VHF Antennas

SearayPaul

SearayPaul

Active Member
Oct 11, 2006
961
Simpsonville, South Carolina
Boat Info
34 Downeaster
Engines
Cummins 5.9 480hp
A couple of people have asked me over the last couple of weeks to post an article on VHF antennas.

My information comes from real world experince, not everyone will agree. I have bought enough antennas with my ham radio over the years to buy Gary's boat and antenna mast.

VHF is line of sight communication and there fore if you can't see it you can't talk to it so get the antenna as high and clear as possible. VHF looses a lot of signal going down the coax so buy a good low loss coax. No barrels, no splices, and no cuts should be used on the coax. Only a single run from antenna to radio.

Antennas commonly available are 3db, 6db, and for those with a lot of room 9db. The db moniker details the gain of the antenna which describes the electrical performance of the antenna. The higher the gain the more amplification of the incoming and outgoing signal and the easier it is for your radio to detect a signal and turn it into usuable information. The higher the gain the more concentrated you signal is in one direction. This is not a big factor in marine antennas but can be a big issue with a phased array. Gary needs one on his antenna mast. I really like his mast.

The gain is obtained by changing the wave length of the antenna from a 1/4 wave to a full wave antenna with the exact length to be determined by the gain of the antenna. If I remember correctly the wavelength is the inverse of the frequency, not sure about that formula. You can lengthen the antenna two ways.

You can use the calculated physical length of the antenna or you can put a loading coil in the antenna. A loading coil is nothing more than wrapping a wire of known length around a spool and feeding it with the coax. A piece of wire is then dropped down the coil to finish the antenna. To tune this type of antenna you move the outsided element up and down the coil and tighten it down when the antenna is adjusted for your frequency. This is usually done at the factory. Guys this is the simple version.

You can pay a little or a lot for an antenna and everyone asks me what you get. The higher priced antennas usually are better constructed than the cheap no name antennas to a point. Those 8 foot antennas are nothing but a piece of wire that needs to be supported by some method. There is no magic powder in the antenna to make it work better. This is about wave propegation and powder does not resonate. The fiberglass is there strickly for strength so that you can run into bridges without breaking the antenna off. The base loaded antennas all have a coil of wire with a covering to protect the coil and an external element. The quality of the coil and the stength of the external element determine the price.

A 3db should antenna should work as good as any other 3db antenna as long as the gain of the antenna has been determined correctly, the same is true for any antenna of similar gain. Spend what you want but 3db is 3db. Sales booklets are a lot like what PT Barnimum said many years ago. As long as you go with a name brand antenna you will be fine.

At a ham radio 2 meter contest we constructed an antenna out of 20 gauge wire, wooden dowels, and an electrical match. We then competed against a fellow with a very expensive commerical antenna. Care to guess who made more contacts? In the interest of fair disclosure we did tinker with that antenna for a long time before we got it to work correctly. I hope this helps someone.
 
Paul;

While I generally agree what you are saying in principle, the only caveat I would make is that the signal is not amplified by a higher gain antenna, but rather, its ERP (Effective Radiated Power) is increased along the characteristic lobes of that antenna, which in effect concentrates the available power in a narrower beam.

If you think of a whip antenna's propagation pattern like a donut lying on its side, the more you flatten the donut, the higher the gain the antenna is. A "flatter donut" will be one that has a longer range along the "core", but less range at the periphery.

In other words, changing the gain of the antenna changes the shape of the antenna's electromagnetic propagation.

So in reality, there is no amplification - the apparent increase is merely due to the available energy being concentrated along narrower path.

One side-effect however, is that with a higher gain antenna having its transmission energy in a smaller cross-sectional area, while it might not affect ship-to-ship communications; it could reduce long-range ship-to-air communications.

By the way:

Wavelength (in meters) = 300/frequency (in hertz)
 
OK... I'm going to hijack the thread... get over it.

If you were given a 480, how would you install a SSB radio/antenna and ground it?
 
AW: you are absolutely correct. I started down that path and then deleted the paragraphs about wave propagation and directional antennas. Thank you sir for providing the formula. With advanced old age setting in I can not remember Jack and that formula I posted just did not look right.

