Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 21-05-2013, 13:23   #1
Registered User

Join Date: Oct 2008
Location: Halifax, Canada
Boat: Hughes 35
Posts: 86
Another VHF antenna problem

I am at lost. Last year, I noticed that the transmission of my VHF was not as it should be. I used the winter to try to figure out the problem. My setup is pretty much standard. One VHF, 10 feet of RG-213 cable to make it to the base of the mast, a female-female coupler, 45 feet of RG-213 cable in the mast and a 3 foot Metz whip antenna at the top. The cable to the base of the mast is new from a year ago. My mast is removed and here is what I did to test/improved the transmission/reception. Fiirst I acquired a SWR tester and measure the standing wave ratio, I got 1.6 (As everybody knows it should be less than 1.4). I tested both cables with an ohm meter for short or open circuit. All was OK, no short or; open circuit when I connected the core and the shield at one end and check the resistance at the other. I replaced and soldered all connectors with shiny new ones. The result is still the same (1.6). I also tried three different VHF, all of them give me the same result (1.6). Before I forget, I must mentionned that my mast is off for the winter and all these tests are done whille the antenna is connected at the end of the first section of the cable in the boat at the base of the mast (which is removed). I tested another Metz antenna and got the same SWR (1.6). I purchased a new Shakespeare antenna and got 2.0 as reading (even worse). I tested the tester on another boat and it read at 1.3 and that system was only using RG-8 cable.

In short, three radios, new cable, new couplers, new antenna and still no joy. I am at lost.

Does anybody have any idea what could cause that poor reading of 1.6?
__________________

__________________
pierrrre is offline   Reply With Quote
Old 21-05-2013, 14:20   #2
Registered User

Join Date: May 2011
Location: Toronto
Boat: Sandpiper 565
Posts: 2,943
Re: Another VHF antenna problem

What frequencies (or channels) are you using for your tests?

The next test I'd try is to put a good 50 ohm VHF load in place of your antenna and repeat the VSWR test. If you still get a poor VSWR into the load, then you have a cable and/or connector problem. Otherwise it's the antenna...

You can also try your tests with a long piece of known-good cable and your radio + antenna; if this setup has good VSWR, then that also indicates there's a problem in your mast or lead-in.
__________________

__________________
Lake-Effect is offline   Reply With Quote
Old 21-05-2013, 15:35   #3
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,888
Images: 4
Re: Another VHF antenna problem

I agree with the 50 Ohm dummy load test.

You might try doing the antenna SWR test at the VHF band edges, and at the middle:
Chan 01A = 156.050 MHz
Chan 74 = 156.725 MHz
Chan 88A = 157.425 MHz

You might find that the SWR is lower at some other frequency. Sometimes an antenna is mistuned, either because the element is (say) too long, or because of proximity to nearby metal. I think the Metz lets you trim the vertical element to tweak the frequency.

If you don't find any cable / connector problems, and can otherwise transmit and receive OK, I wouldn't stress about an SWR of 1.6. That's not great, but not the end of the world either.

There's also the possibility that your SWR meter isn't reading accurately. This can happen if it isn't designed for VHF, or is damaged. Plug the dummy load directly into the meter and see what you get.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 21-05-2013, 15:47   #4
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
Re: Another VHF antenna problem

The SWR readings you'll get with the mast down are not necessarily reflective of the true SWR when the mast is in place.

With the antenna in a horizontal position and in close proximity to the ground, other rigging, etc., the SWR could, indeed, be affected.

Just make sure that there is no short or open in the RG-213, and that the PL-259 UHF connectors are correctly affixed to the coax. There is a bit of an art to doing this correctly, and it takes some time and experience.

If they're OK, I wouldn't worry about it. Step the mast, and try again.

Q: did you use an SWR meter of known reliability and capable of measuring SWR up to 160mHz or so?

Bill
WA6CCA
__________________
btrayfors is offline   Reply With Quote
Old 21-05-2013, 15:49   #5
Senior Cruiser
 
Jim Cate's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 11,471
Re: Another VHF antenna problem

Pierre, I really doubt that a SWR of 1.6:1 could possibly be distinguished from one of 1.3:1 at the receiving end under most (nay, essentially all) conditions. You are talking about a pretty small decrement of radiated power in a mode where line of sight propagation is the general rule.

If you have really suffered from a decrease in effective range of transmission I would suspect a lossy cable in the mast... this would not necessarily show up as a bad SWR. Or barely possible a diminution of transmitter output power... did you check the output level with the SWR meter?

Good luck with tracking it down, though!

Cheers,

Jim
__________________
Jim and Ann s/v Insatiable II , lying Port Cygnet, Tasmania once again
Jim Cate is offline   Reply With Quote
Old 21-05-2013, 17:10   #6
Registered User

Join Date: Oct 2008
Location: Halifax, Canada
Boat: Hughes 35
Posts: 86
Re: Another VHF antenna problem

Thank you for all your reply,

I will test it with a dummy load. At least it would eliminate the antenna variable.

Also, I only did the tests on channel 16

Thanks again
__________________
pierrrre is offline   Reply With Quote
Old 21-05-2013, 18:47   #7
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,888
Images: 4
Re: Another VHF antenna problem

Quote:
Originally Posted by btrayfors View Post
The SWR readings you'll get with the mast down are not necessarily reflective of the true SWR when the mast is in place.

With the antenna in a horizontal position and in close proximity to the ground, other rigging, etc., the SWR could, indeed, be affected.
YES! I forgot to mention that. If the antenna isn't well off the ground (a dozen feet or more), especially if it is horizontal rather than vertical, you will probably get a high SWR. The antenna and cable could be fine.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 22-05-2013, 10:38   #8
Registered User

Join Date: May 2011
Location: Toronto
Boat: Sandpiper 565
Posts: 2,943
Re: Another VHF antenna problem

Quote:
Originally Posted by Paul Elliott View Post
YES! I forgot to mention that. If the antenna isn't well off the ground (a dozen feet or more), especially if it is horizontal rather than vertical, you will probably get a high SWR. The antenna and cable could be fine.
We tested a horizontal mast last week. It was on sawhorses, lots of other masts nearby. I believe it was also a Metz masthead whip, and it measured 1.1 on VSWR. So horizontal isn't always a factor.

Regardless, testing with a good dummy load will eliminate the antenna or antenna position as a variable.
__________________
Lake-Effect is offline   Reply With Quote
Old 22-05-2013, 12:05   #9
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,888
Images: 4
Re: Another VHF antenna problem

Quote:
Originally Posted by Lake-Effect View Post
We tested a horizontal mast last week. It was on sawhorses, lots of other masts nearby. I believe it was also a Metz masthead whip, and it measured 1.1 on VSWR. So horizontal isn't always a factor.
Not always a factor for sure. The wavelength of marine-band VHF is about six feet. Metal stuff beyond a few wavelengths distant won't affect the SWR significantly, and closer items (such as the ground) may not have much effect, depending on the conductivity, and the exact distance. Even a few inches can make a big difference.

Bottom line, it's hard to predict. A good SWR reading is usually a good sign. A poor reading may not be a problem. A dummy load is a great tool for isolating these types of problems.

(I'm completely agreeing with you, just thought I'd mention why.)
__________________

__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Reply

Tags
antenna, vhf

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 08:27.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.