VHF antenna questions

#1
Hello all , good to have the forum back . Changed my name, I used to be iceberg ..themarinator is the boat name but anyways, quick question about Cbs and radios for Powell

I scored a nice old cobra cb radio recently and am going to add that to my boat ,never had one before and soon I will be taking my boys to the pond with me so it will be good to have a fixed mount , I always just brought a small handheld.

So my question is about antennas and Powell . With all of the cliffs and such ....what size should I get ? Aren't VHF all line of sight ? Meaning that a 30 foot antenna will basically be the same as 3 when you are in the canyons ? My boat has never been to any other lake than Powell so keep that in mind , no ocean trips or anything.

They have these nice stainless 3footers , or of course the common 8 footers . Any tips will be appreciated .

- Collin
 

capt.catfish

Well-Known Member
#2
Hey Collin,

Just to be clear, a Citizen's Band (CB) radio isn't the same as having a fixed mount Marine VHF. Not sure if you were referring to an actual CB or just using it as short hand for a fixed mount Marine VHF, but wanted to clarify, since they operate on differently defined frequencies and won't talk to each other. I assume that NPS probably monitors CB channels as well, since there are probably plenty of people on the land in Glenn Canyon NRA that might want to talk to them, but I don't know if you'll be able to raise anyone else on a CB. Not saying it wouldn't be useful, maybe if others in your group also had CBs you could talk to them, but just want to make sure you're getting what you expect.

Other than that, there's a lot of technical stuff about electromagnetism and such that goes into antenna selection. The two main determining factors in range are antenna height (you are correct, it is pretty much a line of sight system, but you can often get transmissions over and around features since an antenna "sees" differently than the human eye) and the transmit power. Marine VHF is limited to 25-Watts of transmit power, but some channels with special uses are limited to 5-Watts (like VHF Channel 13, intended for maneuvering communication from bridge-to-bridge on larger ships, since ships communicating on it should by definition be in visual sight of one another). By going to a fixed system you will definitely get a boost in range, since most of the handheld radios probably have a maximum transmit power of 3-Watts.

Antenna length is a factor of wavelength (Marine VHF operates in the range between 156.025Mhz to 157.425Mhz and is often referred to as the 2-meter band, since that's roughly the average wavelength in that band), and VHF antennas are usually 1/4, 1/2, or 5/8 of that wavelength. The longer the antenna the better the range, at least theoretically, as it narrows the lobes focusing the transmit power and increasing the range; however, the drawback is that if you are rocking, like on a boat at sea, those narrow lobes can miss the target you're trying to contact and, if you're trying to transmit over obstructions, the narrower lobe often won't extend over the top of the feature like the wider lobe of a shorter antenna. Now, the physical length of the antenna may not be the actual antenna length; for example, the 8-ft fiberglass antenna you buy might actually house two 4-ft antennas stacked to increase range by shaping the waveform. I'm realizing that I've gotten off on kind of a tangent, but the point I'm trying to make is that the 8-ft antenna probably won't have a very noticeable difference in performance over the 3-ft antenna; either will be drastically better than the built in antenna on a handheld. I would even venture to say that the 3-ft antenna might give you better range since the lobes are wider and should allow you to transmit over obstructions better.

Just mount the antenna as high as you feasibly can on your boat to improve your line of sight and get an antenna that fits your boat well.
 
#3
Interesting . Yeah mine is basically a trucker one , not a marine specific . I didn't realize that they ran different channels (mine is 1-40)

Guess I will have to get a marine specific model .

Thanks for the info there !
 

capt.catfish

Well-Known Member
#4
I just bought a Uniden off of Amazon a few months ago for $85 or so.

As an additional benefit, it will be designed for the marine environment and will be DSC capable. Just be sure to register online to get your MMSI number and input it into the VHF; that way when you hit the Distress button on the VHF it will broadcast your vessel's information (you can register for free with USCG Auxiliary or SeaTow. you can also register with BoatUS, if you are a member). You'd also want to hook it up to your GPS, if you have one, so that it can automatically send your position with the distress message. You can also buy some radios with built in GPS, but they're a little more expensive.

