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Testing VHF Antenna

Old 03-08-2013, 08:38 PM
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Default Testing VHF Antenna

Years ago when we were heavy into CB radios, we used an SWR (standing wave ratio) meter to test and adjust our antenna installations.

Does this same type of meter work for marine VHF antennas?
Old 03-08-2013, 08:44 PM
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you need one that can test the vhf range. ~150mhz. the Cb testers probably won't work as they are under 30mhz. but same idea. you just need a SWR meter for the correct freq range.
Old 03-08-2013, 09:09 PM
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I've been using one of these for years. This is the latest model.

Shakespeare ART-3 VHF Radio and Antenna Tester
Old 03-08-2013, 09:11 PM
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Originally Posted by polarred21 View Post
Years ago when we were heavy into CB radios, we used an SWR (standing wave ratio) meter to test and adjust our antenna installations.

Does this same type of meter work for marine VHF antennas?
To get accurate measurement of VSWR at 156-MHz requires a better grade of instrumentation. Once you have a VSWR measurement, it will be unlikely you will be able to prune your antenna length. Most marine antennas for the VHF Marine Band are encased in fiberglass, and they offer no means of adjustment.

For everything you ever wanted to know about accuracy ranges in measurement of VSWR see

http://www.aspen-electronics.com/fil...irectivity.pdf
Old 03-08-2013, 10:57 PM
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Agree that "pruning" can't really be done, placement of the antenna and it's feed line can affect the SWR of the typical FG antenna operating without a ground plane (mounted to a FG boat). Also agree that all SWR meters are not created equal. Be sure to get one the is accurate at VHF frequencies.

The "apparent" SWR can vary depending on where the meter is located along the feedline (if there is a miss-match to the coax/radio at the antenna base. For most antenna types, the meter should be at the base of the antenna). The typical FG marine antenna presents a different situation when making SWR measurements. There is not usually a connector at the antenna base - and - the feed line can be used as part of the "match" for best SWR. Given this, it is best to use a short as possible pigtail line between the radio and the meter.

Woody
Old 03-09-2013, 06:06 AM
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Originally Posted by capehaze View Post
Agree that "pruning" can't really be done, placement of the antenna and it's feed line can affect the SWR of the typical FG antenna operating without a ground plane (mounted to a FG boat). Also agree that all SWR meters are not created equal. Be sure to get one the is accurate at VHF frequencies.

The "apparent" SWR can vary depending on where the meter is located along the feedline (if there is a miss-match to the coax/radio at the antenna base. For most antenna types, the meter should be at the base of the antenna). The typical FG marine antenna presents a different situation when making SWR measurements. There is not usually a connector at the antenna base - and - the feed line can be used as part of the "match" for best SWR. Given this, it is best to use a short as possible pigtail line between the radio and the meter.

Woody
The VSWR on a transmission line is not affected by where the instrument used to measure it is placed. This is a myth. The VSWR is affected by only two influences:

--the characteristic impedance of the transmission line
--the impedance of the antenna

Neither of those change at different points in the transmission line.

The transmission line length cannot be pruned for best match. This is another myth. The only way transmission line length can affect the VSWR is through line loss. If the transmission line is made long enough that there is considerable loss in the transmission line, the VSWR measurement made at the far end of the transmission line from the antenna will show a lower VSWR due to line loss.

There is no benefit to be obtained from pruning the transmission line length between a VHF Marine Band radio transmitter and its antenna to a particular length.

In my previous reply, I linked to a document that provides information about the accuracy of VSWR measurements as a function of the quality of the instrument used to measure the VSWR. The document points out that for a typical low-directivity VSWR measure instrument, a reading on the instrument of a VSWR of 1.5:1 could be from an actual VSWR of anything from 1.9:1 to 1.2:1.

The notion that measurement of the VSWR with an inexpensive, low-directivity, VSWR instrument is going to lead to some opportunity to improve the match between the radio and the antenna is specious.

The impedance of the antenna will vary with its location relative to the sea and other conducting portions of the boat. The antenna should be sited for the highest location possible, not for the best VSWR. The notion that someone would move their antenna around the boat looking for the lowest VSWR is specious. Mount the antenna as high as possible.

The Shakespeare antenna tester can be useful to establish that the transmitter is producing some RF power output. However, there are no adjustments on the transmitter that can be made to alter the power output. FCC regulations prohibit boaters from making adjustments to the transmitter. If one measures the power output of their VHF Marine Band radio and finds it is not to specification, the only remedy is to have the radio repaired or replaced.
Old 03-09-2013, 06:13 PM
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Originally Posted by jhebert View Post
The VSWR on a transmission line is not affected by where the instrument used to measure it is placed.
Disagree.