Gary: I would lean towards a marine SSB antenna and an automatic antenna match. You do not say what frequencies you are planning on using but I am sure you can not use just one antenna to cover all of the amatur bands. The antenna probably will be too long to mount on your antenna mast as they appear to be about 18 foot or so and need a support somewhere. In addition to the antenna you will need a matching box or antenna tuner to match your antenna to the radio at the frequency you are working. Without the match you will not be able to transmit or receive and could damage your equipment faster than those goldens do. This box electrically lengthens and shortens the antenna as needed to match your operating frequency. Get the automatic model so that you don't have to remember to adjust.

Check your SSB manual for specific directions on grounding. Without reading your book I would run a ground wire from the antenna match to the ground system of the boat. There probably will be a lug on the back of the antenna tuner marked ground. Some radios also require a chasis ground, if so your instruction manual should tell you. The mistake most people make is making a mole hole into a mountain. Your ground is what it is and there is not much you can do to change it in a boat
Disclaimer I have not put an SSB on a boat but have had lots of antennas in my yard. I can not at this time think of a single reason this won't work.
 
I already have an ICOM IC-M802 SSB radio and the matching tuner and a 16 foot antenna (by Digital Antenna) for the port side of the boat. Just was wondering what your opinion of the underwater grounding plates and copper ribbon connections was... I don't think a 16 foot antenna would look right or work on that mast. ;-)
 
Gary you are ready to go with that setup. The ground is fine and is the best you can do with a boat. I would check the connections to make sure they are not oxidized or corroded and then use it.
 
SSB Ground Plane

Hey Gary:

SSB rdios are the bomb for the serious cruiser. You've got the room and the will to wander, so a SSB is perfect for you to stay in touch and tune in the Vienna Orchastra's live performances.

SSB is an AM TxRx unit and AM needs a ground plane (to push the signal off) efficiently. You probably have one of those bronze BB type ground plates for the VHF already. These wetted surface type ground plates will advertise an incredible SqFt equivilency, but in the reay world all those little BB's get fouled easyily. They work well but must be kept open to the sea water vs. being coated in uyster shel and barnacles.

I have insalled these radios on several fiberglass boats. I've found installing two ground plates, one on each side of the bottom separated by 8-10 feet will give you the efffective Sq.ft. of ground plane to get soild transmissions with SSB. These ground plates should be connected by copper ribbon separately led to the radio equipment and to each other. This ground will give the SSB a solid push.

I know some radio nuts that have had 200 sq ft. of copper screen glassed over the bottom of their hulls to get the biggest ground possible. That is extreme.

On the otherhand the steel and aluminium hulls are set from go.

The Automatic Tuner is essential.

To keep everything working Peak on SSB, keep the cable lugs spotless clean, the antenna connections clean and when Tx, the batteries must be peak. These radios are huge power consumers and its best to curtail other DC draws while using the radio so all the juice is available.
 
This is the ground plate I was thinking of using for the RF ground:

http://www.consumersmarine.com/modp...ounding_Plates&cname=&r=view&i=880008&aID=41M

However, it's not a case of just drilling holes and bolting it on since the bottom of the 480 is cored. I need to get up with Sea Ray on how to do that right. I think they have to bore out a bigger hole on the inside of the hull and glass it in first then drill through the solid glass to mount this... but I need to verify that.

You say two of these are better spread apart? Do you just connect them with the copper ribbon or do you run the ribbon from each plate up to the antenna/tuner and radio individually. I've read somewhere that the ribbons should not daisy chain the equipment but should have independent runs. Is that your understanding?

I've also heard these grounding plates and copper ribbons are lightning rods and people have come back to their boats after a storm and the SSB antenna is "missing". Have you heard that?
 
Paul;

I appreciate your effort to explain antennas - its almost voodoo science to many. And, I know what you mean by getting old and forgetting stuff - believe me, it has happened more than I can remember... :grin:

By the way, here is a little drawing I like to use to explain ERP.

vhf22.jpg


With the flashlight in the Wide angle mode, that can be thought of as a 3dB gain antenna, and when in the spotlight mode, think of it as a 6dB gain antenna.

The ERP will be greater for the 6dB gain antenna as long as you are in the central cone, because the higher gain has the effect of concentrating the energy into a narrower beam.

But the down side is that in the 6dB gain mode, the periphery will have less energy.

