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Building a better buggy whip...

bjdecker

bjdecker

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...also known as the Archer Sportcraft antenna

I posted something along these lines before. A few days ago a friend of mine wanted to modify his SA-001 to get a bit better glide slope reception. Borrowing from @ewumnrr Martin's good work, I set about making another antenna.

Basically the same dimensions as the SA-01, .025" 2024-T3 aluminum - major dimensions are 1 1/2" wide.

The "base" leg was extended to +20" and a 3/4 flange added to facilitate mounting to the aft wing rib ( W-712-L/R (RV-7), W-1012-L/R (RV-10, RV-14), W-812, W-912, etc.)
The "front" leg was modified to include a 3/16" flange
The "top" leg was extended to +19" and will be trimmed further to get the VSWR < 1.5:1 if needed.
The tuned element / gamma match section measures 5 1/2" inside, all angles are @60° forming an equilateral triangle. The width of the tuned element is 3/4". Lacking fiberglass "pc board" material (FR4) of .063" thickness, I substituted a piece of similar thickness Lexan and ran it the full length of the tuned element.
The tuned element measures about 4" and is held in place with 3 nylon screws/nuts. It includes the glide slope element of 9" long.

In order to tune the antenna, I used a MFJ-259D and NanoVNA-F V3.

I mounted the antenna to a large metal plate to serve as the surrogate for the wing rib -- this is a part of the antenna system, similar to a counter poise / ground plane / reflector. It's size/shape/position relative to the tuned element will affect antenna performance ( impedance, VSWR, etc.).

View attachment 119468

I added a little material to the "top" leg to get the VSWR a little better -- I suspect this will change when mounted to the airframe as this leg extends past the test counterpoise.

View attachment 119469

Long cable to simulate what will be installed in the RV-10.

I trimmed the glide slope stub until I got down to a reasonable VSWR.

IMG_0695.jpeg

and a workable impedance:

IMG_0697.jpeg

and the obligatory Smith chart -- I can't read it; but it's pretty cool to see it on the screen:

IMG_0698.jpeg

Yeah, I know - white sneakers -- I won't be wearing them after Labor day :)
 
bjdecker said:
...also known as the Archer Sportcraft antenna

I posted something along these lines before. A few days ago a friend of mine wanted to modify his SA-001 to get a bit better glide slope reception. Borrowing from @ewumnrr Martin's good work, I set about making another antenna.

Basically the same dimensions as the SA-01, .025" 2024-T3 aluminum - major dimensions are 1 1/2" wide.

The "base" leg was extended to +20" and a 3/4 flange added to facilitate mounting to the aft wing rib ( W-712-L/R (RV-7), W-1012-L/R (RV-10, RV-14), W-812, W-912, etc.)
The "front" leg was modified to include a 3/16" flange
The "top" leg was extended to +19" and will be trimmed further to get the VSWR < 1.5:1 if needed.
The tuned element / gamma match section measures 5 1/2" inside, all angles are @60° forming an equilateral triangle. The width of the tuned element is 3/4". Lacking fiberglass "pc board" material (FR4) of .063" thickness, I substituted a piece of similar thickness Lexan and ran it the full length of the tuned element.
The tuned element measures about 4" and is held in place with 3 nylon screws/nuts. It includes the glide slope element of 9" long.

In order to tune the antenna, I used a MFJ-259D and NanoVNA-F V3.

I mounted the antenna to a large metal plate to serve as the surrogate for the wing rib -- this is a part of the antenna system, similar to a counter poise / ground plane / reflector. It's size/shape/position relative to the tuned element will affect antenna performance ( impedance, VSWR, etc.).

View attachment 119468

I added a little material to the "top" leg to get the VSWR a little better -- I suspect this will change when mounted to the airframe as this leg extends past the test counterpoise.

View attachment 119469

Long cable to simulate what will be installed in the RV-10.

I trimmed the glide slope stub until I got down to a reasonable VSWR.

View attachment 119470

and a workable impedance:

View attachment 119471

and the obligatory Smith chart -- I can't read it; but it's pretty cool to see it on the screen:

View attachment 119472

Yeah, I know - white sneakers -- I won't be wearing them after Labor day :)
Click to expand...
If you are experimenting with antennea on your plane have you given a slot antenna some thought? No added weight or drag.
 
Norman CYYJ said:
If you are experimenting with antennea on your plane have you given a slot antenna some thought? No added weight or drag.
Click to expand...
Time to go a Googling :)

Edit: Not practical at the frequencies we operate -- e.g. for LOC/VOR, the slot would need to be >92" long :(
 
Last edited:
Sorry, not sure why, but all I am seeing are pictures of your nice white tennis shoes below the digital displays showing the various readouts of its performance.
 
Nice work. Glad the GS antenna add-on I designed found another user!
In general, the archer antenna is very sensitive to the distance between the arm(s) and the ground plane. It is designed to overlap exactly 1/2 with the wing structure. If that is not honored, the length of the arms may be shortened / lengthened to regain perfect resonance at the center frequency.
One thing to keep in mind when making the perfect NAV/GS antenna: the better is is, the more energy will find its way from the transmitting COM antenna to the NAV radio's front end with the risk of de-sensing the NAV pre-amplifier (signs: your autopilot will disengage on ILS approaches if you key the mic). The lowest COM frequency is right adjacent to the highest NAV frequency, therefore the NAV radio has a hard time filtering this out. If you run your VNA in S21 mode and use the COM antenna as feed and the NAV antenna as input, you can measure the isolation (airplane must not be in or close to hanger). Depending on your COM antenna location, the separation can be quite low (maybe just 30 dB). The FAA uses 53 dB isolation as requirement for airliners. Therefore, with the perfect NAV antenna, you need to do everything to reduce the cross feed from the COM cables to the NAV cables. Use RG400. Don't run the wires closely in parallel. Put a ferrite choke on the coax cable at the antenna base (COM antennas have poor SWR, thus the cable will transmit if not 100% grounded with single digit milliohms at the antenna base).
 
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