SailNet Community - Reply to Topic

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics > VHF antenna issues?
 Not a Member? 


Thread: VHF antenna issues? Reply to Thread
Title:
  

By choosing to post the reply below you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Message:
Trackback:
Send Trackbacks to (Separate multiple URLs with spaces) :
Post Icons
You may choose an icon for your message from the following list:
 

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Additional Options
Miscellaneous Options

Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Topic Review (Newest First)
10-23-2013 01:12 PM
El Rubio
VHF antenna issues?

I should have added that an FDR is the ideal tool for measuring performance and accurately isolating problems at rf frequencies. TDR's have trouble with all but the worst corroded shielding, poorly crimped connectors, kinked or damaged cables short of open or shorted. The FDR gives accurate return loss measurements at operating frequency and can actually show the bump that exposed shield exhibits before it actually fails, something that would otherwise pass the TDR sweep. My only point was the TDR wasn't a good tool in the case presented.

I don't mean to come across as a nit picker correcting anyone. I forget this is a sailing page and such technical minutia is not very useful to non RF Enginners. I read my earlier post and it sounded a bit like that to me. I hope i didn't offend.
10-22-2013 09:04 PM
OPossumTX
Re: VHF antenna issues?

Quote:
Originally Posted by El Rubio View Post
The TDR is not likely to identify the corroded shield either. Tdr's use a dc pulse that isn't effective at isolating rf cable faults like corrosion, kinks, or badly crimped connectors unless they're open or shorted.
The TDR may not show any reflection at all from a cable that is acting as a dummy load.

What it will show is the absence of the expected impedance bump for the antenna connector.

You should have an idea how long the antenna cable is and if you see bumps in places where they should not be or you don't see bumps where they should be, these are indications of a problem.

The common UHF connector makes an impedance bump that should be easily seen even at the end of 100+ feet of RG58. Also, the TDR should spot the connectors at the base of the mast and something from the antenna.

If the cable is properly terminated using impedance matched BNC, N or some other impedance matched connector, there may not be enough of an impedance bump to see them.

Sometimes what you don't see is every bit as important as what you do see.

Antennas and feed lines! I can see why some people think it is all magic.

Sheesh!
O'
10-18-2013 07:34 PM
svinshallah
Re: VHF antenna issues?

I was guessing, without any evidence, that a shield that had corroded open would show up via TDR. As for corrosion that hadn't completely removed the shield I would expect a less accurate diagnosis with TDR. WRT bad connectors, an antenna bridge or SWR meter is your best choice as long as you correct the SWR reading for cable losses.
10-18-2013 01:20 PM
El Rubio
VHF antenna issues?

The TDR is not likely to identify the corroded shield either. Tdr's use a dc pulse that isn't effective at isolating rf cable faults like corrosion, kinks, or badly crimped connectors unless they're open or shorted.
10-15-2013 01:56 AM
OPossumTX
Re: VHF antenna issues?

A geophysical boat had a problem with their Inmarsat terminal. Repeated service calls by various Rep company techs and the factory techs didn't find the problem. I DID!

A little cut in the cable jacket let in saltwater spray. The shield coroded and turned the coax into a very effective dummy load. The only external sign of the damaged cable was a slight increase in diameter, well, that and a complete lack of conductivity to RF.

A time domain reflectometer would have been a welcome addition to my bag of tricks!

Have FUN!
O'
10-11-2013 08:11 PM
svinshallah
Re: VHF antenna issues?

You're right on about the corroded shield. I've only seen it once, but I wasted more than enough time actually finding the problem to have bought a Fluke TDR meter about 3 times.
10-07-2013 12:02 PM
OPossumTX
Re: VHF antenna issues?

Quote:
Originally Posted by svinshallah View Post
In over 50 years of working on radios, the biggest problem by far is that the connectors on the coaxial cable are badly installed. The gold-plated twist-on connectors are worthless in a salt-water environment. Even if the connector doesn't corrode, the wire will. Using an SWR meter on a mast-head mounted antenna is NOT a reliable way to check the antenna/cable. Measuring SWR assumes that there is no loss in the cable. In fact, RG58 cable (the 1/4" diameter stuff that is cheap and easy to run) loses about 80% of the power in 100', thus the SWR will be low even if the antenna is shorted or missing.
You are right a about that! Another less common failure is corrosion of the cable shield due to a small cut or gouge in the jacket allowing water into the shield. The cable looks great to the SWR bridge but nothing much gets to or from the antenna.

About the easiest way to diagnose this problem is to measure the radio output power at the radio and compare that reading to a measurement taken at the antenna. You will always see some drop. But if nearly nothing is getting through the feed line, Check the connections first, if they are good, change the feed line.
10-06-2013 04:48 PM
svinshallah
Re: VHF antenna issues?

In over 50 years of working on radios, the biggest problem by far is that the connectors on the coaxial cable are badly installed. The gold-plated twist-on connectors are worthless in a salt-water environment. Even if the connector doesn't corrode, the wire will. Using an SWR meter on a mast-head mounted antenna is NOT a reliable way to check the antenna/cable. Measuring SWR assumes that there is no loss in the cable. In fact, RG58 cable (the 1/4" diameter stuff that is cheap and easy to run) loses about 80% of the power in 100', thus the SWR will be low even if the antenna is shorted or missing.
08-12-2013 04:09 PM
El Rubio
VHF antenna issues?

I see that kind of thing all the time. You paid for professional installation and they don't even have a proper crimping tool. The bad thing is they probably get away with it most of the time because that will work well enough and nobody will know better. I have seen where the installers will "hide" the poor crimp with heat shrink tubing. The dead giveaway in one case was the use of butyl tape on connectors inside out of the weather. The tape was holding the connection together.
If wrapped up good, your connector should be good for quite some time.
08-02-2013 05:24 PM
GMFL
Re: VHF antenna issues?

Pic of old connector for any interested, just sloppy work imo.

This thread has more than 10 replies. Click here to review the whole thread.

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


All times are GMT -4. The time now is 01:08 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.