Once that is understood, the next figure helps as well.

vhf21.jpg



As I mentioned, a whip VHF Marine antenna has a radiation pattern not unlike a donut on its side. Here is a drawing of the "donut" cut across the vertical plane to show its cross section. A VHF whip antenna like this is known as an omni-directional antenna.

vhf23.jpg


Lastly, you can map the different gains of the VHF antenna, and see how the donut shape and distance change due to the various antenna gains.
 
AW

Great explanation of how an antenna works. It is too bad that most people believe antennas are voodoo science because the antenna companies take full advantage of that. Your drawings are right on the money. If you don't mind the next time I explain antennas I am going to use your flashlight concept before I draw the radiation patterns. People tend to get lost when the drawings come out and that flashlight concept is easy to understand.

Gary needs to get something like this. It is not quite the right frequency but a little customizing and he will be in business. I used a pair of these stacked for 2meter DXing for a while. As long as you new where the target was you were going to make the contact. If you were off by a degree or two you could not hear the station.

http://www.hamcity.com/images/26b2lg.jpg
 
Paul, that is a nice looking pair of stacked Yagis. Methinks you ought to be somewhere up around 17-20 dB gain with that if you can get the matching right. Wish I had a nice set like that cut for FM radio band so I could do some DX radio. There is not much to listen to here in jackrabbit and cowboy land.

The drawings AW has done are quite good. I think the patterns for the 3, 6, and 9 dB antennas are not quite to scale. The 3 dB antenna should reach half as far as the 9 dB antenna, and it doesn't quite do that in the drawing. The reason for that is that a change of 3 dB is half or double power, depending on whether you are going up or down. 6 dB is 1/4 or 4 times the power. When you change the distance from the antenna by a factor of two, you change the power level by a factor of 4, or 6 dB. So to go twice as far takes 4 times as much power, or 6 dB more antenna gain.

Probably nobody cares about the paragraph above, though. What you should see from AW's drawing is what happens if you tilt your antenna at a nice angle to match your radar arch. If you have a 3 dB antenna, you will still be nicely on the lobe of the antenna even with a 30 degree tilt. But if you swap out the 3 dB and put in a 9 dB, you will be off the lobe of the antenna pattern, and you will actually lose effective power in the horizontal plane. The 3 dB will work better than the 9 dB.

Another thing that could be a minor issue is that the more gain you get out of an antenna like we are discussing, the more narrow-band it becomes. So normally it will be easier to keep the reflected power down across the VHF band with a 3 dB antenna than it will with a 9 dB antenna. The stacked Yagis that Paul linked to really would not work for me to play with across the whole FM radio band. You can see from the photo that the antenna has very little taper, or change in length of the cross elements, from front to rear. That makes for good gain, but very narrow bandwidth.

I suspect when Gary talks about SSB, he is referring to the HF and MF bands. These lower frequencies are a whole different beast, I have no experience with them.

You should probably ignore all of this post except the third paragraph.
 
Since I stuck my nose into this post and used the term dB to refer to antenna gain, I should make a disclaimer and state that if someone states antenna gain in dB then they are making a mistake, or an assumption, or maybe trying to fool you. The reason is that dB refers to the DIFFERENCE between two different things. When someone states an antenna gain in dB, they are telling you how much better it is than some reference, but they are not telling you what that reference is.

There are two commonly accepted references. One is an isotropic radiator. It radiates equally in all directions. like a light bulb in free space. North, south, east, west, up, down, all the same. When an antenna is compared to this isotropic radiator, it is proper to list the gain in dBi (for isotropic). Some VHF Marine antenna specs may be using the dBi number when they just say dB.

The other common reference is a dipole. It is not easy to make a real isotropic antenna, it just makes for simple math. But it is easy to make a dipole, so you can test your antenna against a dipole and get real world measurements. When antenna gain is referenced to a dipole, then it is proper to specify the again in, you guess it, dBd! I think the dBd number is most often used. The rub comes in that the number for gain in dBd is 2.15 dB lower than when it is specified in dBi. So an antenna with a gain of 6 dBi has a gain of slightly less than 4 dBd.

More gain is normally considered better. So you can see that some manufactures may want to tell you the gain in dBi, not dBd. And if they conveniently forget to put the 'i' in dBi, you won't know which it is, and may just assume it is dBd. Just the marketing group (those are the ones with the party hats) in action!
 
