Vhf cable / antenna....

dlochner · Dec 8, 2018

The Metz antennas seem to be popular and have been around a long time. I have one and use it for VHF and AIS.

Check those Specs on the coax again. It is rated for 85c, that would be 185 F. If global warming takes us to that temperature, the efficiency of the coax will the least of your problems.

dlochner · Dec 8, 2018

Coffee usually helps.

When you mount the antenna do not mount the bracket directly to the mast. If done correctly the metal bracket should be electrically insulated from the mast. If the metal bracket is connected to the mast it forms a connection between the negative on the battery and the mast and the bonding system. This can accellerate galvanic corrosion. MaineSail has posted a photo somewhere about how to correctly mount the antenna, using a piece of starboard. Mount the bracket to the starboard and then mount the starboard to the mast.

JamesG161 · Dec 8, 2018

@Brian D is one of the experts on radio cable and low dB loss and you were right about 85 degrees, but wrong scale.
I hope to bake some seasonal cookies at 350 degrees and not Centigrade.

Jim...

KD3PC · Dec 8, 2018

I have metz and LMR 400, and would suggest you go with the metz and the LMR, and don't skimp on connectors get the silver/teflon ones as they hold up better in the marine environment. You might look at the specs for the LMR-240 as it is much smaller diameter and at VHF frequencies should be a fine choice.

Brian D · Dec 8, 2018

I wouldn't call myself an expert, but I do understand certain properties of RF.

You talked about the LMR-400. That cable is about 1/2" in diameter. Check the cable you have installed for any markings. My guess is the cable was OEM to the boat, unless you purchased the boat from a PO and they replaced the coax. If you look at LMR cable, the number pretty much denotes to diameter of the cable: LMR-400 (.4") LMR-240 (.25") and so on. So think about size and weight/100 ft. If weight is a concern, then you would want to go with the smaller diameter cable. Now LMR-240 is 3dB loss over 100ft @ 150MHz. So your lost at the antenna would be about 1.6dB. That would be nominal.

Make sure which ever cable you go with, it has an impedance of 50Ω. 75Ω cable won't kill you but it will reduce your output power at the antenna.

Speaking of output power. If you radio produces 25 watts of RF at the connector, that will not be the same as the power at the antenna. Attenuation will reduce the power reaching the antenna. If a cable is rated at x frequency with x db of attenuation/100 ft, just divide the attenuation by 100 and then multiply by the height of the mast. I use the clearance value for bridges as the height. On the Bene 423 that is 55' above water. So for the LMR-400 @150MHz is 1.5dB loss. 1.5/100 = .015/ft. .015 x 55ft = .825db loss. So you are will see less than 1 db loss of energy from the radio. To put it into perspective, 3db loss with be half your power at the antenna. So 12.5 watts of RF energy will reach you antenna if you have a cable with a loss of 3db.

Antenna gain is something everyone should watch out for. Why? The gain is theoretical. The manufactures use an Anechoic Chamber to test their wares. Then they will claim that their antenna has a gain of 6dBi. That is an impressive figure, but again is theoretical. The "i" in dBi mean isotropic. The "real" gain is usually much less, but again, nothing major to get worked up about unless the gain is huge. Most vertical antennas do not really have much gain.

Now it was mentioned above about galvanic corrosion. Yes, one can isolate the base mount of the antenna from the mast. But is it really that bad? I think most mounts are made of aluminum anyway. Some that are used on towers are stainless steel. My antenna mounts at home are stainless steel. I don't know about the boat though as I have never been at the top of the mast. But if you are concerned, then use a Firestik insulator. These can be found everywhere online. They are nylon washers that go between the antenna's SO-239 connector and PL-259 coax connector. They isolate the antenna from the mount. Caveat... The antenna works on Alternating Currents. Meaning the signal is AC. If you isolate the sheild of the coax from the actual antenna you will have issues. So I would say, just leave well enough alone and if Bene mounted the base to the mast just follow suit.

Hope this shines some light on the subject.

Oh... price point is not a measure of quality. It does have some bearing, but some guys here have purchased VHF antennas for as low as $20 and have been very happy with them. Find out what you have now and best to replace same fit, form, and function.

dlochner · Dec 8, 2018

Brian D said:
Oh... price point is not a measure of quality.

