VHF antenna question

[thelm]

I have an 18 foot starcraft aluminum center console. VHF antenna is a 36" stainless whip similar to Glomex RA-109 but older model. Lost the whip last year and don't know how long it was. I have an old CB stainless whip that will fit but not sure what length to cut it. It's 37" right now and I think that is too long. Glomex states 36" but not sure if that includes entire antenna or just the whip. Any thoughts on this?

Also, this type of antenna requires a ground plane. I've always assumed the antenna was grounded to the hull through the mount and that was good enough. Thoughts on this?

I boat on Lake Winnebago in Wisconsin. The lake is 28 miles long by ~ 10 miles wide with several marinas with towers that monitor traffic so 5 miles is plenty of distance.

 

[bruceb58]

Re: VHF antenna question

Don't use the CB antenna...completely different frequency.

 

[thelm]

Re: VHF antenna question

I know that CB runs on a different frequency. Frequency is tunable based on the length of the radiating device. The antenna had a stainless whip on it previously. The CB whip is no different except in length. I just want to know how long the whip should be for VHF marine use.

 

[Splat]

Re: VHF antenna question

The only proper way to do it without blowing the finals in the radio is to tune the antenna with a SWR meter.

Fraction of an inch makes the difference in the total ERP coming out of the antenna, versus the amount being fed back into the radio. The length of the cable also plays a factor in this.

That being said, VHF radios operate at 156-157mhz. This is located in the 2-meter band. Actual size of the electromagnetic wave is something like 1.92 meters crest to crest if i remember correctly. From here you have to option to make either a 1/4, 1/2, 5/8 wavelength antenna, depending on the gain you would like.

The most common VHF marine antenna is 8ft overall, however this is the antennas physical, not electrical length.

Bill

 

[bruceb58]

Re: VHF antenna question

The CB whip is no different except in length.

I understand that but you will have an effective length that is more than just the length of the whip. Does your CB antenna have any coils in it?

 

[waterinthefuel]

Re: VHF antenna question

I picked up a nice 36" marine whip antenna and 15' of cable to go with it already attached for 19.95. You really want to risk frying the radio to make an attempt at a CB to VHF conversion when a really good, brand new antenna costs that much?

I don't think this is worth the risk to be honest with you. You aren't an engineer and missing it by a 1/4" can mean the difference between OK and a cooked radio. You want that?

I sure don't. I wish you luck man. They are two totally different antennas for a reason.