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SWR changes with outside TEMP?

what mount do you recommend for big truck applications?

  • NMO

    Votes: 5 41.7%
  • magnetic

    Votes: 4 33.3%
  • mirror mount "L" bracket style

    Votes: 3 25.0%

  • Total voters
    12
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jm3691175

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setup= mirror mounted dual firestiks with tunable tip, mounted just over 7'6" apart, 18ft cophased coax.

SWR is lower when it is cloudy and colder outside, the brighter the sun the higher the SWR.

Am I missing something obvious here, or am I thinking way too much into this?


out of curiosity please answer my poll question:
 
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Daniel_Boone

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You have to remember that the entire vehicle is part of the antenna system.

Standing wave ratio - Wikipedia, the free encyclopedia

This leads me to believe that maybe your coax is bad......

Since all waves acts the same be it radio or water, just think of a standing wave like a rock in a creek.
In front of the rock - the water dams up and gets higher, while behind the rock the water is lower then in the front.

Think of the wave that is transmitted like two kids splashing in a swimming pool.
The one kid takes their hand and physically pushes the water which creates a wave, that wave will travel out to the edge of the pool and go no further.
Now just think of the reflection as the second child also splashing water in the opposite direction.
As the one pushes the water in one direction and the other pushes the water in the other direction, the two waves crashes into each other and cancels each other out.

The purpose of measuring the standing wave is to try to make the feed line and antenna as efficient as possible - because anytime the wave reflects back towards the transmitter - you get less power going out the antenna because the waves are canceling each other out in the feed line.
The standing wave will cause heat which will melt the coax and will try to take out the first transistor in the front end - if there is not a guard or protector built in - such as was the case with the old tube type CB radios - back in the day that did not have anything built in and when you keyed the mic without a dummy load or a antenna connected - it took out the transistor because the power was reflected back towards the transmitter - because it had no where else to go.
 

Allan_Love_Jr

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With my CB Radio system. It does not matter weather it is cold or hot nor Rain nor Snow. My SWR's always stay low no matter what. That's because I use a Magnetic mount K40 Antenna on the roof of my Truck. But there is one thing that you must remember. ALWAYS do maintenance of your system once a year. A cleaning job to make sure that the Antenna and the connectors are 100% corrosive free. Especially from rust. Plus make sure that the ROD itself is clean. As for Antenna STICKS. Make sure that no moisture gets into the base of the Antenna such as the threads. Take care.
 

jm3691175

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Location
Iowa, USA
doesnt make sense

My higher SWR readings only occur on sunny warmer days.
In the mornings before sunrise or on cold cloudy days my SWR is perfectly fine.
Still puzzled.........
 

jm3691175

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about my poll .....

i mean class 8 big rigs (semi trucks).
ok, the poll question was slightly loaded since a majority of big rigs are aluminum cabs and a magnetic mount will not work.

any other ideas on my SWR changing with the sun and temp?....
 

roadranger

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Athens, Ga
SWR, versus TEMP

My system has no issues with the variance in the temperature. I have nothing to do with it exept adding the radio, with a "little help," and pluging them in.

Using the meter on the radio (gasp!) I always get a 1-1 reading, and the ANT. light rarely glows. I use the Factory installed antennas, which are directly mounted to the mirror, not necessarilly it's mount, which I noticed is grounded to the truck with a piece of copper wire on each mount.

For some reason, I am not allowed to participate in your poll, yet, I prefer the metal "L" brackets, because they are better suited for holding larger antennas and withstand tree strikes. They are necessary for the Ham Stick, as well.

Some drivers will insist on a screwdriver antenna for HF, which may require a more suitable mount, often bolted also to the mirror mounts. If you are going to use any amount of power, mount your antennas as high as you can on the MM and if they are as tall as my Ham Sticks, at least mount 20M and 40M on the passenger side, not to tilt too far out, so that it won't grap EVERY low tree limb, or the occasional low bridge! Mount the taller antennas from the bottom of the bracket, not the top. ( Some mirror mounts are NOT exactly perpandicular to the door, as you might imagine, and angle back a bit, so if you tilt the antenna forward, it WILL stick out away from the truck striking many curbside objects!)

Thank You for taking the time to read my response.

The Road Ranger
 
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jm3691175

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Using the meter on the radio (gasp!) I always get a 1-1 reading, and the ANT. light rarely glows.
The Road Ranger
I have read that using longer runs of coax can make SWR meters read higher than actual because of more resistance? if this is true should I take out my co-phase 18ft and replace it with 9ft co-phase?

ideas anyone?
 

LtDoc

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Dec 4, 2006
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Location
Oklahoma
The only reason for SWR changing is that something in the antenna system has changed, or the surroundings of that antenna system has changed. The 'surroundings' part deals with things fairly close, 6" to 60 feet sort of close, as in just driving down the road. It happens all the time and is completely normal.
The most likely reason is the first one, something in the antenna system, meaning the antenna, the feedline, and probably the mount, has changed with the weather. That typically means one of those things isn't 'normal', has a bad connection, or leaks.
Which is your problem? Beats me, take a look at it and then you tell me. Or, get someone else to take a look at it, you might be too close to the trees to see the forest... sort of.
- 'Doc
 

roadranger

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Athens, Ga
We're half way there!

We only know about half of your station.

We would like to know the following:

What kind of radio?

What kind of truck?

Where on the truck are the antennas mounted?

Can you see through the coaxial casing?

Is the sheild thread count 112, or 144? ( I learned that one toady!)

Are you making measurements with the meter on the radio, or one in conjunction with the station inline between the radio and antennas?

Is the Motorola connector intact on the cophase intact?

Has it been removed, or modified?

Please respond with as much detail as you can muster. THX
 
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jm3691175

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Location
Iowa, USA
Uniden PC78Elite / has been tuned NOT clipped
Kenworth W900L
"L brackets" top and outside of mirror mounts which are stationary cab mounted
36" tunable tip Firestiks mounted on HD springs
black casing on coax
dont know shielding count
measurements have been done with radio and external meter inline
all motorola connectors are intact
no parts have been moved or modified in any way

---im completely puzzled , ready to completely tear out and start from scratch with a single Laird antenna and shortest coax possible
 

jm3691175

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roadranger

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Here's an Idea

Remove the springs and tune it again. Beleive it or not, the extra length in the springs make the antennas longer than they ought to be.
 

mikepdx

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Corbett, OR USA
if so why does my SWR read just fine when it is cold and cloudy?
Think humidity rather than temp.

Warmer = lower humidity (probably)
Cloudy = higher humidity (like in the morning)

Grounds (most connections) tend to be better when the humidity is higher.

You could test it. When the SWR is lousy (warm/clear).
Dump some water around where the mirror mount contacts ground.

I would suggest re-checking grounds (clamps, etc).
Loosen and re-tighten bolts that mount the mirror as well.
Might be dirty.
 
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jm3691175

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Location
Iowa, USA
Been too cold to really mess with the water trick out in the open, but did give truck a quick rinse and SWR has seemed to stabilize.
Maybe I am being to picky....
SWR on both 1 and 40 is always below 2.0 but sometimes antenna light will flicker
 

roadranger

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That's okay.

Been too cold to really mess with the water trick out in the open, but did give truck a quick rinse and SWR has seemed to stabilize.
Maybe I am being to picky....
SWR on both 1 and 40 is always below 2.0 but sometimes antenna light will flicker
I think you are off to a good start!
 
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