[TowerTalk] 75 ohm - v - 50 ohm coax

[Jim Brown]

On 2/19/2013 9:28 AM, Roger D Johnson wrote:
> s not the loss on receive determined by the RX input impedance? IOW 
> It's the LOAD on
> the transmission line. 

Yes, BUT -- signal to noise on receive on the MF and HF bands is nearly 
always determined by band noise (that is, noise picked up on the 
antenna), not circuit noise in the RX.  VHF/UHF is a very different 
story -- circuit noise CAN be the limiting factor for hams living in 
quiet locations, and in those situations, line loss on RX does matter.

Also, the input impedance of most receivers is not well controlled, and 
often differs significantly from its nominal value.

A good rule of thumb is that if the band noise increases by 3dB or more 
when you connect the antenna, your RX system is mostly limited by noise 
picked up on the antenna, and for a 6dB increase is entirely limited by 
external noise.  in those quiet locations where circuit noise is the 
limiting factor, we usually want an RX preamp up on the tower.

73, Jim K9YC