J

jeff beckley

Well Known Member
Installed new 430W but used old GA-56 antenna. Was told that it would work just is not certified. Have 5 feet RG-400 cable to the antenna. Antenna is mounted under the cowl on a plate mounted to the firewall on a RV7A.
Every LPV approach I have tried to do ends up with "Approach downgraded"
I still have the GA-35 if needed but is something else a problem?

Also the data base is expired. Will that keep me from a LPV approach?
 
WAAS downgrade

Don't know effect on LPV's, but install manual for 430W suggests 13 feet of RG-400 for appropriate impedence. I plan to use that length and it certainly gives you latitude for mounting location. Bill
 
13 feet? Could I add connectors and additional cable to make that length?
Otherwise I need to fish out the cable I have installed already.
 
reference for 13' cable?

Bill Dicus said:
...install manual for 430W suggests 13 feet of RG-400 for appropriate impedence. ...
Click to expand...
Hi Bill - cable length isn't ususally used to adjust impedance... but I did find a section of the manual that calls out for a total cable loss (between antenna and 430W) of 3-7dB. The 3dB minimum case might be close to your 13 feet of cable with a couple of connectors or adapters.

The minimum loss is amost certainly to improve inteference rejection (actually intermodulation) of the receiver.... probably not your problem but you never nkow.

I'd bet the problem is the antenna. The WAAS signal in space is the same center frequency as the regular old GPS L1, but it is a wider bandwidth. That is why the antennas are different - the WAAS antenna is specified over a wider frequency range.

I'd verify the type / quality of my antenna & coax connections. If that is all good then I'd swap for the correct antenna.


Good luck!
 
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While a different radio, the GNS 480 required a new antenna to remain WAAS/GPV compliant (which Garmin provided) along with a software upgrade. Don't know if there is a similar deal for the 430W upgrade.
 
GPS

Jeff: I don't know how many you'd need, but would think additional connectors would move in the right direction. My ST Upgrade Installation Manual 400W series Page 9 Revision B paragraph 2.5.4 says: (the manuals) "require that the coaxial cable have a loss of 3 dB to 7 dB. If RG400 is used, 3 dB equates to a length of approximately 13 feet of RG400 cable with a connector on each end." It also notes that a BNC to TNC adaptor (such as P/N1-1478013-0 from AMP (Tyco)) may be used to connect a cable with a BNC connector to an antenna with a TNC connector. This will add 0.5 dB to the overall GPS antenna cable loss. The minimum cable would seem to be 13 feet of RG400 with a connector on each end. I guess you could just splice in additional cable by the radio or the antenna; then you wouldn't have to restring the RG400 already in place. Good luck - let me know how it works out for you. Bill
 
You answered your own question

jeff beckley said:
Installed new 430W but used old GA-56 antenna. Was told that it would work just is not certified. Have 5 feet RG-400 cable to the antenna. Antenna is mounted under the cowl on a plate mounted to the firewall on a RV7A.
Every LPV approach I have tried to do ends up with "Approach downgraded"
I still have the GA-35 if needed but is something else a problem?

Also the data base is expired. Will that keep me from a LPV approach?
Click to expand...

I think you answered your own question. I would install the correct antenna and update the database before I would monkey around with anything else.

Also, it is my understanding that a GPS TSO'd for IFR use is required for an IFR flight plan. Compliance with the TSO includes installation IAW the manufacturer's installation manual, which would include the specified GPS antenna. If IFR is you intended mission, I think you need to install a new antenna.
 
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Did some xpermentin. I grabed an old GA-36 off the shelf and a 10 foot length of 400. Connected it to the 430 and placed the antenna next to me in the cockpit. Everything worked just fine. I think the placement of the antenna on the firewall inside the firewall is causing some problems. I restrung 14 feet of 400 and am waiting for the wass antenna to show up.
 
jeff beckley said:
Installed new 430W but used old GA-56 antenna. Was told that it would work just is not certified. Have 5 feet RG-400 cable to the antenna. Antenna is mounted under the cowl on a plate mounted to the firewall on a RV7A.
Every LPV approach I have tried to do ends up with "Approach downgraded"
I still have the GA-35 if needed but is something else a problem?

Also the data base is expired. Will that keep me from a LPV approach?
Click to expand...

As others have said...that's a No No!

Wrong antenna, wrong coax length, etc.. I'm surprised the GPS worked at all, because most times it won't with the old antenna. Your 430W should have come with the new antenna, along with the appropriate installation instructions.

Put in the right antenna and the right length of coax and I bet you'll be fine!

Cheers,
Stein
 
ummm... I think I would listen to Stein.... well... becuase he does this for a living.. and.. ummm.. yeah... most guys that do this type of stuff for a living tend to know what they are talking about....

This coming from MANY times Duncan has come over and slapped me upside the head and asking me what was I thinking...
 
Adding to what Stein said... my office used to be right outside one of the labs where they did WAAS antenna testing. Those guys sure seemed pretty busy. I bet there's probably a reason they want you to use a new antenna.

mcb
 
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