What's inside the VHF antenna can

[yelkenli]

I am curious as to what is inside the VHF antenna - the can - and if I need to worry about it failing. I have the mast down on this new-to-me boat and can spend time to check it and replace it. I am thinking it could be a never-fail item. And I am curious. I struck out searching on the internet.

 

[dlochner]

It is a loading coil that makes the antenna act like it is much larger. There are of course more technical explanations, for me this is close enough.

Yes they can fail if water gets into the can or the connections corrode. Corrosion, even mild corrosion, can seriously affect the antenna's efficiency. And then there is the coax, it can and will degrade over time. Might as well replace that too.

This article may be helpful.

Easy VHF Terminations - Marine How To

If You Don't Know How to Solder..... If you really don't know how to solder a VHF PL259 connector there are other options as you'll see below. This one is laughable, except that when this owner needed his VHF it was no joke or laughing matter that it failed to transmit. The previous owner or...

marinehowto.com

 

[Don S/V ILLusion]

quick and dirty way to check coil is a simple continuity test.

 

[jssailem]

Oh, the mystery of electronics.
Here is a brief on a VHF Coil that might illuminate your quandary.

Antenna loading coil: Everything You Need to Know

An antenna loading coil is an inductor placed in series with an antenna element in order to lower the antenna’s resonant frequency. A standard dipole antenna is resonant if constructed with a length of one-half wavelength. A vertical antenna (effectively half ... Read More

www.amateur-radio-wiki.net

If the wire is broken, it will not work as desired. If you can transmit a signal and be heard by another VHF radio, you're good.

 

[dlochner]

If the wire is broken, it will not work as desired. If you can transmit a signal and be heard by another VHF radio, you're good.

This is a gross simplification. There is quite a bit of science and engineering that goes into designing and installing an antenna. An antenna may work, but how well is it working? Another boat in the marina may hear you, but what about a boat 1 mile away or 10 miles away? What is the quality of the signal, is it all static or is the voice clear and understandable?

Antennas are also used for reception, a good quality antenna with a good low loss cable will receive weaker signals than an old antenna with old coax.

If you know any amateur radio enthusiasts, one of them would be gland to help you determine the condition of your antenna system and to help with any replacements. They will also have an Standing Wave Ratio (SWR) which is pretty definitive for determining antenna condition and efficiency.

 

[JoeWhite]

Continuity test failing does not indicate a failed antenna. My antenna has a capacitor in series with the coil. Only a SWR meter can indicate its health. If it is replaced it must be tuned after mast is reinstalled.

 

[jviss]

They will also have an Standing Wave Ratio (SWR) which is pretty definitive for determining antenna condition and efficiency.

Yea, there's a lot of voodoo on this thread already. Dave is right. The loading coil must be in good condition. I've seen ones that were flooded and really hampered operation. An SWR meter is essential. Fortunately, they are quite inexpensive.

The loading coil adds inductive reactance to counter the capacitive reactance of the antenna, so the whole thing looks resistive to the cable, and no energy is reflected back, meaning, all of it is used to transmit/receive. A tuned whip without a loading coil at marine VHF frequencies would be impractically long.

You can get a VHF SWR meter on Amazon for about $40. A new Shakespeare 3' VHF whip is about $120. Cheap insurance.

Also, I've encountered many VHF radio problems caused by flooded (wet) coax cable. That's where the meter can help. A bit tougher to correct than a new whip. Make sure you use real self-amalgamating tape to seal connections outside, not just vinyl electrical tape. Or, use adhesive-lined heat shrink if possible. But make sure it's water tight.

 

[Mark Maulden]

Use a 1/4 wave whip without a loading coil. It’s somewhere around 22 inches long. 1/4 wave for ham 2meter is 19”. I actually use my marine vhf for ham 2 meter and it works quite well. (144-148 Mhz).