Notices
Like Tree1Likes

VHF Antenna for a bay boat?

Old 09-05-2017, 01:13 PM
  #21  
Senior Member
 
Join Date: Jun 2010
Posts: 4,458
Default

Originally Posted by Brokenline View Post
20' seems to be a standard length of wire provided. It seems like I've read that certain lengths are better/worse than others. Would shortening this length be adverse? I really only need about 3' or even less.
Most radio manufacturers recommend a minimum of 4' of coax, to get the RF radiation further away from the radio itself. Line-loss on 20', even if just RG-58, will be minimal, and there is nothing wrong with neatly coiling the excess, preferably on a cylinder form, like 3" PVC pipe...even has slight advantage as acting as a common-mode choke. Don't exceed the minimum bending radius for whatever coax the antenna uses, however...you may need a 4" or even 5" form, and keep each turn parallel to adjacent turn. For examples, see:

https://www.google.com/search?q=comm...iw=873&bih=404
Karl in NY is offline  
Old 09-05-2017, 08:39 PM
  #22  
Senior Member
 
Join Date: Dec 2002
Location: Southeast Michigan
Posts: 2,626
Default

[QUOTE=Brokenline;10645024]I'm also curious [about] trimming.../QUOTE]

GAM ELECTRONICS sells a specific VHF Marine Band antenna version which is already tuned for 156.800-MHz and the whip has been pre-cut. This is mentioned in the article I wrote and linked-to in my initial reply.

If you get a whip that is not pre-cut, the antenna maker usually supplies a cutting chart. You just measure the whip with a ruler and cut it to the proper length based on the frequency you plan to use it.

The whip for a quality antenna like the GAM is made from a hard and springy steel, so you may have to cut it with a little grinding wheel, like found on a Dremel tool, or use a hacksaw blade that can actually cut hard steel--not the 25-year-old blade in the hacksaw you borrow from your neighbor.

The GAM SS-2 antenna whip is very flexible and does not take a set when it is deflected from its normal position, although I image you could do that it you really set out to bend it up.
jhebert is offline  
Old 09-05-2017, 08:46 PM
  #23  
Senior Member
 
Join Date: Dec 2002
Location: Southeast Michigan
Posts: 2,626
Default

Originally Posted by Brokenline View Post
...It seems like I've read that certain lengths are better [or] worse than others. Would shortening this length be adverse? I really only need about 3' or even less.
There is plenty of very wrong information passed around about antennas and their transmission line lengths. In an antenna like the GAM SS-2, the transmission line is matched to the antenna, and the length of the transmission line does not affect the antenna performance. It can be cut to any length, but I would not cut it shorter than about four feet.

It is more important to lead the transmission line away from the antenna and keep it inline with the vertical radiator before you begin to run it horizontally. The transmission line should be kept out of the main lobe field of the antenna radiation, otherwise the transmission line, like any conductor in the near field of the antenna, will have antenna current flowing on it. Those currents will affect the radiation pattern--and so will any other conductors near the antenna. This situation is another reason why I like to mount the GAM SS-2 atop a four-foot extension mast and to run the transmission line down the extension mast center. It gets the transmission line out of the antenna field as much as possible.

And, as KARL mentions, I coil the excess transmission into a solenoidal coil so that it might act as a choke balun. But that is another topic and too technical for this discussion.
jhebert is offline  
Old 09-06-2017, 08:20 AM
  #24  
Senior MemberCaptains Club Member
 
Join Date: Jun 2002
Location: S. FL then Erie Canal, NY, next NE FL
Posts: 1,625
Default

Originally Posted by Karl in NY View Post
Most radio manufacturers recommend a minimum of 4' of coax, to get the RF radiation further away from the radio itself. Line-loss on 20', even if just RG-58, will be minimal, and there is nothing wrong with neatly coiling the excess, preferably on a cylinder form, like 3" PVC pipe...even has slight advantage as acting as a common-mode choke. Don't exceed the minimum bending radius for whatever coax the antenna uses, however...you may need a 4" or even 5" form, and keep each turn parallel to adjacent turn. For examples, see:

https://www.google.com/search?q=comm...iw=873&bih=404
Take this with a grain of salt, but I suspect air wound chokes are hard to make work at VHF and above. Too much turn to turn capacitance. Ferrite beads work great, so do coaxial line chokes (no turns). Even turns around torroids are problematic at VHF and above.
diver dave is offline  
Old 09-08-2017, 03:24 AM
  #25  
Senior Member
Thread Starter
 
Join Date: Oct 2013
Location: South LA
Posts: 802
Default

Thank you jhebert, and everyone else for their help. I'll get my antenna and radio ordered, and I'm sure I'll come back with another question or two. (that I'm sure are already answered in jhebert's article (that I THOUGHT I read, but evidently just scanned.)

Not sure I dare ask if there is a particular radio that I should/should not consider.
Brokenline is offline  
Old 10-07-2017, 11:07 PM
  #26  
Senior Member
 
Join Date: Mar 2017
Posts: 486
Default

Brokenline,
By all means, ask!
At the risk of hijacking the thread, there is some great dialogue going on here.
This is one of the best threads ever on THT, involving really knowledgeable input from people who are not just blowing their horns!
Thanks to JHebert and all that have participated.
nuffaredy is offline  
Old 10-08-2017, 07:26 AM
  #27  
Senior Member
Thread Starter
 
Join Date: Oct 2013
Location: South LA
Posts: 802
Default

Well, I bought a cheap radio, a Uniden UM415 Oceanus D for like $115. Maybe should've spent more or done more research, but it's easily changed if it doesn't perform well enough... the antenna, on the other hand, not as easily changed.

I've got everything installed, except getting the PL-259 antenna coax connector on the coax. I messed one up (grounded it out), and realized this when I tested it the resistance (as I learned on youtube)... BEFORE I tried it on the radio. So, just haven't got over to the boat to finish that... maybe today. Weather and life has interfered with fishing, so just hasn't been an urgent deal.

I used the 4' whip antenna that is blogged about above... so we'll see! (trimmed to the proper length!)

Comments on cheap radio are welcome. Again, this is on a bay boat, so it will be well tested inshore before venturing out too far.

New t-top with antenna installed...


Last edited by Brokenline; 10-08-2017 at 07:47 AM.
Brokenline is offline  
Old 03-02-2018, 02:54 PM
  #28  
Senior Member
Thread Starter
 
Join Date: Oct 2013
Location: South LA
Posts: 802
Default

jhebert, still around?

I did go with the Gam SS-2 whip antenna. Unfortunately, not even sure how it performed...

Evidently, I didn't tighten the set screw in it tight enough, as I lost it on the first trip or two.

Any chance you know where I might could just purchase the whip part of the unit?
Brokenline is offline  
Old 03-03-2018, 07:55 AM
  #29  
Admirals Club Admiral's Club Member
 
Join Date: Oct 2015
Location: Phoenix, AZ
Posts: 748
Default

Just going from a vague memory but I think Defender sells them. Or you could contact GAM directly.
JiminAZ is offline  

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 On
Refbacks are Off


Thread Tools
Search this Thread