Vhf cable / antenna....

[jssailem]

Greg,
I own the XB8000, GPS antenna, splitter, LMR-400 coax cable and VHF/AIS antenna. I used the standard PL-259 Connector with soldered connections.

I ran one continuous coax thru the deck from antenna to splitter, in order to minimize loss. I do not take my mast down yearly. Hopefully I will not have to do it again. If I do, I'll have to consider cutting and splicing the coax at that time.

I connect the Vesper to my SeatalkNG network backbone. I use an iPad as cockpit display and my laptop at the nav station connected by wi-fi to the Vesper gateway.

 

[dlochner]

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

Ken, my understanding is the Metz and Shakespeare SS whip antennas are ½ wave. The whip itself is a ¼ wave with a coil in the base making up the other ¼ wave. While physically they are ¼ wave antennas, electrically they are ½ wave antennas and don't need a ground plane.

 

[Brian D]

Yeah, most whip antennas are electrically shorten to conserve space, but what they gain is compactness they lose in efficiency. If one thinks about it, a halfwave @150MHz is only 37 inches. I think one can extend their mast by that much and still clear a bridge.

But I am going to have to agree with Ken to bond the antenna to the mast. I still do not think there will be any galvanic corrosion to be concerned about. I think when the OP goes and changes his antenna he can take a photo and post it here to see how much, if any, corrosion is present.

 

[NYSail]

Yeah, most whip antennas are electrically shorten to conserve space, but what they gain is compactness they lose in efficiency. If one thinks about it, a halfwave @150MHz is only 37 inches. I think one can extend their mast by that much and still clear a bridge.

But I am going to have to agree with Ken to bond the antenna to the mast. I still do not think there will be any galvanic corrosion to be concerned about. I think when the OP goes and changes his antenna he can take a photo and post it here to see how much, if any, corrosion is present.

Will do!

 

[jssailem]

I think one can extend their mast by that much and still clear a bridge.

I know I do, but I added 4ft to my estimated mart length. I do not use it as a feeler gauge to test bridge heights. I do not want to have to climb the mast to replace the whip antenna.

 

[dlochner]

@Brian D and @Ken Cross thank you for your insights and spurring me on to review some of what I used to know about antennas. And reminding me of why I dislike ground planes, it was cutting the grass around my father's antennas and those horizontal ground plane aluminum poles.

So, the two of you left me with a dilemma, 2 trusted sources with differing opinions on insulating the VHF antenna from an aluminum mast. One factor that kept nudging me was wooden masts. If the antenna needed a ground plane formed by the aluminum mast, how do they work on wooden masts and carbon fiber masts?

To resolve this dilemma, I contacted Metz Antennas Tech support. My question:

I have a question about mounting a Metz Manta antenna on my sailboat. There seems to be 2 theories about mounting the antenna on an aluminum mast. One theory says mounting directly to the mast provides a ground plane that improves the antenna's performance. Another theory says the antenna and mount should be electrically isolated from the mast in order to prevent stray current and galvanic corrosion. What does Metz recommend and in layman's terms why?

How does this work on a wooden mast? Is a ground plane necessary?

Thanks,

Dave

Their answer:

Hi Dave, MCC Model #206 Manta 6 Marine Antenna does not require any type of ground plane.Antenna covers frequency range of 156 thru 163Mhz, . It may be mounted on any surface type, Stray current not a issue, corrosion will also not be a problem.Regards Dick Metz

Out of an abundance of caution, I would remove and insulate the mount from the antenna as it should be simple and inexpensive on a new install. However, the SS screws on my antenna mount have become one with the aluminum mast.

 

[Ken Cross]

That tells me it's a 1/2 wave antenna (also good on wooden masts.) Either the antenna is tall or it's built with a loading coil. (Think of it as coiling up the excess antenna length. ) Otherwise on wood some add a wire from the antenna base going down the mast. It only needs to be the length of the antenns.

I'm glad to hear you have it resolved.

