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Antenna conundrum

RVG8tor

RVG8tor

Well Known Member
I am going with two bent whip antennas near the wing spar under the foot wells like I have seen on many RV-8s. My issue is with the transponder antenna. I have not paid attention to where RV-8 drivers are putting these, so please tell me where you put yours and QC what I have pictured below. I have the GRX-327, it states to keep the transponder antenna 3 feet from other antennas and 3 feet from the transponder unit. It also lists a 6.6 foot limit on RG400 cable, there are other cables listed that I have not heard of that would allow longer runs. The location I would like to use due to the cable length limit is just forward of the rear seat on the left near the floor rib. This location meets all of the criteria but it put it 22" from the left VHF antenna. I can't remember but I thought someone has used this location with success. My alternative is to move it aft, of the baggage area about even with the bell crank, this will require me to find the fancy cable and appropriately size BNC connectors.

Please let me know what locations you have used for the transponder antenna, if you exceeded the 6.6 foot limit with RG400 cable. Thanks for the help. In the picture you can see all three doublers set in place but not drilled.

Cheers

35ibygx.jpg
 
My suggestion...

The Comm antenna locations look great. But, I would mount the transponder antenna further back in the fuselage and off to one side or the other to get it out of the flow of the exhaust. I don't know where you get the 6.6' max requirement for the Xponder RG-400 coax. My GTX-327 Installation Manual states a "recommended" 8.8' max with RG-400 coax. If you route the cable from behind the instrument panel over to the side of the fuselage and back from there you can save several feet vs. routing it forward, down and back. You could gain even more distance if you go with RG-303 or RG-394 coax, but as you said, that may require different connectors/tools. I have heard of many who have exceeded the 8.8' max cable length with no claimed adverse effects, but I cannot recommend anything that conflicts with the Garmin installation manual.

Actually, The Garmin 8.8' length is a recommendation. The actual requirement is no more than 1.5dB of cable loss. The published attenuation of RG-400 coax cable at 1090 Mhz is 15.372 dB/100ft. so you should be good with a run of just under 10 ft., but Garmin's recommendation may include connector loss.

Just my $.02...
 
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Is it me

Nearly 24 hours and only one reply and not from an RV-8 person, but Sparky thanks for the input. Why is it some subjects one gets a blast of responses and other issues like this one, its dead calm with regard to responses?

BTW Sparky, I assume I have the latest version of the GTX-327 manual which has a maximum run of 6 ft 1.3 in for RG-400, this assumes 18dB/100ft loss plus .2 dB loss for each connector. I have heard others talk about the 8.8 ft, must be from a previous version.

Back to the questions though. I know many are flying RV-8s out there and read these posts, please spill the beans on where you located your transponder antennas and any issues, complaints or things you would do differently.

Thanks
 
Mike: I'm watching with you and hoping for good input since haven't installed antennae yet. Please post your decision. Thanks. Bill
 
Hey Mike, you're not being ignored - some answers just need seasoning....:)

My transponder antenna is as far aft on the lower fuselage-to-wing skin section as I could get it and not interfere with the flap. This kept it on the fuselage, but out it as far OUT and AFT as I could get it. It's worked just great there.

My single belly Comm antenna is in the center bay of the fuselage, a little right of center. If I had it to do over again, I'd move it outboard so I could put two of them on (and forget the wingtip Comm antenna), very similar to what you have done. Some might say they are to close to the gear legs, but I suspect they'll work just fine for you. If it were me, I'd probably install them where you have them.

Paul
 
ON the belly skin extension

Paul,

I am trying to visualize where you put the transponder antenna. Are you talking just outside the fuselage side but attached to the belly skin that extends out from the fuselage? I never thought of that area, it sure would meet the distance requirements better.


Ironflight said:
Hey Mike, you're not being ignored - some answers just need seasoning....:)

My transponder antenna is as far aft on the lower fuselage-to-wing skin section as I could get it and not interfere with the flap. This kept it on the fuselage, but out it as far OUT and AFT as I could get it. It's worked just great there.

My single belly Comm antenna is in the center bay of the fuselage, a little right of center. If I had it to do over again, I'd move it outboard so I could put two of them on (and forget the wingtip Comm antenna), very similar to what you have done. Some might say they are to close to the gear legs, but I suspect they'll work just fine for you. If it were me, I'd probably install them where you have them.

Paul
Click to expand...
 
Yup - exactly what I mean Mike - the part that ends up as the lower "fairing" between the fuse and wing (and that we all spent the entire fuse build worrying about accidentally bending...).

Paul
 
Mike,

I put my transponder antenna about 10 inches behind the rear seat back, under the baggage floor. Useing RG400 cable it seems to have plenty of power to the antenna.
 
I like

I kind of like this area as well. My floor will be riveted in, so I would have to make some type of access for inspections and installations. This could be as simple as two small holes large enough to get a nut driver on the nuts, with slack the cable could be removed once the antenna drops out the bottom. I would just plug the two holes with the same low profile plastic plugs you can get from the home stores. Hmmm more to think about, I will finish up my VHFs then make a decision on the transponder.

