Trying to find the perfect VHF antenna

[stormalong]

Originally Posted by TreblePlink

But I have never heard of isolating the antenna bracket from the mast. Is there a reference advocating this? The mast and stays form part of the antenna and can't be ignored. Some "no ground plane required" antennas can work with just the coax outer conductor forming the "other half" of the dipole, but most mast top antennas need the stays and or mast connection. From a lightning protection standpoint (nearby, not direct mast strike where not much helps) bonding at the top and close to the base is best.

http://www.cruisersforum.com/forums/...-193349-2.html

I agree. The mast is the ground plane of the VHF antenna. No point in isolating the mast mount.

[CarinaPDX]

Exactly. Most marine VHF antennas are base-loaded 1/4 wave whip antennas and require a ground plane in order to radiate. If it is isolated and the coax is inside of an aluminum mast that will be a problem.

Greg

[belizesailor]

Originally Posted by Auspicious

Nothing is perfect. I like the Metz Manta for many reasons.

Me too. I refurbed my 18 year old Metz a couple of years ago. It was still in very good condition. The white plastic cover and a big oring at the bottom were the only degraded parts and were easy to replace with a cut down PVC pipe end cap and standard oring. Solid product.


https://www.bluemarinestore.com/metz...-whip-antenna/

[TreblePlink]

Originally Posted by CarinaPDX

Exactly. Most marine VHF antennas are base-loaded 1/4 wave whip antennas and require a ground plane in order to radiate. If it is isolated and the coax is inside of an aluminum mast that will be a problem.

Greg

1/4 wave can work fine, but most commercial mast top antennas are 1/2 wave - a bit more gain. The difference in vertical radiation pattern is not that much.

[IslandHopper]

Originally Posted by belizesailor

Me too. I refurbed my 18 year old Metz a couple of years ago. It was still in very good condition. The white plastic cover and a big oring at the bottom were the only degraded parts and were easy to replace with a cut down PVC pipe end cap and standard oring. Solid product.


https://www.bluemarinestore.com/metz...-whip-antenna/

Me three...

Just make sure the retainer for the antenna (hidden below the red cover) is nice and tight before installation and descending the mast, I didn't and my brand new one wasn't

[CarinaPDX]

Originally Posted by TreblePlink

1/4 wave can work fine, but most commercial mast top antennas are 1/2 wave - a bit more gain. The difference in vertical radiation pattern is not that much.

Duly corrected. I think that technically it is a 5/8 wave antenna - 1/2 would be a problem. Too busy today to spend the time to work on this - heading over the mountains now.

Greg

[belizesailor]

Originally Posted by IslandHopper

Me three...



Just make sure the retainer for the antenna (hidden below the red cover) is nice and tight before installation and descending the mast, I didn't and my brand new one wasn't

Oops!

Mine is tight and packed w dialectric grease.

[Auspicious]

Originally Posted by TreblePlink

But I have never heard of isolating the antenna bracket from the mast. Is there a reference advocating this? The mast and stays form part of the antenna and can't be ignored. Some "no ground plane required" antennas can work with just the coax outer conductor forming the "other half" of the dipole, but most mast top antennas need the stays and or mast connection. From a lightning protection standpoint (nearby, not direct mast strike where not much helps) bonding at the top and close to the base is best.

Most of the antennas I see seem to be either 5/8 wave or 1/2 wave end-fed antennas. They aren't dipoles and don't need "another half."

The issue is ground loops. If you have a boat with bonded rigging (most US/ABYC boats) you have two ground paths from the antenna: the coax shield and the mast. If you isolate the antenna bracket you break the ground loop. This often reduces the RF noise floor at the radio. Enough to really matter? That depends on the radio and the boat. I've solved noisy radio problems this way. Testing is easy - just plug one of the emergency antennas into your radio and see if the noise is different. If it is, move the emergency antenna to the masthead (you may need a barrel connector) and test again. If the noise goes up you probably have something being picked up by poor quality coax. If it doesn't but the attached masthead antenna is noise the problem is likely a ground loop.

[TreblePlink]

All antennas are two terminal; a dipole works as a mostly balanced antenna and a vertical monopole works with the vertical element "against" either a ground plane or other metallic structure. For techies, think of it as a resonant capacitor-inductor, with the resistive component being the actual radiated power. A capacitor in series with a coil, in series with a 50 ohm resistor is a pretty good model.



They sell 1/2 wave antennas (end fed) which are marketed as not requiring any ground plane, but when you install them you read the fine print indicating that a certain minimum amount of coax needs to be placed near the base in a certain way. Obviously the vertical element is using the outer conductor of the coax as a (poor) ground plane. Generally these currents on the coax shield are detrimental to the antenna pattern. In more advanced designs the coax is decoupled to reduce this.



You can easily tell (for the mast-top antennas) if it is 1/4, 1/2, or 5/8 wave by the length: Approximately:




1/4 wave is 18 inches
1/2 wave is 36" ... a bit more gain.

5/8 wave is 45" ... a tiny bit more gain.



The taller antennas seen on sport fisherman are essentially a collinear antenna, (more gain) end-fed with a decoupling sleeve. Their theory is more complex.



