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what is your vswr at the antenna?

Alexis Mercado

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May 17, 2016
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freecell, you are right, we agree with that. The longer the cable and the higher the frequency, the more attenuation will be present, but that’s another story.
 

Marconi

Usually if I can hear em' I can talk to em'.
Oct 23, 2005
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"VSWR should not be affected by length of coax cable." Fortunately it is, anytime the line is not terminated in a load = to the characteristic impedance (Zo) of the feedline or in the case of a feedline with excessive line attenuation losses.

A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna. If the load impedance is anything but a perfect match, additional power will be "displaced" due to reflection from the load back down the line.

As a follow-up to the thread "New V4K vs Conventional J-Pole, I just got through adding a feed line to the Vector model. So, in order to compare it, I added a feed line to an old Startuster model with my idea for the hub to specs.

These two models showed me something I realized happens in a sort of vague way for me...showing how much adding a feed line can effect the gain reported.

I used modest lengths of pretty good coax that a lot of folk use RG-213, but I don't think it would be consider as low loss.

I was surprised at the results for these two models, so I decided not to publish the idea, because I figured nobody would believe such an idea. But, Free Cell was talking about Feed Line issues in this thread and Bob I think you made mention about loss of gain on adding a feed line in the real world.too, in another thread above. So, the models are posted in the PDF file below.

BTW the SD'r feed line is 648" inches long, and the Vector is only 432" inches. At this point I tend to believe the difference may be due to the gamma being more lossy than the direct fed Starduster. There is a 3* degree difference in the maximum TOA in favor of the taller Vector, and that is worth considering.

If these cables were the same length the difference would likely show some more gain for the Starduster. That said, I doubt these differences, and the higher angle for the SD'r...might tend to make things about equal and hard to tell...just using you radio.

FC, posts above suggest that a loss in gain of 70% might be expected, but these models show the loss in gain with a FL added is less that, but still significant.




 

Attachments

  • Vector vs. Starduster both with feed lines and Overlays.pdf
    3.6 MB · Views: 10
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Marconi

Usually if I can hear em' I can talk to em'.
Oct 23, 2005
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"posts above suggest that a loss in gain of 70%...."
i was speaking of 70% power loss over 500 ft. of RG-213 due to line attenuation, nothing to do with any antenna.

Must have missed that. Some times I get up to got the kitchen for a glass of water, and when I get there, I forget why.

Short time memory is fading...a symptom of Covid19 and my age.

Would you think this value of loss in the models is close?
 
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freecell

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lines are not able to be compared. RG-213 wishes it had a line loss figure of .6 dB. per 100 feet and for the starduster the line loss is given as .0 dB, per 100 feet, that's impossible. the data output of these programs is only as accurate as the data input.

648 inches = 54 feet, 432 inches = 36 feet. @ 1.1 dB. per 100 feet @ 30 MHz. this is close enough.

1.1 dB. per 100 ft. = 0.011 dB. per foot. 54 ft. X 0.011 dB. = .594 dB. of loss, 36 ft. X 0.011 dB. = .396 dB. of loss. the .198 dB. difference is negligible and undetectable.
 
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kopcicle

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A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna
Q) At what frequency?
If the load impedance is anything but a perfect match, additional power will be "displaced" due to reflection from the load back down the line.
Q) Displaced?

In our efforts to obtain low feed-line SWR values of 1.1, 1.2, or even 1.5 to 1, we have gone far past the diminishing-returns point with respect to efficient power transfer, even for single-frequency operation. It is like installing no. 4 or no. 6 wire in a house-wiring run where no. 12 wire is sufficient.
Reference to the basic transmission-line equations, which have always been readily available in engineering texts and handbooks verify this analogy. In addition, such references make it clearly apparent that authors who simply insist on low SWR, or find 1.5:1 or 2:1 objectionably high, have failed to comprehend the true relationship between reflected and dissipated power. From the viewpoint of amateur communications, it can be shown mathematically, and easily verified in practice, that the difference in power transferred through any coaxial line with an SWR of 2:1 is imperceptible compared to having a perfectly matched 1:1 termination. This is true no matter what the length or attenuation of the line. Further, it can be shown that many typical coaxial feed lines we use on the HF bands with an SWR of 3 or 4, and often as high as 5 to 1, have an equally imperceptible difference at the receiving end. When feed-line attenuation is low, allowing such higher values of SWR permits operating over reasonably wide-frequency excursions from the self-resonant frequency of the antenna with the imperceptible power loss just described,
~Maxwell
[sarc] ...so Ive been doing this wrong for over 40 years. Thank you for opening my eyes! [/sarc]

So I'll add to this train wreck once again. The above passage is from "Reflections III" by M. Walter Maxwell, W2DU.The first chapter is titled "Too Low an SWR Can Kill You ". I won't spend any further time here painting generalities with a broad brush. I'll just link to the publication and let any and all read it. Read it all as Maxwell taken out of context is often seen word for word describing electrical principals that are just plain wrong. While on the subject of inaccuracies, Maxwell is far from perfect. It's up to the reader to do due diligence and experimentation.

http://www.w3pga.org/Antenna Books/Reflections III.pdf
 

The DB

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A 500' line of RG-213 will lose 70% of the power produced by the transmitter before it is delivered to the antenna.
Q) At what frequency?

100 feet of RG-213 at CB frequencies is a little over 1 dB of loss (or about 20%), so at first glance his 70% sounds about right for upper HF frequencies to me. But since you directly asked, lets see what we can figure out.

Lets start with a decibel calculator, like this one. Set it to power, then set the input to 100 and the output to 30, this simulates 70% power loss, when you click calculate dB you get a result of -5.228787.

Next we need to calculate coax losses to that loss amount. So I used this coaxial loss calculator. I chose Belden 8267 (RG-213) and set the length to 500 feet and cycled frequencies until I got to 5.229 dB (rounded from above) of "Matched Loss". "Matched Loss" on this tool is simply the losses on the coax between the radio and antenna, assuming the antenna is a perfect match. I don't really like the name they gave it, but what can I do about it?

What frequency was it? 27.025 MHz, which is channel 6 on the CB band, which makes sense as this is a post on a CB forum...

The rest of it you guys can hash out yourselves, I'll just say I'm in Maxwell's camp and leave it at that.


The DB
 

Handy Andy

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the ability of the electrical half wave line to mirror and repeat the complex load impedance seen at the antenna feedpoint to the transmitter or analyzer (opposite) end of the line is indisputable.
giphy.gif

...And they just keep pounding away...​
 
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