[TowerTalk] 75 ohm - v - 50 ohm coax

[Jim Brown]

On 2/19/2013 6:21 AM, Jim Thomson wrote:
> ##  Fine for your to recommend  the use of 1” 75 ohm coax for a high dipole,

Sometimes folks don't read very carefully, especially when the ideas 
expressed are different from their preconceptions.

I said I was using Belden 8213 to feed my high dipoles. 8213 is a low 
loss RG11 with a foam dielectric. 130 ft of it weighs 11.3#.  I said I 
am using tuned lengths of 1/2-in 75 ohm CATV hard line to feed monoband 
Yagis for 20, 15, and 10M.  That hard line is laying on the ground.

When I started in ham radio 57 years ago, most hams fed dipoles with 75 
ohm coax and verticals with 50 ohm coax. Our transmitters had some form 
of tuned output, usually a pi-network, that could match a range of 
output impedances. Fifty years ago, the manufacturers of ham 
transceivers standardized on 50 ohms for the design load impedance of 
their output stages. When that happened, hams saw 50 ohms on the data 
sheet and assumed that they should be using 50 ohm coax with it. That 
was, and continues to be a mistake -- the loss in a transmission line is 
determined by the match between the ANTENNA and the LINE, NOT the match 
to the transmitter.

>Lets get practical for once.

Yes, let's do. That starts with studying and understanding the 
fundamentals of transmission lines and antennas, understanding that 
every design problem is different, and that one size does not fit all. 
As I observed in yesterday's post, low dipoles are generally a closer 
match to 50 ohm coax, and high dipoles to 75 ohm coax.

73, Jim K9YC