The Cessna 120-140 Association Forum

Hello,

I need your advice about an upgrade to my antenna system.

LV-NGL has the same antenna installation that was in place when I got the airplane, twenty years ago. Is time to upgrade.

One existing antenna is certified, services the main radio, a King 175, and is mounted via three screws on the left side. See pic OldLeftAntenna.

The second one is home made, services the auxiliary radio, an Icom A21, and is mounted via one central hole and porcelain isolators. See pic OldRightAntenna.

This is my idea:

1. Install the new Comant Antenna on the right side, to service the main radio.
2. Keep the old left antenna in place on the left side to serve the aux radio.
3. In this way I will minimize the holes on the wing skin.
4. Change all the old cables from RG58-U to certified RG400 modern cable.
5. Change all the four connectors from old Bendix screwed model to modern Amphenol Connex crimped PN 11-01802

I already got all the materials from ASpruce.

When I look at the actual mounting I notice that the left antenna, that seems to be original, is mounted close to the upper window line of screws.
This antenna is locked and serviced from within the cabin.
But the right antenna is mounted, as you can see, some three inches far from that line.
Is unreacheable from the cabin, must be fixed and serviced from the wing, removing the aluminium skin close to the right door.

Now the question is:

To fix the new Comant to the right wing I see two options.

6. Drill the four holes near the place where the old home antenna is now, and service from the exterior of the airplane,
7. Drill closer, service from the cabin, and close the hole of the home antenna some way.

If I proceed as 6, the installation will remain asymmetrical, but one hole is saved,
If I proceed as 7, the installation will remain symmetrical, but one circular hole, about 3/4 inch needs to be closed. How ?

Thanks for your continued help !

Have a very nice week end,
Horacio

Don't know about antenna, but your fuel cap might not be correct?

Hi Ray,

First of all, thanks for your observation.

My mechanic changed the caps some six to seven years ago.
I remember he removed the rubber seal on that caps, which are certified for another series 100 Cessna, and made a log book entry with that.
Both caps do vent.

If that is incorrect, please give me a reading to get the arguments and discuss that with him.

Thanks again !
Best regards,
Horacio

I would try and get the correct caps. As long as the caps you have do vent, then they are safe. The correct caps have two small holes that provide venting to allow the fuel to flow from the tanks. Some say the holes will allow water into the tanks if the airplane is subject to rain. Some people have made cap covers out of tin cans to prevent that from happening. In any case, be careful with fuel caps... or cap covers that you might forget to remove.

I forgot to add that there is an excellent article by Neal Wright on fuel caps under the Technical Articles part of this site.

Thanks again, Ray.

I was already aware of excellent study of Neil about this matter. I have downloaded all his articles to have them handy, years ago.
When my mechanic discarded the old caps that were rusty and did not close well, he discussed the installation of the "half vented caps" with our local authority inspector, in order to get his approval to do so. He agreed and signed the log book with that modification.
I remember, I was at his hangar, when we received the caps from A Spruce. They did not sell at that moment, the original ones, so we decided to go with the half vented.
We discarded the two orange-red valves, and the caps worked as full vent since then.
I never let NGL sleep outdoors, so rain is not an issue.

Thank you very much for your help !
Regards,
Horacio

Thanks for taking the time to respond. Based on your previous posts I should have known that you would be fully informed. Best regards to you and safe flying!

To actually answer your question. I believe that the existing antennas are OK. The performance of the one on the right will be improved with the new antenna. It performs better than the wire antenna. If you can put the three holes where they are accessible between the fuselage and the wing, that will work fine.

If I understand correctly, both antennas are nav/com antennas. As such, they operate in the 108-125 mHz range. They should be a minimum of 1/2 wavelength apart to prevent interference. (The "other" antenna will interfere with the "in use" antenna even if the radio to which it is connected is not turned on, in fact, it doesn't need to be connected to anything.) The wave length in feet is 984/frequency in mHz, so, 1/2 wave length for that frequency band is 48"-55" , or 122 - 139 cm (I think). So, I'm guessing they're already too close together.

Hello,

Thanks Randy and John for your valuable information, I still conserve the King 175 Install manual, so I will respect separation between antennas.
With reference to caps, I am sure that observations relative to security, which are on the safe side, are always welcome. Thanks Ray.

Have a nice Sunday !
Horacio