VHF - DC Grounded vs Ungrounded Antennas

[Mark42]

What is the difference between antennas that are grounded vs the ungrounded antennas. They seem to have the same specs, and I'm wondering if there is a disadvantage in mounting/grounding one or the other.

I'm interested in this: Shakespeare 5206-N VHF 6dB 8 Ft Fiberglass Antenna

But they also have this grounded model: Shakespeare 5101 8' VHF Antenna

Another antenna that fits the budget is this TRAM 8-ft VHF Marine Antenna 1620-HC. And it comes with the nylon ratchet mount. Doesn't say if its grounded or not.

Wondering if these are good antennas for their low price (between $32 and $38).

Anyone use these?

 

[dingbat]

Re: VHF - DC Grounded vs Ungrounded Antennas

I can't answer your question about ground/ no ground. Doubt has anything to do with the price since your comparing a mid level antenna to an entry-level antenna. The difference in the quality of the materials and construction between the two is noticeably

The 5201 is a 6 db, mid-priced /performance antenna. It has brass and copper elements, a heavy outside shell and nominal SWR of 1.5:1 at 156.8 MHz.

The 5206 is a 6 db, budget antenna. It has copper elements, a flimsy outside shell and a nominal SWR of 1.3:1 at 156.8 MHz

Tram ?Cannot find a manufactures page or performance specifications. All that I can find is that it?s a 6-db antenna. Personally, I would stay away from the Tram based on the pretense that you get what you pay for in antennas.

FWIW: I would also recommend against using a plastic antenna mount. Especially if you use a good high quality antenna that has some weight ot it. The first and only one I ever used did not make it a season before it broke. It?s been stainless steel ever since.

 

[Mark42]

Re: VHF - DC Grounded vs Ungrounded Antennas

This seems to be a popular stainless mount, the Shakespeare 4187 Stainless Ratchet Mount . Is this the one you use?

 

[dingbat]

Re: VHF - DC Grounded vs Ungrounded Antennas

I'm running a pair of Galaxy 5225XT antennas on the hardtop. Decided to use the 4187-HD version for a couple bucks more.

The 4187 should be fine for your application.

 

[Boatist]

Re: VHF - DC Grounded vs Ungrounded Antennas

I am also a fan of the 5225-XT antenna. I had a 8 foot 6 db gain Shakespeer antenna. It had the plastic mount and antenna was quite flexable. After about 10 years the plastic at the base of the antenna broke. When I replace it with a 5225-XT I could not belive the difference. I could here twice as many station. Fishing offshore in open water when near another boat I could here their radio picking up Boats I could not here. After I replaced it with the 5225-XT now I can here everything and I never here another boat radio on a station I do not here.

Plastic or nylon mounts will last about 10 years if at the end of the day you cover the mount and antenna. Maybe longer it you store the boat in the garage. Out in the direct hot summer sun do not expected it to last more than about 5 years before it cracks.

 

[Mark42]

Re: VHF - DC Grounded vs Ungrounded Antennas

One thing that surprised me is this warning in the icom documentation. The bold is by me. Who is able to stay 3 meters away from thier antenna? Heck, my boat is only 16 feet long. No matter where I attach it, if I am at the helm I will be withing 3 meters of the antenna.

Whoopee, we're all gonna die!

Icom requires the radio operator to meet the
FCC Requirements for Radio Frequency Exposure.
An omnidirectional antenna with gain not
greater than 9 dBi must be mounted a minimum
of 5 meters (measured from the lowest point of
the antenna) vertically above the main deck and
all possible personnel. This is the minimum safe separation
distance estimated to meet all RF exposure compliance requirements.
This 5 meter distance is based on the FCC Safe
Maximum Permissible Exposure (MPE) distance of 3 meters
added to the height of an adult (2 meters) and is appropriate
for all vessels.
For watercraft without suitable structures, the antenna must
be mounted so as to maintain a minimum of 1 meter vertically
between the antenna, (measured from the lowest point of the
antenna), to the heads of all persons AND all persons must
stay outside of the 3 meter MPE radius.Do not transmit with radio and antenna when persons are within the MPE radius of the antenna, unless such persons (such as driver or radio operator) are shielded from antenna
field by a grounded metallic barrier. The MPE Radius is the
minimum distance from the antenna axis that person should
maintain in order to avoid RF exposure higher than the allowable
MPE level set by FCC.

 

[Boatist]

Re: VHF - DC Grounded vs Ungrounded Antennas

Mark
It not just their antennas it is all antennas. All the VHF marine antennas, VHF handhelds likely the most dangerous, also Cell Phones.
No one really knows but cell phones held tight to the head will probably be the ones that prove or disprove if they cause cancer or damage the brain.

If your sinking off shore in 50 degree water or have a fire on board, or dead in the water and drifting to a rocky shore with the surf pounding against the rocks then I would take my chances with the radiation from the radio antenna.

 

[Mark42]

Re: VHF - DC Grounded vs Ungrounded Antennas

Mark
It not just their antennas it is all antennas. All the VHF marine antennas, VHF handhelds likely the most dangerous, also Cell Phones.
No one really knows but cell phones held tight to the head will probably be the ones that prove or disprove if they cause cancer or damage the brain.

If your sinking off shore in 50 degree water or have a fire on board, or dead in the water and drifting to a rocky shore with the surf pounding against the rocks then I would take my chances with the radiation from the radio antenna.

Yeah, the FCC puts limits on everything. I have a panel antenna in my attic for mobil broad band that is running through an amplifier. That frequency is very close to micro waves and you really don't want to stand in front of it. Thats one reason its up in the attic, so the kids won't mess with it and get cooked.

 

[Splat]

Re: VHF - DC Grounded vs Ungrounded Antennas

Cool little science trick

Key your radio up one day and hold a small fluorescent bulb up to it. I'll bet you it lights up!!!

Bill

 

[jhebert]

Re: VHF - DC Grounded vs Ungrounded Antennas

What is the difference between antennas that are grounded vs the ungrounded antennas.

Generally these end-fed VHF Marine Band antennas which show a DC continuity between the center conductor and shield of their coaxial input connector are using some type of internal matching network to a high impedance feed point. The DC continuity generally comes from having a shunt inductance. Sometime the inductance is in the form of a shorted stub.

About the only reasonable inferences to be made are:

--an antenna operating at DC ground potential might bleed off static discharges if you happened to be operating in unusual conditions where there was a static charge in the atmosphere, but that is probably rare and not much of a consideration.

--it is likely the antenna is using some sort of impedance matching that involves a shunt element, either an inductor or a shorted stub. This really does not lead to any other significant conclusion about the antenna itself.

I do tend to favor antennas that do not show any DC continuity at their feed point, as I think this implies a simpler feed system with less critical tuning. A shunt-fed antenna often has a higher Q and the tuning of the feed system is more critical. If given a choice, I'd go with a series fed antenna, that is, one that shows no DC continuity.