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Old 30-06-2022, 02:57   #1
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2.1:1 SWR on VHF

Hi all,

I loaned my SWR meter to a fellow cruiser today so that he could test his recently installed VHF. The meter is a Diamond SX-200. A reasonable bit of kit.

On my installation it records a SWR of 1.4:1 for the masthead antenna. Forward and reflected power are as you’d expect. I have an Icom VHF with a GME masthead antenna. Around 20 meters of antenna cable.

On his installation, same antenna, cable length and radio as it turned out, it recorded a SWR of 2.1:1. The SWR reading was also swinging around a bit, starting at 2:1. It slowly climbed and fell over a few seconds before settling on 2.1:1.

I am not sure exactly which cable he installed. But I’ve been looking at the catalogue of the place where he bought it and I think it may have been either RG6 or RG59.

I don’t want to rain on his parade but I get the feeling that those kinds of cable and that SWR ratio are going to lead to disappointment.

Thoughts? Is he likely to have trouble with the radio?

Matt
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Old 30-06-2022, 03:02   #2
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Re: 2.1:1 SWR on VHF

If he has used 75 ohm tv cable I would replace it.

2.1 :1 is generally usable and within ok range. I would have expected much higher with wrong cable. So it might be right cable with a poor end.
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Old 30-06-2022, 03:05   #3
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2.1:1 SWR on VHF

Quote:
Originally Posted by smac999 View Post
If he has used 75 ohm tv cable I would replace it.

2.1 :1 is generally usable and within ok range. I would have expected much higher with wrong cable. So it might be right cable with a poor end.


That’s good to know, thank you. Yes, I did gently suggest he think back and ask himself if he had any doubts about the ends. I didn’t visit the top of the mast but the soldered end at cabin level looked adequate. A bit cooked but not too bad.
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Old 30-06-2022, 05:12   #4
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Re: 2.1:1 SWR on VHF

VHF radios have a have an output impedance of 50 ohms, and operate in the frequency range of 156Mhz to 164Mhz. The three most commonly used coax types are RG-58U, RG-8X, and RG-213 (which replaced the older RG-8U). Which one you choose, is a trade-off between the amount of signal loss in the cable, the cost and the wire size.
RG-59 and RG-6 are 75 Ohm coaxial cables, designed for cable television applications. This impedance miss-match will lead to greatly increased cable losses.
RG-6 and RG-11 both have an aluminum shield material, that can be quite difficult to solder. Attaching a PL-259 connector to the cable, by soldering the braid to the barrel of the connector, will often result in a weak connection. that will corrode, or break, in time.
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Old 30-06-2022, 08:26   #5
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Re: 2.1:1 SWR on VHF

I use crimp PL-259 connectors
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Old 30-06-2022, 08:44   #6
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Re: 2.1:1 SWR on VHF

SWR has to do with length of antenna, tuning...

https://turbofuture.com/consumer-ele...am-CB-Antennas
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Old 30-06-2022, 08:46   #7
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Re: 2.1:1 SWR on VHF

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Originally Posted by goboatingnow View Post
I use crimp PL-259 connectors
Good luck with those.

As to the coax, it seems like an obvious question easily answered - what are the markings on the cable?

If 70 ohm coax, that would account for the mismatch but not the fluctuation observed. That variation is manifested by either 1. A poor connection or 2. Moisture in the coax. Or both.
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Old 30-06-2022, 09:37   #8
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Re: 2.1:1 SWR on VHF

2.1:1 SWR is OK for VHF but I would be more concerned about it "swinging around a bit". An unstable SWR says bad connection to me. Check out both connectors carefully and do continuity checks while flexing coax at the connectors.
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Old 30-06-2022, 13:50   #9
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Re: 2.1:1 SWR on VHF

SWR (Standing Wave Ratio) is about the energy reflected back into the radio output, as opposed to radiated from the antenna. Any flaws in the transmission line will cause reflections - the fact at the root of TDR (Time Domain Reflectometry). TDR has been used for decades to find flaws in cables, by transmitting a signal and measuring the return over time, similar to radar. A poor PL259 - barrel connector - PL259 connection is going to be reflecting energy back into the radio. Of course the antenna itself could be defective; I have one that has a high SWR from the factory (which needs to be sent back on the lifetime warranty). And the wrong coax is another problem. But if I were to bet I would bet on a poor PL-259 installation.

Years ago when the ethernet was done with coax (remember "thicknet"?) my brother was doing a lot of network consulting. When he was called in to fix networking problems as a matter of course he replaced the (non-factory) PL-259 connectors before looking for other causes - and that fixed the problem most of the time. It turns out that most IT "professionals" are not electronic technicians and do not have the skills to reliably make up PL-259 connectors - but they think that they do and proceed to screw things up anyway. And pity the poor outside consultant that has to explain that their networking problems were a result of their poor workmanship. I find that the world of cruising is not a lot different...

If the correct version of a PL-259 is used (with adapter as necessary) and the person doing it knows what they are doing and has a powerful enough soldering pencil, a good solder installation is possible. If a crimp style PL-259 is used with the proper crimper (adjusted properly) a very reliable connection is (almost) guaranteed; these crimpers used to be expensive but I suspect a serviceable one can be bought from HF, AliExpress, or similar. Most importantly the braid needs to be trimmed back carefully - a good nail clipper is the best way to accomplish this - before assembly.

Before tearing everything apart I would remove the antenna from the masthead and connect it through the SWR meter to the radio, using a good bit of the proper coax between antenna and SWR meter, then check the antenna's SWR. (You need the long coax because the shield is the counterpoise for the antenna.)

