VHF Antenna Upgrades

[rslifkin]

I'm planning out some upgrades to my antennas and wanted to tap into the wealth of radio knowledge here.

Currently, I have 2 VHFs on board and just finished up an AIS install. The VHFs each have the common 8' 6db gain antenna (Shakespeare 5225-XT) on them with the antennas mounted approximately midship (on opposite sides of the deckhouse, so they're about 10 feet apart). The new AIS antenna is mounted to the top of the dinghy davits (13 feet aft of the other antennas) and with about1.5 feet of extension between the davits and the base of the antenna. That antenna is a 6db Morad (159mhz center tuning).

Current antenna heights above waterline to the tips of the antennas are just under 17 feet for the VHF antennas, about 15 feet for the AIS antenna. I don't want to raise the AIS antenna any higher, as the goal is to keep that and a possible third VHF antenna (which would go on the other side of the davits 10 feet away from the AIS antenna) at a height where I never have to lower them anywhere we go (lowest bridge I'm accounting for is 15'6").

VHF performance currently is at least good, but I know it can be better. AIS performance seems quite good, as in open water I was tracking freighters reliably at 20 nm and had signal from some as far as 45 nm. Sailboats with Class B (CSTDMA) transceivers were showing up fine from 6 - 7 nm. It seems like not many in our area have B+ (SOTDMA) transceivers, so I only got a small test of how far those would come in, but I picked up one at around 12 miles and another was fine out to about 8 miles (that's as far away as they got before sailing back).

The pictures below show the placement of the antennas. The AIS antenna is well clear of any metal horizontally from it (nearest thing is several feet away, well beyond 1/4 wavelength). The VHF antennas, however, are a bit close to the windshields and bimini frame.

Based on the metal proximity concerns and this boat also being a bit of a lively ride at times, I'm debating between going for the big guns and putting a tall set of 8 or 9db antennas on or getting another pair of the 6db Morads (with 156mhz tuning this time) and putting them on extension poles to get the tips up around 20' above the waterline (which would put the bottom of the antenna above any metal and above a future radar install when the bimini becomes a hard top at some point).

What does everyone here think would be the most effective and practical setup? If I can keep overall height around 20 feet that will reduce how often the big antennas need to be lowered (any travel through the Oswego and eastern Erie canals limits us to 21' clearance), but there's no hard requirement for maximum height beyond not doing anything that would require an impractical mounting setup.

 

[Bob Cofer]

I run the Morad 156HD for VHF and am vey happy with the performance, SWR is below 1.2 throughout the range. The SS whip is the radiating element on these and mine is mounted so it is just above the Anchor Light.

 

[psneeld]

All my commercial captain career I seemed to have boats that had the 15-17 foot Galaxy Antennas (9 db) that definitely out performed the 6 db antennas also on the boats. That is what I put on my trawler along with a 6 db 8' antenna on a mast that made them look nearly identical on either side of the flying bridge.

I have heard all the "theoretical" arguments against the bigger antenna, but the only one I can back-up is that they do cost more.

 

[rslifkin]

I run the Morad 156HD for VHF and am vey happy with the performance, SWR is below 1.2 throughout the range. The SS whip is the radiating element on these and mine is mounted so it is just above the Anchor Light.

That's exactly the antenna I was referencing with the thought of possibly doing 6db Morads on big poles. The plan would be to put the bottom of the antenna around 15 - 15.5 ft off the water which would get it well above any future obstructions on the boat including radar (design target is no fixed objects above 14'6") and would put the tip around 20 ft off the water give or take a little. I'm using the 159mhz tuned version of that antenna for the newly installed AIS antenna and it seems to be a great performer so far.

All my commercial captain career I seemed to have boats that had the 15-17 foot Galaxy Antennas (9 db) that definitely out performed the 6 db antennas also on the boats. That is what I put on my trawler along with a 6 db 8' antenna on a mast that made them look nearly identical on either side of the flying bridge.

I have heard all the "theoretical" arguments against the bigger antenna, but the only one I can back-up is that they do cost more.

The concerns around the overly flat radiation pattern of the big 14+ foot 8+ db antennas is a minor one in my mind. The bigger concern is that it would be challenging to get one up high enough that it won't have any nearby metal around it (like the bimini frame and windshields). That's where I'm wondering if I'm better off with 6db gain but mounted up high and completely free of obstructions.

