Coax question
 

ALl I have available in my areas is RG 6 or RG58. I know 58 is 50 ohm
and 6 is 75. SOmeone said using 75 ohm would cause my swr meter not to
read right. I only need about 10 feet of cable to "test" some antennas
I am going to build this weekend. It is for 2 meters and 440.
Should I just use rg 58?

Hi,

ALl I have available in my areas is RG 6 or RG58. I know 58 is 50 ohm
and 6 is 75. SOmeone said using 75 ohm would cause my swr meter not to
read right. I only need about 10 feet of cable to "test" some antennas
I am going to build this weekend. It is for 2 meters and 440.
Should I just use rg 58?

You are between a rock and a 'hard place' as both RG-6 and RG-58 will
cause incorrect readings on your SWR meter. The first, simply because it
will introduce a mismatch and the second because higher cable losses
will make the SWR appear to be lower than it actually is. However, if
you are just interested in seeing how well you can communicate, use the
lower loss RG-6 and work out what the meter readings mean later. Of
course, any matching adjustments at the antenna end will have to be
remade when you get some low-loss 50-ohm cable. If you were to make the
75-ohm feeder an odd number of half wavelengths down to the bridge
though, the antenna's impedance would be roughly duplicated there.

You can calculate the effect that feeder losses have on SWR from the
equations in the ARRL Handbook. If you are just going for a minimum then
these won't matter as long as you take the final figures with a pinch of
salt. Remember that cable losses act twice as far as SWR is concerned
and that mismatch loss is higher than straight cable loss.


Cheers - Joe, G3LLV

... If you were to make the
75-ohm feeder an odd number of half wavelengths down to the bridge
though, the antenna's impedance would be roughly duplicated there.

OOPS - That should read 'A MULTIPLE' of half wavelengths...


Cheers - Joe :-(

Joe McElvenney wrote:

Hi,


ALl I have available in my areas is RG 6 or RG58. I know 58 is 50 ohm
and 6 is 75. SOmeone said using 75 ohm would cause my swr meter not to
read right. I only need about 10 feet of cable to "test" some antennas
I am going to build this weekend. It is for 2 meters and 440.
Should I just use rg 58?


You are between a rock and a 'hard place' as both RG-6 and RG-58 will
cause incorrect readings on your SWR meter. The first, simply because it
will introduce a mismatch and the second because higher cable losses
will make the SWR appear to be lower than it actually is. However, if
you are just interested in seeing how well you can communicate, use the
lower loss RG-6 and work out what the meter readings mean later. Of
course, any matching adjustments at the antenna end will have to be
remade when you get some low-loss 50-ohm cable. If you were to make the
75-ohm feeder an odd number of half wavelengths down to the bridge
though, the antenna's impedance would be roughly duplicated there.

You can calculate the effect that feeder losses have on SWR from the
equations in the ARRL Handbook. If you are just going for a minimum then
these won't matter as long as you take the final figures with a pinch of
salt. Remember that cable losses act twice as far as SWR is concerned
and that mismatch loss is higher than straight cable loss.


Cheers - Joe, G3LLV





10 feet of good quality RG58 shouldn't make too much difference,
particularly on 2 meters, but even 70cm should be OK.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I absolutely agree with Tim. 10 feet of RG-58 coax isn't going to mean
squat for loss on either 2 or 440. The only difficulty I see is that 10
feet doesn't get YOU too far out of the picture to make a difference in the
VSWR readings, especially on 2 meters. Lessee now, 2 meters is about 6
feet, so you are much less than 2 wavelengths away from what you are trying
to measure. If you are just looking for rough cut, it may not be too bad,
but if you are looking for some fine grain detail, I'd get a bit further
away -- like 3 or 4 wavelengths if you have the space.

Jim



"Tim Wescott" wrote in message
...
Joe McElvenney wrote:

Hi,

I only need about 10 feet of cable to "test" some antennas
I am going to build this weekend. It is for 2 meters and 440.
Should I just use rg 58?

You are between a rock and a 'hard place' as both RG-6 and RG-58 will
cause incorrect readings on your SWR meter.
10 feet of good quality RG58 shouldn't make too much difference,
particularly on 2 meters, but even 70cm should be OK.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

most car mag mounts come with about 10 feet of RG58 on so it must be ok?

m0eme

"RST Engineering" wrote in message
...
I absolutely agree with Tim. 10 feet of RG-58 coax isn't going to mean
squat for loss on either 2 or 440. The only difficulty I see is that 10
feet doesn't get YOU too far out of the picture to make a difference in the
VSWR readings, especially on 2 meters. Lessee now, 2 meters is about 6
feet, so you are much less than 2 wavelengths away from what you are trying
to measure. If you are just looking for rough cut, it may not be too bad,
but if you are looking for some fine grain detail, I'd get a bit further
away -- like 3 or 4 wavelengths if you have the space.

Jim



"Tim Wescott" wrote in message
...
Joe McElvenney wrote:

Hi,

I only need about 10 feet of cable to "test" some antennas
I am going to build this weekend. It is for 2 meters and 440.
Should I just use rg 58?

You are between a rock and a 'hard place' as both RG-6 and RG-58 will
cause incorrect readings on your SWR meter.
10 feet of good quality RG58 shouldn't make too much difference,
particularly on 2 meters, but even 70cm should be OK.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Thu, 20 Jan 2005 15:18:11 -0800, Paul wrote:

most car mag mounts come with about 10 feet of RG58 on so it must be ok?

They do that because they're cheap ;-)

--
Gregg "t3h g33k"