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SWR - wtf?



 
 
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  #1  
Old June 27th 05, 06:48 PM
john d
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Default SWR - wtf?

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=48 699

(not advertising it just using it for the picture) ... Anyone know how to
use the Land Matic LM-50 ... it was pretty cheap like $16.00 Cdn I think - I
got it new but it didn't have any instructions - I don't even know the
wattage I should be setting it on. I'm assuming that the antenna goes into
the back on the side with ANT and the radio goes into the side marked
TRANS - but I'm not sure - does anyone know of any webpages that provide
some sort of instructions on how to use SWR meters?

TIA
jdd


  #2  
Old June 27th 05, 11:31 PM
Frank Gilliland
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On Mon, 27 Jun 2005 13:48:34 -0400, "john d"
wrote in :

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=48 699

(not advertising it just using it for the picture) ... Anyone know how to
use the Land Matic LM-50 ... it was pretty cheap like $16.00 Cdn I think - I
got it new but it didn't have any instructions - I don't even know the
wattage I should be setting it on. I'm assuming that the antenna goes into
the back on the side with ANT and the radio goes into the side marked
TRANS - but I'm not sure - does anyone know of any webpages that provide
some sort of instructions on how to use SWR meters?



They're pretty easy. The cables hook up just like you figured.

Start by setting your radio to ch 18 AM (not SSB). Switch the matcher
off (you won't even need it), set the power switch for 10W, the top
switch to FWD CAL and the SWR/CAL switch to CAL. Key up and adjust the
slider so the meter reads full scale. Then unkey, switch the SWR/CAL
to SWR, key up again and read the meter. Adjust your antenna to get
the lowest SWR -- the lower the better. Then check SWR at both ends of
the band. An SWR of 2:1 or lower across the band is fine. If you can't
get it below 3:1 on any channel then there's something wrong with your
system, like a bad ground or coax cable.

If you want to play with the matcher, make your initial adjustments
while listening to the noise in receive. Start with TUNE all the way
left and sweep LOAD until the noise peaks. Check SWR. Then turn TUNE a
little bit to the right, sweep through LOAD again for the noise peak,
and check SWR again. If the SWR is better then keep going through the
process until it starts getting worse. Eventually you will find a spot
for both knobs that work to give you the best SWR. But remember that
when the matcher is tuned it's tuned very sharply to that freq -- you
will need to retune the matcher every time you move more than a couple
channels away.




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  #3  
Old June 28th 05, 06:11 AM
Scott in Baltimore
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So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this



What makes you think 18 feet of coax is even a half wave?

At 27.185 MHz (ch 19) a half wave is 17.21 feet.

At 66% velocity factor, an electrical half wave is 11.36 feet.

At 77% velocity factor, an electrical half wave is 13.25 feet.

What's so special about a half wavelength of coax?

It's the point that the SWR at the feedpoint is reflected to the other
end of the coax. At any other point in the coax, the phase angle affects
the apparent SWR seen by a Voltage reading SWR bridge. You can swap in
different lengths of coax to see this in action for your self. If your
antenna feedpoint is 50 ohms NONREACTIVE, coax length does not matter.
If there is a reactive component to your antenna system, the reflection
travels back through the coax and at different points the voltage and
current will affect the reading on the meter. A matchbox does not fix the
mismatch of the antenna, it only fixes what the load sees the antenna as.

The mismatch does not go away just because you've adjusted a few knobs.
  #4  
Old June 28th 05, 01:29 PM
Vinnie S.
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On Tue, 28 Jun 2005 01:11:31 -0400, Scott in Baltimore
wrote:

So ... I have an older CB - Cobra 21 LTD Classic with weather stns etc. ...
a 102" Shakespeare Antenna - 18' of cable - and this



What makes you think 18 feet of coax is even a half wave?

At 27.185 MHz (ch 19) a half wave is 17.21 feet.

At 66% velocity factor, an electrical half wave is 11.36 feet.

At 77% velocity factor, an electrical half wave is 13.25 feet.

What's so special about a half wavelength of coax?


The Mobile antenna websites practically tell you to keep the coax at 18 feet, or
else. I thought that was true, until numerous people at this group and several
websites said that is nonsense.



It's the point that the SWR at the feedpoint is reflected to the other
end of the coax. At any other point in the coax, the phase angle affects
the apparent SWR seen by a Voltage reading SWR bridge. You can swap in
different lengths of coax to see this in action for your self. If your
antenna feedpoint is 50 ohms NONREACTIVE, coax length does not matter.
If there is a reactive component to your antenna system, the reflection
travels back through the coax and at different points the voltage and
current will affect the reading on the meter. A matchbox does not fix the
mismatch of the antenna, it only fixes what the load sees the antenna as.