If a conventional VSWR meter is placed at the transmitter, with some appreciable length of coax between it and the antenna, the reflected power will appear to be lower due to the loss in the coax - both from the power loss on the way to the antenna, and the power loss of the reflected return; the measurement will consequently appear to be better match than it is. In fact, a VSWR meter at the base of a tall tower can show a flat (matched, VSWR=1:1) line - when the antenna is actually well out of tune and badly mismatched - due simply to the length of the feedline.

The error introduced by a typical length of RG58 on a boat is unlikely to make a large difference but - if it were feasible - only a measurement at the base of the antenna would be correct.

"An SWR meter should be connected to the line as close as possible to the antenna: All practical transmission lines have a certain amount of loss, which causes the reflected wave to be attenuated as it travels back along the line." http://en.wikipedia.org/wiki/SWR_meter

Last edited by GTBecker; 03-09-2013 at 06:37 PM.
Old 03-09-2013, 06:33 PM
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Originally Posted by GTBecker View Post
Disagree.

If a conventional reflectometer (the common VSWR meter) is placed at the transmitter, with some appreciable length of coax between it and the antenna, the reflected power will appear to be lower due to the loss in the coax - both from the power loss on the way to the antenna, and the power loss of the reflected return; the measurement will consequently appear to be better match than it is. In fact, a reflectometer at the base of a tall tower can show a flat (matched, VSWR=1:1) line - when the antenna is actually well out of tune and badly mismatched - due simply to the length of the feedline.

The error introduced by a typical length of RG58 on a boat is unlikely to make a large difference but - if it were feasible - only a measurement at the base of the antenna would be correct.
I already accounted for the influence of line loss.

The VSWR is the same at any place on the transmission line, other than the influence of line loss.

There typically is not enough loss in 10-feet to 15-feet of transmission line in the typical boat installation to affect the VSWR measurement very much.

The greatest error comes from the cheap VSWR instrument.

In general, I don't see much value in making only a roughly accurate measurement of VSWR with a cheap instrument. I don't have any idea what one might do with the inaccurate information obtained. You can't adjust the antenna. You can't adjust the transmitter. Exactly what are you going to do with the data provided?

If you want to make accurate measurements, you have spend more than the cost of your radio and antenna combined.
Old 03-09-2013, 08:01 PM
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Great dialogue. Ideally I stumbled across one of the old Micronta SWR meters we used to use on e-bay for $10 and was just curious if this would work on a VHF marine radio. Sounds like it won't and I figured as much.

I would not mind having a test meter since I am a tool junkie but would not spend much on one due to lack of need. My antenna is an 8' fiberglass with no adjustment as mentioned, but I do have a length of cable coiled up that I could possibly shorten. Would be nice to see a before and after test with a test meter just to see what the change would be.

Last edited by polarred21; 03-10-2013 at 01:49 AM.
Old 03-10-2013, 08:55 AM
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I don't recommend using a $10 instrument to make a decision on the condition of a $300 radio and a $200 antenna. Typically really accurate test equipment costs much more than the device it is going to test.
Old 03-10-2013, 12:46 PM
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Originally Posted by jhebert View Post
... really accurate test equipment costs much more than the device it is going to test.
Largely true. I am a retired broadcast engineer who needed to know that the feedline impedance was 51.67 Ohms, -j0.15 because 50,000 Watts, if not radiated, can do serious damage to the transmission system and, thus, to station revenues. Only an expensive network analyzer can provide that confidence.

Most boaters, though, don't need the impedance numbers, nor that degree of accuracy nor the concern for potential harm. Instead, he needs only know that maybe 80% of the apparently-25-Watt transmitter output power is _not_ reflected back to the transmitter and is, therefore, probably radiated. An inexpensive instrument can do that. I use a Workman 104. http://www.ebay.com/itm/Workman-Mode...-/230933205691
Old 03-10-2013, 03:56 PM
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Originally Posted by GTBecker View Post
I am a retired broadcast engineer...
Me, too.

Let us investigate a hypothetical situation. You insert a $10 meter in your transmission line and make a measurement of VSWR. Please describe what action you take based on the reading. For example, if the VSWR is 1.8:1, what action do you take? Just tell me the decision points at which your action changes course. I would like to know what the typical boater is supposed to do when he gets a measurement of the VSWR with his $10 meter. It is all fine and good to take the measurement with a $10 meter, but I don't know what the next step is supposed to be.

