Rainier,
What about double shielded RG400 coax, such as this.....
https://fieldcomponents.com/RG-400-...MI0biCr-uV4AIVDoTICh3IpAKWEAQYBCABEgKa8_D_BwE
...at $2/ft, its cheaper than regular RG400 from Spruce etal....tie the outer shield to the radio case, leave the other end floating, and no live shield to act as an antenna, no?
Sadly our experiences with RG400 show that it is not a plaster to fix antenna problems. It is good cable with very low loss as long as it is used correctly. To be honest, provided you have a well matched antenna, there is not much difference to be had between a good quality RG58 and RG400 at VHF frequencies for our use.
It is often thought (and that included me) that the extra shield somehow isolates the inner cable from interference etc. But that does not happen this way (sadly).
Essentially the inner core and the shield forms a transmission line. If all if perfectly done the signal energy travels as a wave via the center conductor and the capacitively and inductively matched shield assists in the wave traveling without radiating anywhere. The result is that all the energy arrives at the destination.
If the shield is not correctly terminated at any of the two ends (the antenna side often being the problem area) the shield does not work correctly. More to the point it becomes "live" - part of the signal traveling in the inner core is now free to couple to the shield which also gets any reflections from the antenna - just to make things worse. Of course since it has some length it forms an antenna that will resonate at a couple of frequencies based on its length but also on how it is routed (you can create shorter antenna stubs in all sorts of ways here) - the result is usually a very complex antenna (not a very efficient antenna most of the time). All of this works for receive as well - the shield receives RF and couples it to the inner core.
A live shield is what often results in radio interference - other instruments going crazy when you transmit and easily receiving interference from strobes to anything digital (in particular if it has a big LCD display).
Using an SWR meter is often used to check the quality of your installation and this is a good way - don't be fooled by statements like "2.0 is good enough". That's fine for radio amateurs 30 meter rig but not for your installation. You should really try and get it below 1.4 if you have any interference.
Now, just a word of warning - I have now come across installations using dipols with typically three ferrites mounted at the base of the antenna over the shield as some sort of balun. These arrangements tend give a very good SWR but actually do not work !!! Instead the reflected energy of the mismatched antenna is simply prevented from returning by the ferrites giving the impression that all is well. Instead, from an RF point of view, the shield is disconnected at the antenna. A quick check with a field strength meter shows that effectively the entire antenna cable is your antenna (the shield is really very live) and the half of the dipol that is attached to the core does some radiation as well - the other half does nothing much.
These antennas are becoming popular in some aircraft as you can hide them but in my experience are a really bad idea.
Back to using antennas like this for NAV (dipols with proper baluns or matching transformers or shaped to present a good match by other means) - generally they work OK. RX is somewhat less critical than TX but you still need to be aware of interference which can affect your NAV radio.
You will likely have your NAV radio plugged into an intercom system.
Switch all of your systems on (except the engine) and scan through all channels starting at 108 Mhz while listening to the background noise. Make sure none of the channels gives you interference - this should be audible. If you find a channel - switch off equipment one by one. If all is off (except the radio and intercom and nothing changes - the interference likely comes from external to your aircraft (Florescent tubes in your hangar perhaps ?).
If you do not find a single channel with interference you cannot live with - you're antenna should be good to go.
You can do a final check with engine running - just to make sure your ignition system is not a potential interference source.
Rainier
CEO MGL Avionics