The mismatch does not go away just because you've adjusted a few knobs.



Vinnie S.
  #5  
Old June 28th 05, 01:33 PM
Steveo
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Vinnie S. wrote:
On Tue, 28 Jun 2005 01:11:31 -0400, Scott in Baltimore
wrote:

So ... I have an older CB - Cobra 21 LTD Classic with weather stns
etc. ... a 102" Shakespeare Antenna - 18' of cable - and this



What makes you think 18 feet of coax is even a half wave?

At 27.185 MHz (ch 19) a half wave is 17.21 feet.

At 66% velocity factor, an electrical half wave is 11.36 feet.

At 77% velocity factor, an electrical half wave is 13.25 feet.

What's so special about a half wavelength of coax?


The Mobile antenna websites practically tell you to keep the coax at 18
feet, or else. I thought that was true, until numerous people at this
group and several websites said that is nonsense.

Have you ever heard from the coax length police? Real sticklers when it
comes to that.
  #6  
Old June 28th 05, 06:43 PM
Scott in Baltimore
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Posts: n/a
Default

The Mobile antenna websites practically tell you to keep the coax at 18 feet, or
else. I thought that was true, until numerous people at this group and several
websites said that is nonsense.



It's just a convenient premade length of coax. My RadioShack 19-210 2 meter
antenna came with the same junky 18 foot length of crappy RG-58 with a crimped
on connector. After removing it from the magnet and drilling a hole to properly
mount it, I cut off the extra cable and put a solder on end on it. If it didn't
have a propietary connector going into the base of the antenna, I would have
replaced it with something better. RG-58 is barely good at 27 MHZ and even
leakier at 146 MHz. The shorter the better. Coax length does not matter if
the SWR is at 1.5:1 or less. I haven't bothered to check my SWR on my CB
antenna since I got my ticket. The CB still works just fine with the RG-8X
and 225 amp on it!!!
  #7  
Old June 28th 05, 07:10 PM
Scott in Baltimore
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The Mobile antenna websites practically tell you to keep the coax at 18 feet, or
else. I thought that was true, until numerous people at this group and several
websites said that is nonsense.


For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.
  #8  
Old June 28th 05, 07:36 PM
Scott in Baltimore
Guest
 
Posts: n/a
Default

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.



And one last thing,

The speed of the signal INSIDE the coax (the velocity factor) is slower
then the speed of the signal OUTSIDE (on the shield). While 17.21 feet
is a quarter wave on the outside of the shield, the inside 1/4 wave is
shorter. If you want to see the actual SWR at the feedpoint, then use
a 1/2 wave electrical length of coax. This will shift the phase of the
mismatch back into it's original position at the other end of the feedline.

(I learned all this stuff while I was still a single bander, and still
laugh at all the ham's that still believe the coax length BS.)

I don't dislike CB. It's another band to use. I dislike all the noise on it now!
If I can't find someone to talk to on one band, I've got others to use now.

BTW, repeaters suck! I've only got to abide by Part 97, not what some control op
thinks his interpretation of the rules are. I'm simplex only these days.

(Sorry, no letters or warnings have been recieved by me! I just said **** it.)
And I didn't say that on the air!!!!!!!!!!!!!
  #9  
Old June 28th 05, 07:52 PM
james
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Default

On Tue, 28 Jun 2005 14:10:10 -0400, Scott in Baltimore
wrote:

For a NGP antenna, the shield is the counterpoise, and if you change the
length of that, you'll detune the antenna. If you can alter things by
moving the coax around, you've got a problem. Think of coax as a
"signal hose". The RF should stay inside of the coax, not run along
the outside and affect things. If things change, start by fixing your
ground. That's one reason I took my antenna off the magnet. The other
was so that it won't get knocked over by a low branch on the trail.

I immediately noticed the antenna worked better with a real ground.

****

If the shield is the counterpoise for the antenna, then the antenna is
installed with no ground and thus inproperly installed period. In a
properly install antenna system you should not have common mode
currents residing on the shield of the coaxial transmission line.

All to often on cars today, there is far to much plastic and not
enough metal to offer a sufficient RF ground. The vehicle frame and
body, if metal, should be the uppper plate of a capacitor that is
formed with the Earth below. The metal under the antenna is important
for radiation. The more the better.

james
  #10  
Old June 28th 05, 07:58 PM
Scott in Baltimore
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Default

I immediately noticed the antenna worked better with a real ground.
****

If the shield is the counterpoise for the antenna, then the antenna is
installed with no ground and thus inproperly installed period. In a
properly install antenna system you should not have common mode
currents residing on the shield of the coaxial transmission line.



It worked good as a magnet mount, it just works better with a real ground!
 




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