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Discussion in 'CB Antennas' started by colt45, Apr 21, 2011.
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.
Click this video:
YouTube - Marconi Demonstrates Antenna SWR Bandwidth Curve
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.
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 .
well, ya got part of it correct,............................ the only thing changing is the impedance.:headbang
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.
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.
Here is my example of what happens when I added 6'+ in the form of a choke to my Sigma4. As I said the frequency changed and went down due to the antenna being slightly reactive. The affect is as we would expect when adding length to the antenna and we know that with a reactive antenna the feed line becomes part of the radiating system.
So hookedon6 does the frequency change or not?
Maybe Colt45 can check his rig out and see if his frequency drops when he adds the jumpers.
View attachment Affect of adding feedline to antenna.pdf
All vSWR is, is just a ratio of power output of the radio to power reflected back to the radio. Of forward to reflected power.
For something that causes power to be reflected back, its often refered to as a reflection. Bad connectors, a kink in the coax, water intrusion, or an antenna thats electrical length is not tuned to the operating frequency of the transmitter, all cause a reflection.
If you want to test the coax, connectors, amp,etc by themselves you need a 50 or 52 ohm load, aka dummy load or cantenna. use it in place of the antenna, in theory that paint bucket with the connector on it is the perfect antenna, should give a 1:1 SWR on any freq from 300khz to 30Ghz.
Also there is going to be some loss and reflection at the amp. Shoulnt hurt the radio so long as it is below a 3:1.
Tune the antenna without the amp, use a female to female adapter in its place but with same jumpers and all. Is SWR good?
You might also have a problem with lack of ground plane, seeing how most RV's are fiberglass shells. The metal body acts as other half your antenna, so without metal body the coax acts as the other half, where part of the trimming coax myths come from, cut the coax and you actually shorten the antenna.
what "frequency" are you refering to?
lets see,.... you added a "choke coil" @ the feed point in an attempt to uncouple the line from the antenna. obviously that isn't effective. there ARE better methods available to uncouple a feedline.
put the VSWR meter AT the FEEDPOINT (where it belongs) and you will see NO change.
let me ask you a question,....... has any radio ever been damaged by a "high" VSWR????
the answer is no,....... because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.
VSWR doesn't actually exist,... it's just a math ratio.
the ONLY thing a radio "sees" is impedance.
you really should brush up on Smith Charts and you will find out that the "VSWR circle" is really an elipse.
so finals cant be blown by high vswr ????????????
hookedon6, in this case I suggest you look at the graph represented by the data from my SWR meter, and my analyzer, to see the low point for SWR noted. Also note in the data fields at the top of the graph, where the analyzer data is recorded, that the impedance within a modest range of resonance (or lowest SWR) did not change as you have suggested. For me it is obvious that the frequency did change somewhat dramatically, from about 27.405 down to about 27.105. IMO, this is the reason Colt45 saw a change in SWR for the system.
Again, I would claim that the SWR measured at the feed point of a reactive antenna, if measure before and after, would remain similar, while showing a change at the TX'er end of the feed line WON'T, it will appear to change due to transformation of the feed line.
The only real way to change the SWR for an antenna is to make a physical change to the antenna, tuner, or the environment around it. This is also why I recommend to always, when possible, tune at the feed point and at installed height.
I did add the choke to check to see if it would help to decouple the feed point from the feed line and it did seem to work in lowering the noise, but I admit that perception can be purely subjective in its determination. Otherwise I noted no other positive responses. I did note the change in resonance as noted above, and had I constructed the choke using the existing feed line instead, I doubt I would have seen the change in frequency.
I agree and that is my point in a nutshell.
IMO, your last statement is a typical straw man argument, to use the words never, no way, shape or form does SWR ever affect the operations or the lack, with your transmitter. What about the cut back circuit that attempts to control the power under a high SWR condition?
I thought you said VSWR did not exists, but now it appears on the Smith Chart? That is interesting. Why don't you do a Smith Chart indicating the addition of more feed line to a vertical 1/4 wave antenna that is matched perfectly and one that is not. That should be easy to do, and interesting to see what it is that you see in doing so. I gave you an example of what I see, so how about it?