RG400 to RG58

[vonjet]

I want to replace all my RG58 coax in the airplane with RG400. The problem is that one of my comm antennas is not accessible anymore. So can I just clip the Coax at a point as close to the antenna as I can get and put a connector there with the new RG400 wire?
So basically I would have a short run of RG58 coming off the antenna to a connector of RG400 that then runs thru the fuselage and to the radio.
Is this going to work?
Bryan

[scard]

sure it will work.

[apatti]

Yes, you can do this. However, there will be an insertion loss associated with the extra connector. I don't know how much it is, but I have heard it is on the order of .5 db (you will want to verify this figure). So, depending on how long the run is and the frequency of the signals, the extra loss associated with additional connector may more than offset any gains from the lower loss RG400.

[Birkelbach]

I found the loss per foot to be around 0.05 dB/ft for RG58 and 0.045 dB/ft for RG400. The rule of thumb for the connectors is 0.5 dB but this is really conservative. Find the dB loss of the connector that you are planning to use and then figure out how many feet at 0.005 dB/ft (The difference between the two types of cables) it would take to equal that loss. That is your break even point. Anything shorter than that and you are going backwards.

If the loss in your connector is 0.1 dB then you'd need to be replacing 20 feet of cable just to break even. Of course if you need to replace the cable because it is damaged then you'll have to do what you have to do and might as well use the better cable. Otherwise, my guess is that you'd be better off leaving it like it is.

[vonjet]

What about shielding from other wires. I was planning to replace the cable because I wanted better shielding and I read that RG400 is double shielded.
Will this help? Or am I wasting time and money?

[Garage Guy]

Originally Posted by vonjet

The problem is that one of my comm antennas is not accessible anymore.

Totally inaccessible? Then you should slap whoever built it upside the haid.

Seriously though, what if you unscrew the base of the antenna from the outside? Can you get access to the cable end that way?

Personally I think replacing RG-58 with RG-400 is worth it if you can do it for the whole run. Not so much for less loss than RG-58, but for better shielding and for better handling (pulling teflon jacketed cable vs PVC cable through a full conduit... no contest).

But in your case if you have a particular problem that you think might be bad shielding, I would try clamp on ferrite cores first... that's pretty easy to do.

--Paul

[Birkelbach]

Are you having noise problems? What 'other wires'?

I can tell you from experience that you'll pick up more noise from that connector than you will through all of RG58 cable you can put in the plane. It may be worth doing on the rest of the radios but if you can't get to that one comm antenna, I'd leave it alone.

[vonjet]

Its completely inaccessible because its a Bob Archer Antenna and I have a Lancair so its buried in fiberglass inside the vertical tail.
As far as other wires, my avionics guy ran my Autopilot servo wires, my Magnetometer wires and other power wires for trim, flap and hyd. motor all in the same bundle.
So I turned on my EFIS and the raw magnetic heading was awol when I PTT. Also my Digiflight IIVS Autopilot heading did the same.
So I have removed the 2 comm cables and Transponder cable from the bundle and now they seem to be ok but while I had them out I figured maybe I should replace them with RG400.
My #2 comm is easy, I can replace that entire run as well as the transponder.
The #1 comm cant be entirely replaced but the portion of RG58 that will be left will not be next to any other wires.
There is also one other small issue. There is 10" of RG58 coming out of the back of each Comm because I have ICOM 200's. I hook up the antenna wires to these.
I think I just want the better shielding in the ares where my wires run close together.

[Garage Guy]

Sounds like your problem is solved, if you can permanently keep the comm cables bundled away from the others.

If so, replacing a section of the RG-58 with RG-400 is just going to be work without noticeable benefit. I wouldn't bother.

--Paul

[Norman CYYJ]

I wouldn't waste the time or money changing the coax. You won't notice any difference in the receive or transmit signals. The extra signal loss and point of problems created by adding another connector is just not worth the hassle or bother.