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Author Topic: Checking SWR  (Read 614 times)
KE5OKQ
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Posts: 193




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« on: March 04, 2008, 04:56:45 PM »


I have a MFJ259 antenna analyzer to check SWR.  To obtain the most accurate reading of SWR for my ground mounted vertical where should I check SWR, at the feedpoint of the antenna or at the end of the coax where the radio would be with the other end hooked to the antenna?
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K7PEH
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« Reply #1 on: March 04, 2008, 04:59:38 PM »

If you are concerned what SWR your radio will see then you should check at the end of the feedline that you connect to your rig.  If you are more interested in analyzing the behavior of your antenna system then you might want to check SWR at the antenna feed point.  

If I were doing it, I would check everything just because I would be interested in seeing how the feedline affects the overall impedance.
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N1QOQ
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« Reply #2 on: March 04, 2008, 05:11:33 PM »

Yup yup me too.
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W5FYI
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« Reply #3 on: March 04, 2008, 05:23:00 PM »

I concur with Phillip. The SWR for a theoretical half-wave vertical over a perfect ground, as seen from a 50 ohm system, is 1.388889:1 (1.4:1 is close enough on your meter). This, of course, is the measurement at the antenna feed point. The SWR would be lower if measured at the rig, because the transmission line introduces losses. With a long enough lossy line, you would be able to read close to 1:1 at the rig, but it would still be 1.4:1 at the antenna.

Most of us read the SWR meter at the rig, simply because it is a whole lot more convenient. We can always tune for lowest SWR when we change bands, and if we see an abnormal SWR reading, we know immediately that something is probably amiss.

FYI, Stew
 ---
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VK1OD
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« Reply #4 on: March 04, 2008, 05:48:21 PM »


The VSWR on a length of (uniform) transmission line is affected by line loss, it decreases smoothly towards the source, and the reduction can be determined from the line loss, or conversely implies the line loss.

For example, if the VSWR at the tx end of 100m of RG213 at 7MHz was 1.2, the VSWR at the antenna end of the line would be 1.3.

You can measure the VSWR where convenient and use knowledge of the line loss to imply the VSWR at another point.

For example, the spec on a 70cm vertical says the feed point VSWR is less than 1.5, and I measure 1.3 in the shack at the end of 30m of RG213, is it within spec?

No, the VSWR at the antenna end of the line would be around 2.3!

This depends on knowing the loss in the line, and this should be measured from time to time as it may develop a fault which increases it loss and which will mask high VSWR at the antenna end.

Owen

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WA3SKN
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« Reply #5 on: March 05, 2008, 05:02:56 AM »

Measure BOTH!
You have the time.  You have the meter.  You have the inclination.
The answers alone will tell you something!
73s.

-Mike.
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KE5OKQ
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Posts: 193




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« Reply #6 on: March 05, 2008, 09:59:15 AM »

Very mystical answer!
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N5EG
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« Reply #7 on: March 05, 2008, 07:27:32 PM »

You don't mention what band(s) you are trying to check. If you are checking at HF, then the next question is: do you have radials installed on the ground-mounted vertical antenna?

Normally at HF, the coax is not too lossy, the SWR should be the same at either location.

However if you do not have any radials, then your feedline will try to act as the other half of your vertical, and you can get really strange readings along the coax.

If you have the type of HF vertical that does not need radials, then you may still have to RF-choke the feedline before you can get consistent readings along the length.

  -- Tom, N5EG


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VK1OD
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« Reply #8 on: March 05, 2008, 10:07:31 PM »


"However if you do not have any radials, then your feedline will try to act as the other half of your vertical, and you can get really strange readings along the coax."

The effect you are referring to is usually due to changing the configuration when making the measurements.

If you preserve the length and physical route of the coaxial line, the VSWR decreases smoothly from the load to the source.


Owen
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KE5OKQ
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« Reply #9 on: March 06, 2008, 06:13:08 PM »

I checked my vertical antenna at the feedpoint with the MFJ-259 to tune the vertical radiator and radials.  I then checked the coax with the analyzer at one end and open at the other for coax loss.

The reason for my initial inquiry is I spoke to someone operating a Superantenna MP 1 who was checking SWR with the coax hooked to the antenna at one end and the analyzer at the other.  He was getting readings as high as 17:1 and only got worse with radials.  That begs the question, why? The way he was checking it , were the readings accurate?
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VK1OD
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« Reply #10 on: March 06, 2008, 09:38:46 PM »

"I checked my vertical antenna at the feedpoint with the MFJ-259 to tune the vertical radiator and radials. I then checked the coax with the analyzer at one end and open at the other for coax loss."

So, did the measured loss reconcile with the expected loss?

"The reason for my initial inquiry is I spoke to someone operating a Superantenna MP 1 who was checking SWR with the coax hooked to the antenna at one end and the analyzer at the other. He was getting readings as high as 17:1 and only got worse with radials. That begs the question, why? The way he was checking it , were the readings accurate?"

Too little detail to answer, and second hand info is suspect.

The accuracy of the analyser at 17:1 is questionable.

Did you know that the VSWR at 14MHz at the source end of 22m of RG213 with an open circuit at the far end is around 17:1?

Coming back to your own configuration, did the coax loss reconcile, was your VSWR at one end or the other in the range expected.

A couple of calculators that might be helpful in analysing your configuration are:
http://www.vk1od.net/tl/vswrc.php
http://www.vk1od.net/tl/tllc.php

Owen
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VK1OD
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« Reply #11 on: March 06, 2008, 10:23:52 PM »

"I checked my vertical antenna at the feedpoint with the MFJ-259 to tune the vertical radiator and radials. I then checked the coax with the analyzer at one end and open at the other for coax loss."

I meant to stress that if you disconnect the feedline outer conductor to make that measurement, you may have changed the configuration. The outer of your feedline may  not be isolated for RF in its normal connection, and measuring with the coax outer disconnected at either end may disturb the thing you are trying to measure.

Lots of people insist that the VSWR at one point cannot be predicted from knowing the VSWR at another point and the line loss between the points... but the usual reason for observing so is inadvertently changing the configuration.

Owen
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KE5OKQ
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Posts: 193




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« Reply #12 on: March 07, 2008, 03:25:28 PM »

The MFJ 256 manual says to "be sure the distant end of the component you are testing is not terminated in any resistance" when testing coax loss. Doesn't that mean unplugged from the antenna?
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VK1OD
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« Reply #13 on: March 07, 2008, 07:41:30 PM »

" The MFJ 256 manual says to "be sure the distant end of the component you are testing is not terminated in any resistance" when testing coax loss. Doesn't that mean unplugged from the antenna?"

Yes. My discussion was not related to the '259B test for coax loss.

The point I was making is that the outer surface of the coaxial cable is potentially a relevant conductor in your antenna system. If you want to get consistent load impedance readings (considering line loss) at both ends of the feedline, you need to maintain the current path from the feed point to the outer of the coax, and similarly at the other end of the coax where it connects to the station ground, transceivers, AC ground etc.

If you disconnect the coax from the feedpoint and measure the feedpoint with the '259B you may not be measuring the same configuration as you transmit on, and so what is the value of the measurement.

Owen
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