There was a time when CBs were cheap in comparison to Marine VHFs and a lot of boats carried them instead. As a result, the USCG monitored the civilian bands, but I think they discontinued that practice in the late 1980s.
 
#5
The three-foot antennas transmission and reception will be shorter than an 8 foot antenna. If you don't want an 8 foot antenna then look at the Morad 4-1/2 foot antenna. It has the same DB as an 8 foot antenna. The Coast Guard uses them on the West Coast . Don't buy a cheap antenna just to save money it is more important than the radio. Part #4120.
 
Last edited:

potter water

Well-Known Member
#6
Vhf marine frequencies are in the 100 megahertz plus frequency range and are line of sight. But higher frequency than CB, (CB is in the 27 mhz freq range), and therefore will go further on less power line of sight. Don't bother mounting a CB fixed mount. They are worthless on the lake. Marine VHF is your best bet, but even then, there are many places where even through the park service repeater, you won't be able to make contact. We use a fixed mount marine radio and carry a cheap handheld to keep track of shore excursions. The longer and higher the antenna, the much much better will be your com with marine radios. Get it as high as you can and as long as you can. A longer shakespear antenna has the ability to increase you effective power from your radio.
 

capt.catfish

Well-Known Member
#7
All other things being equal, the difference between a 3-ft antenna and an 8-ft antenna is the transmit and receive pattern (i.e. how the antenna "sees"). This is often referenced as dB of gain in sales literature; the higher the number the better the range of the antenna, but I don’t think that accurately depicts what is going on.

You can picture the transmit/receive pattern extending out from the center of the antenna and rotated around the antenna in a uniform toroid or "donut" like shape. If you view a cross-section of the transmit/receive pattern, you would see what I would refer to as a lobe (my background is more in acoustics for sonars and my terminology may not always be accurate, but it translates to electromagnetic transmissions as well). As you make the antenna longer, the transmit/receive pattern will become narrower and more focused; in effect you are putting more power into a narrower area, producing longer range, like the beam of a spot light as opposed to a flood light. The lobe of a shorter antenna would be more diffuse and broader with a shorter maximum range (like a flood light), while a longer antenna would have a narrower pattern with greater overall range (like a spot light). Please excuse my rudimentary Paint diagram, but the blue lobe would be that of a 3-ft antenna and the green lobe would be that of an 8-ft antenna.



The increased range with the more focused pattern is great, but it can present an issue on Lake Powell and I think might actually adversely affect the range. Diffuse patterns like the 3-ft antenna can overlap and "see" over obstructions, like canyon walls; the narrower pattern of the longer antenna may not "see" over the obstruction. The area inside that lobe is not only the area that the antenna can transmit its signal, but it is also the only area where it will detect another transmitted signal. Your antenna doesn’t have to “see” the other antenna to communicate with it, their transmit/receive lobes just have to overlap. Again, please excuse the crude Paint illustration.



The other issue with longer antennas are that on smaller boats that pitch and roll a lot, the lobe will move up and down and not point perfectly horizontal, which can mean that you do not get steady reception with other stations. A shorter antenna's broader lobe isn't as critical to keep level since its wider pattern covers the area even when the boat is heeled or pitching. Also, 8-ft antennas are more prone to damage and can be difficult to mount on a smaller boat. On a houseboat, an 8-ft antenna is feasible, but on a smaller boat I would opt for a shorter 4-ft antenna or even the 3-ft antenna.

Additionally, it’s important to note that the physical length of an antenna may not be the antenna length. For example, the 8-ft fiberglass antenna you buy might actually house two 4-ft antennas stacked atop one another. This is done to get benefits of what I would refer to as "beam forming" to improve performance of the antenna, but I'm not sure I fully grasp how they are "beam forming" and I don't really want to do the research.

My point is that the 8-ft antenna probably won't have a very noticeable improvement in performance over the 3-ft antenna on Lake Powell. I’m not advocating getting the cheapest antenna you can, you want to get a good quality antenna, I’m just saying that length shouldn’t be a driving factor in choosing the antenna. It’s more important to find an antenna that fits your application well (i.e. the size of your vessel) and I took the original poster to be referring to a small trailer-able boat.
 
Top