Topband: "Is your feedline also an antenna?"

[MW Comercial]

GM,
After an investigation I did during the weeeknend I still receive some
signals on the coax, but there is a lot of improvement. The noise sources
were:

1. TX antenna. When my INV Vee is not connected to ANT1 socket in my IC736,
the coax connected to ANT2 socket is few db more silent. So I have to do
some decoupling of TX antenna.

2. The main reason (as Tom W8JI suggested) was the switchbox  with shields
connections trough relays. When I connected all shields together the local
AM station on 227 khz, which is 200km from my QTH is 15-18db down! Now the
switcher is modified and makes no difference if the coax is connected
directly to TRX or trough this switchbox.

3. I have 3 "parts" of the coax feeding my beverage's:
a) from TRX to the outside of the house (25m)
b) from the house to the switcher (30m)
c) from the switcher to antenna (<90m)

I found the most noisy is part "b". Maybe I'm wrong, but for example the
last 90m of the cable, when connected to the system produces 6-8 db more
signals, while those 30m from part "b" produces S7 more... I dont't know
exactely how to check the cable, anyway when a dummy load is connected, it
shows 51 ohms. What is the best way to check the coax? (I'd like to be 100%
sure before replacing it, because it's already buried and will be hard job
to remove it).

73's
Mac SP2XF