Last edited by jhebert; 03-10-2013 at 04:14 PM.
Old 03-10-2013, 04:15 PM
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Originally Posted by jhebert View Post
The VSWR on a transmission line is not affected by where the instrument used to measure it is placed. This is a myth. The VSWR is affected by only two influences:

--the characteristic impedance of the transmission line
--the impedance of the antenna
.

it's been a few years since school but I'm pretty sure this is wrong. the waves going out and reflecting waves coming back will not be in phase. and you end up with some wierd signal voltage amplitudes along the line as the 2 combine. no longer a pure wave. and you'll end up with different power numbers as you move up and down the line. repeating itself every wavelengh distance.

though we were testing with wave guides. so maybe coax is different but I doubt it since it's still a wave..
Old 03-10-2013, 04:23 PM
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Originally Posted by jhebert View Post
Me, too.

Let us investigate a hypothetical situation. You insert a $10 meter in your transmission line and make a measurement of VSWR. Please describe what action you take based on the reading. For example, if the VSWR is 1.8:1, what action do you take? Just tell me the decision points at which your action changes course. I would like to know what the typical boater is supposed to do when he gets a measurement of the VSWR with his $10 meter. It is all fine and good to take the measurement with a $10 meter, but I don't know what the next step is supposed to be.

I run a test length of cable from vhf to ant, or ant joint and test again. then I replace ant with spare ant and test again... and see if one makes it better. if not, just leave it..

1.8 isn't poor. I would count that as fine. basically the cheaper meters just tell you if you're OK, or bad. it's not going to tell you perfect. if a line is shorted, open, or a bad ant. you're going to get a really high reading. and it's easy to diagnose those with a cheap meter. which on a pleasure boat is really the only senorios you're going to run into that you can do anything about. other then a bad radio. or bad connections / joints of cables.

I had a birdmeter at my past job, but now just use the shakespeare one.
Old 03-10-2013, 05:06 PM
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Originally Posted by jhebert View Post
... if the VSWR is 1.8:1...
Well, I would recut the antenna since mine are rebuilt old Shakesperes; I just use the fiberglass "radome" to house my J-pole on the boat or a coaxial dipole on the roof.

Were I Joe Boater, though, like smac999 would, I'd be less than really happy but probably do nothing but note that it's not 3:1, where I would take action. I'd likely bite a bullet and buy a better replacement - and return it if it, too, was > 2:1 or so.

Ah, and if it's really bad, I'd be looking for a coax crimp, loose or corroded connectors... etc.
Old 03-10-2013, 06:25 PM
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Originally Posted by GTBecker View Post
...probably do nothing but note that it's not 3:1, where I would take action.
If I understand what you just said, you recommend boaters take measurement of the VSWR of their antenna. If the measurement is below 3:1, then take no action.

If the VSWR is above 3:1, what does the average boater do?
Old 03-10-2013, 06:28 PM
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Originally Posted by smac999 View Post
it's been a few years since school but I'm pretty sure this is wrong. the waves going out and reflecting waves coming back will not be in phase. and you end up with some wierd signal voltage amplitudes along the line as the 2 combine. no longer a pure wave. and you'll end up with different power numbers as you move up and down the line. repeating itself every wavelengh distance.

though we were testing with wave guides. so maybe coax is different but I doubt it since it's still a wave..
I am sure you are wrong. The VSWR does not change. The VSWR is affected only by the antenna impedance and the transmission line impedance.

If you get a different reading when you move your measuring instrument, your measuring instrument is not reliable.
Old 03-10-2013, 06:30 PM
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Originally Posted by jhebert View Post
If the VSWR is above 3:1, what does the average boater do?
If the antenna is relatively new and one finds no obvious self-induced problems, return the antenna. If it is old, replace it.
Old 03-10-2013, 06:40 PM
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Originally Posted by GTBecker View Post
If the antenna is relatively new and one finds no obvious self-induced problems, return the antenna. If it is old, replace it.
I agree that if a boater uses a $10 instrument and measures a VSWR of 3:1 there is likely a significant problem with his antenna, his transmission line, or the connection of the transmission line to his transmitter.
Old 03-10-2013, 07:08 PM
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BTW, just not to mislead anyone, I am aware of no $10 VHF VSWR meters although one might be found at a garage sale, swap meet or ebay auction. I suspect you use that price metaphorically.

Still, one needn't be costly; I believe I paid ~$30 for my Workman 104.

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