 

[Bob Cofer]

That's exactly the antenna I was referencing with the thought of possibly doing 6db Morads on big poles. The plan would be to put the bottom of the antenna around 15 - 15.5 ft off the water which would get it well above any future obstructions on the boat including radar (design target is no fixed objects above 14'6") and would put the tip around 20 ft off the water give or take a little. I'm using the 159mhz tuned version of that antenna for the newly installed AIS antenna and it seems to be a great performer so far.

 

[Bob Cofer]

The pole extensions are high quality. I use the RG8X cable from Morad as it is quite supple and easy to manage. It made for a very clean and quite efficient installation. Nearly every commercial and military vessel around here uses them. The also have a 10db gain version:

• VHF 156 10db Part # 9117 (Marine VHF Radio)

 

[psneeld]

That's exactly the antenna I was referencing with the thought of possibly doing 6db Morads on big poles. The plan would be to put the bottom of the antenna around 15 - 15.5 ft off the water which would get it well above any future obstructions on the boat including radar (design target is no fixed objects above 14'6") and would put the tip around 20 ft off the water give or take a little. I'm using the 159mhz tuned version of that antenna for the newly installed AIS antenna and it seems to be a great performer so far.


The concerns around the overly flat radiation pattern of the big 14+ foot 8+ db antennas is a minor one in my mind. The bigger concern is that it would be challenging to get one up high enough that it won't have any nearby metal around it (like the bimini frame and windshields). That's where I'm wondering if I'm better off with 6db gain but mounted up high and completely free of obstructions.

Have thousands of hours on small, narrow boats (like older Shamrock 26' keel drives, natural rocking chairs) with no ill effects from the "flatter" radiation pattern...like I said...my experience is in the field...not theory.

The small metal amount from biminis/railings are no big deal, though... on the assistance toing boats I ran, there was a large aluminum mast withing a few feet of the antennas, 9db on one side, 6 db on the other.

Plus why I suggested one and one.

 

[rslifkin]

I know within a few feet isn't a big concern, but anything within 1/4 wave (about 20 inches for marine VHF) is usually a significant issue for antennas. In my case, some of the metal framing is within 3 - 4 inches of the antennas currently. The only way to avoid that is to move the antennas up by several feet (at least 4).

Generally my concerns with the big antennas come down to whether they suffer too much performance degradation with a worst case design scenario of pitching or rolling +/- 15* or so. It sounds like that may not be much of an issue. But I also worry about a sturdy enough mounting setup for one of those big, heavy antennas on an extension mast where I'd potentially have the top of the antenna 30 feet off the water (and would then be looking at potentially something like 20 feet above the highest available point to support it). I'm not sure most of those antennas are strong enough to handle that without a support higher up.

On the other hand, if I go with something like the Morad 6db antennas on poles, I'd only be looking at something like 10 feet above the supports, and the top 2.5 feet of that is just the stainless whip, so much less weight and structural concern for the overall assembly.

As an interesting note on range, while sitting in our slip about 3/4 of a mile up a river and with some land obstructions around, the new AIS setup was tracking a freighter on the lake out to 60nm (at which point it hadn't lost the signal but I had other stuff to do and stopped watching) with only occasional signal drops once they got to 35+ nm away. That ship seems to have an unusually good AIS antenna setup though as most others have needed to be much closer for me to receive them. Knowing that their antenna height can't be more than ~116 ft (due to St. Lawrence Seaway clearance limits) and my AIS antenna is only 15 feet off the water (to the tip) I'm finding that very impressive.

 

[psneeld]

I had my 15 footers supported within 3 feet of base to standoff.... base was at the bottom of the flybridge fairing, standoff at the top... never a problem in 20,000+ miles. Were right up against the bimini frame and railing around the fly bridge. But I get your concerns.

Guess I just got used to those big antennas and was very thankful for their performance/reliable comms when offshore and needed to "phone home" to dispatch.

I used to pick up VHF transmissions from the USCG in Southern NJ from Maine, Cape Hatteras and Baltimore. Not all the time but under good atmospherics. Not enough experience with AIS to notice the ranges it did/didn't work at with different antennas.

 

[Bud]

A 9db gain antenna will perform better than a 6db gain antenna in calm sea conditions. When the vessel is subject to rolling from sea conditions the 9db will become choppy, the higher up the antenna is on the vessel the more extreme the roll hence the choppier the communication.