Back in the days of my (mis)guided youth we used an abandoned Long Island Lighting Co. power line as an antenna for our pirate radio station Used a treble hook to connect to the line. The FCC was quite impressed with our signal when we were busted.

Brian D said:
Now it was mentioned above about galvanic corrosion. Yes, one can isolate the base mount of the antenna from the mast.

The issue with corrosion is not with the connectors and the mounting bracket. The coax shield is connected to the frame of the radio which is connected to the battery negative or another ground. This allows a pathway for stray DC current, battery -> radio->mast->bonding system-> common ground on the engine. It is best to have only one pathway to the common ground, everything to a negative busbar, then to the engine, AC ground, DC negative, and Bonding system.

Brian D · Dec 8, 2018

In theory, most modern radios have what is called a DC Block installed at the antenna connector. That will allow the AC current to pass but block the DC component. This could be at the connector or it could be at the final output stage of the amplifier in the radio. This is the same technique used for SSB in which the DC component "ground" path of the antenna tuner is blocked with capacitors to prevent corrosion. So there really should be no 12VDC on that connector. In theory.

Charlie Jones s/v Tehani · Dec 8, 2018

dlochner said:
Back in the days of my (mis)guided youth we used an abandoned Long Island Lighting Co. power line as an antenna for our pirate radio station Used a treble hook to connect to the line. The FCC was quite impressed with our signal when we were busted.

The issue with corrosion is not with the connectors and the mounting bracket. The coax shield is connected to the frame of the radio which is connected to the battery negative or another ground. This allows a pathway for stray DC current, battery -> radio->mast->bonding system-> common ground on the engine. It is best to have only one pathway to the common ground, everything to a negative busbar, then to the engine, AC ground, DC negative, and Bonding system.

LOL one nice thing about a wooden mast

dlochner · Dec 8, 2018

Brian D said:
In theory, most modern radios have what is called a DC Block installed at the antenna connector. That will allow the AC current to pass but block the DC component. This could be at the connector or it could be at the final output stage of the amplifier in the radio. This is the same technique used for SSB in which the DC component "ground" path of the antenna tuner is blocked with capacitors to prevent corrosion. So there really should be no 12VDC on that connector. In theory.

So, if I put a multimeter between the connector (shield side) and the DC ground it read as an open circuit?

I'll try that next spring, if I remember.

Brian D · Dec 8, 2018

No, the shield and the ground are the same. Ground is Ground and everything electrical must have a return path. We are talking AC vs DC ground. However, if you put the multimeter between the center conductor of the connector and the shield, you will see an open circuit. If you do the same for the coax cable that is not connected to anything, you will see an open. If you connect the multimeter to an antenna you will see a closed loop circuit (like resistance) and get some kind of reading. The same holds true for a speaker. The speaker is a load and the component of sound is alternating current. Just like RF. Since we are dealing with RF, we are dealing with Impedance.

In regards to the SSB Radio and Tuner, the coax can be used to power the turner with a Bias-T. 12VDC is inserted after the radio and then blocked after the tuner. In this case one would want to use a "unbonded" water ground such as a mushroom through hull that is NOT connected to any other ground in the boat. But here we do not want any DC voltage to be present.

Sorry, it is very complicated. There should be tons of reading material about blocking DC in a radio system's RF path.

jssailem · Dec 8, 2018

Greg, Dealing with the same problem I purchased the antenna sold by Vesper along with the other gear. I have been pleased with the results. Also used LMR-400 cable in my refit.

dlochner · Dec 8, 2018

Brian D said:
No, the shield and the ground are the same. Ground is Ground and everything electrical must have a return path. We are talking AC vs DC ground. However, if you put the multimeter between the center conductor of the connector and the shield, you will see an open circuit. If you do the same for the coax cable that is not connected to anything, you will see an open. If you connect the multimeter to an antenna you will see a closed loop circuit (like resistance) and get some kind of reading. The same holds true for a speaker. The speaker is a load and the component of sound is alternating current. Just like RF. Since we are dealing with RF, we are dealing with Impedance.

In regards to the SSB Radio and Tuner, the coax can be used to power the turner with a Bias-T. 12VDC is inserted after the radio and then blocked after the tuner. In this case one would want to use a "unbonded" water ground such as a mushroom through hull that is NOT connected to any other ground in the boat. But here we do not want any DC voltage to be present.

