RadioBanter - View Single Post - Off topic post regarding antenna feedline

September 9th 04, 05:22 AM

In article ,
(Steve) wrote:

This is off topic, but I'm going to ask it anyway...

I'm don't have a lot of technical expertise and I'm wondering how one
determines what kind of coax to use as a feedline. I currently have
about 50 feet of fairly thin (RG174) coax that I'm using to feed an
outdoor active antenna that's mounted on the fire escape outside my
window. I'm thinking about mounting the antenna on the roof of my
apartment building and suspect 50 ft of feed line might just be enough
to allow me to do that. However, a friend of mine told me that 50 ft
of RG174 is going to allow too much signal loss. So, there you have my
first question: Is 50 ft of RG174 coax a bad feedline?

It depends on the frequency you will use it on and how much signal you
are willing to lose.

RG174
MHz dB loss for 50 foot
50 -2.9
100 -4.2
200 -6.25
400 -9.5
1000 -17

3 dB is half your power so you will lose half your signal around 50 MHz
and it gets worse the higher in frequency you go.

If you are using this cable for HF then you can plot these points in
Excel. The line should be fairly linear and the loss will not be as bad.

The second question is this. My friend seemed to think that the 50 ft
run to my roof would result in greater signal loss than the much
shorter run to my fire escape, even though we're talking about 50 ft
of coax either way. This didn't make sense to me. 50 ft of coax is 50
ft of coax, it seems to me, whether it's coiled and leading just
outside my window or straight and leading all the way up to the roof.
Am I wrong about this?

The loss is over the length of the cable, it does not matter where it is
routed. If the fire escape is closer and you cut the coax for that
shortened distance then you will experience less loss.

--
Telamon
Ventura, California