Ken

 

[Ken Cross]

@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?

One thing that frustrates folks with a digital meter is the high internal impedance of the meter. The result is you can't always believe what the meter says. This is particularly bad in non DC ground situations. A cheap and simple answer is a battery poweted circuit tester that incorporates a battery and flashlight bulb.

Ken

 

[Brian D]

So I am going out on a wooden limb here. There is no such thing as a "no ground plane antenna" Why? But first, did you notice that Metz did not answer you question. All they did was to try and sell you one of their antennas.

OK, why is there no such thing as a no ground plane antenna. Because, remember we said that the shield of the radio connector is grounded to the chassis of the radio? That is why. Also remember that there are two components at work here. DC ground and AC currents. The DC voltage is blocked at the source, which is at the radio by a DC Blocking capacitor. Look at any amplifier schematic and you will see at the output of the final stage is a capacitor.

OK, so now we see the DC voltage is blocked, this leave the AC current. Where will it go if it does not have something to work on? The only place it can go is on the shield of the cable. So in essence, the other side of the RF signal is the shield of the coax. In the HF world this can be a serious problem. It is called Common Mode Currents and can harm the operator and the equipment. To prevent this Common Mode Current one inserts a Balun at the feed of the antenna. But that is getting way ahead of our selves here. If you want to learn more, Google Common Mode Currents.

Now, in some antenna designs, part of the feed cable is used to "extend" the RF length of the antenna. In the case of an antenna that is tuned to a halfwave length, we do not want common mode voltage or current. It is bad juju. This is why we like to have the antenna attached to the aluminum mast. The RF will see the mast and all its associated connections as a "counter poise". A long one, but one none the less.

On the DC side, the ground of the chassis, hence the ground of the boat's DC can be used as the other side of the RF signal. Boats with SSB can use this but they should have a good RF AC side other than DC ground. I mentioned this in my first post regarding the SSB tuner ground being isolated.

This is all based on my readings and theory, and practical applications. My HF station right now uses a long wire as my antenna (35 feet) and my "other side" is just a small bundle of wires spewed about haphazardly. It works.

That's my limb and I hope it doesn't break.

 

[Don S/V ILLusion]

Brian is correct. I've been experimenting with antennas for 50 years and stumbled across a lot of myths, this being one.

Antenna theory is beyond the scope of this forum so suffice it to say lots of commercial antenna manufacturers advertise "no ground plane needed" which isn't elementary physics. About the only design which comes close to that claim is a vertical dipole. Half wave and loaded full wave verticals (among others) all depend on a ground plane to maximize their radiation efficiency.

 

[4arch]

Just a word of caution - I attempted to replace the RG-8X coax in my Beneteau 400's Sparcraft mast with LMR-400. I determined (and a rigger concurred) that it's all but impossible to do this with the stick up as there is not enough extra space in the cable penetrations at the top and bottom of the mast to accommodate the thicker coax. Enlarging them from a bosun's chair would be difficult and feeding the cable down without the ability to fish it through even trickier. So I'd be careful about trying to significantly increase coax diameter if you're not planning to drop the stick.

That said, I was able to significantly improve my VHF performance by re-terminating the connections on my RG-8X coax. I'm getting 15-20 mile TX/RX and am happy with that until the next time I need to take the rig down.

 

[Ken Cross]

The problem with using the coax as a ground plane is also frustrated by the coax usually being internal to the mast. It's electrostatically shielded. This does all kinds of nasties to the characteristic impedance of the antenna.

Ken

 

[dlochner]

This is all based on my readings and theory, and practical applications. My HF station right now uses a long wire as my antenna (35 feet) and my "other side" is just a small bundle of wires spewed about haphazardly. It works.

Maybe we are not really talking about the same type of antenna. It is my understanding that the Metz Manta with a ~36" whip has a coil in the base, wouldn't that serve as the counterpoise to the whip? I think the similar model by Shakespeare has a coil in the base too.