Good luck,

Greg
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Old 30-06-2022, 16:19   #10
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Re: 2.1:1 SWR on VHF

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Originally Posted by CarinaPDX View Post
SWR (Standing Wave Ratio) is about the energy reflected back into the radio output, as opposed to radiated from the antenna.
. (You need the long coax because the shield is the counterpoise for the antenna.)

Good luck,

Greg
Ah, no. All that need be said here is check the coax for moisture and the connectors for integrity.

The re-elected power doesn’t “go back into the radio”. Rather, it returns to the antenna minus the cable loss each travel cycle.

The shield is simply that, not a counterpoise which as a practical matter is whatever the vertical is mounted on.
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Old 30-06-2022, 16:50   #11
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Re: 2.1:1 SWR on VHF

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Originally Posted by smac999 View Post
If he has used 75 ohm tv cable I would replace it.

2.1 :1 is generally usable and within ok range. I would have expected much higher with wrong cable. So it might be right cable with a poor end.
I disagree, 2.1:1 is pretty bad. Even worse when you consider that a long lossy cable will create false readings on a meter that read better than the true match at the antenna.

I suspect a bad connection or improperly installed connector. Neither RG 6 or RG 59 would be the right cable to use. It should be RG-213 or RG-214. If it's a very short run, RG8x can be used, but that is lossy and will produce the results mentioned. The VSWR will display better on the meter than reality.

There are better cables even than RG-213 or 214, but we can skip that discussion.
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Old 30-06-2022, 16:59   #12
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Re: 2.1:1 SWR on VHF

Quote:
Originally Posted by CarinaPDX View Post
<snip>
Years ago when the ethernet was done with coax (remember "thicknet"?) my brother was doing a lot of network consulting. When he was called in to fix networking problems as a matter of course he replaced the (non-factory) PL-259 connectors before looking for other causes - and that fixed the problem most of the time. It turns out that most IT "professionals" are not electronic technicians and do not have the skills to reliably make up PL-259 connectors - but they think that they do and proceed to screw things up anyway. And pity the poor outside consultant that has to explain that their networking problems were a result of their poor workmanship. I find that the world of cruising is not a lot different...
</snip>
Good luck,

Greg
This needs to be emphasized. I always suggest a professional radio installer terminate the cable ends. Everyone seems to think they can do a good job, and are convinced of it when done. But often that is the issue.
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Old 30-06-2022, 17:08   #13
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Re: 2.1:1 SWR on VHF

Matt,
As others have written here, it is most likely that your friend has "the wrong cable", and/or "improperly-installed connectors".

If you can convince him to look at the cable, please do so.

Fyi....a VSWR of 2.1:1 by itself isn't an issue....although measuring this at the radio end of the transmission line, it should be understood that the VSWR at the antenna end is higher as the coax cable attenuation reduces both the forward power that reaches the antenna AND the reflected power that is being measured by the meter down-below....So, this alone is an indication to me that there might be serious issues with the cable / cable connections / antenna.

And, Matt if you add into this your assumption that he bought "TV" cable (75-ohm coax) like RG-6 (or RG-59, yikes....haven't seen RG-59 sold, except for baseband video cable, in decades....so, this could be decades old cable!), I'd say that your friend really needs to look at his cable and connectors!



And, s/v Illusion....thank you for this!
Quote:
Originally Posted by S/V Illusion View Post
Ah, no. All that need be said here is check the coax for moisture and the connectors for integrity.

The re-elected power doesn’t “go back into the radio”. Rather, it returns to the antenna minus the cable loss each travel cycle.

The shield is simply that, not a counterpoise which as a practical matter is whatever the vertical is mounted on.
I'm happy that I don't have talk about this!

BTW, I was friends with Walt Maxwell, W2DU (not close friends, but friends nonetheless)....and I've got some cool stories he related to me over the years....but, I digress...

I still smile and cringe, when I see folks (some even learned hams) talk of reflected power being lost in the radio (or in the amp, or tuner, etc.)....Ugh!
Of course some power does get "lost" in the feedline (attenuation), and some minor amount may be "lost" in heating of some components, but many times you just cannot get past the old wives' tales!


Fair winds (and 73) to all.

John
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Old 30-06-2022, 17:14   #14
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Re: 2.1:1 SWR on VHF

Quote:
Originally Posted by S/V Illusion View Post
Ah, no. All that need be said here is check the coax for moisture and the connectors for integrity.

The re-elected power doesn’t “go back into the radio”. Rather, it returns to the antenna minus the cable loss each travel cycle.

The shield is simply that, not a counterpoise which as a practical matter is whatever the vertical is mounted on.
It's unusual for the average person to check the moisture content of coaxial cable, or to check the connectors for integrity in any direct sense. That's why previous posts had helpful advice as to checking the performance of the system with common instruments.

With the SWR meter connected to radio with a short length of coax, nearly all
of the indicated reflected power does go back into the radio. That's why higher power good quality radios, like ICOM HF rigs, have circuitry that detects the reflected power and reduces the power to the final stage amplifiers if SWR is too high.

Your point about conterpoise is correct, but the OP will have the antenna down from the mast for a check, so he may not be able to mount it on an equivalent metal structure.
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Old 30-06-2022, 17:58   #15
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Re: 2.1:1 SWR on VHF

Well, this all seems pretty indicative that he will have trouble.

Thank you to those that remarked on the variable SWR, that one was a first to me and I wondered about the connections.

Does anyone want to hazard a guess on the range he could expect with the current setup? The boat is a Sun 43.3, so I figure something like 17 or 18 meters of elevation at the top of the mast.

He decided to replace the original wire (which he thinks was factory fitted and appeared to be RG8 to my untrained eye, whatever it was it was very skinny.) because he was only getting about 3 nautical miles range.
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