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Author Topic: High SWR after replacing coax  (Read 2912 times)
W1RML
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Posts: 13




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« on: March 17, 2012, 10:04:17 AM »

When I purchased my first HF rig about a year ago, I bought a 40 meter dipole antenna and 100 feet of RG8x coax.  I have been able to use the antenna without a tuner on 40 and 15 meters with relatively low SWR, and on all bands between 40 and 10 meters using the FT-950's internal tuner.  Since 100 feet of coax was more than I needed, I just purchased 50 feet of flex9913 coax and hooked it up.  The result: SWR on 40 and 15 meters is very high (2-2.5:1) and the tuner will not tune on any other band.  I know that the shorter and better coax will have less loss, but the SWR results on 40 meters make no sense to me.  Any ideas?  Thanks in advance for any responses.
73 de W1RML
« Last Edit: March 17, 2012, 10:54:12 AM by KB1VAU » Logged
KA4POL
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Posts: 1962




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« Reply #1 on: March 17, 2012, 10:17:06 AM »

Was it a ready made cable or did you solder the connectors onto it? Check it out for any shorts or even mechanical damage.
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W1RML
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Posts: 13




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« Reply #2 on: March 17, 2012, 10:41:11 AM »

The connectors came pre-installed and appear to be mechanically sound.
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W5FYI
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Posts: 1044




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« Reply #3 on: March 17, 2012, 11:02:53 AM »

Put a 50Ω non-inductive dummy resistor on the antenna end of the coax and report what kind of SWR you see. If the coax and connectors are alright, it should be 1:1. If not, then I'd suspect a bad connector fitting.

Another thing, really lossy cable can 'improve" SWR a lot, but doesn't do anything to help get the signal to the antenna.  Is your SWR equally poor across the band with your new cable, or does it seem to be better at one end rather than the other?  If it's the latter, then you may want to retrim the antenna.  GL
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WB6BYU
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« Reply #4 on: March 17, 2012, 11:04:44 AM »

There are two likely solutions:

1) the SWR at the dipole was always that high, but the losses in the coax made it
lower at the rig end of the cable.  Increasing cable loss is one way to lower SWR,
but though it looks better at the rig, less of your RF is being radiated.

In this case, as long as the coax is in good shape and hasn't been contaminated by
moisture or other issues, the losses are low enough that an SWR of 2.2 at the rig
though the 9913 would mean 2.27 : 1 at the antenna, which would read just a hair
under 2 : 1 at the rig.  (You can use VK1OD's transmission line calculator for this:
http://www.vk1od.net/calc/tl/tllc.php)

2) there are common mode currents on the feedline due to not using an effective
balun with the dipole.  In that case, your feedline IS part of your antenna, so any
changes to it will change the SWR.  One or the other piece of coax is of such a length
that more current flows on it than on the other piece, which changes the feedpoint
impedance of the antenna, and thence the SWR.
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W1RML
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Posts: 13




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« Reply #5 on: March 17, 2012, 01:22:53 PM »

Thanks for the replies.  I took some more SWR readings. On 40 meters, running only 5 watts, I measure 2:1 at 7.020 and over 3:1 at 7.295.  At 100 watts, the readings are much worse.  I think i can assume (but correct me if I am wrong, which I usually am) that a commercially made 40 meter dipole with a 1:1 balun, which I trimmed further when I first erected it and which has performed well for a year, should not have readings like this.  I would also add that the signal reports i have gotten today have not been as good as usual.  So how can I test  the coax to determine if that is the problem?  I do not have a dummy load.  Thanks!
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WB2WIK
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« Reply #6 on: March 17, 2012, 02:04:57 PM »

Thanks for the replies.  I took some more SWR readings. On 40 meters, running only 5 watts, I measure 2:1 at 7.020 and over 3:1 at 7.295.  At 100 watts, the readings are much worse.

Sounds like you have two things going on:  (1) Your dipole is too long and needs to be trimmed a bit shorter; and (2) Whatever you're using to measure SWR isn't very good, since SWR can't change with applied power unless something blows up.  SWR is what it is, and has nothing to do with power.

Quote
I think i can assume (but correct me if I am wrong, which I usually am) that a commercially made 40 meter dipole with a 1:1 balun, which I trimmed further when I first erected it and which has performed well for a year, should not have readings like this.  I would also add that the signal reports i have gotten today have not been as good as usual.  So how can I test  the coax to determine if that is the problem?  I do not have a dummy load.  Thanks!

Not sure about signal reports, but of course band conditions change by the day, hour and minute.  However, a "commercially made dipole" isn't different from a homebrew dipole, it's just two pieces of wire and a center insulator.
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W1RML
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« Reply #7 on: March 17, 2012, 05:58:09 PM »

Thanks for the replies.  I took some more SWR readings. On 40 meters, running only 5 watts, I measure 2:1 at 7.020 and over 3:1 at 7.295.  At 100 watts, the readings are much worse.

Sounds like you have two things going on:  (1) Your dipole is too long and needs to be trimmed a bit shorter; and (2) Whatever you're using to measure SWR isn't very good, since SWR can't change with applied power unless something blows up.  SWR is what it is, and has nothing to do with power.

