[AMRadio] 50 ohms simple question


Thomas Adams quixote2 at ix.netcom.com
Thu Apr 5 19:46:26 EDT 2007


>>>  There is a very acceptable range of impedance (ohms)
that will work just fine for you. <<<

Right on! That's why Gawd (and the receiver makers) made
antenna trimmer capacitors!  <<grin>>

In recieve only antennas, I tend to save a few bucks by 
designing around Mil surplus RG11/U coax (70 ohms to begin
with) as the transmission line to feed them...  not that 
it really makes a whit of difference in reception if you
use 50 or 70 ohm coax.

I used to make 50 to 70 ohm broadband transformers for the
reciever end of the line, but finally decided it wasn't
worth the trouble; most receivers, especially old ones,
don't REALLY have a 50 ohm input impedance anyway, no
matter WHAT the manual says!


>>> If you want to split hairs (or ohms) then get an 
SWR meter or some form of analyzer and either adjust the 
length of your antenna to something near 50 ohms or use a 
tuner that can match the feedline and the antenna to 50 
ohms. There are several possible ways to transfer power to 
an antenna in a ratio of 50 volts per ampere of current
that many transmitters like to see. 

Transmitters manufacturers have chosen 50 ohms based
on the most commercially available and popular
transmission lines on the market with consideration to
the many different antenna systems in use other than
just a theoretical dipole. <<<

Quite true...   but with TRANSMITTING antennas you wind
up with a different problem; ya ever seen a Bird 43 with
a 70 ohm line section?  In that case, 50 ohms is a sort
of mandatory thing unless you keep a pocket calculator 
next to your Bird...  so for transmitting antennas I go
with the more traditional 50 ohm lines. It's just easier
that way.

I have to wonder about that tho; I'm a broadcast engineer, 
and in 3.125" hard line I keep encountering older stuff
(mostly made by RCA) that is marked as 51.3 ohms!!! Never
could figure out what's up with THAT!

In any case... for receiving antennas, just don't sweat 
the entire issue. My ricebox TX doesn't like 70 ohm lines, 
but my boatanchor rigs don't even notice the difference;
Valiants, DX-100s, and Globe King 500s will eat ANYTHING!


Mr. T., W9LBB

  






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