Please if you have not disclosed where you put your transponder antenna on you -8, post it here I am sure more than just myself will benefit from the information. Silence I guess is golden I guess, maybe I should ask if those of you had trouble with your transponder, where was it located!

ronschreck said:
Mike,

I put my transponder antenna about 10 inches behind the rear seat back, under the baggage floor. Useing RG400 cable it seems to have plenty of power to the antenna.
Click to expand...
 
I put mine the same place as Smokey (though I'm not flying yet). Seems like a good place and easy to get to. Dave
 
Pretty much the same place as Mr. Schreck. Mine is about 8 inches behind the last bulkhead below the pass seat, left bay. Was going to put insp plate in baggage floor, but decided to try and test without and have no problems removing ant dropping it enough to unscrew connector, so didn't install one.
 
Which Bay

Those of you who put the transponder antenna behind the back seat under the baggage floor, is it in the same bay as the rudder cable? I was thinking the middle bay, the one with the push rod connected to the bell crank. The travel of that rod will not interfere with the antenna cable if it is off to the left or right. I could put it int he left bay under the step support, looks like it will take just over 8 feet of cable to put it back there. I have conduit so changing the cable to something that will allow a longer run will be easy if there ends up being a problem with ATC picking up the transponder. Thanks for the inputs, I guess I can always put it someplace else and patch the hole if it does not work out.

I am going to go someplace just aft of the F807 bulkhead, this meets all of the parameters for antenna placement except for the length of RG400 cable, seems like no one is having issues with it this far back.

Cheers
 
RVG8tor said:
Those of you who put the transponder antenna behind the back seat under the baggage floor, is it in the same bay as the rudder cable?

Cheers
Click to expand...

Yes. It is at the very front of that bay. No way can it interfere with the rudder cable.
 
bump for recent input

I am bumping this thread to see if there is any recent information before I start cutting holes in my fuselage.

I am considering the positions in the pic of the OP. Two bent whips, one on each side of the fuselage, just forward of the spar. Transponder antenna further aft, perhaps in the position recommended by Paul Dye. Archer nav antenna in the wingtip. That leaves the marker beacon antenna that I have not yet thought through.

I am building an 8A.

Do any of you who are flying have any reason not to go with the above positions?

Thanks
 
We also mounted the transponder antenna (shark fin) under the baggage compartment floor. I used RG-400 coax and a low-profile 90-degree adapter at the antenna end to help keep the coax and connector well clear of any moving parts back there.

The com antenna is a bent whip, also mounted on the belly, just behind the trailing edge of the wings and about 18-20 inches forward of the transponder shark fin.

You can see them both in this pic, taken when I was departing from Osh. The thought of mounting both these antennas so far aft, was to help keep their propagation patterns from getting "shrouded" by the wings so much. They seem to work great and have a good omnidirectional propagation to them. On the way home from Oshkosh, I was able to keep talking to my buddies as they approached the DFW metroplex while I was just crossing the Kansas/OK border just north of Enid, OK.
http://www.airport-data.com/aircraft/photo/658556L.html

We've also got a Bob Archer VOR antenna in the left wingtip, and a homemade glideslope antenna made from a thin aluminum strip in the right wingtip.

We don't need no steekin' marker beacon :D
 
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Neal;
Could you give some detail on the glideslope ant?
I've go the Bob Archer Nav Ant going in the left wing, and a wireway to the right wing, so I think you have a good set up that I would like to copy.
I have an ADF Ant in the aft fuse, behind the elevator bellcrank, so I'm thinking the Com Ant can match yours, but the Transponder will go under the rear footwell.
 
In my RV8A I have the GTX-327 txpndr antenna in the same place as Paul Dye and I have yet to have anyone complain. Access is also easy as it is just under the ring root fairing. This seemed the simplest solution given the limitations.

My single comm is just left outboard of the center section inside of the left rear foot well. Again, no complaints and I pick up signals at long range as well.
 
Having done a fair amount of transponder testing I have found antenna location is more important than cable length. I have tested the 327 with probably a 15 ft RG400 run and power was still over 200W. If you shadow the antenna with gear legs/exhaust/other antennas/etc. that always causes problems for me when groung testing and sometimes makes it hard to get a transponder to pass the test (both MTL and power).
 
SHIPCHIEF said:
Neal;
Could you give some detail on the glideslope ant?
Click to expand...

I ripped off the idea from Paul Dye, who used a piece of coax with 16" of shield stripped away as a glideslope antenna.

Since glideslope is a horizontal polarized transmission, and 16" is the length that Paul's homemade antenna was working well with (half-wavelength?), I simply glued a 16" strip of 0.020 aluminum (about 3/8 inch wide) to the bottom inside of the right wingtip, extending outward as straight as I could, but angled rearward somewhat due to the available size of the "floor" of the wingtip. At the inboard end of the aluminum strip, I drilled a hole and connected the center lead of the coax with a crimped-on ring terminal and a 4-40 screw and nut, and grounded the shield of the coax to the outboard end of the wing itself. In Paul's post with his homemade coax GS antenna he said he was able to pick up the glideslope as far out as 20 miles.... we're only getting about 10 miles, but Paul and others who've made homemade GS antennas like this are not grounding the coax shield at the antenna end, only at the radio end. Someday I plan on disconnecting the shield ground out on the wingtip end and see if that improves reception.
 