Dave

dbI and dbD. Now you have exposed the voodoo in antennas. Marketing loves voodoo and magic powder to sell their expensive antennas. If I remember correctly db is also a logarithmic function this also exsaserbates the reported ERP problem since a log is not a linear function.

You electrical engineers are approaching the end of my knowledge on this subject. The cob webs in the ole brain are being knocked around.
 
Great info, thanks all.

I don't think I have a range issue, but the static and squealing is so bad that I have to turn up the squelch or turn down the volume so the radio is useless. I just noticed that Shakespeare has an 8' Galaxy 5225-FLT that has a filter to suppress out-of-band signals. Would that help, or is it just hype? I would put it where my current 8 footer is.
 
Nobody else has answered yet, so I will take a shot at it. But my opinion is of less value than someone who has lots of experience with marine radios and antennas. Mine is with ground based stuff.

You obviously should not have all that static and squealing. So there are a couple of questions you might ask. First, what is the source of all the noise and static? If it is something on your boat that needs fixing, fix that first. Try turning things off and see if you can determine the source.

Second, how is it getting into your radio? If it is coming in through the antenna, then a new antenna may help. But if it is coming in through the coax instead of the antenna, a new antenna won't help. The reason it might be coming in through the coax has already been mentioned by others. Someone may have cut it, and maybe did a poor job of splicing it back together. Maybe the connectors are not well seated. Maybe the coax itself is getting old, cracked, frayed conductors. IMHO, the PL-259 is the second worst connector known to mankind, being beaten out by the "F" connector used to connect a coax to the back of your TV where the center conductor is just the coax center wire sticking out. Problems with the PL-259 are that it has poor contact reliability of the outer ring, or ground side of the connection, between the connector and the antenna or whatever you mate it to. The connector is often built by hand, and the center conductor and coax shield have to be soldered to the connector. But the connector has so much thermal mass that by the time the connector metal and the coax conductor are up to solder flowing temperature, you are starting to melt the coax dielectric between the center conductor and the outer shield. So there is a tendency to either melt the dielectric or, trying to avoid that, not use enough heat, and therefore getting a poor solder job between the coax and the connector. If you get a cold solder joint here, then you will bet intermittent operation, intermittenly bad VSWR, and susceptability to external radio noise. I just ran across this link verifying I had the number correct in the connector designation. The link is worth a look if you are dealing with this sort of issue.

Third, if all the above are OK, then I would advise that it is the job of the radio to filter out noise, not the antennas job. You should then be asking what is wrong with your radio. In a good radio, the first thing that the signal hits when it comes into the radio is what is called a pre-selector. This is a filter, and its job is to get rid of all the signals and noise that are not in the frequency range you will be receiving. So I think you should not have to duplicate this function in the antenna.

So that is my 2 cents worth. Guys who deal with this regularly may give you a more 'real world' answer.

OK, here is 2 more cents worth. I just gotta say this. For most boat applications, you will probably find that the coax to the antenna is RG-58. This is 50 ohm coax, don't confuse it with RG-59 which looks the same but is 75 ohm. RG-58 is very common, and is made in different qualities from nearly worthless, minimal number of strands in the outer shield, to very good coax that may even be double shielded. You should have a good quality coax, properly terminated in it connectors, rated for marine use.

If you looked at the first link, you would see that the author states the PL-259 is designed for RG-8, not RG-58. RG-8 cable is larger than RG-58, has lower loss and normally better shielding. The disadvantage on a boat is that it is heavier, bulkier and stiffer than RG-58. So where RG-58 will easily flex if you adjust the position of your radio or antenna, RG-8 will be more resistant to change. But the RG-8 is a step up in coax from RG-58. I would also hope to say you could step up to a type 'N' connector instead of a UHF (PL-259), but I don't find any marine VHF antennas with that connector offered.

OK, rant off. Sorry for long post.
 
I maybe exaggerated the lobes a bit to prove the point, but Paul, you are more than welcome to use the drawings.

Here are a couple more - should be self-explaintory.....

vhf24.jpg



(these are actually from an article I wrote awhile back)
 
And then put that second boat up on plane with the bow towards the sky and he could have great converstaion with either Davey Jones or my Uncle Martin.... :grin:

Great info guys...appreciate the time it took to post...
 
You must log in or register to reply here.

Forum statistics

Threads
113,274
Messages
1,429,835
Members
61,147
Latest member
Boudica
Back
Top