VHF marine antenna

[smoke blower]

I have a Question I have a 102" whip that the top has 4" missing off it. Is it possible I could cut the rest of it to the length needed for a vhf Marine radio on my boat

[MDYoungblood]

The antenna in question is an old CB marine antenna?

3's

Greg

[TNT1450]

In short, yes. But you're going to be cutting off far more than is left. You only need 1.49 feet, or roughly 1 1/2 feet of antenna to be resonant on Channel 16 (156.800), the international distress, safety, and calling channel. You could even cut it to be a 5/8 wave antenna, but I don't know the formula offhand to figure the length needed.

*edit* Just looked up the formula for a 5/8 wave antenna, and it's 585/156.8, so the length is 3.73 feet, or roughly 3 feet 9 inches.

[443 Arizona]

I'm assuming you have one of the white fiberglass whips (for example;~8'er's)made by Shakespere ? theyare a bipole design.
i think you would have to shorten from both ends.

[smoke blower]

No the antenna I have is the steal cb 102" whip it came off a guys jeep but some where for some reason 4" has been chopped off . I need a marine antenna so I was just wondering if I could cut that to length it needed to be and use for vhf.

[TNT1450]

That's what I was thinking, steel whip. In short, yes, you can cut any antenna for a higher frequency. It's just a lot harder to lengthen an antenna for a lower frequency.

[smoke blower]

Ok thanks for everyone's help. I will take and cut the antenna 3' 9" and see what happens. It's a shame someone cut 4" off but anyway that's why I figured I would just make it the length for marine

[TNT1450]

Yeah, 5/8 wave will give you a little bit more gain than a standard 1/4 wave whip. Just be sure to check the SWR once you're done.

[smoke blower]

Ok.sure will thanks and everyone please stay safe!

[Grinder74]

At 5/8 you will need a matching circuit to get to 50ohms. 1/4 wave does not need this.

[smoke blower]

A matching circuit made from what I don't understand what is needed here. Basically I just want this to listen to the boats in the creek around me within 4 or 5 miles of me so I hope just the antenna will be good enough I don't want to put a pile of money into this it's just to listen I am not worried about talking on all the time . I would think cutting the steel whip to the right length and be good to go

[smoke blower]

So will cutting the steel whip 3' 9" work for my marine antenna or do I have to do all this circuit matching to have it work or what . Confusing answers here

[MDYoungblood]

If you just want to listen you could cut the whip to 6' 3", that is a full wave for the marine band. If you have already cut it shorter than that then try 3" 1 5/8" for half wave. You could make a matching network out of some 75ohm coax and a little math.

3's

Greg

[smoke blower]

Ok so if I cut the antenna to 6'3" could it be talked on if needed . I don't plan on doing alot of talking but I want to know if I had to for some odd reason would it work fine

[MDYoungblood]

The impedance is close at a full wave so I would say a short keyup or two would blow the radio depending on what brand it is. You will need to get a frequency chart so you don't go on the CG freqs and keep the power low so as not to interfere with ship traffic.

3's

Greg

[smoke blower]

I think I will just leave the whip alone and buy a tram mobile marine radio antenna probably more simple and safe to use

[MDYoungblood]

That is the best idea, an antenna made for the band will work better.

3's

Greg

[TNT1450]

At 5/8 you will need a matching circuit to get to 50ohms. 1/4 wave does not need this.

I was wondering about that. I had a 5/8 wave antenna for 2m on my truck years ago but I didn't know if it had some sort of matching network in it or not. I wasn't sure what the impedance at 5/8 wave was.

[MDYoungblood]

At 5/8 you will need a matching circuit to get to 50ohms. 1/4 wave does not need this.

I was wondering about that. I had a 5/8 wave antenna for 2m on my truck years ago but I didn't know if it had some sort of matching network in it or not. I wasn't sure what the impedance at 5/8 wave was.

Most manufactured antennas use a shunted load of some sort to compensate for impedance mismatches.

3's

Greg

[TNT1450]

Yes, there must be something in the base of this antenna like that.