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Which antenna to use?

XLS+PAT

Active Member
I've got a Garmin GTN 750 installed in my RV7. We never put in an antenna for the VOR and ILS feature of the 750. Since we are doing a condition check I figured this would be a good time to do it.
I'm unsure about the capabilities of the "Archer antenna" is it VOR only or can I use it to operate the ILS feature of the GTN750 with a splitter?

Any thoughts on which way to go?

Thanks

RV7
KHAF
 
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For the 750 (or 650), you just connect the VOR antenna to the tray. The 750 does the needed split internally.

The wingtip antenna will do everything you need.

Carl
 
+1 On the Archer wingtip.

One small thing to note however, If you fly an ILS/LOC approach using the GTN -> autopilot, your aircraft will be offset left or right of the radio course centerline, depending upon which side the antenna is located

...It's that good...
 
+1 On the Archer wingtip.

One small thing to note however, If you fly an ILS/LOC approach using the GTN -> autopilot, your aircraft will be offset left or right of the radio course centerline, depending upon which side the antenna is located

...It's that good...

You don’t need an autopilot to demonstrate this....if you’re that good(!).
 
I'm unsure about the capabilities of the "Archer antenna" is it VOR only or can I use it to operate the ILS feature of the GTN750 with a splitter?
It "works" and is a huge compromise. I am not sure of the legality or safety of using a wing tip internal antenna for VOR {with marginal performance) with a splitter for GS.... If you are serious about shooting VOR/LOC/GS/ILS approaches by all means get cat whiskers (dipole). Mount it on belly way back on the bottom of fuselage on tail dragger, or top of vertical stabilizer if "A" model. That is my opinion. The drag is small (fraction of a knot loss at cruise speed) but the RF or navigation performance will be way better.
 
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It "works" and is a huge compromise. If you are serious about shooting VOR/LOC/GS/ILS approaches by all means get cat whiskers (dipole). Mount it on belly way back on the bottom of fuselage if tail dragger, or top of vertical stabilizer if "A" model. That is my opinion. The drag is small and the RF performance will be way better.

Exactly not my experience. My wingtip VOR antenna works at 100+nmi for VORs, and ILS/GS far beyond practical range. The kicker is it cost me ~$2 of hardware beyond some left over aluminum strips.

And - you don’t have “eyeball pickers” - as is present when mounting whiskers on the bottom of the tail (A models) or on top of the VS (tail draggers).

Put it in the wingtip and never look back.

Carl
 
Carl, I thought mine were working fine also until I occasionally lost the GS signal when i banked. Happened with both radios and both wingtip antennas. The external dipole is the way to go if you are serious about IFR
 
Again, not my experience.

A properly installed and tuned wingtip VOR antenna will do everything needed in any IFR environment.

Carl
 
Carl, apparently I was getting some blockage from the aircraft structure which was causing the gs to fail in a turn. It did not happen all the time, but enough to convince me to abandon the wingtip antenna. BTW, the vor reception was great as was the localizer. But as you probably know, the GS freq being much higher and in the approximately 330 MHz range, does not result in as good a match as the approximate 110 MHz center freq of the nav side.

Just another data point......
 
I'm an antenna guy. The archer antenna is an elegant solution for the VOR/Localizer band, designed for and delivering a good impedance match in the 108-118 Mhz area.

The "cat's whisker" antenna is a pretty standard dipole design, which works great at the primary 108-118 frequency, but also, due to its special nature, also resonates and provides a good impedance match at triple that frequency - the GS band. That's why it has been the standard antenna for many years.

I've not seen any statement, measurement, or analysis of the performance of the archer antenna at the 330 MHz frequency range. It could be acceptable or pretty awful. Anecdotal performance reports are all we have.

Of course, one could always build a 1/3 scale archer for the GS and mount it in the other wingtip ...

Also, one can use either a (frequency selective) diplexer or a plain splitter to divide the cat's whisker's output to separate vor/gs receivers. The receivers do not need any further help with selectivity.
 
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I'm an antenna guy...

Cool! Very glad to see you on the forums!

The Archer antenna, and similar design from the Aerolectric Connection, is a Gamma-match Yagi (the driven element is DC isolated from array, but couples to elements in the array.)

Anecdotally, it works really well for VOR/LOC Frequencies & GS Frequencies. However, others have had bad/negative experiences that are no less real...

Perhaps my mounting strategy changed the aperture so GS frequencies captured/resonated better, I dunno...

I built mine per figure 13-12 from Aerolectric Connection and tuned it with some excellent support from Carl F. using the MFJ-259D.

