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Why adding coax changes swr

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Old 04-21-2011, 01:15 PM
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Default Why adding coax changes swr


On my rv rig I have a good swr reading on a 102 whip 1.2 to 1 and on a 4 foot top load ant it's 1.2 to 1.4 or so why does swr go up when I add swr meter or amp with jumper or if I add amp and swr meter including 2 jumpers swr goes even higher...What would be the problem here ?antenna? or amp or ground plane? Tonight I'll add 3 jumpers 3 foot each with out swr meter or amp and see what that does. I'm reading swr also on the meter on the cobra 29 I just had a shop check it all out and both worked together on the bench so problem must be in antenna setup or rf feedback being close to antenna but that would'nt matter with just swr meter causing higher swr reading. I'm getting about ready to give up on this amp better antenna stuff, I get good swr with just 18ft coax and whip so does'nt that mean the antenna is all correct?


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Old 04-21-2011, 01:47 PM
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My first guess would be an insufficient ground plane. Forgive my ignorance, but it's mounted on your RV, which I think is a big mobile home thing, is this made of fiberglass by any chance? How and where is the antenna mounted?

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Old 04-21-2011, 01:59 PM
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It is my opinion that the SWR for the antenna is not actually changing, but when you add some length to the system you are seeing the SWR change. It doesn't seem to make sense, but it does.

What is happening is the frequency is changing due to the transformational affect which can happen in a system where the antenna is not set perfectly at the frequency of choice. Either or both the resistance or the reactance is off a bit, which means your system is a bit reactive.

You are making the change and then checking out the SWR, but missing the fact that the lowest SWR reading has shifted in frequency a bit, maybe 10-20 channels when I see it happen. That is why you're often told to ignore it, that the meter is just being fooled or is lying to you, which is CB BS. If the meter is close to correct it is just showing what it's supposed to at the frequency where you take the reading, and where the meter is located in the feed line.

If you bother to take a simple bandwidth reading, record the info, and then make a graph...you will be able to see the BW curves', best SWR, move around according to frequency.

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What causes this can be anything that makes your antenna tune off a bit, tune, construction, installation, ground, objects close by that can cause reflections.
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Old 04-21-2011, 02:55 PM
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"why does swr go up when I add swr meter or amp with jumper or if I add amp and swr meter including 2 jumpers swr goes even higher"

your post reads like youre using the meter on the radio since your vswr is going up when you ADD YOUR METER , AMP AND COAX . you dont want any amps or meters or filters or anything else between the swr meter and your antenna besides coax . i also would not use a radios built in meter to tune swr . always use a external swr meter .

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Old 04-21-2011, 02:57 PM
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Quote:
Originally Posted by Marconi View Post
...It is my opinion that the SWR for the antenna is not actually changing, but when you add some length to the system you are seeing the SWR change. It doesn't seem to make sense,...
well, ya got part of it correct,............................ the only thing changing is the impedance.

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Old 04-21-2011, 03:20 PM
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colt45,

are you saying that you are using those double female barrel connectors and coax jumpers to add length to your coax?

if so, they could well be the source of your problems.

you need one uninterrupted length of coax going from the antenna to the output of whatever meter/amp you have in line.

i cant say for sure that these things are messing you up, but if it were my setup, thats what i would do.
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Old 04-21-2011, 05:25 PM
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Quote:
Originally Posted by loosecannon View Post
colt45,

are you saying that you are using those double female barrel connectors and coax jumpers to add length to your coax?

if so, they could well be the source of your problems.

you need one uninterrupted length of coax going from the antenna to the output of whatever meter/amp you have in line.

i cant say for sure that these things are messing you up, but if it were my setup, thats what i would do.
LC
One uninterrupted length of coax is the ideal, but as long as the sections of coax have the same characteristic impedance, a few (say 5) ohms, there's no problem with one or two barrel splices. The length of the "discontinuity" is so short (relative to a wavelength at HF), that for all practical and measurable purposes, it isn't there. Just be sure to weather-seal each such splice properly.
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Old 04-21-2011, 06:27 PM
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The total reactances of the feed line and the antenna are what make up SWR when measured at the transmitter end of the feed line. If you add/subtract coax it changes that total 'mix' to change the SWR. But that's only really gonna happen if the antenna isn't presenting a 50 ohms, non-reactive load, meaning it is really tuned incorrectly. That's NOT an uncommon thing to have a mis-tuned antenna, quite common if only an SWR meter is used to do that tuning. An SWR meter wouldn't know a reactance from a strawberry malt.
- 'Doc

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