If this is the case, why was the dipole not too long when I used the RG8X coax?  Can swapping out coax really result in the SWR on a given frequency going from 1:1 to 3:1?  As for measuring the SWR, I am using the meter on the FT-950.  But if SWR doesn't change with applied power, can you please explain to me the basis for the previous comment that the lossy RG8X masked the high SWR?  I assumed this meant that the loss of power through the coax resulted in a lower apparent SWR, but if power loss does not affect SWR, why would higher loss coax mask higher SWR?
 
Quote
I think i can assume (but correct me if I am wrong, which I usually am) that a commercially made 40 meter dipole with a 1:1 balun, which I trimmed further when I first erected it and which has performed well for a year, should not have readings like this.  I would also add that the signal reports i have gotten today have not been as good as usual.  So how can I test  the coax to determine if that is the problem?  I do not have a dummy load.  Thanks!

Not sure about signal reports, but of course band conditions change by the day, hour and minute.  However, a "commercially made dipole" isn't different from a homebrew dipole, it's just two pieces of wire and a center insulator.
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K8AXW
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« Reply #8 on: March 17, 2012, 06:10:11 PM »

Your antenna just happened to give you a good SWR reading with 100ft of cable.  One poster commented that the antenna needs to be shortened.  I think it needs to be lengthened.  The SWR is better at the low frequency than the high.  A longer antenna will continue to lower the SWR at the lower frequency.  Your 2:1 at 7.020 will shift on up the band.

The suggestion of checking the coax with a dummy on the end is very good.  It IS possible you have a bad connector/connection.

It would be great if you could put an antenna analyzer on it.  You could find out very quickly what is what.
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AC5UP
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Posts: 3835




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« Reply #9 on: March 17, 2012, 06:17:41 PM »

why would higher loss coax mask higher SWR?

Because you're measuring it at the radio... through the length of the coax... meaning that what you see at the radio is whatever power is reflected back from the antenna minus whatever portion of that power is lost in the coax. Looking at it another way, let's say the coax is bad enough to lose half the power you put into it. SWR meter at the radio sees 100 watts. Antenna sees 50 watts. Reflected power might be 10 watts, but since the coax loses half that, you see 5 watts.

SWR meter sees 100 watts going in, 5 watts reflecting back, and you measure a 5% loss in your antenna system. Not bad, ehhhhh?

Until you upgrade the coax and see the real numbers.  Wink
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WB6BYU
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« Reply #10 on: March 17, 2012, 06:26:52 PM »

Quote from: KB1VAU
...On 40 meters, running only 5 watts, I measure 2:1 at 7.020 and over 3:1 at 7.295.  At 100 watts, the readings are much worse...


What are you using for an SWR meter?  Do you have to set the sensitivity to full scale at low
power on 40m?

I have a number of SWR meters designed for CB use.  They work fine down to 80m, but require
more power to get a full scale reading on the lower bands.  It takes about 5 watts on 40m and
25 watts on 80m.  With the sensitivity at maximum there is less voltage across the detector
diode in the meter (especially on the reflected power side, where the voltage is lower).  Because
of the inherent voltage drop in the diode, the reflected power (and hence the SWR) will read
low.  As you increase your output power the diode voltage drop becomes less of a percentage
of the voltage across the diode, so the error diminishes.

There could be other explanations - such as an arc that only happens above a certain power
level, or the ferrite core in the balun saturating.  But if it is a strip-line type SWR meter designed
for CB, this is likely the reason for the difference in the SWR readings between low and high
power.

If I model a 40m inverted vee with the wire length such that the SWR is 2.0 : 1 at 7.000 MHz,
the SWR rises to 2.8 : 1 at 7.1 MHz and 5 : 1 at 7.3 MHz.  That's without taking into account
any feedline loss.  (It would drop from 5 : 1 to about 4.5 : 1 due to the coax loss.)
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STAYVERTICAL
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Posts: 854




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« Reply #11 on: March 18, 2012, 02:17:06 AM »

If everything was ok before, and the only change is the coax, and the connectors are solid, and the balun is rated for this power and swr, then:
Your old coax was masking the real SWR by having such high losses that the reflected energy returning to the rig was very low.

The same thing happened to me when I went from RG58 ( a lousy brand and quality) to RG213.
My SWR shot up, but my signal was much better getting out.
Also, my SWR meter was much "livelier" in that it showed less frequency range before the SWR went up.
This is partly due to the SWR meter's "suck in" effect, where the low SWR point is in a "valley" which will not register much change due to the knee voltage of the SWR meters rectifier diodes.
Also, the losses in the feedline will be added to the radiation resistance of the antenna, swamping the antennnas changes in radiation resistance with frequency, which makes your antenna appear to more broadband. This is similar to having a dummy load at the end of the feedline with a wire attached to one end.
The same effect holds true in an inefficient antenna with high losses - it appears much more broadband than a low loss antenna.

This is another demonstration of the inefficiency of using SWR as a yardstick to the efficiency of an antenna, rather than considering the entire feedline/antenna as a unit - and maximising the efficiency of each part.

Good luck and 73s
« Last Edit: March 18, 2012, 02:19:52 AM by STAYVERTICAL » Logged
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