I would LOVE to see an analysis of the thing given the dimensions in the attached drawing.

Cheers!

B
 

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For hard core IMC- rain, light snow (yes, NOT a FIKI plane type), etc- is there any p-static effect on an Archer in a wingtip vs. whiskers or towel bar or blades on the tail?
 
It "works" and is a huge compromise. I am not sure of the legality or safety of using a wing tip internal antenna for VOR {with marginal performance) with a splitter for GS.... If you are serious about shooting VOR/LOC/GS/ILS approaches by all means get cat whiskers (dipole). Mount it on belly way back on the bottom of fuselage on tail dragger, or top of vertical stabilizer if "A" model. That is my opinion. The drag is small (fraction of a knot loss at cruise speed) but the RF or navigation performance will be way better.

Not the experience I’ve had. I use my archer antenna and shoot ILS down to minimums with no issues.
 
Disclaimer: I'm in the "happy user" camp. VOR reception works out as far as I've ever tried (SL-30), and I've never had the GS or localized flag. (nor, for the other poster, have I ever encountered P-static issues in rain. Can't say for snow).

Carl: Since you have the test equipment, have you ever looked at 330 MHz, to see how well (or not well) the impedance/phase match is, at 330 MHz?

General: I have seen some photos of unbelievably bad installations. Builders who moved wires that were nowhere near the antenna to go over it, because they mis-understood the instructions. Builder who installed the ground leg 8" out from the end rib, and then ran a few wires between it and the rib. And finally, the builder who reported "disappointing results"; his photo showed the ground leg floating in the wing tip, with the radiating leg attached to the end rib!
 
Again, not my experience.

A properly installed and tuned wingtip VOR antenna will do everything needed in any IFR environment.

Carl

Mine has worked well for 23+ years and over 3,400+ flying hours. Never had any issues but then again I knew the designer and have visited with him many times when I lived near him.

When flying with other airplanes, I would typically pick up VORs at longer ranges than the people flying with "cat whiskers" antennae.

Never had any GS issues flying IFR and using an ILS when I lived in SoCal. Have flown more GPS approaches than ILS but the SL-30 and Archer wingtip nav antenna worked perfect with the on ILS.
 
The drawing provided is slightly different than the archer I've seen, but still electrically similar. I'll try to build a model in EZnec and check performance at the GS frequencies.

Regarding precipitation static, most antennas and or radios keep the element at dc ground (to bleed off charge). This is about all you can do, short of adding static wicks ...
 
Exactly not my experience. My wingtip VOR antenna works at 100+nmi for VORs, and ILS/GS far beyond practical range. The kicker is it cost me ~$2 of hardware beyond some left over aluminum strips.

And - you don’t have “eyeball pickers” - as is present when mounting whiskers on the bottom of the tail (A models) or on top of the VS (tail draggers).

Put it in the wingtip and never look back.

Carl

+1

I have archers on both planes and fly IFR in both. I also get VORs out to 80+ miles and have never not received a localizer or GS signal no matter how far out I was vectored onto the approach course. My 10 also uses a diplexer for the GS. I do not consider this to be a compromised installation. It is a well engineered antenna.

Larry
 
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Just to close the loop on a few details.

For the wingtip antenna you have two choices, buy the Archer product or build. If you build, you must have a way to tune the antenna, such as this: https://www.gigaparts.com/mfj-259d-...CYWXbLeLy0HnL7BaDog5gy3BBNft1ecRoCGk4QAvD_BwE

I suggest however even if you buy, you should use this or similar antenna analyzer to verify operation - of all your antennas. This is a perfect shared tool for builders or an EAA chapter.

If you build, use the nice diagram on page 13-16 of the AeroElectric publication as a model. From that however you can achieve better performance by optimizing all the space you have in the wingtip. This translates to holding the “triangle” constant, extending the arm from the end rip do that the antenna reaches all the way into the wingtip, then shortening the trailing arm inside the wingtip about the same amount as you extended it from the rib. Make this last piece a couple of inches long so you have material to trim as you tune it. Same for the gama match - make it a little bigger to allow for tuning. When you tune, you trim the end of the trailing piece to achieve the desired resonant frequency. After that you trim the gama match to achieve the best impedance match. This is an iterative process that sounds harder than it is.

