View Single Post
  #3   Report Post  
Old January 20th 05, 07:48 PM
 
Posts: n/a
Default


Fred McKenzie wrote:
Is there a way to make it work or do I need to take the pl 259
connectors back? I found some r6 8/u but it is 78 cents a foot! I am
just wanting something to test some antennas but am afraid of

inacurate
swr readings.

Stryped-

AOL doesn't have your other message about RG-6 vs RG-58, so I'll try

to address
both.

When testing antennas, one measure is standing wave ratio (SWR),

often read
using a "reflectometer" SWR bridge, that actually measures relative

forward and
reflected power but has a scale calibrated in SWR. You may find more
information about this at the Bird Electronics web site,
http://www.bird-electronic.com/.

Such an instrument is accurate only if the cable between the bridge

and the
antenna has the same characteristic impedance as the bridge. For 50

Ohm cable,
you need a 50 Ohm bridge. In this case, a resonant antenna with an

impedance
of 50 Ohms, has an SWR of 1.00:1 (one-to-one).

If you do not have 50 Ohm co-ax, simply make your measurements with

the SWR
bridge AT the antenna. Even if you have 50 Ohm co-ax, loss due to

heating of
the cable at higher frequencies will cause the SWR to look better

than it
really is. Moving the bridge closer to the antenna has an advantage

in that
case as well. However, using relatively short lengths should not

cause much of
a problem up to about 150 MHz.

I don't know how to attach PL-259 connectors to RG-6. To attach them

to RG-58,
you need an adapter that matches the outer diameter of the cable to

the inner
diameter of the connector. Since the PL-259 is so commonly used with

radio
equipment, I suggest you keep them and learn about attaching them to

RG-58 and
RG-8 cables.

The RG-6 is a low-loss cable commonly used in the cable TV industry.

As nice
as it is, its 75 Ohm impedance will cause serious errors in your

antenna
measurements if used between the instruments and the antenna. I

suggest you
steer clear of it for your experimental work. (If you have a 75 Ohm

bridge and
are working with a 75 Ohm antenna, that is another story!)

73, Fred, K4DII

Forgive my ignormance, but what are you calling "bridge" the radio?