I have worked with ground loop problems many times. They usually result from using some chassis or structural metal as the negative return for some load. This produces current flow in the mast, and the coax can share some of that current, raising the possibility of induced noise.


For all mast lighting, etc, it is always best to provide a dedicated negative return wire so no (DC) current flows through the mast or stays. No current, no ground loops.

[continuouswave]

Re antenna radiator length:

I do not agree with the statement that most VHF Marine Band antennas are a quarter-wavelength with ground plane. At 156.8-MHz the wavelength in free space is about 75-inches. A quarter wavelength would be just under 19-inches.

The most common VHF Marine Band antenna in my experience is a half-wavelength vertical monopole with end feed. A half-wavelength in free space would be 37.6-inches, and in most of these antennas the whip length is slightly shorter, about 35.5-inches. The impedance at the feed point is high and must be matched to 50-Ohms with some sort of transformer.

A half-wavelength vertical may not need a ground plane but it does required some counterpoise. The counterpoise provides a path for the RF current to be a complete circuit.

There are several half wavelength antennas with metal whip radiators and a base transformer. A common element of the base transformer is to be enclosed in a metal cylinder. You can see this on the Metz antenna and also on the very similar GAM SS-2 antennas.

In most of these antennas, inside them you will find the metal cylinder encloses a single winding coil connected to the antenna base and the coaxial connector outer conductor, with the coaxial feed line center conductor tapped onto the coil with a turns ratio of about 3:10. The impedance ratio is the square of the turn ratio, so you'd expect about 1:11 impedance ratio. That sounds too low, implying the antenna impedance would be about 580-Ohm, which is below what would be predicted for an end-fed half-wavelength. The pitch of the windings on the coil are not uniform, so that may also be affecting the matching characteristics.

The whip length is made slightly shorter than half-wavelength due to having some effective length of the antenna reside inside the matching transformer enclosure, and also perhaps for tuning of the matching network.

I also suspect that the metal cylinder enclosure is important and provides part of the counterpoise. I suspect the design was reached by trial and error rather than by theoretical analysis. And these antennas seem to work well.

Re isolating the antenna coaxial connector shield from the mast:

Decoupling an antenna from common-mode noise currents helps to reduce noise induced into the antenna and sent down the feed line, and if isolating the antenna shield from the mast helps, then that is great. However, there will be some antenna currents flowing on any conductive surface near the antenna.

Re transmission line types and connectors: I would avoid coaxial cables made with foam dielectric for two reasons: the center conductor tends to migrate into the foam at cable bends, and the foam tends to melt when soldering to the cable.

A proper high-quality PL-259 connector properly soldered to Mil-Spec RG-213/U cable and properly wrapped with real Scotch 3M 35 Vinyl electrical tape will be very environmentally resistant.

Avoid installations in which the PL-259 connector-to-cable joint becomes a load bearing connection and must support the weight of a long length of transmission line hanging from the connector.

I do not see much advantage of RG-214/U cable. It is just a double-shielded version of RG-213/U. It is more expensive, weighs more, but has the same loss characteristics. Generally there are not other transmission lines run along side the VHF Marine Band radio transmission line, so signal ingress into the cable is not particularly a problem. Double-shielded flexible coaxial cable is usually used in environments where there are many other cables with many other signals around. Also in a sailboat the cable is usually run inside a metal mast, so additional shielding is not really needed. There is one advantage to RG-214/U: generally it is made only in very high quality and will usually exceed the quality of any run-of-the-mill off-brand RG-8 cable, and its shield conductors are tinned copper, not bare copper.

Re advantage of height over antenna gain: many studies have shown that increase in height produces better signals at distant receivers than would be predicted by simple analysis. I discuss four models that predict this in more detail at

VHF Radio Propagation Over Water
continuousWave: Whaler: Reference: VHF Radio Propagation Over Water

See the section under the subheading "Effect of Antenna Height".

Re dielectric-grease: this is a grease that resists conducting electricity. Its principal purpose is to be a grease. As a grease it is often used to enhance the effects of seals to keep out contaminants. It does not IMPROVE electrical conductivity but rather prevents it.

[Auspicious]

Originally Posted by continuouswave

Re antenna radiator length:

I do not agree with the statement that most VHF Marine Band antennas are a quarter-wavelength with ground plane. At 156.8-MHz the wavelength in free space is about 75-inches. A quarter wavelength would be just under 19-inches.

I agree.

Generally I see end-fed half wavelength antennas, followed by end-fed 5/8 wavelength antennas, with the rare end-fed quarter wavelength antenna on sailboats. Powerboats use collinear arrays more often than not which are a bunch of stacked center fed dipoles.

In my experience the mast and rigging make unpredictable counterpoises. Note that the "ground plane" of some antennas is better described as a counterpoise than a ground. The coax shield makes a fine counterpoise, including when inside a metal mast with an insulated bracket.

Also note that the benefit of a 5/8 wave antenna over a 1/2 wave antenna is not the miniscule additional gain but the lower take-off angle.

Noise from ground loops is the second most common VHF noise problem I see on boats. The most common is cheap LED lights.