Coax question

Coax question

Post by stry.. » Fri, 21 Jan 2005 22:23:23

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?
 
 
 

Coax question

Post by Joe McElvenne » Fri, 21 Jan 2005 23:44:09


Hi,

Quote:> 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

 
 
 

Coax question

Post by Joe McElvenne » Fri, 21 Jan 2005 23:55:11

Quote:>                                         ... 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 :-(

 
 
 

Coax question

Post by Tim Wescot » Sat, 22 Jan 2005 01:24:15


> 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

 
 
 

Coax question

Post by RST Engineerin » Sat, 22 Jan 2005 01:33:42

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



>> Hi,

 I only need about 10 feet of cable to "test" some antennas
Quote:>>>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

 
 
 

Coax question

Post by Paul » Sat, 22 Jan 2005 08:18:11

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

m0eme


>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




>>> 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

 
 
 

Coax question

Post by geek » Sat, 22 Jan 2005 11:20:26


> 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"