(Steve) wrote:

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

We need more "off topic" posts like this.... :-)


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.

You are going to get a lot of bull on this subject, largely built on
mythology.

Below 30 MHz, i.e. shortwave, and with only a 50 foot cable length,
cable loss simply isn't an issue.

What *is* important, however, is that your cable be capable of
surviving conditions outside. So, the cable should be UV resistant,
"non-contaminating," meaning that water won't get in and destroy the
dielectric between the shield and the center conductor. Finally, it
should be "direct burial" if you plan to, well, bury it.

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?

Yes, but not because of the loss. At 30 MHz, the highest frequency
you'll be using, it has a loss of less than 3dB. However, it it not
UV resistent or non-contaminating, so it'll get waterlogged and break
down relatively quickly.

Almost any coax would be better, given those characteristics. My
personal favorite is RG-8X, because it is extremely flexible. RG-58
has higher losses (who cares?) and is less flexible, but the
most-easily obtained RG-58 is radio shack cable, which, not to put too
fine a point on it, is junk.

You can obtain RG-8X from cable-X-perts (www.cablexperts.com) among
other sources.


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?

No, you are exactly correct. Furthermore, the distance is so short.

Now if you want to get into scanners, that's a whole different ball
game. Much better cable is necessary, and losses are important.

Likewise, if you start getting into transmitting, or start running
very long cable runs, you need better cable.

A final thought: Just to give you an idea on how absurd the RG-174
argument is, I have several GPS antennas, all of which are fed with
RG-174. One of them has a 25 foot feed. GPS uses 1.5 GHz. At 1.5
GHz, RG-174 is wet string. Yet, somehow, it does the job just fine,
despite losing over 99% of the signal over a short run. (Preamps help
overcome this problem in most external GPS antennas.)

Good luck with your new antenna mounting!

Regards,


Steve

--
Eric F. Richards,
"Making me root for a sanctimonious statist blowhard like Kerry isn't
the worst thing Bush has done to the country. But it's the offense
that I take most personally."
-- http://www.reason.com/links/links071304.shtml