Topband: Silent Feedline for RX antennas

[Tom Rauch]

> improved shielding (e.g. buried feedline). * As a last resort, you can
> increase S/N by placing the preamp at the antenna end, before the
> feedline effects are added.

Good advice Gary, but moving the preamp will not affect the system 
unless the feedline loss is so high the noise floor is established by 
the preamp and not the antenna.

By far the most common problem is common-mode ingress into the 
antenna from the feedline, and with that problem moving the preamp 
won't change the system. The reason for no change is the common mode 
impedance of virtually all preamps are nearly zero ohms, so the 
common mode system is affected by the amplifier move.

If we don't change the common mode impedance, we won't change the 
effects of the feedline. (This is very similar to the incorrect but 
popular notion that moving a balun from the output to the input of a 
ground floated tuner somehow improves system balance or relaxes balun 
impedance requirements.) 

Adding a ground isolated input or output winding on a preamp input or 
output transformer might help, but I am not aware of any published 
designs like that. In any event, it would be no better than just 
adding a passive isolation transformer and keeping the amp in the 
house.

 

73, Tom W8JI
W8JI@contesting.com