ccsmith51

ccsmith51

Well Known Member
On my RV-4 I have a SL-40. When I bought the plane it had the el-cheapo bent whip antenna, you know, the kind with the threaded stud to connect the coax.

It has served me well for the last three years, with good reception (at 2,000' I can hear CATF calls from 50 miles away) and transmission.

Recently I purchased a used Comant CI-122 antenna, thinking that it looked better and might work better.

Today I installed it and everything was much, much worse. I could barely hear the AWOS at 2,000' only five miles away. I asked for a radio check and the only person that responded was a Lake in the pattern with me.

I landed and checked all the connections, especially the new BNC connector I installed to mate with the antenna. I checked all the way back to the SL-40 and everything seemed good.

So, any thoughts on what could have happened that would make this higher-end antenna perform so much worse than my old one?

Thanks,
 
There is a 99% chance you have an issue with the installation (somehow). Either your BNC crimp connection or your coax cable, or perhaps the physical installation of the antenna itself....and an outside slim/minor chance it could be the antenna.

Cheers,
Stein
 
Hi Stein,

Thanks for the response.

For the coax I just cut off the end and installed a new connector. It is one from Radio Shack that does not need a crimp. You kind of screw it on and then tighten a set screw on the side to penetrate the insulation and make contact with the braid. I removed the set screw to ensure that the insulation had been penetrated...

http://www.radioshack.com/product/i...ce=CAT&znt_medium=RSCOM&znt_content=CT2032231

After I landed I removed the covers and got access to the complete cable run and checked all connectors. There is the one that I just installed, at the other end of that run is a male BNC. That connects to a female BNC that is on the end of the pigtail from the SL-40 tray. Everything seemed to be solid.

Is there an easy way to check with a multimeter the quality of the connections at all the BNC's?

I bought the antenna from someone on VAF that had just removed it from his RV-4 and said it was working fine.

Thanks again.
 
Stein, can you elaborate a bit on the physical installation of the antenna? I simply removed the one that was there and installed the new one in the same place....
 
ccsmith51 said:
For the coax I just cut off the end and installed a new connector. It is one from Radio Shack that does not need a crimp. You kind of screw it on and then tighten a set screw on the side to penetrate the insulation and make contact with the braid.
Click to expand...

Yikes! You need to install a real crimp-on BNC connector (http://www.steinair.com/storedetail.cfm?productid=139)
CON-SA-1010M_X.jpg


Every time I see one of those RS "solderless" connectors on an airplane, more of my hair turns gray and falls out. Please save my hair :p

The original builder of my RV-6 had a copper foil adhesive strip antenna stuck to the inside of the windshield and it had about a 10-20 mile range tops with the SL-40 radio. I installed a RAMI AV-17 antenna, essentially a less-expensive copy of the Comant CI-122, and my useful range instantly increased to typically 100 miles or better aircraft-to-aircraft and can now easily pick up AWOS transmissions 30-40 miles away or further... and I didn't even use the good RG-400 coax, I used plain old RG-58 because I had a piece of that already on hand.

I'll bet it's that el-cheapo connector... The Comant CI-122 should yield identical performance as my RAMI AV-17.
 
To check the coax, disconnect both ends and first measure across one end, center contact to the outer shell of the connector, with an ohmmeter and make sure you have no short. Next measure from center contact on one end to center contact on the other to make sure you have continuity. Then do the same from outer shell to outer shell.

The antenna should be mounted ideally on the belly of the airplane, use a doubler plate on the inner side of the belly skin to help give added strength to the installation. The mounting screws need to "dig in" to the metal of the die-cast antenna base so that a good contact is made. Sometimes using small "outside-star" lockwashers helps and you need to strip away some paint off the inside of the belly skin where it contacts the doubler plate so that a good electrical connection is made to ground the antenna to airframe.
 
ccsmith51 said:
Neal, thanks, I'll buy one from Stein. Does it require a special crimping tool?
Click to expand...

Yep it needs a proper crimping tool. You might try to see if you could borrow one from someone around your airport first if you'll only ever need it this once...

Stein sells the ratcheting crimper tool frame for $30 and the BNC coax dies (jaws) for it for $12, that makes it $42 plus shipping for a tool you'll likely only use once. If you do buy one, I'd recommend to also buy the $12 standard wire terminal crimper dies for the frame since that would make the tool useful for crimping regular wire terminal ends such as ring terminals, fast-on terminals, etc.
 
Hi Chris,

Indeed Neal is correct. I certainly wasn't intending to sell you a BNC, but now that I see what you used I'd strongly encourage you do get a decent one (from anywhere, even a local avionics shop) because there is a very good chance your problem is with that particular connector....to be blunt (and not insulting you at all, just for others who may consider that solution) - those specific bnc's from Radio Shack are not a good for this type of installation!

Cheers,
Stein

PS, this is intended with levity - when you remove that existing connector, take all of the individual pieces and throw in them in separate garbage containers - preferably spread across the city (after you smash them with a hammer) so that nobody can accidentally assemble or use it again! :)
 
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Thanks everyone.

Tomorrow or Saturday I will remove the antenna, ensure that there is good bare metal between the backing plate and the fuselage belly, and ensure that there is good contact between the mounting screws and the antenna base.

I will try to find a better BNC connector somewhere at the airport. If I can't then I will order one from Stein.

I am flying with some other RV's to Cecil Field tomorrow to see Fifi and some other warbirds, and then lunch. The radio works well enough for that as I will not be lead.

But on April 12th my girlfriend and I are flying to the RAF weekend camping at Blackwater (8FD3) and I have to have it in good shape by then. I will be at Sun 'n Fun from Monday - Sunday next week, perhaps I can find a connector there?

Thanks again for all the help.
 
ccsmith51 said:
Thanks everyone.

Tomorrow or Saturday I will remove the antenna, ensure that there is good bare metal between the backing plate and the fuselage belly, and ensure that there is good contact between the mounting screws and the antenna base.

I will try to find a better BNC connector somewhere at the airport. If I can't then I will order one from Stein.

I am flying with some other RV's to Cecil Field tomorrow to see Fifi and some other warbirds, and then lunch. The radio works well enough for that as I will not be lead.

But on April 12th my girlfriend and I are flying to the RAF weekend camping at Blackwater (8FD3) and I have to have it in good shape by then. I will be at Sun 'n Fun from Monday - Sunday next week, perhaps I can find a connector there?

Thanks again for all the help.
Click to expand...

At a pinch you can get one from Frys and an el cheapo crimper there too.

there are other BNC connectors that don't use a crimp, rather you solder the center connector and the outer piece is threaded and clamps down that way (Amphenol makes one). Not sure you'll get that from Frys, so if you have to wait for shipping go to stein.

if you go that route be careful, that center insulator melts like ice cream.

Get the good crimper from stein with the replaceable dies.
 
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