenna, except horizontally polarized.

Click to expand...

Interesting observation. A J-Pole at the right frequencies would need to be more than 6' long.

I'm not an RF engineer, so I have no idea how Bob Archer created a well tuned performing antenna which is small enough to fit in an airplane wingtip.

The updated design documentation should increase the success factor of anyone who decides to go the DIY route.

humptybump said:

Interesting observation. A J-Pole at the right frequencies would need to be more than 6' long.

I'm not an RF engineer, so I have no idea how Bob Archer created a well tuned performing antenna which is small enough to fit in an airplane wingtip.

The updated design documentation should increase the success factor of anyone who decides to go the DIY route.

Click to expand...

In its simplest form it's a quarter-wave (around 27") antenna over a ground plane. But it's bent (both to fit in the wingtip, and also to reduce directionality) and is fed with what's sometimes called a delta (or half of a T) match. It's important to get that parallel plate capacitor right-use the specified material, and copy length, width, and spacing exactly. If you have a ham friend with an SWR meter you can use that to tweak to get the best possible match.

BobTurner said:

It's important to get that parallel plate capacitor right. Use the specified material, and copy length, width, and spacing exactly.

Click to expand...

Thankfully I found all the necessary materials, tools, and instrumentation in my shop. (amazing what 30 years of hording will get you.)

J-pole

... is an end- (voltage)-fed half-wave antenna fed with a quarter wave stub (for 3/4 wave overall length) and a gamma match (I think that's the correct term). It requires no ground plane whatsoever.

The Archer is a bent, current-fed, grounded quarter-wave with a shunt match and requires coupling to an effective ground plane. Its overall size is less than 1/4 wavelength due to the folded design. Much better suited to proper polarization in the confines of a wingtip.

My $.02, and a bargain at half the price.

Bill Boyd said:

The Archer is a bent, current-fed, grounded quarter-wave with a shunt match and requires coupling to an effective ground plane. Its overall size is less than 1/4 wavelength due to the folded design. Much better suited to proper polarization in the confines of a wingtip.

Click to expand...

This seems like a good time to quote Paul, "I'm not an RF engineer, I just listen to guys who are". (Smart armature radio guys count.)

-10 canopy top

I'm convinced I can fit a resonant copper foil Vee dipole inside the canopy top using it as a fiberglass radome. Just need to work out the feed/match details. For VOR reception I'm guessing a tiny balun on a suitable toroid would work - and could probably be skipped altogether with no noticeable effect, as it's receive-only (so VSWR-be-danged.)

I also want to hide a vertical copper foil quarter-wave comm antenna in the back of the canopy top over the baggage area (C-pillar, if you will). Zero drag, unknown effectiveness, close proximity to rear seat pax. In my daydream, it would complement the under-wing bent comm as a second comm antenna, and rarely see use.

Mounting the GPS pucks inside the overhead console, I hope to have only the ELT and Comm 1 antennas as pokey-outside types

A lot of older Cessnas used a half-wave dipole near the top of the windscreen for glide slope antennas, and they worked okay. But the GS transmitter is always in front of you. For VOR use, the sensitivity off the ends is poor (theoretically zero).