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vonjet

Well Known Member
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
 
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.
 
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.
 
What about the shielding?

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?
 
vonjet said:
The problem is that one of my comm antennas is not accessible anymore.
Click to expand...

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
 
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.
 
Inaccessible yes.

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.
 
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
 
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.
 
Transmit and signal strength are fine

The motivation for replacing the 58 with the 400 was to get better sheilding as to avoid getting RF interference into the autopilot and magnetometer.
Would I achieve this goal by doing this or no?
 
Maybe a little; you will get more from re-routing the wire than by changing the coax type.

If you can simply pull the wire BACK from the front and then route around the side away from the tunnel, that alone might give significant improvement. You could then leave the existing cable intact and simply add an extender if needed. It is ok to mix types. I fubared all my connections, it might be worth your while to have a length made up for you by Stein.

And I think you already read the thread on the RF filter - have you tried that?
 
I bought the filter and got it today. I will be plugging that in this afternoon.
I really cant get my wires routed much different anymore since my interior is done. They are no longer bundled with the other wires but they still run a few inchs away down the center tunnel.
I'm getting close to solving it but I still have the issue of where to place my Magnetometer. Haven't found a spot in the airplane that doesn't cause mass RF problems yet.
 
Coax can be a funny animal...

...Although we commonly refer to the outer conductor as a "shield", it is not a shield. The outer conductor is just that...it is the outer conductor.

Coax is constructed with a "center conductor" surrounded by an insulated layer, and then the outer-surround conductor, followed by (optionally) an outer insulated layer. The inner insulated layer is called a "dielectric" separator that keeps the outer conductor and the inner conductor a specific distance from each other.

The combination of inner conductor size, the dielectric material thickness and electrical properties, and the outer conductor size and placement distance from the inner conductor all combine to create the characteristic impedance of the coax cable ( for us, usually 50 ohms).

The outer conductor can be one or several layers of braided wire, or the coax can be of a rigid type...resembling a water pipe (high power applications).

The choice of coax, which incorporates two braided layers in the outer conductor, is generally of a higher quality than a coax with only a single braided outer conductor. Whenever a coax cable is severely bent (at or near its minimum bend radius) the braided outer conductor can be distorted and the individual braid strands can move. This causes the cable impedance to change at that bend position. This is not desirable for the following reason:

As a signal is launched from a transmitter, the Tx circuits are designed for a 50 ohm (in this case) load. The coax transmission line is expected to be 50 ohms, and the target end (the antenna) is also expected to be 50 ohms. If any part of this turns out to not be 50 ohms, then a mismatch is "felt" by the signals, and a "mode" change is the result (technical terms). This mode change point is what allows the signal to "escape" from the transmission line (coax) at some undesirable point, and is also the point at which outside signals can be brought into the coax (think radiated interference).

So, to minimize these negative effects, use gentle bends, as few connectors as possible, and be sure to "match" all items for the proper characteristic impedance.

If you are inclined to shop at your local blister-pack audio store, just be aware that their coax connectors can be 50 or 75 ohm items. They are not marked as to which they are, so you may end up with the wrong impedance stuff, and you will suffer mode change problems.
 
vonjet said:
I bought the filter and got it today. I will be plugging that in this afternoon.
I really cant get my wires routed much different anymore since my interior is done. They are no longer bundled with the other wires but they still run a few inchs away down the center tunnel.
I'm getting close to solving it but I still have the issue of where to place my Magnetometer. Haven't found a spot in the airplane that doesn't cause mass RF problems yet.
Click to expand...

See your pm's since this is not an "RV Solution..."

:)
 
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