 

[Don S/V ILLusion]

The problem with using the coax as a ground plane is also frustrated by the coax usually being internal to the mast. It's electrostatically shielded. This does all kinds of nasties to the characteristic impedance of the antenna.

Ken

If you are 'seeing' that, I suspect there is something else other than the mast causing it. I have done some playing with my antenna analyzer (VHF model rather than the stock picture attached) to compare different installations on/in the mast and can find no effect of running the coax internally. Observations of resonant frequency, SWR, L, X, R and C characteristics are unchanged regardless.



I could understand what you say only if your antenna is not above the mast but not solely a function of coax placement. It should make no difference.

If it did, those of us with ham radio towers would never be able to run the coax up the tower.

 

[JamesG161]

A bit rusty on antennae and zero experience on a boat, but the PO was a Ham and my boat was Ham ready. I should have bought his, but thats another story.

Vertical antennae with a coil in the base. The coil is for the other half of a dipole [1/2 wave or 5/8th is common].
It tough to have a "ground plane" on a vertically polarized dipole antennae.

SWR is just an RF impedance match between the coax and the antennae. They should be matched or loss of RF radiated power.

RF to DC is possible and should be blocked. Normally done at the radio.

I am learning counterpoise, but is it just the balancing of RF induced on the rigging and other DC wire.

So...
Match your coax to the antennae, 50 or 75 ohm.
The dB loss on a short run of coax is minimal, so get what fits the mast the best.

Retired Ham...
Jim...

 

[Brian D]

Maybe we are not really talking about the same type of antenna. It is my understanding that the Metz Manta with a ~36" whip has a coil in the base, wouldn't that serve as the counterpoise to the whip? I think the similar model by Shakespeare has a coil in the base too.

Dave, I think mobile antenna design is not that different among manufactures. Probably the choice of materials being the major difference. The coil in the antenna I believe is used for matching the whip with the coax impedance. I believe the coil is center tapped with the lower portion of the coil connected to the shield. I do not believe it is used as a counterpoise/ground plane. But I do not know so I will have to deffer to someone more knowledgeable.

 

[NYSail]

Just a word of caution - I attempted to replace the RG-8X coax in my Beneteau 400's Sparcraft mast with LMR-400. I determined (and a rigger concurred) that it's all but impossible to do this with the stick up as there is not enough extra space in the cable penetrations at the top and bottom of the mast to accommodate the thicker coax. Enlarging them from a bosun's chair would be difficult and feeding the cable down without the ability to fish it through even trickier. So I'd be careful about trying to significantly increase coax diameter if you're not planning to drop the stick.

That said, I was able to significantly improve my VHF performance by re-terminating the connections on my RG-8X coax. I'm getting 15-20 mile TX/RX and am happy with that until the next time I need to take the rig down.

Yes the plan is to take the rig down in the spring for a few days. Was ping to do it over the winter however there was a bit of confusion with yard and actually works better with my winter cover. On the 423 there is more than enough space at the bottom and the top if I recall also is good. But definetlysomerhingbto consider!

Thanks for the input!

 

[Ken Cross]

If you are 'seeing' that, I suspect there is something else other than the mast causing it. I have done some playing with my antenna analyzer (VHF model rather than the stock picture attached) to compare different installations on/in the mast and can find no effect of running the coax internally. Observations of resonant frequency, SWR, L, X, R and C characteristics are unchanged regardless.

View attachment 159230

I could understand what you say only if your antenna is not above the mast but not solely a function of coax placement. It should make no difference.

If it did, those of us with ham radio towers would never be able to run the coax up the tower.

If the antenna is greater than 1/4 wave you won't see an effect from running the coax in the mast, It's just if you are using a 1/4 wave antenna and the mount not bonded to the mast where problems occur. The OP seems to have a 1/2 wave antenna so this is not an issue for them. Only if the installation tries to use the coax shield as a ground plane would this apply.
Ken