Side notes
- As with any bent whip type antenna the majority of the “work” is done by the first third or so (the high current part). This is a driving reason to extend this part as far into the wingtip as possible. The rest is there to bring the antenna into resonance.
- LOC and GS signals are relatively strong. If you tune the antenna for VOR frequencies LOC and GS reception range will be way beyond practical use.
- While if I could put the airplane on an instrumented test stand I suspect I could measure aspect dependency on signal strength (the same for any antenna). I suggest that my extending the antenna way out into the wingtip aids in my not experiencing any aspect “shadowing”.

Attached photo shows how I mount my antennas:
5-B5-A30-B5-B9-C1-4-A3-A-9280-B16-E2183-C321.jpg


Note three things:
- I do not run the Nav/Strobe wire along the antenna arm as I’ve seen on some Archer installs. I can only assume the Archer instructions have this for those installs that “must” do so.
- I move the antenna back a bit - keeps NAV/Strobe RFI down.
- I mount the antenna on the rib, not the wingtip. The wingtip just slides over it. Not shown in the photo is a piece of 1/4” split plastic wire conduit (Harbor Freight) along the outside edge of the antenna. The outside edge nestles into the wingtip - this little piece of plastic keeps it from moving.

Carl
 
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I have created a model of the Archer antenna using EZnec - a version of the standard _Numerical Electromagnetics Code_
https://en.wikipedia.org/wiki/Numerical_Electromagnetics_Code

I have analyzed both the impedance and the azimuthal pattern at both the VOR/LOC frequencies and GS frequencies. The impedance data is usually pretty accurate compared to real-world measurements, the pattern data somewhat less so.

My conclusions:

The antenna performs well at VOR/LOC frequencies 108-118 MHz. VSWR is less than 2:1 with the gamma match capacitor. Feedline loss is held to less than 1 dB for 15 feet. The pattern is good, giving broad frontal coverage.

At Glideslope frequencies, it's a mixed bag. The antenna presents a poor impedance match, with VSWR at 13.5:1. This introduces an additional 6 dB of loss. Also, due to the impedance mismatch the feedline loss rises from a normal 1 dB to more than 3 dB, putting the total additional losses at 8-9 dB.

The azimuthal pattern, while not ideal, is not too bad.

So the overall performance of the antenna at GS frequencies is somewhere between acceptable and pretty awful. This seems consistent with anecdotal reports.

Four images are included - 2 respective smith charts showing impedance data at VOR/LOC and at Glideslope frequencies. Purple is the raw impedance at the feed point, while blue is the actual impedance presented to the feedline (with the gamma capacitor)

2 model pictorials are shown with azimuthal patterns overlaid. The small circle depicts the feed point. The underlying structure forms the ground plane similar to the end of the wing.

It should be noted that 8-9 dB additional loss may produce acceptable glide slope signal-to-noise ratios in modern receivers.
 

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Mine has worked well for 23+ years and over 3,400+ flying hours. Never had any issues but then again I knew the designer and have visited with him many times when I lived near him.

When flying with other airplanes, I would typically pick up VORs at longer ranges than the people flying with "cat whiskers" antennae.

Never had any GS issues flying IFR and using an ILS when I lived in SoCal. Have flown more GPS approaches than ILS but the SL-30 and Archer wingtip nav antenna worked perfect with the on ILS.
So you are saying you are biased. Ha ha.

So you say your antenna performs better than other VOR antennas of buddies planes you flew besides? With out details that anecdotal evidence is suspect and possible (likely) the other plane has a very poorly matched antenna. Builders don't understand that some VOR dipole antennas are balanced and coaxial is unbalanced, requiring an external BALAN (*BALAN is like a matching transformer and means BALanced to UNbalanced and is often accomplished with a coaxial loop.) Some VOR/LOC/GS antennas have an internal BALUN and work on VOR/LOC and GS Freqs. This is a typical VOR/LOC/GS antenna https://www.rami.com/product/av-532l/

Range of proper VOR dipole out in the open unobstructed by most of the aircraft properly connected (BALUN) is going to have a more omni directional pattern and have more sensitivity (better range) than a wing tip antenna.

A wing tip NAV antenna in aluminum structure is not going to have the omni directional capability of VOR antenna out in the open unobstructed by most of the aircraft. I don't think the designer of this antenna makes any claim of out performing an external dipole.

Archer antennas "work" for basic NAV function to get 0.2 mph increase in speed. It is up to the builder. Not a big deal, however I put my VOR on the tail bottom of fuselage. When I wanted to go racing I took it off. Two screws. Secured coaxial and put speed tape over the hole.

The Archer Com antenna, I would not recommend. It is an even bigger compromise in my opinion. However again up to builder.
 
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