 

[rslifkin]

I had my 15 footers supported within 3 feet of base to standoff.... base was at the bottom of the flybridge fairing, standoff at the top... never a problem in 20,000+ miles. Were right up against the bimini frame and railing around the fly bridge. But I get your concerns.

Guess I just got used to those big antennas and was very thankful for their performance/reliable comms when offshore and needed to "phone home" to dispatch.

I used to pick up VHF transmissions from the USCG in Southern NJ from Maine, Cape Hatteras and Baltimore. Not all the time but under good atmospherics. Not enough experience with AIS to notice the ranges it did/didn't work at with different antennas.

That's good info on the mounting and gives me some food for thought.

In general, I'm not looking for "what's the best setup at any cost?" but more of trying to find the point where I'm unlikely to ever wish I had done more, and where getting a small further improvement in performance would be significantly more expensive or significantly more effort to install.

 

[psneeld]

Just responding to most "effective and practical" in the OP.

I think one of each db is most effective for those with 2 radios but not necessarily practical and certainly more cost...but if it makes that one bit of difference on a MAYDAY call.......

I still hear the "not effective or poor" in rough conditions as an issue...well I gotta say as an assistance tower, I was out in pretty crappy conditions quite a bit and still swear by the 9db antennas. I never seemed to have issues due to sea conditions.

I also am saying have one of each and use the one that IS working the best and many times I could get through on the 9 db because of distance or other chuckleheads chatting on the radio I could walk over when needed for work/emergencies.

 

[rslifkin]

If I do end up going for the 9db option, it would be 2 of them. Regardless of which route I take for the 2 main antennas, at some point I'll add a 3rd VHF with a lower mounted 6db on the davits opposite the AIS antenna (to give me a non-handheld VHF that's still usable in places where I have to lower the big antennas).

 

[psneeld]

I put outrigger laydown hooks on the rear of my flybridge and could lower both of my tall boys in probably less than 30 seconds. Even with the 9db laid down, I could often use it to talk to the bridges I was approaching (so much for the rough water worry as I would be in a 90 degree knockdown at that angle).

Not all bridges, but it was maybe 1 out of 10 and their antenna may just have been at a bad angle to mine...the same was true of the tall boy on my assistance tow boat.... that one got exercised by a LOT of bridges while laying down.

Not sure why you want 2 - 9 dbs...that does get expensive....

To look balanced, I just put a 6 db on an extension (really was just an old 9 db bottom mast someone was throwing away after breaking the top part). It kept the 2 within inches of each other for height.

I am not trying to convince anyone of exactly what to do, just tossing out decades of high radio use experience in variable conditions and possibilities of choices for their boats.

 

[rslifkin]

Not sure why you want 2 - 9 dbs...that does get expensive....

I just find having the 2 primary VHFs being functionally interchangeable to be convenient, so it only makes sense in my mind to give them identical antenna setups.

 

[psneeld]

I just find having the 2 primary VHFs being functionally interchangeable to be convenient, so it only makes sense in my mind to give them identical antenna setups.

Might consider a separate battery back up for one.

Did that on my sportfish sorta by accident, some electronics would drop when starting the engines, the battery solved that and with an isolator was a reserve power source for the electronics.

 

[rslifkin]

Might consider a separate battery back up for one.

Did that on my sportfish sorta by accident, some electronics would drop when starting the engines, the battery solved that and with an isolator was a reserve power source for the electronics.

In my case the electronics all feed from the house bank, so nothing drops out during engine start. There are 2 separate power feeds (on separate breakers) for helm electronics, so currently I have 1 VHF on each feed to reduce the chance of a fault interrupting both at the same time. Plus I keep a handheld on board (mostly for use in the dinghy). All power comes from the same main house bus in the end though. In a pinch the electronics feeds and such could be jumpered over to one of the engine start batteries if I lost house power for some reason and couldn't recover it.

 

[psneeld]

 

[rslifkin]

On the topic of the big 8, 9, and 10db antennas, what are the preferences for any that are known to perform better or be more durable? Shakespeare makes a 14' 8db, a 15'2" 9db, and a 17'6" 9db. Digital and Morad make 10db antennas at 16' and 18'5" respectively (and they cost more than the Shakespeares). The cheapest option for more than 6dbi gain appears to be a GAM TG-5-SH at 11.5 feet long and 7.1 dbi gain (plus a lot of the antenna length is a stainless whip, so less weight way up high to stress the mounts as the boat rolls around).