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archer nav antenna functions

Just when I thought that I had my antenna plan pretty much under control, new information arises.

One issue is the archer nav antenna that I will have in one wingtip. I thought that antenna recieved VOR, Localizer and Glideslope information. Then I read Neals post regarding the Paul Dye improvised glideslope antenna. I know that Paul was using an archer nav antenna.

So, does the archer nav antenna provide glideslope information as well as VOR and localizer?

If I use a separate glideslope antenna, where do I connect it to my SL30??
 
tonyjohnson said:
So, does the archer nav antenna provide glideslope information as well as VOR and localizer?

If I use a separate glideslope antenna, where do I connect it to my SL30??
Click to expand...

The SL30 just has one single nav antenna jack, and has an internal splitter circuit that separates the VOR/LOC signal from the GS signal.

If you have a nav/com that has separate antenna jacks for VOR/LOC and GS (like the GNS430/530 series) then you can use an outboard antenna splitter to separate the signals... or a use a separate dedicated GS antenna. That's what I did.
 
glideslope

The only nav/com I have is the SL30. If the archer provides VOR/LOC and glideslope, then it seems that I am alright.

If the archer does not provide glideslope info what are my options?

Edit: a little searching turned up what seems to be the answer. I now understand what Neal was saying. Paul has the VOR/LOC information from the archer go to his 430 and the glideslope go to his SL30. It appears that the SL30, as Neal points out, can split the signals accepted from the single input.

So, the bottom line seems to be that I can attach the archer nav antenna to my SL30 antenna input and have VOR/LOC/and GS.

excellent
 
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Correct. The Archer nav antenna (or any other nav antenna such as the good old fashioned v-shaped catwhiskers common on spamcans) simply picks up whatever RF is in the air that is reasonably within the range of its tuning or harmonics thereof. The nav and gs receiver circuitry in the SL30 splits up the various signals to process them for the appropriate displays. In my case, we've only got a a single 430, which has a separate gs antenna jack from the vor/loc antenna jack, and a good external nav/gs splitter (diplexer) costs about a hundred bucks. The homemade gs antenna cost me virtually nothing :D and if it would've been a complete flop, I'd have shelled out the money for a splitter, but it seems to be working alright.
 
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Hi,
I read with great interest all of your threads and I try to find the best...My last opinion is I will install the xponder antenna as Paul said, far aft and out of the fuselage to wing section. But concerning the VHF antenna I would install It out side the fuselage at the opposite to be sure There is at least 36 " between the 2 antennas. Am I wrong doing this?
Thanks
 
trackdom said:
Hi,
I read with great interest all of your threads and I try to find the best...My last opinion is I will install the xponder antenna as Paul said, far aft and out of the fuselage to wing section. But concerning the VHF antenna I would install It out side the fuselage at the opposite to be sure There is at least 36 " between the 2 antennas. Am I wrong doing this?
Thanks
Click to expand...

Dom,

You are not wrong on the transponder antenna. That will work just fine.

However, how many VHF comm antennas will you mount on the belly? If just one, then somewhere in the middle under the pilot's seat is a good location.

If you are mounting two antennas, you need to be concerned about their spacing. At 120MHZ, a wavelength is 98.425 inches. A quarter wavelength is 24.60 inches. A radiated signal from antenna 1 gets picked up by antenna 2 and re-radiated. If they are a quarter wavelength apart then the re-radiated signal from ant 2 arrives back at antenna 1 with a phase difference of exactly 180 degrees (1/2 wavelength). This creates a null in the direction of a line drawn between the two antennas.

Any two antennas in the same frequency range will exhibit this behavior. The frequency, spacing, and position will determine the effect of any nulls that may occur. Of course you must consider the entire frequency range in which you operate. We have limited options to where we put our antennas, so sometimes we just have to live with what we have.

I hope this helps, and please excuse the inches vs meters. The principles are the same.

Bon Chance,
Don
 
Hi Don,
I am sure I will install just one vhf antenna , so outside the fuselage seems good, as it is more than 36 " away from transponder antenna ..if I install the com antenna in the fuselage I can't be far enough, is it a problem? I will take care to find a place where it doesn't disturb the wing fixation or aileron pushrod ...at the front of the wing Spar There is room enough I suppose...
Ps: I am ok with inches and feet etc...as I build a RV8 There is no other way, and in fact it's not at all a problem...an american Man was able to conceive a beautifull aircraft family, so it is a minimum to me to speak english and use english mesurments....
 
Placing the comm antenna just left or right of center, aft of the wing spar, and having the transponder antenna on the belly left of (or right or under )the rear seat will cause no problems. That is where mine are located with no known issues.

The frequencies are well separated, as are the physical distances.

Don
 
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