Sorry, it is very complicated. There should be tons of reading material about blocking DC in a radio system's RF path.

@Brian D The question I asked was asked too simply. By way of background I am the son of a ham, K2ROG and KH6DYX, who grew up with a forest of aluminum poles in the back yard and miles of wire strung between the trees. I won't claim to have the knowledge that you and other Hams have, however, I'm not unfamiliar with some of the basics. So back to my question about DC in the antenna cable.

If the VHF has a circuit to block DC current entering the antenna cable, either the shield side or the center conductor then I would think that measuring resistance between the shield and the boat's DC negative should show an open circuit because the Multimeter's DC current is being blocked. If the DC blocking circuit has failed or is nonexistent, then the circuit would be closed and the MM should read some sort of minimal resistance, because the shield would be electrically connected to the VHF chassis which would be connected to the DC negative.

My thinking is that the VHF DC blocking circuit is similar to a Galvanic Isolator which blocks low level DC in an AC circuit. If I measure resistance between the ground and neutral on a circuit with a GI I should get a reading of infinite resistance or an open circuit. Taking a measurement in the same way on an AC circuit without a GI, should yield some minimal resistance as those 2 wires are connected at the AC panel.

Does this clarify my question?

CharlieSea · Dec 8, 2018

'Was going through zincs on a monthly/bi-monthly basis. Then three things happened at almost exactly the same time. 1. Two of my canal neighbors sold their boats and bought new ones. 2. Installed an Inner/Outer DC block on the antenna cable. 3. Changed the bilge pump and float switch. The present zinc has been on since June and is still OK. Seems anyone or all of the three could have been the cause.

Brian D · Dec 8, 2018

Dave, what I think is happening is I am including too many items in the circuit. So I think I will limit the discussion to the VHF radio only. Per you statement above...

dlochner said:
...
If the VHF has a circuit to block DC current entering the antenna cable, either the shield side or the center conductor then I would think that measuring resistance between the shield and the boat's DC negative should show an open circuit because the Multimeter's DC current is being blocked.

That is correct. But we know that the SO-239 connector on the radio is connected directly to the chassis of the radio, as is the PCB. Therefore, the shield will show the return voltage of the 12VDC system. The only time one would not see a voltage is if the entire path from radio to antenna is isolated. Remove the antenna from the mast and the DC voltage will go away. However, (and I hate to include this but...) the antennas used on our boats have loading coils in their base. This will act as a return path if one uses a MM to test the cable and antenna only. One will see some value of resistance.

If the DC blocking circuit has failed or is nonexistent, then the circuit would be closed and the MM should read some sort of minimal resistance, because the shield would be electrically connected to the VHF chassis which would be connected to the DC negative.

That is a true statement also, if there was a physical connection between the SO-239 and the chassis.

A good test to see what is happening is to use the MM between the shield of the coax and any bare spot on the mast. There should be NO resistance, or 0Ω. If one gets infinity, then the two items are possibly on independent grounds. To check for that, one would change the settings of the MM to voltage and see if there is voltage present between the shield and mast. If so, then current is flowing and a ground loop is present. Very bad news.

Don S/V ILLusion · Dec 9, 2018

Two suggestions for you guys:
1. LMR cables contain a foam dielectric so use caution to follow the manufacturer's bending limitations, and
2. Read the numerous cautions in contemplating using splitters. They can and do fail (as well as add a nominal insertion loss) with no outward indication.

Ken Cross · Dec 9, 2018

dlochner said:
Coffee usually helps.
When you mount the antenna do not mount the bracket directly to the mast.

NOOOO!!!

Sorry but this is bad advice. If the antenna is a 1/4 wave antenna (as most are) you need to electrically bond the mount to your mast to have good VSWR. If not you risk damaging the final output stage of your transmitter. Not so with half wave antennas, but I've never seen one mounted on a mast. The 1/4 wave antenna uses the ground plane as the other half of the antenna and if it's missing you have a worthless installation. Just basic radio electronics.
Many power boats mount the antenna on fiberglass so that's why they use that giant beam pole antenna. Without a ground plane, they need a 1/2 wave antenna.
Here is a link:
http://www.boat-project.com/tutorials/vhfant.htm

Ken